| 1. |
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| 2. |
- Andersson, Magnus, 1983-
(författare)
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Turbulence Descriptors in Arterial Flows : Patient-Specific Computational Hemodynamics
- 2021
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- At this very moment, there are literally millions of people who suffer from various types of cardiovascular diseases (CVDs), many of whom will experience reduced quality of life or premature lift expectancy. The detailed underlying pathogenic processes behind many of these disorders are not well understood, but were abnormal dynamics of the blood flow (hemodynamics) are believed to play an important role, especially atypical flow-mediated frictional forces on the intraluminal wall (i.e. the wall shear stress, WSS). Under normal physiological conditions, the flow is relatively stable and regular (smooth and laminar), which helps to maintain critical vascular functions. When these flows encounter various unfavorable anatomical obstructions, the flow can become highly unstable and irregular (turbulent), giving rise to abnormal fluctuating hemodynamic forces, which increase the bloodstream pressure losses, can damage the cells within the blood, as well as impair essential structural and functional regulatory mechanisms. Over a prolonged time, these disturbed flow conditions may promote severe pathological responses and are therefore essential to foresee as early as possible.Clinical measurements of blood flow characteristics are often performed non-invasively by modalities such as ultrasound and magnetic resonance imaging (MRI). High-fidelity MRI techniques may be used to attain a general view of the overall large-scale flow features in the heart and larger vessels but cannot be used for estimating small-scale flow variations nor capture the WSS characteristics. Since the era of modern computers, fluid motion can now also be predicted by computational fluid dynamics (CFD)simulations, which can provide discrete mathematical approximations of the flow field with much higher details (resolution) and accuracy compared to other modalities. CFD simulations rely on the same fundamental principles as weather forecasts, the physical laws of fluid motion, and thus can not only be used to assess the current flow state but also to predict (foresee) important outcome scenarios in e.g. intervention planning. To enable blood flow simulations within certain cardiovascular segments, these CFD models are usually reconstructed from MRI-based anatomical and flow image-data. Today, patient-specific computational hemodynamics are essentially only performed within the research field, where much emphasis is dedicated towards understanding normal/abnormal blood flow physiology, developing better individual-based diagnostics/treatments, and evaluating the results reliability/generality in order to approach clinical applicability.In this thesis, advanced CFD methods were adopted to simulate realistic patient-specific turbulent hemodynamics in constricted arteries reconstructed from MRI data. The main focus was to investigate novel, comprehensive ways to characterize these abnormal flow conditions, in the pursuit of better clinical decision-making tools; from more in-depth analyzes of various turbulence-related tensor characteristics to descriptors that evaluate the hemodynamics more globally in the domain. Results from the studies in this thesis suggest that these turbulence descriptors can be useful to: i) target cardiovascular sites prone to specific turbulence characteristics, both in the bulk flow and on the intraluminal wall, ii) provide a more extensive view of the general flow severity within malformed vascular regions, and iii) evaluated and potentially improve cardiovascular modeling strategies and MRI-measured turbulence data.The benefit of these descriptors is that they all, in principle, can be measured by different MRI procedures, making them more accessible from a clinical perspective. Although the significance of these suggested flow-mediated phenotypes has not yet been evaluated clinically, this work opens many doors of opportunities for making more thorough and longitudinal patient-specific studies, including large cohorts of patients with various CVDs susceptible to turbulent-like conditions, as well as performing more in-depth CFD-MRI validation analyzes.
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| 3. |
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| 4. |
- Berntsson, Fredrik, 1971-, et al.
(författare)
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A Modification to the Kirchhoff Conditions at a Bifurcation and Loss Coefficients
- 2018
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- One dimensional models for fluid flow in tubes are frequently used tomodel complex systems, such as the arterial tree where a large numberof vessels are linked together at bifurcations. At the junctions transmission conditions are needed. One popular option is the classic Kirchhoffconditions which means conservation of mass at the bifurcation andprescribes a continuous pressure at the joint.In reality the boundary layer phenomena predicts fast local changesto both velocity and pressure inside the bifurcation. Thus it is not appropriate for a one dimensional model to assume a continuous pressure. In this work we present a modification to the classic Kirchhoff condi-tions, with a symmetric pressure drop matrix, that is more suitable forone dimensional flow models. An asymptotic analysis, that has beencarried out previously shows that the new transmission conditions hasen exponentially small error.The modified transmission conditions take the geometry of the bifurcation into account and can treat two outlets differently. The conditions can also be written in a form that is suitable for implementationin a finite difference solver. Also, by appropriate choice of the pressuredrop matrix we show that the new transmission conditions can producehead loss coefficients similar to experimentally obtained ones.
