| 1. |
- Altimira, Mireia, et al.
(författare)
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Teaching Research Methodologies
- 2016
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Ingår i: INTED2016 Proceedings. - : IATED. - 9788460856177 ; , s. 3019-3027
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Konferensbidrag (refereegranskat)abstract
- This paper presents the methodology employed in the Research Methodology course, part of the Master Program in Engineering Mechanics of the Royal Institute of Technology (KTH). As a higher education institution, KTH aims at excellence in both generation and dissemination of knowledge. Even though these two activities are traditionally seen as independent –or even mutually exclusive-, there is a positive correlation between performance in research andin pedagogical activities, reinforced by the fact that inquiry-based or researchbased learning activities enhance deep learning among the students. The course Research Methodology in Engineering Mechanics poses a unique opportunity to engage the students to research in different areas through diversity-oriented learning activities.The course’s main learning outcome is that the students become acquainted with the most common concepts and research methodologies used in the fields of Fundamental Mechanics, Solid Mechanics, Fluid Mechanics, Acoustics and Biomechanics. After the completion of the course, the student should also be able to identify and analyze the methodologies in a given published work.The course consists of 9 lectures and a group project, with an estimation of the total dedication time of 80h (3hp). One lecture is focused on research ethics;while in the other 8 researchers from KTH present their areas of expertise, introducing the most relevant methodologies applied. The attendance is about40 students.In the last two years, and based on the course assessment survey, the structure of the course has been continuously shifting from traditional lectures to cover a broader range of teaching activities. In this way, different learning styles are covered and the learning outcomes can be achieved by as many students as possible. In this regard, traditional lectures are combined with problem-based or case-based lectures, and role-play. Additionally, in the content of the lectures we also try to keep a balance between experimental and numerical research methods of the different disciplines, in such a way that the students get a holistic view of the research in that particular field.This project involves reading a research journal article in the subject of engineering mechanics and presenting, in written and oral form, a critical analysis of the methodologies employed. In the beginning of their project, the students give an oral presentation of their article to another group. At the end of this activity, each group is asked to mention a positive aspect of the presentation they just heard and something that they believe should be improved. As an additional task, each group is asked to peer-review another group’s report. To do so, the students are given a document with some guidelines and evaluation criteria. Special instructions are given to make sure positive feedback is also included in the review. With this, the students get to read the work of others, learn to apply quality criteria and give feedback, and self-reflect on their own work after the review process.
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| 2. |
- Ananthaseshan, S., et al.
(författare)
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Red blood cell distribution width is associated with increased interactions of blood cells with vascular wall
- 2022
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Ingår i: Scientific Reports. - : Springer Nature. - 2045-2322. ; 12:1
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Tidskriftsartikel (refereegranskat)abstract
- The mechanism underlying the association between elevated red cell distribution width (RDW) and poor prognosis in variety of diseases is unknown although many researchers consider RDW a marker of inflammation. We hypothesized that RDW directly affects intravascular hemodynamics, interactions between circulating cells and vessel wall, inducing local changes predisposing to atherothrombosis. We applied different human and animal models to verify our hypothesis. Carotid plaques harvested from patients with high RDW had increased expression of genes and proteins associated with accelerated atherosclerosis as compared to subjects with low RDW. In microfluidic channels samples of blood from high RDW subjects showed flow pattern facilitating direct interaction with vessel wall. Flow pattern was also dependent on RDW value in mouse carotid arteries analyzed with Magnetic Resonance Imaging. In different mouse models of elevated RDW accelerated development of atherosclerotic lesions in aortas was observed. Therefore, comprehensive biological, fluid physics and optics studies showed that variation of red blood cells size measured by RDW results in increased interactions between vascular wall and circulating morphotic elements which contribute to vascular pathology.
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| 3. |
- Belliato, M., et al.
(författare)
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An experimental model of veno-venous arterial extracorporeal membrane oxygenation
- 2019
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Ingår i: International Journal of Artificial Organs. - : SAGE Publications Ltd. - 0391-3988 .- 1724-6040.
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Tidskriftsartikel (refereegranskat)abstract
- Introduction: Veno-venous arterial extracorporeal membrane oxygenation is a hybrid-modality of extracorporeal membrane oxygenation combining veno-venous and veno-arterial extracorporeal membrane oxygenation. It may be applied to patients with both respiratory and cardio-circulatory failure. Aim: To describe a computational spreadsheet regarding an ex vivo experimental model of veno-venous arterial extracorporeal membrane oxygenation to determine the return of cannula pairs in a single pump–driven circuit. Methods: We developed an ex vivo model of veno-venous arterial extracorporeal membrane oxygenation with a single pump and two outflow cannulas, and a glucose solution was used to mimic the features of blood. We maintained a fixed aortic impedance and physiological pulmonary resistance. Both flow and pressure data were collected while testing different pairs of outflow cannulas. Six simulations of different cannula pairs were performed, and data were analysed by a custom-made spreadsheet, which was able to predict the flow partition at different flow levels. Results: In all simulations, the flow in the arterial cannula gradually increased differently depending on the cannula pair. The best cannula pair was a 19-Fr/18-cm arterial with a 17-Fr/50-cm venous cannula, where we observed an equal flow split and acceptable flow into the arterial cannula at a lower flow rate of 4 L/min. Conclusion: Our computational spreadsheet identifies the suitable cannula pairing set for correctly splitting the outlet blood flow into the arterial and venous return cannulas in a veno-venous arterial extracorporeal membrane oxygenation configuration without the use of external throttles. Several limitations were reported regarding fixed aortic impedance, central venous pressure and the types of cannulas tested; therefore, further studies are mandatory to confirm our findings.
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| 4. |
- Berg, Niclas, 1988-
(författare)
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Blood flow and cell transport in arteries and medical assist devices
- 2018
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The cardiovascular system is responsible for transport of nutrients, oxygen, as well as the cells and molecules making up the immune system. Through the hemostatic system, the body maintains the integrity of the blood vessels, and prevents bleeding. The biochemical and physical processes governing the circulation interact, and take place at a large range of time and length scales - from those related to the individual cells up to the large scale flow structures. Dysfunctions of the heart or the circulatory system may have severe consequences. Cardiovascular diseases (CVD) is a heterogeneous group of diseases, responsible for about 50% of all death cases in the western world.Patients with severe but transient heart and/or lung disease may require the assistance of a heart-lung machine to bridge over the period required for the affected organ to recover. One such system is the Extracorporeal Membrane Oxygenator (ECMO) circuit, consting of a blood pump, a membrane oxygenator, cannulae and tubing system. While the therapy is life-saving, it is associated with relatively frequent thromboembolic (blood clotting and/or bleeding) events. Modeling of the flow in some components of the ECMO circuit was undertaken. The flow data was used together with models for platelet activation to assess the risk for thrombus formation. The results indicated locations of elevated risk of thrombosis in the centrifugal blood pump, the ECMO cannulae and the pipe connectors. The identified locations agreed well with clinical observations. The results lead to a direct recommendation to minimize the use of tube connectors. Further study of the sensitivity of the platelet activation models to uncertainties and errors was carried out. Some recommendations for improved modeling were proposed.Arteriosclerosis develops slowly over a long period of time (years or decades). It manifests initially at some common sites; arteries of certain sizes with relatively strong flow rate, as well as near artery bifurcations and locations of strong vessel curvature. The location specificity indicates that the blood flow plays a central role in the arteriosclerotic process. Being able to predict the future development of arteriosclerotic lesion and its location for an individual patient would imply that pre-emptive actions could be taken. This idea was the foundation of some of the numerical simulations in this thesis. A stenoted patient specific renal artery was considered, and was reconstructed to a non-pathological state by removing the stenosis using different segmentation methods. We could then evaluate if common stenosis markers based on functions of time-averages of the Wall Shear-Stress (WSS) could be use as predictive parameters. It was shown that these markers are not adequate as predictive tools. Furthermore, it was shown that the sensitivity to reconstruction technique was at least of the same order as the effect of the choice of blood rheology model. The rheology of blood was further studied through detailed simulations resolving the blood plasma flow and its interaction with the red blood cells (RBC) and the platelets. A hybrid Immersed boundary-Lattice Boltzmann method was applied, and the rheological data was compared to the Quemada model. It was found that the Quemada model could underpredict the effective viscosity by as much as 50%. The same methodology was applied to study the transport of RBCs and platelets, and the influence of RBC polydispersity. An increased degree of variability in RBC volume was found, under certain circumstances, to lead to an increase of the transport of platelets to the vessel wall (margination).