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| 5. |
- Bolger, Ann F, 1957-, et al.
(författare)
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Transit of blood flow through thehuman left ventricle mapped by cardiovascular magnetic resonance
- 2007
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Ingår i: Journal of Cardiovascular Magnetic Resonance. - : Informa UK Limited. - 1097-6647 .- 1532-429X. ; 9:5, s. 741-747
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Tidskriftsartikel (refereegranskat)abstract
- BACKGROUND:The transit of blood through the beating heart is a basic aspect of cardiovascular physiology which remains incompletely studied. Quantification of the components of multidirectional flow in the normal left ventricle (LV) is lacking, making it difficult to put the changes observed with LV dysfunction and cardiac surgery into context.METHODS:Three dimensional, three directional, time resolved magnetic resonance phase-contrast velocity mapping was performed at 1.5 Tesla in 17 normal subjects, 6 female, aged 44+/-14 years (mean+/-SD). We visualized and measured the relative volumes of LV flow components and the diastolic changes in inflowing kinetic energy (KE). Of total diastolic inflow volume, 44+/-11% followed a direct, albeit curved route to systolic ejection (videos 1 and 2), in contrast to 11% in a subject with mildly dilated cardiomyopathy (DCM), who was included for preliminary comparison (video 3). In normals, 16+/-8% of the KE of inflow was conserved to the end of diastole, compared with 5% in the DCM patient. Blood following the direct route lost or transferred less of its KE during diastole than blood that was retained until the next beat (1.6+/-1.0 millijoules vs 8.2+/-1.9 millijoules, p<0.05); whereas, in the DCM patient, the reduction in KE of retained inflow was 18-fold greater than that of the blood tracing the direct route.CONCLUSION:Multidimensional flow mapping can measure the paths, compartmentalization and kinetic energy changes of blood flowing into the LV, demonstrating differences of KE loss between compartments, and potentially between the flows in normal and dilated left ventricles.
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| 6. |
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| 7. |
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| 8. |
- Carlhäll, Carljohan, 1973-, et al.
(författare)
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Contribution of mitral annular dynamics to LV diastolic filling with alteration in preload and inotropic state
- 2007
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Ingår i: American Journal of Physiology. Heart and Circulatory Physiology. - : American Physiological Society. - 0363-6135 .- 1522-1539. ; 293:3, s. G1473-H1479
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Tidskriftsartikel (refereegranskat)abstract
- Mitral annular (MA) excursion during diastole encompasses a volume that is part of total left ventricular (LV) filling volume (LVFV). Altered excursion or area variation of the MA due to changes in preload or inotropic state could affect LV filling. We hypothesized that changes in LV preload and inotropic state would not alter the contribution of MA dynamics to LVFV. Six sheep underwent marker implantation in the LV wall and around the MA. After 7–10 days, biplane fluoroscopy was used to obtain three-dimensional marker dynamics from sedated, closed-chest animals during control conditions, inotropic augmentation with calcium (Ca), preload reduction with nitroprusside (N), and vena caval occlusion (VCO). The contribution of MA dynamics to total LVFV was assessed using volume estimates based on multiple tetrahedra defined by the three-dimensional marker positions. Neither the absolute nor the relative contribution of MA dynamics to LVFV changed with Ca or N, although MA area decreased (Ca, P < 0.01; and N, P < 0.05) and excursion increased (Ca, P < 0.01). During VCO, the absolute contribution of MA dynamics to LVFV decreased (P < 0.001), based on a reduction in both area (P < 0.001) and excursion (P < 0.01), but the relative contribution to LVFV increased from 18 ± 4 to 45 ± 13% (P < 0.001). Thus MA dynamics contribute substantially to LV diastolic filling. Although MA excursion and mean area change with moderate preload reduction and inotropic augmentation, the contribution of MA dynamics to total LVFV is constant with sizeable magnitude. With marked preload reduction (VCO), the contribution of MA dynamics to LVFV becomes even more important.
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| 9. |
- Carlhäll, Carljohan, 1973-, et al.
(författare)
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Contribution of mitral annular excursion and shape dynamics to total left ventricular volume change
- 2004
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Ingår i: American Journal of Physiology. Heart and Circulatory Physiology. - : American Physiological Society. - 0363-6135 .- 1522-1539. ; 287:4, s. H1836-H1841
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Tidskriftsartikel (refereegranskat)abstract
- The mitral annulus (MA) has a complex shape and motion, and its excursion has been correlated to left ventricular (LV) function. During the cardiac cycle the annulus’ excursion encompasses a volume that is part of the total LV volume change during both filling and emptying. Our objective was to evaluate the contribution of MA excursion and shape variation to total LV volume change. Nine healthy subjects aged 56 ± 11 (means ± SD) years underwent transesophageal echocardiography (TEE). The MA was outlined in all time frames, and a four-dimensional (4-D) Fourier series was fitted to the MA coordinates (3-D+time) and divided into segments. The annular excursion volume (AEV) was calculated based on the temporally integrated product of the segments’ area and their incremental excursion. The 3-D LV volumes were calculated by tracing the endocardial border in six coaxial planes. The AEV (10 ± 2 ml) represented 19 ± 3% of the total LV stroke volume (52 ± 12 ml). The AEV correlated strongly with LV stroke volume (r = 0.73; P < 0.05). Peak MA area occurred during middiastole, and 91 ± 7% of reduction in area from peak to minimum occurred before the onset of LV systole. The excursion of the MA accounts for an important portion of the total LV filling and emptying in humans. These data suggest an atriogenic influence on MA physiology and also a sphincter-like action of the MA that may facilitate ventricular filling and aid competent valve closure. This 4-D TEE method is the first to allow noninvasive measurement of AEV and may be used to investigate the impact of physiological and pathological conditions on this important aspect of LV performance.
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| 10. |
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| 11. |
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| 12. |
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| 13. |
- Ebbers, Tino, 1972-, et al.
(författare)
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Estimation of relative cardiovascular pressures using time-resolved three-dimensional phase contrast MRI
- 2001
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Ingår i: Magnetic Resonance in Medicine. - : Wiley. - 0740-3194 .- 1522-2594. ; 45:5, s. 872-879
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Tidskriftsartikel (refereegranskat)abstract
- Accurate, easy-to-use, noninvasive cardiovascular pressure registration would be an important addition to the diagnostic armamentarium for assessment of cardiac function. A novel noninvasive and three-dimensional (3D) technique for estimation of relative cardiovascular pressures is presented. The relative pressure is calculated using the Navier-Stokes equations along user-defined lines placed within a time-resolved 3D phase contrast MRI dataset. The lines may be either straight or curved to follow an actual streamline. The technique is validated in an in vitro model and tested on in vivo cases of normal and abnormal transmitral pressure differences and intraaortic flow. The method supplements an intuitive visualization technique for cardiovascular flow, 3D particle trace visualization, with a quantifiable diagnostic parameter estimated from the same dataset.
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| 14. |
- Ebbers, Tino, 1972-, et al.
(författare)
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Noninvasive measurement of time-varying three-dimensional relative pressure fields within the human heart
- 2002
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Ingår i: Journal of Biomechanical Engineering. - : ASME International. - 0148-0731 .- 1528-8951. ; 124:3, s. 288-293
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Tidskriftsartikel (refereegranskat)abstract
- Understanding cardiac blood flow patterns is important in the assessment of cardiovascular function. Three-dimensional flow and relative pressure fields within the human left ventricle are demonstrated by combining velocity measurements with computational fluid mechanics methods. The velocity field throughout the left atrium and ventricle of a normal human heart is measured using time-resolved three-dimensional phase-contrast MRL. Subsequently, the time-resolved three-dimensional relative pressure is calculated from this velocity field using the pressure Poisson equation. Noninvasive simultaneous assessment of cardiac pressure and flow phenomena is an important new tool for studying cardiac fluid dynamics.
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| 15. |
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| 16. |
- Ekman, Petter, 1988-, et al.
(författare)
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Accuracy and Speed for Scale-Resolving Simulations of the DrivAer Reference Model
- 2019
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Ingår i: WCX SAE World Congress Experience. - 400 Commonwealth Drive, Warrendale, PA, United States : SAE International.