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| 5. |
- Berg, Niclas, 1988-, et al.
(författare)
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Blood flow simulations of the renal arteries - effect of segmentation and stenosis removal
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Ingår i: Flow Turbulence and Combustion. - 1386-6184 .- 1573-1987.
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Tidskriftsartikel (refereegranskat)abstract
- Patient specic based simulation of blood flows in arteries has been proposed as a future approach for better diagnostics and treatment of arterial diseases.The outcome of theoretical simulations strongly depends on the accuracy in describing the problem (the geometry, material properties of the artery and of the blood, flow conditions and the boundary conditions). In this study, the uncertainties associated with the approach for a priori assessment of reconstructive surgery of stenoted arteries are investigated. It is shown that strong curvature in the reconstructed artery leads to large spatial- and temporal-peaks in the wall shear-stress. Such peaks can be removed by appropriate reconstruction that also handles the post-stenotic dilatation of the artery. Moreover, it is shown that the effects of the segmentation approach can be equally important as the effects of using advanced rheological models. Unfortunately, this fact has not been recognized in the literature up to this point, making patient specic simulations potentially less reliable.
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| 6. |
- Berg, Niclas, 1988-, et al.
(författare)
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Blood Flow Simulations of the Renal Arteries - Effect of Segmentation and Stenosis Removal
- 2019
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Ingår i: Flow Turbulence and Combustion. - : SPRINGER. - 1386-6184 .- 1573-1987. ; 102:1, s. 27-41
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Tidskriftsartikel (refereegranskat)abstract
- Patient specific based simulation of blood flows in arteries has been proposed as a future approach for better diagnostics and treatment of arterial diseases. The outcome of theoretical simulations strongly depends on the accuracy in describing the problem (the geometry, material properties of the artery and of the blood, flow conditions and the boundary conditions). In this study, the uncertainties associated with the approach for a priori assessment of reconstructive surgery of stenoted arteries are investigated. It is shown that strong curvature in the reconstructed artery leads to large spatial- and temporal-peaks in the wall shear-stress. Such peaks can be removed by appropriate reconstruction that also handles the post-stenotic dilatation of the artery. Moreover, it is shown that the effects of the segmentation approach can be equally important as the effects of using advanced rheological models. This fact has not been recognized in the literature up to this point, making patient specific simulations potentially less reliable.
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| 7. |
- Berg, Niclas, 1988-, et al.
(författare)
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Flow characteristics and coherent structures in a centrifugal blood pump
- 2019
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Ingår i: Flow Turbulence and Combustion. - : Springer Nature. - 1386-6184 .- 1573-1987. ; 102:2, s. 469-483
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Tidskriftsartikel (refereegranskat)abstract
- Blood clot formation can be initiated by local flow conditions where regions of high shear and long residence time regions, such as flow separation and stagnation, have been identified as risk factors. This study highlights coherent structures,some of which not yet considered in the literature that may contribute to blood clot formation in the ECMO (Extra Corporeal Membrane Oxygenator) circuit. The centrifugal ECMO pump investigated in this study is compact and delivers adequate volume of blood with relatively high pressure in order to compensate for the large pressure drop in the membrane oxygenator. These requirements lead to regions with high shear in several different parts of the pump. In the narrow gap between the pump house and the impeller body (the magnet) a Taylor-Couette-like flow is observed with azimuthally aligned wavy vortices, which are also pushed towards the bottom of the pump-house by the flow generated by the blades. At the bottom gap between the impeller house and the pump house one finds spiraling flow structures, due to the rotation of the former structure. Separation bubbles are found near the tongue of the pump and at the lee sides of the blades. Such vortical structures have in literature been identified as regions where platelets may be activated whereby clots may develop.
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| 8. |
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| 9. |
- Broman, Lars Mikael, et al.
(författare)
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Pressure and flow properties of cannulae for extracorporeal membrane oxygenation I : return (arterial) cannulae
- 2019
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Ingår i: Perfusion. - : SAGE PUBLICATIONS LTD. - 0267-6591 .- 1477-111X. ; 34, s. 58-64
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Tidskriftsartikel (refereegranskat)abstract
- Adequate extracorporeal membrane oxygenation support in the adult requires cannulae permitting blood flows up to 6-8 L/minute. In accordance with Poiseuille's law, flow is proportional to the fourth power of cannula inner diameter and inversely proportional to its length. Poiseuille's law can be applied to obtain the pressure drop of an incompressible, Newtonian fluid (such as water) flowing in a cylindrical tube. However, as blood is a pseudoplastic non-Newtonian fluid, the validity of Poiseuille's law is questionable for prediction of cannula properties in clinical practice. Pressure-flow charts with non-Newtonian fluids, such as blood, are typically not provided by the manufacturers. A standardized laboratory test of return (arterial) cannulae for extracorporeal membrane oxygenation was performed. The aim was to determine pressure-flow data with human whole blood in addition to manufacturers' water tests to facilitate an appropriate choice of cannula for the desired flow range. In total, 14 cannulae from three manufacturers were tested. Data concerning design, characteristics, and performance were graphically presented for each tested cannula. Measured blood flows were in most cases 3-21% lower than those provided by manufacturers. This was most pronounced in the narrow cannulae (15-17 Fr) where the reduction ranged from 27% to 40% at low flows and 5-15% in the upper flow range. These differences were less apparent with increasing cannula diameter. There was a marked disparity between manufacturers. Based on the measured results, testing of cannulae including whole blood flows in a standardized bench test would be recommended.
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| 10. |
- Broman, Lars Mikael, et al.
(författare)
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Pressure and flow properties of cannulae for extracorporeal membrane oxygenation II : drainage (venous) cannulae
- 2019
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Ingår i: Perfusion. - : SAGE PUBLICATIONS LTD. - 0267-6591 .- 1477-111X. ; 34, s. 65-73
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Tidskriftsartikel (refereegranskat)abstract
- The use of extracorporeal life support devices such as extracorporeal membrane oxygenation in adults requires cannulation of the patient's vessels with comparatively large diameter cannulae to allow circulation of large volumes of blood (>5 L/min). The cannula diameter and length are the major determinants for extracorporeal membrane oxygenation flow. Manufacturing companies present pressure-flow charts for the cannulae; however, these tests are performed with water. Aims of this study were 1. to investigate the specified pressure-flow charts obtained when using human blood as the circulating medium and 2. to support extracorporeal membrane oxygenation providers with pressure-flow data for correct choice of the cannula to reach an optimal flow with optimal hydrodynamic performance. Eighteen extracorporeal membrane oxygenation drainage cannulae, donated by the manufacturers (n = 6), were studied in a centrifugal pump driven mock loop. Pressure-flow properties and cannula features were described. The results showed that when blood with a hematocrit of 27% was used, the drainage pressure was consistently higher for a given flow (range 10%-350%) than when water was used (data from each respective manufacturer's product information). It is concluded that the information provided by manufacturers in line with regulatory guidelines does not correspond to clinical performance and therefore may not provide the best guidance for clinicians.
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| 11. |
- Broman, L. M., et al.
(författare)
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Pressure and flow properties of dual-lumen cannulae for extracorporeal membrane oxygenation
- 2020
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Ingår i: Perfusion. - : SAGE Publications Ltd. - 0267-6591 .- 1477-111X.
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Tidskriftsartikel (refereegranskat)abstract
- Introduction: In the last decade, dual-lumen cannulae have been increasingly applied in patients undergoing extracorporeal life support. Well-performing vascular access is crucial for efficient extracorporeal membrane oxygenation support; thus, guidance for proper cannulae size is required. Pressure–flow charts provided by manufacturers are often based on tests performed using water, rarely blood. However, blood is a shear-thinning and viscoelastic fluid characterized by different flow properties than water. Methods: We performed a study evaluating pressure–flow curves during standardized conditions using human whole blood in two commonly available dual-lumen cannulae used in neonates, pediatric, and adult patients. Results were merged and compared with the manufacturer’s corresponding curves obtained from the public domain. Results: The results showed that using blood as compared with water predominantly influenced drainage flow. A 10-80% higher pressure-drop was needed to obtain same drainage flow (hematocrit of 26%) compared with manufacturer’s water charts in 13-31 Fr bi-caval dual-lumen cannulae. The same net difference was found in cavo-atrial cannulae (16-32 Fr), where a lower drainage pressure was required (Hct of 26%) compared with the manufacturer’s test using blood with an Hct of 33%. Return pressure–flow data were similar, independent whether pumping blood or water, to the data reported by manufacturers. Conclusion: Non-standardized testing of pressure–flow properties of extracorporeal membrane oxygenation dual-lumen cannulae prevents an adequate prediction of pressure–flow results when these cannulae are used in patients. Properties of dual-lumen cannulae may vary between sizes within same cannula family, in particular concerning the drainage flow.
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| 12. |
- Drevhammar, T., et al.
(författare)
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Flows and function of the Infant Flow neonatal continuous positive airway pressure device investigated with computational fluid dynamics
- 2020
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Ingår i: Acta Paediatrica. - : Wiley. - 0803-5253 .- 1651-2227.
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Tidskriftsartikel (refereegranskat)abstract
- Aim: The first dedicated neonatal continuous positive airway pressure (CPAP) device using variable flow was the Infant Flow. The system was pressure stable with a low resistance to breathing. The aim of this study was to describe the flow and function of the Infant Flow geometry using simulated breathing and computational fluid dynamics. Method: The original Infant Flow geometry was used with simulated term infant breathing at three levels of CPAP. The large eddy simulation methodology was applied in combination with the WALE sub-grid scale model. Results: The simulation fully resolved the flow phenomena in the Infant Flow geometry. The main flow feature during inspiration was support by gas entrainment and mixing. During expiration, the jet deflected towards the outlet with unstable impingement of the jet at the opposing edge. Conclusion: The proposed mechanism has previously been based on theoretical reasoning, and our results present the first detailed description of the Infant Flow. The pressure stability was based on a jet supporting inspiration by gas entrainment and then being deflected during exhalation. This confirmed previously assumed principles of function and flows within the geometry and provided a base for further developments.
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| 13. |
- Dsouza, Shaima Magdaline, et al.
(författare)
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Multi-Fidelity Gaussian Process Surrogate Modeling for Flow Through Stenosis
- 2023
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Ingår i: UNCECOMP 2023. - : National Technical University of Athens.
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Konferensbidrag (refereegranskat)abstract
- The blood flow characteristics found in our larger vessels are unsteady, particularly around the heart valves and bifurcations. In the case of stenosis, or narrowing of the vessels, the flow may transition to turbulence. To understand the dynamics of the forces acting on the blood components and the vessel wall, simulations using computational fluid dynamics (CFD) are commonly applied. The severity of the stenosis can be determined by accurately assessing the fluid flow, which can also serve as a risk indicator for potential thromboembolic events. Motivated by the vessel’s geometry being a factor that highly influences the flow characteristics, we investigate here the impact of changes in geometry on turbulence using multi-fidelity models, which are based on Gaussian processes. The objective is to develop a multi-fidelity model to construct a high-fidelity estimate by combining numerical simulations from spectral element-based direct numerical simulations (DNS) and finite volume-based Reynolds-Averaged Navier-Stokes (RANS) simulations. Specifically, a co-kriging-based model with Gaussian process is used to combine various levels of fidelity (RANS, DNS). To vary the blood vessel geometry, the stenosis’s severity and eccentricity are considered uncertain input parameters. A multi-fidelity model is then used to predict the consequences of said uncertainties on the mean pressure drop across the vessel and the wall shear stress, the quantities of interest directly linked to the biological activity of the vessel. Using data of different accuracy, the multi-fidelity technique allows us to optimize the accuracy and cost of predictions.
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| 14. |
- Fiusco, Francesco, 1994-, et al.
(författare)
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Blood pumps for extracorporeal membrane oxygenation : Platelet activation during different operating conditions
- 2021
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Ingår i: ASAIO journal (1992). - : Ovid Technologies (Wolters Kluwer Health). - 1058-2916 .- 1538-943X. ; Publish Ahead of Print
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Tidskriftsartikel (refereegranskat)abstract
- Extracorporeal membrane oxygenation (ECMO) is a therapy used in severe cardiopulmonary failure. Blood is pumped through an artificial circuit exposing it to nonphysiologic conditions, which promote platelet activation and coagulation. Centrifugal pumps used at lower flow rates than their design point may lose pump efficiency and increase the risk of hemolysis. In this study, thrombogenic properties of two ECMO pumps designed for adult and neonatal use were evaluated using simulations in different flow scenarios. Three scenarios, adult pump in adult mode (4 L/min), adult pump in baby mode (300 ml/min), and neonatal pump used in its design point (300 ml/min), were simulated using computational fluid dynamics. The flow was numerically seeded with platelets, whose activation state was computed considering the stress history that acted along their respective path lines. Statistical distributions of activation state and residence time were drawn. The results showed that using the adult pump in baby mode increased the fraction of platelets with higher activation state confirming that low-pump flow rate impacts thrombogenicity. The neonatal pump showed a backflow at the inlet, which carried platelets in a retrograde motion contributing to an increased thrombogenic potential compared with the adult mode scenario.
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| 15. |
- Fiusco, Francesco, 1994-
(författare)
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Computational modelling of blood flow in medical assist devices
- 2023
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Extracorporeal membrane oxygenation (ECMO) is a life-saving support treat-ment in case of pulmonary and/or cardiac failure. An artificial extracorporealcircuit is used to offload the function of lungs and/or heart. Patient blood is drained through a drainage cannula, pumped with a centrifugal pump, oxygenated in a membrane lung and returned to the body through a reinfusion cannula. Tubing and connectors complete the circuit. However, its use canlead to thromboembolic and haemolytic complications, which are related to mechanical stresses arising in the flow of blood through its components. Numerical simulations of some of the pumps and cannulae used in the circuit were performed to investigate the flow structures developing in these components and their relation to measures of blood damage in the form of platelet activation state (PAS) and haemolysis index (HI). Simulations of two magnetically levitated centrifugal ECMO pumps were performed both in on- and off-label conditions with flow rates compatible with adult and neonatal use. The results showed that off-label low flow rate can be damaging due to an increase of residence time of the particles, which exposed them for longer to non-physiological stress. This held true for both passive tracers and inertial particles subjected to lift and drag. The neonatal pump showed a backflow structure with flow swirling back to the inlet tubing over its whole labelled range. Simulations of a lighthouse drainage cannula were undertaken to assess drainage characteristics at different haematocrits and flow rate ratios. The results indicated that the flow field was dominated by a jet in crossflow type of structure, with the most proximal holes draining the largest amount of fluid in all the studied cases and for all the considered haematocrits. The effects due to non-Newtonian behaviour of blood were less relevant in the drainage area, allowing to use a Reynolds number analogy to bridge between water and blood results.A lighthouse cannula in return configuration was also considered in both a centred and a tilted position. A characteristic confined jet configuration was found, with a backflow developing at the vessel wall, increasing residence time. In the tilted case, a group of small vortical structures developed at the holes close to the wall, which behaved as an obstacle to the vessel flow and increased both residence time and stress. This led to locally increased haemolysis which, however, did not impact haemolysis at large due to the low flow exposed to this area. The use of different viscosity models in this case led to small variations in the results, which were minor compared to the uncertainty introduced by the use of different model coefficients in the computation of the haemolysis index.
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| 16. |
- Fiusco, Francesco, 1994-, et al.
(författare)
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Effect of low rate ratio and positioning on a lighthouse tip ECMO return cannula
- 2023
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Ingår i: Biomechanics and Modeling in Mechanobiology. - 1617-7959 .- 1617-7940.
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- Extracorporeal membrane oxygenation is a life-saving support therapy in the case of cardiopulmonary refractory failure. Its use is associated to complications due to the presence of artificial surfaces and supraphysiological stress conditions. Thus, knowledge of the fluid structures associated to each component can give insight into sources of blood damage. In this study, an experimentally validated numerical study of a conventional lighthouse tip cannula in return configuration was carried out to characterize the flow structures using water or a Newtonian blood analog with different flow rate ratios and cannula positioning and their influence on hemolysis. The results showed that strong shear layers developed where the jets from the side holes met the co-flow. Stationary backflow regions at the vessel wall were also present downstream of the cannula. In the tilted case, the recirculation was much more pronounced on the wide side and almost absent on the narrow side. Small vortical backflow structures developed at the side holes which behaved like obstacles to the co-flow, creating pairs of counter-rotating vortices, which induced locally higher risk of hemolysis. However, global hemolysis index did not show significant deviations. Across the examined flow rate ratios, the holes on the narrow side consistently reinfused a larger fraction of fluid. A radial force developed in the tilted case in a direction so as to recenter the cannula in the vessel.