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Konferensbidrag (refereegranskat)abstract
- In aerodynamic development of ground vehicles, the use of Computational Fluid Dynamics (CFD) is crucial for improving the aerodynamic performance, stability and comfort of the vehicle. Simulation time and accuracy are two key factors of a well working CFD procedure. Using scale-resolving simulations, accurate predictions of the flow field and aerodynamic forces are possible, but often leads to long simulation time. For a given solver, one of the most significant aspects of the simulation time/cost is the temporal resolution. In this study, this aspect is investigated using the realistic vehicle model DrivAer with the notchback geometry as the test case. To ensure a direct and accurate comparison with wind tunnel measurements, performed at TU Berlin, a large section of the wind tunnel is included in the simulation domain. All simulations are performed at a Reynolds number of 3.12 million, based on the vehicle length. Three spatial resolutions were compared, where it could be seen that a hybrid element mesh consisting of 102 million cells only revealed small differences to the finest mesh investigated, well as showing excellent agreement with wind tunnel measurements. An investigation of the temporal resolution is performed, in order to see its effect on the simulation time/cost and accuracy of the results. The finest temporal resolution resulted in a Courant-Friedrichs-Lewy number less than unity, while the coarsest reached a CFL number of around 100. From these results, it is seen that it is possible to reduce the simulation time with more than 90 % (CFL 20) and still keep sufficient accuracy of the forces and important features of the flow field.
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| 17. |
- Ekman, Petter, 1988-, et al.
(författare)
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Aerodynamic Drag Reduction - from Conceptual Design on a Simplified Generic Model to Full-Scale Road Tests
- 2015
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Ingår i: SAE 2015 World Congress & Exhibition. - 400 Commonwealth Drive, Warrendale, PA, United States : SAE International.
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Konferensbidrag (refereegranskat)abstract
- Road transportation by trucks is the major part of the goods transportations system in the European Union (EU), and there is a need for increased fuel efficiency. While truck manufacturers already spend significant resources in order to reduce the emissions from their vehicles, most truck manufacturers do not control the shape of the trailer and/or swap bodies. These devices are usually manufactured by different companies that cannot consider the overall aerodynamics around the complete vehicle.By use of Computational Fluid Dynamics (CFD) and previous wind tunnel experiments, the flow around a simplified generic tractor-trailer model has been investigated. With better understanding of the flow features around the tractor with attached trailer or swap bodies, an improved design of the trailer and swap body can be achieved, which is the aim for the project. Special emphasis is put on achieving simple, easy to install or implement drag-reducing geometrical modifications to the trailer or swap bodies that can be mounted on existing trucks.Reynolds-Averaged Navier-Stokes (RANS) simulations were used for the conceptual development phase where trends in drag reduction due to the modified geometries were studied using a parameter study, while more advanced scale resolving simulations (SRS) were used in order to investigate the details of the flow fields.The investigation indicates that aerodynamic drag reduction is possible with quite simple geometrical modifications. Some of the results have also been verified through road tests of vehicles in commercial use, which has shown reduced fuel consumption of up to 5%.
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| 18. |
- Ekman, Petter, 1988-, et al.
(författare)
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Aerodynamic Drag Reduction of a Light Truck - from Conceptual Design to Full Scale Road Tests
- 2016
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Ingår i: SAE 2016 World Congress and Exhibition. - 400 Commonwealth Drive, Warrendale, PA, United States : SAE International.
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Konferensbidrag (refereegranskat)abstract
- Considerable amounts of the everyday goods transports are done using light trucks. In the last ten years (2005-2015), the number of light trucks has increased by 33 % in Sweden. The majority of these light trucks are fitted with a swap body and encounter the same problem as many other truck configurations, namely that several different manufacturers contribute to the final shape of the vehicle. Due to this, the aerodynamics of the final vehicle is often not fully considered. Hence there appears to be room for improving the aerodynamic performance. In this study the flow around a swap body fitted to a light truck has been investigated using Computational Fluid Dynamics. The focus has been on improving the shape of the swap body in order to reduce both the aerodynamic drag and fuel consumption, while still keeping it usable for daily operations. Reynolds-Averaged Navier-Stokes simulations were used for concept evaluation while more advanced Detached Eddy Simulations were performed on the best concept in order to investigate details of the flow. Various concepts were evaluated from which it could be seen that a more streamlined top of the swap body together with a lowered top trailing edge had a significant positive effect on the aerodynamic drag. A full scale light truck was equipped with a swap body with with these modifications for road tests. During a test period, a mean fuel consumption reduction of 12 % was measured, thus indicating a significantly reduced aerodynamic drag.