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| 17. |
- Fiusco, Francesco, 1994-
(författare)
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Hemodynamics of artificial devices used in extracorporeal life support
- 2021
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Extracorporeal Membrane Oxygenation (ECMO) is a life-saving therapy usedfor support in critical heart and/or lung failure. Patient’s blood is pumped viaan artificial lung for oxygenation outside of the body. The circuit is composedof a blood pump, cannulae for drainage and reinfusion, a membrane lung,tubing and connectors. Its use is associated with thromboembolic complicationsand hemolytic damage. Detailed numerical studies of two blood pumps anda lighthouse tip drainage cannula were undertaken to characterize the flowstructures in different scenarios and their link to platelet activation. The pumpsimulations were modelled according to manufacturer’s proclaimed use but alsoin off-design conditions with flow rates used in adult and neonatal patients.Lagrangian Particle Tracking (LPT) was used to simulate the injection ofparticles similar in size to platelets to compute platelet activation state (PAS).The results indicated that low flow rates impacted PAS similarly to high flowrates due to increased residence time leading to prolonged exposure to shearstress despite the fact that shear per se was lower at low flow rate. Regardingthe cannula, the results showed that a flow pattern similar to a jet in crossflowdeveloped at the side holes. A parameter study was conducted to quantifydrainage characteristics in terms of flow rate distribution across the holes wheninput variables of flow rate, modelled fluid, and hematocrit were altered. Thefindings showed, across all the cases, that the most proximal hole row drainedthe largest fraction of fluid. The effects due to the non-Newtonian nature ofblood were confined to regions far from the cannula holes and the flow structuresshowed very limited dependence on the hematocrit. A scaling law was found tobridge the global drainage performance of fluid between water and blood.
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| 18. |
- Fiusco, Francesco, 1994-, et al.
(författare)
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Numerical and experimental investigation of a lighthouse tip drainage cannula used in extracorporeal membrane oxygenation
- 2023
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Ingår i: Artificial Organs. - : Wiley. - 0160-564X .- 1525-1594. ; 47:2, s. 330-341
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Tidskriftsartikel (refereegranskat)abstract
- Extracorporeal membrane oxygenation is a life saving therapy used in case of acute respiratory/circulatory failure. Exposure of blood to non-physiological surfaces and high shear stresses is related to hemolytic damage and platelet activation. An investigation of the flow structures developing in a conventional single-staged drainage cannula was performed with cross-validated computational fluid dynamics and particle image velocimetry. The aim was to quantify the variation in drainage performance and stress levels induced by different fluid models, hematocrit and vessel-to-cannula flow rate ratios. The results indicated that the 90◦ bends of the flow through the side holes created a recirculation zone potentially increasing the residence time and flow structures developing inside the cannula resembling a jet in a crossflow. The use of different hematocrits did not induce a considerable effect on the drainage performance, with the most proximal set of holes from the tip draining the largest fraction of fluid. However, different flow rate ratios altered the flow rate drained through the tip. The use of 2D data led to a 50% underestimation of shear rate levels, and a Reynolds-number scaling was applied to capture the velocity profiles and flow rates through the side holes.
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| 19. |
- Fiusco, Francesco, 1994-, et al.
(författare)
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Sensitivity of hemolysis modelling in a tiltedlighthouse tip cannula
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Extracorporeal membrane oxygenation is a life-saving treatment used insevere cardiac/lung failure. The occurrence of non-physiological stresses and thepresence of artificial surfaces gives rise to complications as thrombus formationand hemolysis. While CFD can be a powerful tool to assess risks associatedto mechanical stresses, the use of models to compute blood flow and predictblood damage entails uncertainties on the results that need to be quantified. Inthis work, the geometry of a ECMO return cannula in a shifted position wasused as a benchmark to evaluate the effects of model coefficients for hemolysiscomputation and the effect of different types of viscosity modelling at differenthematocrits. The results showed that the largest uncertainty was induced bythe chosen model coefficients, with variability of up to two orders of magnitude.Using a Newtonian analog led to similar global hemolysis indices compared toa non-Newtonian model, if the viscosity value was based on the asymptoticviscosity of the non-Newtonian model. Considering the local values of hemolysis,differences were observed in stagnation areas, with variations of more than 50%.The inclusion of a simple red blood cell transport model did not significantlyaffect time-averaged results, but it introduced larger time variability.
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| 20. |
- Fuchs, Alexander, 1985-
(författare)
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Assessment of predicting blood flow and atherosclerosis in the aorta and renal arteries
- 2020
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Cardiovascular diseases (CVD) are the most common cause of death in large parts of the world. Atherosclerosis (AS) has a major part in most CVDs. AS is a slowly developingdisease which is dependent on multiple factors such as genetics and life style (food, smoking, and physical activities). AS is primarily a disease of the arterial wall and develops preferentially at certain locations (such as arterial branches and in certain vessels like thecoronary arteries). The close relation between AS sites and blood flow has been well established over the years. However, due to multi-factorial causes, there exist no early prognostic tools for identifying individuals that should be treated prophylactically or followed up. The underlying hypothesis of this thesis was to determine if it is possible to use bloodflow simulations of patient-specific cases in order to identify individuals with risk for developing AS. CT scans from patients with renal artery stenosis (RAS) were used to get the affected vessels geometry. Blood flow in original and “reconstructed” arteries were simulated. Commonly used wall shear stress (WSS) related indicators of AS were studied to assess their use as risk indicators for developing AS. Divergent results indicated urgent need to assess the impact ofsimulation related factors on results. Altogether, blood flow in the following vessels was studied: The whole aorta with branches from the aortic arch and the abdominal aorta, abdominal aorta as well as the renal arteries, and separately the thoracic aorta with the three main branching arteries from the aortic arch. The impact of geometrical reconstruction, employed boundary conditions (BCs), effects of flow-rate, heart-rate and models of blood viscosity as function of local hematocrit (red blood cell, RBC, concentration) and shear-rate were studied in some detail. In addition to common WSS-related indicators, we suggested the use of endothelial activation models as a further risk indicator. The simulations data was used to extract not only the WSS-related data but also the impact of flow-rate on the extent of retrograde flow in the aorta and close to its walls. The formation of helical motion and flow instabilities (which at high flow- and heart-rate lead to turbulence) was also considered.Results:A large number of simulations (more than 100) were carried out. These simulations assessed the use of flow-rate specified BCs, pressure based BCs or so called windkessel (WK) outlet BCs that simulate effects of peripheral arterial compliance. The results showed high sensitivity of the flow to BCs. For example, the deceleration phase of the flow-rate is more prone to flow instabilities (as also expressed in terms of multiple inflection points in the streamwise velocity profile) as well as leading to retrograde flow. In contrast, the acceleration phase leads to uni-directional and more stable flow. As WSS unsteadiness was found to be pro-AS, it was important to assess the effect flow-rate deceleration, under physiological and pathological conditions. Peaks of retrograde flow occur at local temporal minima in flow-rate. WK BCs require ad-hoc adjusted parameters and are therefore useful only when fully patient specific (i.e. all information is valid for a particular patient at a particular point of time) data is available. Helical flows which are considered as atheroprotective, are formed naturally, depending primarily on the geometry (due to the bends in the thoracic aorta). Helical flow was also observed in the major aortic branches. The helical motion is weaker during flow deceleration and diastole when it may locally also change direction. Most common existing blood viscosity models are based on hematocrit and shear-rate. These models show strong variation of blood (mixture) viscosity. With strong shear-rate blood viscosity is lowest and is almost constant. The impact of blood viscosity in terms of dissipation is counter balanced by the shear-rate; At low shear-rate the blood has larger viscosity and at high shear-rate it is the opposite. This effect and due to the temporal variations in the local flow conditions the effect of blood rheology on the WSS indicators is weak. Tracking of blood components and clot-models shows that the retrograde motion and the flow near branches may have so strong curvature that centrifugal force can become important. This effect may lead to the transport of a thrombus from the descending aorta back to the branches of the aortic arch and could cause embolic stroke. The latter results confirm clinical observation of the risk of stroke due to transport of emboli from the proximal part of the descending aorta upstream to the vessels branching from the aortic arch and which lead blood to the brain.Conclusions:The main reasons for not being able to propose an early predictive tool for future developmentof AS are four-folded:i. At present, the mechanisms behind AS are not adequately understood to enable to define aset of parameters that are sensitive and specific enough to be predictive of its development.ii. The lack of accurate patient-specific data (BC:s) over the whole physiological “envelop”allows only limited number of flow simulations which may not be adequate for patientspecificpredictive purposes.iii. The shortcomings of current models with respect to material properties of blood andarterial walls (for patient-specific space- and time-variations) are lacking.iv. There is a need for better simulation data processing, i.e. tools that enable deducinggeneral predictive atherosclerotic parameters from a limited number of simulations, throughe.g. extending reduced modeling and/or deep learning.The results do show, however, that blood flow simulations may produce very useful data thatenhances understanding of clinically observed processes such as explaining helical- andretrograde flows and the transport of blood components and emboli in larger arteries.
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| 21. |
- Fuchs, Alexander, 1985-, et al.
(författare)
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Assessment of Rheological Models Applied to Blood Flow in Human Thoracic Aorta
- 2023
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Ingår i: Bioengineering. - : MDPI AG. - 2306-5354. ; 10:11
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Tidskriftsartikel (refereegranskat)abstract
- Purpose: The purpose of this study is to assess the importance of non-Newtonian rheological models on blood flow in the human thoracic aorta. Methods: The pulsatile flow in the aorta is simulated using the models of Casson, Quemada and Walburn-Schneck in addition to a case of fixed (Newtonian) viscosity. The impact of the four rheological models (using constant hematocrit) was assessed with respect to (i) magnitude and deviation of the viscosity relative to a reference value (the Newtonian case); (ii) wall shear stress (WSS) and its time derivative; (iii) common WSS-related indicators, OSI, TAWSS and RRT; (iv) relative volume and surface-based retrograde flow; and (v) the impact of rheological models on the transport of small particles in the thoracic aorta. Results: The time-dependent flow in the thoracic aorta implies relatively large variations in the instantaneous WSS, due to variations in the instantaneous viscosity by as much as an order of magnitude. The largest effect was observed for low shear rates (tens s-1). The different viscosity models had a small impact in terms of time- and spaced-averaged quantities. The significance of the rheological models was clearly demonstrated in the instantaneous WSS, for the space-averaged WSS (about 10%) and the corresponding temporal derivative of WSS (up to 20%). The longer-term accumulated effect of the rheological model was observed for the transport of spherical particles of 2 mm and 2 mm in diameter (density of 1200 kg/m3). Large particles' total residence time in the brachiocephalic artery was 60% longer compared to the smaller particles. For the left common carotid artery, the opposite was observed: the smaller particles resided considerably longer than their larger counterparts. Conclusions: The dependence on the non-Newtonian properties of blood is mostly important at low shear regions (near walls, stagnation regions). Time- and space-averaging parameters of interest reduce the impact of the rheological model and may thereby lead to under-estimation of viscous effects. The rheological model affects the local WSS and its temporal derivative. In addition, the transport of small particles includes the accumulated effect of the blood rheological model as the several forces (e.g., drag, added mass and lift) acting on the particles are viscosity dependent. Mass transport is an essential factor for the development of pathologies in the arterial wall, implying that rheological models are important for assessing such risks.
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| 22. |
- Fuchs, Alexander, 1985-, et al.
(författare)
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Blood rheology modeling effects in aortic flow simulations
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Purpose: The purpose of the study is to assess the importance of non-Newtonian rheological models onblood flow in the human thoracic aorta.Methods: The pulsatile flow in the aorta is simulated using the models of Casson, Quemada and Walburn-Schneck in addition to a case of fixed (Newtonian) viscosity. The impact of the four rheological models wasassessed with respect to the following quantities:i. Magnitude of the viscosity relative to a reference value (the Newtonian case) and the relative mean deviation from that value.ii. Mechanical kinetic energy,vorticity, viscous dissipation rate.iii. WSS and its time derivative.iv. WSS-related indicators; OSI, TAWSS and RRT.Results: The flow in the thoracic aorta is characterized by shear-rates leading to an increase in viscosity by afactor of up to six. The different models had negligible impact on the kinetic energy and viscous dissipationrate. The effect on WSS related parameters was quantified and was found to be modest. Largest effect wasobserved for low shear-rates (below 100 s-2).Conclusions: The choice of a non-Newtonian model is important whenever the flow is viscosity dominated.Blood flow in larger arteries is weakly dependent on viscosity and can be handled by a model with weakdependence on shear-rate (e.g. Quemada or Newtonian). Blood flows with regions with low shear-rate andstrong temporal variation requires rheological models that better account for low shear and explicitlyincludes temporal variation effects.
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| 23. |
- Fuchs, Alexander, 1985-, et al.
(författare)
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Fluid mechanical aspects of blood flow in the thoracic aorta
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Arterial blood flow contains structures known to be associated with arterial wall pathologies (such as atherosclerosis and aneurysms) but also with helical motion reported to be atheroprotective. Numerical simulation of the flow in a typical human thoracic aorta model was carried out for several heart- and flow-rates. The aim was to explore the presence and the underlying mechanism of the formation of helical flow, retrograde motion and the formation of smaller scale unsteady flow structures. The main findings of the paper are as follows:- Retrograde flow is induced during flow deceleration. Reversed flow may persist throughout the cardiac cycle in parts of the descending aorta. Retrograde flow may lead enhanced risk of upstream transport of thrombi from the descending aorta to the branches of the aortic arch.- Helical flows are induced by bend and torsion of the aorta and through non-uniformity in the spatial distribution of the inlet flow (aortic valve plane).- Amplification of axial vorticity was shown to occur in the thoracic aorta. This convective instability is enhanced in the descending aorta.- Transitional and turbulent flow may occur in the thoracic aorta under elevated flow- and heart-rate conditions also in healthy individuals.- Under normal conditions, healthy individuals do not develop turbulent flow in the thoracic aorta.A hypothesis for a possible mechanism for the atheroprotective effect of helical flow is suggested.
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| 24. |
- Fuchs, Alexander, 1985-, et al.
(författare)
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On the modelling of cell and lipoprotein transport in the thoracic aorta
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Purpose: The purpose of the study is to compare and assess modeling of transport cells and lipoproteins by the blood in the human thoracic aorta.Methods: In the continuum framework, three flux models were considered; Fickian, Zydney-Colton (Z-C) and Leighton-Acrivos (L-A). The transport of spherical particles (cells and lipoprotein of different sizes and densities) under pulsatile flow condition were simulated. The effect of local red blood cell (RBC) concentration (hematocrit) on blood viscosity wasconsidered through Quemada’s model. Lagrangian particle transport (LPT) was assessed and compared to the continuum models. Contribution to RBC flux (diffusion) due to gradients inhematocrit, mixture density and viscosity was assessed. Results were extracted in terms ofmean and variations in concentrations, residence time and path lengths of RBC and six othercells and lipoproteins.Results: The effects of local hematocrit variations on the local blood viscosity is large (a factor of more than 2) but the effect on wall shear stress (WSS) indicators is much more modest (few percent). In terms of mean concentration, the three continuum transport modelsyield local viscosity that deviate by a factor between about 1.3 to 2, as compared to a constantviscosity case. The main contribution to the mass (RBC) flux in the L-A model is from the shear-rate gradient term, followed by the viscosity gradient term and least by the RBC concentration gradient term (low flow rate). The inflow and wall boundary conditions play an important role on the details of the mass transport. The LPT result do converge to the expected concentration at the different outflow boundaries. However, the convergence rate isslow and require more than 30 cardiac periods to get below 2% in outflow hematocrit.Detailed analysis of the RBC paths shows large variations. For the outlet from the thoracicaorta RBC path length and residence time ranging from 0.333 m to 0.0.791 m and from lessthan one to about four cardiac cycles, respectively. The corresponding values for the LCCA are about 0.2 m to more than 0.5 m and about a quarter to about four cardiac cycles,respectively. The LPT results also show that particles are subject to a lift force driven bystrong path curvature and particle to fluid density difference. A simulation with injection ofparticles in the descending aorta indicated the possibility of upstream transport of particlesinto the three main arteries branching from the aortic arch.Conclusions: Continuum transport models depend strongly on calibrated model parametersand the imposed boundary conditions. Counter gradient diffusion may occur as the fluxes aredependent on gradients of shear rate, concentration, and viscosity. LPT has the advantage ofaccounting for temporal effect and are most appropriate for dilute particle suspensions such ascells (except for RBC) and lipoproteins. LPT though, may require substantially longercomputational time when statistical data is sought.
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| 25. |
- Fuchs, Alexander, 1985-, et al.