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| 19. |
- Ekman, Petter, 1988-, et al.
(författare)
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Assessment of hybrid RANS-LES methods for accurate automotive aerodynamic simulations
- 2020
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Ingår i: Journal of Wind Engineering and Industrial Aerodynamics. - : Elsevier BV. - 0167-6105 .- 1872-8197. ; 206
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Tidskriftsartikel (refereegranskat)abstract
- The introduction of the Harmonized Light Vehicles Test Procedure causes a significant challenge for the automotive industry, as it increases the importance of efficient aerodynamics and demands how variations of optional extras affect the car’s fuel consumption and emissions. This may lead to a huge number of combinations of optional extras that may need to be aerodynamically analyzed and possibly optimized, being to resource-consuming to be done with wind tunnel testing merely. Reynolds Average Navier-Stoles (RANS) coupled with Large Eddy Simulations (LES) have shown potential for accurate simulation for automotive applications for reasonable computational cost. In this paper, three hybrid RANS-LES models are investigated on the DrivAer notchback and fastback car bodies and compared to wind tunnel measurements. Several yaw angles are investigated to see the model’s ability to capture small and large changes of the flow field. It is seen that the models generally are in good agreement with the measurement, but only one model is able to capture the behavior seen in the measurements consistently. This is connected to the complex flow over the rear window, which is important to capture for accurate force predictions.
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| 20. |
- Ekman, Petter, 1988-, et al.
(författare)
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Importance of Sub-Grid Scale Modeling for Accurate Aerodynamic Simulations
- 2021
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Ingår i: Journal of Fluids Engineering. - : ASME. - 0098-2202 .- 1528-901X. ; 143:1
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Tidskriftsartikel (refereegranskat)abstract
- The Ahmed body is one of the most well-investigated vehicle bodies for aerodynamic purposes. Despite its simple geometry, the flow around the body, especially at the rear, is very complex as it is dominated by a large wake with strong interaction between vortical structures. In this study, the flow around the 25 deg Ahmed body has been investigated using large eddy simulations and compared to high-resolution particle image velocimetry (PIV) measurements. Special emphasis was put on studying three commonly used sub-grid scale (SGS) models and their ability to capture vortical structures around the Ahmed body. The ability of the SGS models to capture the near-wall behavior and small-scale dissipation is crucial for capturing the correct flow field. Very good agreement between simulations and PIV measurements were seen when using the dynamic Smagorinsky-Lilly and the wall-adopting local eddy-viscosity SGS models, respectively. However, the standard Smagorinsky-Lilly model was not able to capture the flow patterns when compared to the PIV measurements due to shortcomings in the near-wall modeling in the standard Smagorinsky-Lilly model, resulting in overpredicted separation.