(författare)
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Pulsatile aortic blood flow – A critical assessment of boundary conditions
- 2020
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Ingår i: ASME Journal of Engineering and Science in Medical Diagnostics and Therapy (JESMDT). - : ASME International. - 2572-7958 .- 2572-7966.
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Tidskriftsartikel (refereegranskat)abstract
- Patient specific (PS) blood flow studies have become popular in recent years but have thus far had limited clinical impact. This is possibly due to uncertainties and errors in the underlying models and simulations set-up. This study focuses on the sensitivity of simulation results due to in- and outflow boundary conditions (BC:s). Nine different inlet- and seven different outlet BC:s were applied to two variants of a healthy subject’s thoracic aorta. Temporal development of the flow is essential for the formation and development of helical/spiralling flow where the commonly observed clockwise helical motion may change direction during the heart-cycle. The sensitivity to temporal and spatial variations in the inlet conditions is significant both when expressed in terms of mean and maximal wall shear stress (WSS) and its different averaged variables, e.g. Time-Averaged WSS (TAWSS), Oscillating Shear Index (OSI) and Relative Residence Time (RRT). The simulation results are highly sensitive to BC. For example, the maximal WSS may vary over 3 orders of magnitude (1 to 1000 Pa) depending on particular combinations of BC:s. Moreover, certain formulations of outlet boundary conditions may be inconsistent with the computed flow field if the underlying assumptions of the space-time dependence are violated. The results of this study show that CFD simulations can reveal flow details that can enhance understanding of blood flows. However, the results also demonstrate the potential difficulties in mimicking blood flow in clinical situations.
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| 26. |
- Fuchs, Alexander, et al.
(författare)
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Stenosis Indicators Applied to Patient-Specific Renal Arteries without and with Stenosis
- 2019
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Ingår i: Fluids. - : MDPI. - 2311-5521. ; 4:1
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Tidskriftsartikel (refereegranskat)abstract
- Pulsatile flow in the abdominal aorta and the renal arteries of three patients was studied numerically. Two of the patients had renal artery stenosis. The aim of the study was to assess the use of four types of indicators for determining the risk of new stenosis after revascularization of the affected arteries. The four indicators considered include the time averaged wall shear stress (TAWSS), the oscillatory shear index (OSI), the relative reference time (RRT) and a power law model based in platelet activation modeling but applied to the endothelium, named endothelium activation indicator (EAI). The results show that the indicators can detect the existing stenosis but are less successful in the revascularized cases. The TAWSS and, more clearly, the EAI approach seem to be better in predicting the risk for stenosis relapse at the original location and close to the post-stenotic dilatation. The shortcomings of the respective indicators are discussed along with potential improvements to endothelial activation modeling and its use as an indicator for risks of restenosis.
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| 27. |
- Fuchs, Alexander, 1985-, et al.
(författare)
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The impact of heart rate and cardiac output on the flow inthe human thoracic aorta
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Purpose: The purpose of the study is to determine the effects of heart rate (HR) and cardiac output(CO), in the temporal variation of CO on flow structures and related biomechanical markers.Methods: The pulsatile flow in the thoracic aorta was simulated for 15 combinations of HR (60-150 beats per minutes, BPM), CO and cardiac temporal profiles. In all cases, the Quemada viscositymodel was used. The results were analyzed in terms of biomechanical markers such as extent ofretrograde flow in the lumen and close to the wall, helicity parameters, commonly used wall shearstress (WSS) indicators along with proposed Endothelial Activation Indices (EAIs).Results: The simulations demonstrated the presence of helical motion in all cases. The helicalmotion depends on the spatial distribution of the flow by the aortic valve. Time- and space-averagedhelicity indices were found to have smallest values in the aortic arch and largest in the descendingpart of the aorta. For all cases, retrograde flow was observed. The extent of separated flow close tothe aortic wall depended strongly on the rate of decelerating CO during late systole as well aspossible axial flow deceleration periods during diastole. At high HR and CO, small scale flowstructures developed, indicating transition to turbulence. Time averaged WSS-related indicatorswere less distinctive in assessing the spatial and temporal impact as compared to the EAI indicators(EAI_Nobili and EAI_Soares) accounting for both accumulated stress and the temporal behavior of thestress.Conclusions: The results underpin the importance of temporal variation of the cardiac flowrate andthe impact of the deceleration phase of systole on retrograde flow and formation of helical flowstructures. As retrograde and helical flow has been found to be related to atherosclerosis, thetemporal contribution of the flowrate must be maintained, since time averaged biomechanicalindicators filter out information of potential diagnostic importance. Temporal flow behavior, up tocell response frequency, needs to be reflected by the biomechanical indicators as in the proposed EAI_Soares indicator.
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| 28. |
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| 29. |
- Fuchs, Gabriel, et al.
(författare)
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Detection of Thrombosis in the Extracorporeal Membrane Oxygenation Circuit by Infrasound : Proof of Concept
- 2017
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Ingår i: Artificial Organs. - : John Wiley & Sons. - 0160-564X .- 1525-1594. ; 41:6, s. 573-579
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Tidskriftsartikel (refereegranskat)abstract
- As of today, there exist no reliable, objective methods for early detection of thrombi in the extracorporeal membrane oxygenators (ECMO) system. Within the ECMO system, thrombi are not always fixed to a certain component or location in the circuit. Thus, clot fragments of different shapes and consistencies may circulate and give rise to vibrations and sound generation. By bedside sound measurements and additional laboratory experiments (although not detailed herein), we found that the presence of particles (clots or aggregates and fragments of clots) can be detected by analyzing the strength of infra-sound (< 20 Hz) modes of the spectrum near the inlet and outlet of the centrifugal pump in the ECMO circuit. For the few patients that were considered in this study, no clear false positive or negative examples were found when comparing the spectral approach with clinical observations. A laboratory setup provided insight to the flow in and out of the pump, confirming that in the presence of particles a low-amplitude low-frequency signal is strongly amplified, enabling the identification of a clot.
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| 30. |
- Fuchs, Gabriel, et al.
(författare)
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Flow-induced platelet activation in components of the extracorporeal membrane oxygenation circuit
- 2018
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Ingår i: Scientific Reports. - : NATURE PUBLISHING GROUP. - 2045-2322. ; 8
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Tidskriftsartikel (refereegranskat)abstract
- Extracorporeal membrane oxygenation (ECMO) is used for rescue in severe respiratory and/or circulatory failure. The patient's blood is pumped over artificial surfaces in the ECMO circuit. A platelet activation model was applied to study the potential thrombogenicity of ECMO circuit components: the centrifugal blood pump, cannulae, and tubing connectors. Based on the accumulated effect of the scalar form of the stress acting on the platelet over time, the activation model enables assessment of platelet activation and pinpoints regions of elevated activation risk in a component. Numerical simulations of the flow in different components of the ECMO circuit was carried out where the activation level is a function of the impact of local stress and its history along the path that the platelets follow. The results showed that the pump carried the largest risk for platelet activation followed by the reinfusion cannula and lastly the connectors. Pump thrombogenicity was mainly due to long residence time and high shear-rate while the connector showed a high level of non-stationary shear-rate that in turn may contribute to the formation of aggregates through direct platelet activation or through high shear-rate modulation of the vWF multimers.
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| 31. |
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| 32. |
- Fuchs, Gabriel, et al.
(författare)
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Modeling sensitivity and uncertainties in platelet activation models applied on centrifugal pumps for extracorporeal life support
- 2019
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Ingår i: Scientific Reports. - : Springer Nature. - 2045-2322. ; 9
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Tidskriftsartikel (refereegranskat)abstract
- Two platelet activation models were studied with respect to uncertainties of model parameters and variables. The sensitivity was assessed using two direct/deterministic approaches as well as the statistical Monte Carlo method. The first two, are linear in character whereas the latter is non-linear. The platelet activation models were applied on platelets moving within an extracorporeal centrifugal blood pump. The phenomenological, Lagrangian stress- and time-based power law-based models under consideration, have experimentally calibrated parameters and the stress expressed in a scalar form. The sensitivity of the model with respect to model parameters and the expression of the scalar stress was examined focusing on a smaller group of platelets associated with an elevated risk of activation. The results showed a high disparity between the models in terms of platelet activation state, found to depend on the platelets’ trajectory in the pump and the expression used for the scalar stress. Monte Carlo statistics was applied to the platelets at risk for activation and not to the entire platelet population. The method reveals the non-linear sensitivity of the activation models. The results imply that power-law based models have a restricted range of validity. The conclusions of this study apply to both platelet activation and hemolysis models.
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| 33. |
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| 34. |
- Hsu, C. -P, et al.
(författare)
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Roughness-dependent clogging of particle suspensions flowing into a constriction
- 2021
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Ingår i: Soft Matter. - : Royal Society of Chemistry (RSC). - 1744-683X .- 1744-6848. ; 17:31, s. 7252-7259
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Tidskriftsartikel (refereegranskat)abstract
- When concentrated particle suspensions flow into a constricting channel, the suspended particles may either smoothly flow through the constriction or jam and clog the channel. These clogging events are typically detrimental to technological processes, such as in the printing of dense pastes or in filtration, but can also be exploited in micro-separation applications. Many studies have to date focused on important parameters influencing the occurrence of clogs, such as flow velocity, particle concentration, and channel geometry. However, the investigation of the role played by the particle surface properties has surprisingly received little attention so far. Here, we study the effect of surface roughness on the clogging of suspensions of silica particles under pressure-driven flows along a microchannel presenting a constriction. We synthesize micron-sized particles with uniform surface chemistry and tunable roughness and determine the occurrence of clogging events as a function of velocity and volume fraction for a given surface topography. Our results show that there is a clear correlation between surface roughness and flow rate, indicating that rougher particles are more likely to jam at the constriction for slower flows. These findings identify surface roughness as an essential parameter to consider in the formulation of particulate suspensions for applications where clogging plays an important role.
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| 35. |
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| 36. |
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| 37. |
- Håkansson, Karl, et al.
(författare)
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Hydrodynamic alignment and assembly of nanofibrils resulting in strong cellulose filaments
- 2014
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Ingår i: Nature Communications. - : Springer Science and Business Media LLC. - 2041-1723. ; 5, s. 4018-
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Tidskriftsartikel (refereegranskat)abstract
- Cellulose nanofibrils can be obtained from trees and have considerable potential as a building block for biobased materials. In order to achieve good properties of these materials, the nanostructure must be controlled. Here we present a process combining hydrodynamic alignment with a dispersion-gel transition that produces homogeneous and smooth filaments from a low-concentration dispersion of cellulose nanofibrils in water. The preferential fibril orientation along the filament direction can be controlled by the process parameters. The specific ultimate strength is considerably higher than previously reported filaments made of cellulose nanofibrils. The strength is even in line with the strongest cellulose pulp fibres extracted from wood with the same degree of fibril alignment. Successful nanoscale alignment before gelation demands a proper separation of the timescales involved. Somewhat surprisingly, the device must not be too small if this is to be achieved.
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| 38. |
- Håkansson, Karl M. O., et al.
(författare)
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Measurement of width and intensity of particle streaks in turbulent flows
- 2013
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Ingår i: Experiments in Fluids. - : Springer Science and Business Media LLC. - 0723-4864 .- 1432-1114. ; 54:6, s. 1555-
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Tidskriftsartikel (refereegranskat)abstract
- Fibre streaks are observed in experiments with fibre suspensions in a turbulent half-channel flow. The preferential concentration methods, most commonly used to quantify preferential particle concentration, are in one dimension found to be concentration dependent. Two different new streak quantification methods are evaluated, one based on Voronoi analysis and the other based on artificial particles with an assigned fixed width. The width of the particle streaks and a measure of the intensity of the streaks, i.e. streakiness, are sought. Both methods are based on the auto-correlation of a signal, generated by summing images in the direction of the streaks. Common for both methods is a severe concentration dependency, verified in experiments keeping the flow conditions constant while the (very dilute) concentration of fibres is altered. The fixed width method is shown to be the most suitable method, being more robust and less computationally expensive. By assuming the concentration dependence to be related to random noise, an expression is derived, which is shown to make the streak width and the streakiness independent of the concentration even at as low concentrations as 0.05 particles per pixel column in an image. The streakiness is obtained by applying an artificial particle width equal to 20 % of the streak width. This artificial particle width is in this study found to be large enough to smoothen the correlation without altering the streakiness nor the streak width. It is concluded that in order to make quantitative comparisons between different experiments or simulations, the evaluation has to be performed with care and be very well documented.
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| 39. |
- Håkansson, Karl M. O., et al.
(författare)
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Nanofibril Alignment in Flow Focusing : Measurements and Calculations
- 2016
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Ingår i: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1520-6106 .- 1520-5207. ; 120:27, s. 6674-6686
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Tidskriftsartikel (refereegranskat)abstract
- Alignment of anisotropic supermolecular building blocks is crucial to control the properties of many novel materials. In this study, the alignment process of cellulose nanofibrils (CNFs) in a flow-focusing channel has been investigated using small-angle X-ray scattering (SAXS) and modeled using the Smoluchowski equation, which requires a known flow field as input. This flow field was investigated experimentally using microparticle-tracking velocimetry and by numerically applying the two-fluid level set method. A semidilute dispersion of CNFs was modeled as a continuous phase, with a higher viscosity as compared to that of water. Furthermore, implementation of the Smoluchowski equation also needed the rotational Brownian diffusion coefficient, which was experimentally determined in a shear viscosity measurement. The order of the nanofibrils was found to increase during extension in the flow-focusing channel, after which rotational diffusion acted on the orientation distribution, driving the orientation of the fibrils toward isotropy. The main features of the alignment and dealignment processes were well predicted by the numerical model, but the model overpredicted the alignment at higher rates of extension. The apparent rotational diffusion coefficient was seen to increase steeply as the degree of alignment increased. Thus, the combination of SAXS measurements and modeling provides the necessary framework for quantified studies of hydrodynamic alignment, followed by relaxation toward isotropy.
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| 40. |
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| 41. |
- Håkansson, Karl, 1984-
(författare)
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Orientation of elongated, macro and nano-sized particles in macroscopic flows
- 2014
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Non-spherical particles are present all around us, in biological, industrial and environmental processes. Making predictions of their impact on us and systems in our vicinity can make life better for everyone here on earth. For example, the ash particles from a volcano eruption are non-spherical and their spreading in the atmosphere can hugely impact the air traffic, as was also proven in 2010. Furthermore, the orientation of the wood fibres in a paper sheet influences the final properties of the paper, and the cause of a specific fibre orientation can be traced back to the fluid flows during the manufacturing process of the paper.In this thesis, experimental and numerical work is presented with the goal to understand and utilize the behavior of elongated particles in fluid flows. Two different experimental setups are used. The first one, a turbulent half channel flow, aims at increasing the understanding of how particles with non-zero inertia behave in turbulence. The second setup is an attempt to design a flow field with the purpose to align nanofibrils and create high performance cellulose filaments.Experiments were performed in a turbulent half channel flow at different flow set- tings with dilute suspensions of cellulose acetate fibres having three different aspect ratios (length to width ratio). The two main results were firstly that the fibres agglom- erated in streamwise streaks, believed to be due to the turbulent velocity structures in the flow. Secondly, the orientation of the fibres was observed to be determined by the aspect ratio and the mean shear, not the turbulence. Short fibres were oriented in the spanwise direction while long fibres were oriented in the streamwise direction.In order to utilize the impressive properties (stiffness comparable to Kevlar) of the cellulose nanofibril in a macroscopic material, the alignment of the fibrils must be controlled. Here, a flow focusing device (resulting in an extensional flow), designed to align the fibrils, is used to create a cellulose filament with aligned fibrils. The principle is based on a separation of the alignment and the assembly of the fibrils, i.e. first align the fibrils and then lock the aligned structure. With this process, continuous filaments were created, with properties similar to that of the wood fibre at the same fibril alignment. However, the highest alignment (lowest angle) of the fibrils in a filament created was only 31o from the filament axis, and the next step is to increase the alignment. This thesis includes modeling of the alignment process with the Smoluchowski equation and a rotary diffusion. Finding a model that correctly describes the alignment process should in the end make it possible to create a filament with fully aligned fibrils.