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| 21. |
- Ekman, Petter, 1988-
(författare)
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Important Factors for Accurate Scale-Resolving Simulations of Automotive Aerodynamics
- 2020
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Road transports are responsible for almost 18 % of the greenhouse gas emission in Europe and are today the leading cause of air pollution in cities. Aerodynamic resistance has a significant effect on fuel consumption and hence the emission of vehicles. For electric vehicles, emissions are not affected by the aerodynamics as such but instead have a significant effect on the effective range of the vehicle.In 2017, a new measurement procedure was introduced, Worldwide Harmonized Light Vehicles Test Procedure (WLTP), for measuring emissions, fuel consumption, and range. This procedure includes a new test cycle with increased average driving speed compared to the former procedure, which thereby increases the importance of the aerodynamic resistance, as it drastically increases with speed. A second effect is that the exact car configuration sold to the customer needs to be certified in terms of fuel consumption and emissions. The result is that every possible combination of optional extras, which might affect the aerodynamic resistance, needs to be aerodynamically analyzed and possibly improved. From 2021, the European Commission will introduce stricter emission regulations for new passenger cars, with the fleet-wide average lowered to 95 grams CO2=km, which puts an even higher demand on achieving efficient aerodynamics.Virtual development of the aerodynamics of road vehicles is today used to a great extent, using Computational Fluid Dynamics, as it enables faster and cheaper development. However, achieving high accuracy for the prediction of the flow field and aerodynamic forces is challenging, especially given the complexity of both the vehicle geometry in itself and the surrounding flow field. Even for a simplified generic bluff body, accurately predicting the flow field and aerodynamic forces is a challenge. The main reason for this challenge of achieving results with high accuracy is the prediction of the complex behavior of turbulence. Scale-resolving simulation (SRS) methods, such as Large Eddy Simulation (LES), where most of the turbulent structures are resolved has in many studies shown high accuracy but unfortunately to a very high computational cost. It is primarily the small turbulent structures within the near-wall region that requires a _ne resolution in both space (the mesh) and in time. This fine resolution is the reason for the very high computational cost and makes LES unfeasible for practical use in industrial aerodynamic development at present and in the near future. By modeling the turbulent structures within the near-wall region using a Reynolds-Averaged Navier-Stokes (RANS) model, and resolving the turbulence outside the region with a LES model, a coarser resolution is possible to use, resulting in significantly lower computational cost. Which used RANS model is of high importance, and especially how much turbulent viscosity the model generates, as too high values can result in suppression of the resolved turbulence.The transitioning between the RANS and LES regions have a significant effect on the results. Faster transition enables more resolved turbulence, favorable for higher accuracy, but needs to be balanced with sufficient shielding of the RANS region. If resolving the turbulence occurs within the near-wall region, and the mesh is not sufficiently fine, it can result in poor accuracy.By increasing the time-step size and disregarding best-practice guides, the computational cost can be significantly reduced. The accuracy is reasonably insensitive to the larger time step sizes until a certain degree, thereby enabling computationally cheaper SRS to achieve high accuracy of aerodynamic predictions needed to meet present and future emission regulations.
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| 22. |
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| 23. |
- Fattahi, Sadegh, et al.
(författare)
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Influence of the Timber Shape on the Aerodynamics of a Timber Truck
- 2021
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Ingår i: SAE technical paper series. - 400 Commonwealth Drive, Warrendale, PA, United States : SAE International. - 0148-7191.
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Tidskriftsartikel (refereegranskat)abstract
- The aerodynamic improvement and efficiency of regular goods transportation trucks have been a topic of current interest; however, the timber transport industry has not been receiving as much attention. This is due to the small portion of timber transportation vehicles, compared to regular trucks, not justifying the cost of investigating these vehicles experimentally. Since these vehicles travel large parts of their journey at around 80 km/h, their fuel consumption is heavily affected by the aerodynamic resistance. In Sweden in 2015, there were around 2000 vehicles in operation transporting 6 billion ton-km with an average of 0.025 liter Diesel per ton-km. To understand these vehicles’ aerodynamics, and improve on these in the future, the modelling of the timber stacks is of utmost importance. Computational Fluid Dynamics (CFD) simulations have been utilized to conduct this investigation due to recent advancements and the relatively low cost of these simulations compared to an experimental approach. By investigating the influence of geometrical modifications of the stacks on the flow features and accumulated drag, a generic timber stack was created representative of a real stack for a loaded baseline vehicle. It was found that the shorter log length and a shuffling of the logs in the stack exhibit important flow features contributing to drag not present in the other cases. Based on this, a new baseline loaded truck configuration was created with all stacks being identical to each other. This generic stack was built with logs that were 4.25 m long and 0.35 m in diameter, had a smooth surface, and were stacked with a certain displacement in the lengthwise direction.