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| 42. |
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| 43. |
- Imani Jajarmi, Ramin
(författare)
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Acoustic separation and electrostatic sampling of submicron particles suspended in air
- 2016
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- We investigate experimentally the effects of acoustic forces on submicron aerosol in a channel flow. This technique can potentially overcome some of the limitations of conventional separation systems and provide advanced manipulation capabilities such as sorting according to size or density. The theoretical framework for acoustophoresis at such small length scales where molecular effects are expected to be significant is still incomplete and in need of experimental validation. The main objectives of this thesis are to identify the physical limitations and capabilities of acoustophoretic manipulation for submicron aerosol particles.Two sets of experiments were carried out: first, qualitative results revealed that acoustic manipulation is possible for submicron particles in air and that the acoustic force follows the trend expected by theoretical models developed for particles in inviscid fluids. The acoustic force on submicron particles was estimated in a second set of measurements performed with quantitative diagnostic tools. Comparison of these results with available theoretical models for the acoustic radiation forces demonstrates that for such small particles additional forces have to be considered. At submicron length scales, the magnitude of the forces observed is orders of magnitude higher than the predictions from the inviscid theory.One potential application for acoustophoresis is specifically investigated in this thesis: assist electrostatic precipitation (ESP) samplers to target very small aerosols, such as those carrying airborne viruses. To identify the shortcomings of ESP samplers that acoustophoresis should overcome, two ESP designs have been investigated to quantify capture efficiency as a function of the particle size and of the air velocity in a wind tunnel. The results reveal that both designs have limitations when it comes to sampling submicron aerosol particles. When exposed to polydispersed suspensions they behave as low-pass filters.
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| 44. |
- Jadoon, Asim, et al.
(författare)
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Dynamic interaction of fixed dual spheres for several configurations and inflow conditions
- 2010
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Ingår i: European journal of mechanics. B, Fluids. - : Elsevier BV. - 0997-7546 .- 1873-7390. ; 29:1, s. 43-52
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Tidskriftsartikel (refereegranskat)abstract
- The changes in force characteristics as well as the shedding patterns for various dual sphere configurations are studied. The Reynolds numbers considered are 300, 600 and two different inflow conditions are used: steady and pulsating. The sphere formations are defined by the separation distance D-0 between the spheres and the angle between the line connecting the centres of the spheres and the main flow direction, gamma. The position of one of the spheres is varied in the range 0 degrees-90 degrees using a 15 degrees increment. Two separation distances are studied; 1.5D and 3D. The method used for the simulations is the Volume of Solid (VOS) approach, a method based on Volume of Fluid (VOF). A major conclusion from this work is that the sphere interaction alters the wake dynamics by obstructing the vortex shedding (generating a steady wake or a wake with lower Strouhal number) and by changing the direction of the lift force so that it in most cases is directed in the plane containing the sphere centres. The results also show that changing the inflow condition gives the same relative change in drag and lift as for a single sphere. The drag is substantially reduced by placing the sphere downstream in a tandem arrangement and slightly increased in a side-by-side arrangement. However, the effect is decreased by increasing separation distance and increasing Reynolds number.
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| 45. |
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| 46. |
- Kékesi, Timea, et al.
(författare)
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Drop deformation and breakup
- 2014
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Ingår i: International Journal of Multiphase Flow. - : Elsevier. - 0301-9322 .- 1879-3533. ; 66, s. 1-10
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Tidskriftsartikel (refereegranskat)abstract
- A Volume of Fluid (VOF) method is applied to investigate the deformation and breakup of an initially spherical drop in the bag- and shear breakup regimes, induced by steady disturbances. The onset of breakup is sought by studying steady-shape deformations while increasing the Weber number until breakup occurs. A parameter study is carried out applying different material properties and a wide range of drop Reynolds numbers in the steady wake regime. Density ratios of liquid to gas of 20, 40, and 80, viscosity ratios in the range 0.5-50, and Reynolds numbers between 20 and 200 are investigated for a constant Weber number of 20. The critical Weber number is found to be 12, in agreement with observations of earlier studies. For Weber number of 20 varying density, viscosity ratios and Reynolds numbers, interesting mixed breakup modes are discovered. Moreover, a new regime map including all modes observed is presented. A criterion for the transition between bag-and shear breakup is defined relating the competing inertial and shear forces appearing in the flow. Furthermore, results on breakup times and the time history of the drag coefficient are presented; the latter is concluded to be a potential parameter to indicate the occurrence of breakup. (C) 2014 Elsevier Ltd. All rights reserved.
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| 47. |
- Kekesi, Timea, et al.
(författare)
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Drop deformation and breakup in flows with shear
- 2016
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Ingår i: Chemical Engineering Science. - : Elsevier. - 0009-2509 .- 1873-4405. ; 140, s. 319-329
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Tidskriftsartikel (refereegranskat)abstract
- A Volume of Fluid (VOF) method is applied to study the deformation and breakup of a single liquid drop in shear flows superimposed on uniform flow. The effect of shearing on the breakup mechanism is investigated as a function of the shear rate. Sequential images are compared for the parameter range studied; density ratios of liquid to gas of 20, 40, and 80, viscosity ratios in the range 0.5-50, Reynolds numbers between 20, a constant Weber number of 20, and the non-dimensional shear rate of the flow G = 0-2.1875. It is found that while shear breakup remains similar for all values of shear rate considered, other breakup modes observed for uniform flows are remarkably modified with increasing shear rate. The time required for breakup is significantly decreased in strong shear flows. A simple model predicting the breakup time as a function of the shear rate and the breakup time observed in uniform flows is suggested.
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| 48. |
- Kékesi, Timea, et al.
(författare)
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Interaction between two deforming liquid drops in tandem and various off-axis arrangements subject to uniform flow
- 2019
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Ingår i: International Journal of Multiphase Flow. - : Elsevier Ltd. - 0301-9322 .- 1879-3533. ; , s. 193-218
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Tidskriftsartikel (refereegranskat)abstract
- A Volume of Fluid (VOF) method is applied to study the interaction between two liquid drops with the same initial diameter in uniform flow. Various arrangements of the drops are studied, based on two parameters, namely the initial separation distance and the angle between the line connecting the centres of the drops and the free-stream direction. Initial separation distances of 1.5–5 drop diameters, and angles between β=0 ∘ and 90° are considered. Simulations for a Weber number of We=20, two Reynolds numbers Re=20 and 50, and density and viscosity ratios in the range ρ * =20–80 and μ * =0.5–50 are performed. The movement of the secondary drop with respect to the primary drop, and estimates on the time required for the breakup of the secondary drop as compared to those observed for single drops are evaluated. It is found that the drops collide only in cases corresponding to the shortest initial displacements, while in others they deform and break up independently, similarly or identically to single drops. The same behaviour is reflected in the time required for breakup. Cases where the drops behave independently show breakup times close to those observed for single drops.
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| 49. |
- Kékesi, Tímea, 1986-, et al.
(författare)
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Interaction between two deforming liquid drops in tandem and various off-axis arrangements subject to uniform flow
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- A Volume of Fluid (VOF) method is applied to study the interaction between two liquid drops with the same initial diameter in uniform flow. Various arrangements of the drops are studied, based on two parameters, namely the initial separation distance and the angle between the line connecting the centres of the drops and the free-stream direction. Specifically, initial separation distances of l = 1.5 − 5D drop diamters, and angles between β = 0◦ − 90◦ are considered. Simulations for a Weber number of W e = 20, two Reynolds numbers Re = 20 and 50, and density and viscosity ratios in the range ρ∗ = 20 − 80 and μ∗ = 0.5 − 50 are performed. The movement of the secondary drop with respect to the primary drop, and the time required for the breakup of the secondary drop as compared to those observed for single drops are evaluated. It is found that the drops collide only in cases corresponding to the shortest initial displacements, while in others they deform and break up independently, similar or identical to single drops. The same behaviour is reflected in the time required for breakup. Cases where the drops behave independently show breakup times close to those observed for single drops.
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| 50. |
- Kékesi, Tímea, 1986-, et al.
(författare)
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Interaction between two deforming liquid drops in tandem subject to uniform flow
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- A Volume of Fluid (VOF) method is applied to study the interaction between two liquid drops in uniform flow, where the drops have the same initial diameter and are placed in tandem, for initial separation distances of l = 1.5 − 20D (drop diameters). Simulations have been performed for a We- ber number of W e = 20, Reynolds numbers in the range Re = 20 − 200, and density and viscosity ratios of ρ∗ = 20 − 80 and μ∗ = 0.5 − 50. The movement of the back drop with respect to the front drop is evaluated and presented in the form of a regime map within the adopted parameter range. Besides the initial separation distance most commonly addressed in literature in the topic, additional parameters such as the Reynolds number and density and viscosity ratios determine whether the drops collide or not. Furthermore, sequential im- ages along with the analyses of the wake around the interacting drops suggest that the time required for breakup is also crucial regarding the outcome of the interaction. Data on the length of the wake region behind a deforming drop is provided, and together with the breakup time of single drops is used to identify, and define criteria for the regimes of the various observed interaction scenarios.
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