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| 24. |
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| 26. |
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| 27. |
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| 28. |
- Ghosh, Arpan, 1987-
(författare)
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Mathematical modelling of flow through thin curved pipes with application to hemodynamics
- 2019
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The problem of mathematical modelling of incompressible flows with low velocities through narrow curvilinear pipes is addressed in this thesis. The main motivation for this modelling task is to eventually model the human circulatory system in a simple way that can facilitate the medical practitioners to efficiently diagnose any abnormality in the system. The thesis comprises of four articles.In the first article, a two-dimensional model describing the elastic behaviour of the wall of a thin, curved, exible pipe is presented. The wall is assumed to have a laminate structure consisting of several anisotropic layers of varying thickness. The width of the channel is allowed to vary along the pipe. The two-dimensional model takes the interactions of the wall with any surrounding material and the fluid flow into account and is obtained through a dimension reduction procedure. Examples of canonical shapes of pipes and their walls are provided with explicit systems of differential equations at the end.In the second article, a one-dimensional model describing the blood flow through a moderately curved, elastic blood vessel is presented. The two-dimensional model presented in the first paper is used to model the vessel wall while linearized Navier-Stokes equations are used to model the flow through the channel. Surrounding muscle tissues and presence of external forces other than gravity are taken into account. The model is again obtained via a dimension reduction procedure based on the assumption of thinness of the vessel relative to its length. Results of numerical simulations are presented to highlight the influence of different factors on the blood flow.The one-dimensional model described in the second paper is used to derive a simplified one-dimensional model of a false aneurysm which forms the subject of the third article. A false aneurysm is an accumulation of blood outside a blood vessel but confined by the surrounding muscle tissue. Numerical simulations are presented which demonstrate different characteristics associated with a false aneurysm.In the final article, a modified Reynolds equation, along with its derivation from Stokes equations through asymptotic methods, is presented. The equation governs the steady flow of a fluid with low Reynolds number through a narrow, curvilinear tube. The channel considered may have large curvature and torsion. Approximations of the velocity and the pressure of the fluid inside the channel are constructed. These approximations satisfy a modified Poiseuille equation. A justification for the approximations is provided along with a comparison with a simpler case.
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| 29. |
- Gupta, Vikas, et al.
(författare)
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Automated three-dimensional tracking of the left ventricular myocardium in time-resolved and dose-modulated cardiac CT images using deformable image registration
- 2018
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Ingår i: Journal of Cardiovascular Computed Tomography. - : Elsevier. - 1934-5925. ; 12:2, s. 139-148
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Tidskriftsartikel (refereegranskat)abstract
- Background Assessment of myocardial deformation from time-resolved cardiac computed tomography (4D CT) would augment the already available functional information from such an examination without incurring any additional costs. A deformable image registration (DIR) based approach is proposed to allow fast and automatic myocardial tracking in clinical 4D CT images.Methods Left ventricular myocardial tissue displacement through a cardiac cycle was tracked using a B-spline transformation based DIR. Gradient of such displacements allowed Lagrangian strain estimation with respect to end-diastole in clinical 4D CT data from ten subjects with suspected coronary artery disease. Dice similarity coefficient (DSC), point-to-curve error (PTC), and tracking error were used to assess the tracking accuracy. Wilcoxon signed rank test provided significance of tracking errors. Topology preservation was verified using Jacobian of the deformation. Reliability of estimated strains and torsion (normalized twist angle) was tested in subjects with normal function by comparing them with normal strain in the literature.Results Comparison with manual tracking showed high accuracy (DSC: 0.99± 0.05; PTC: 0.56mm± 0.47 mm) and resulted in determinant(Jacobian) > 0 for all subjects, indicating preservation of topology. Average radial (0.13 mm), angular (0.64) and longitudinal (0.10 mm) tracking errors for the entire cohort were not significant (p > 0.9). For patients with normal function, average strain [circumferential, radial, longitudinal] and peak torsion estimates were: [-23.5%, 31.1%, −17.2%] and 7.22°, respectively. These estimates were in conformity with the reported normal ranges in the existing literature.Conclusions Accurate wall deformation tracking and subsequent strain estimation are feasible with the proposed method using only routine time-resolved 3D cardiac CT.
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| 30. |
- Gårdhagen, Roland, 1978-, et al.
(författare)
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Assessment of Geometrical Influence on WSS Estimation in the Human Aorta
- 2006
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Ingår i: WSEAS Transactions on Fluid Mechanics. - 1790-5087. ; 4:1, s. 318-326
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Tidskriftsartikel (refereegranskat)abstract
- Computational fluid dynamics simulations were performed on a stenosed human aorta with poststenotic dilatation, in order to estimate wall shear stress (WSS). WSS is important due to its correlation with atherosclerosis. Both steady-state and non-stationary simulations were conducted. Three different models were created from a set of MRI images. Comparison of geometrically different models was accomplished by using geometrical landmarks and a comparison parameter. Geometrical differences had larger influence on WSS magnitude than inflow rotation in steady-state results for the models used. In non-stationary flow the largest differences in WSS are found when the flow velocity near the wall is low e.g. when the inflow is low or in recirculation regions.
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| 38. |
- Gårdhagen, Roland, 1978-, et al.
(författare)
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Subject Specific Wall Shear Stress in the Human Thoracic Aorta
- 2006
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Ingår i: WSEAS Transaction on biology and biomedicine. - 1109-9518. ; 10:3, s. 609-614
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Tidskriftsartikel (refereegranskat)abstract
- Numerous studies have shown a correlation between Wall Shear Stress (WSS) and atherosclerosis, but few have evaluated the reliability of estimation methods and measures used to assessWSS, which is the subject of this work. A subject specific vessel model of the aortic arch and thoracic aorta is created fromMRI images and used for CFD simulations with MRI velocity measurements as inlet boundary condition. WSS is computed from the simulation results. Aortic WSS shows significant spatial as well as temporal variation during a cardiac cycle, which makes circumferential values very uninformative, and approximate estimates using Hagen-Poiseuille fails predict the averageWSS. Highly asymmetric flow, especially in the arch, causes the spatial WSS variations.
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| 39. |
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| 40. |
- Heiberg, Einar, 1973-, et al.
(författare)
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Three-dimensional flow characterization using vector pattern matching
- 2003
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Ingår i: IEEE Transactions on Visualization and Computer Graphics. - 1077-2626 .- 1941-0506. ; 9:3, s. 313-319
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Tidskriftsartikel (refereegranskat)abstract
- This paper describes a novel method for regional characterization of three-dimensional vector fields using a pattern matching approach. Given a three-dimensional vector field, the goal is to automatically locate, identify, and visualize a selected set of classes of structures or features. Rather than analytically defining the properties that must be fulfilled in a region in order to be classified as a specific structure, a set of idealized patterns for each structure type is constructed. Similarity to these patterns is then defined and calculated. Examples of structures of interest include vortices, swirling flow, diverging or converging flow, and parallel flow. Both medical and aerodynamic applications are presented in this paper.
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| 41. |
- Huge-Brodin, Maria, 1966-, et al.
(författare)
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Sustainable Logistics Service Providers : A strategic perspective on green logistics service provision
- 2015
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Ingår i: Proceedings from the 20th Annual Logistics Research Network Conference. - Derby : The Chartered Institute of Logistics and Transport & Derby University. - 9781904564508 ; , s. 1-8
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Konferensbidrag (refereegranskat)abstract
- Purpose: Environmental sustainability is of increasing importance. The Logistics Service Industry is of specific interest: its impact from mainly transports is still increasing. However, the sustainability of any company also includes a long-term prosperous business. The purpose of this paper is to demonstrate how a hauler, as an example of a logistics service provider (LSP), through approaching environmental sustainability from an overall strategic business perspective, can succeed long-term.Research Approach: The paper builds on an in-depth case study of a medium sized privately held LSP. The case was selected as it stands out compared to its competitors with respect to environmental engagement and long-term commitment to sustainability related issues. Starting from a framework based on RBT (Resource Based Theory), the case is analysed regarding how various types of resources and dynamic capabilities can contribute to sustainable competitive advantage.Findings and Originality: The case study illustrates how an LSP can put various resources to work and how resources can be combined. It is demonstrated that these combinations of resources are valuable, rare and perfectly inimitable, and that they to a high extent are well established in the company, and hence not rely in certain individuals. The cases study further demonstrates how an LSP can sense possibilities, seize opportunities and, when needed, reshape its business accordingly, and hence possess critical dynamic capabilities. This research contributes by analysing an LSP that, in contrast to most of its competitors, has succeeded in making its sustainable development a strategic asset. This asset reaches beyond the commonly acknowledged efficiency gains, and is an interesting example of how an LSP through its dynamic capabilities can gain a unique position.Research Impact: The strategic perspective through an RBT lens has previously been applied to LSPs mainly in survey based research. The results of this paper are rare, as they build on a real-case situation of a successful and sustainable LSP, while the extant research of success among LSPs going greener mostly suggest future development to become successful. This paper suggests how sustainable development can become a strategic asset for an LSP, and how dynamic capabilities can support that. Theories on dynamic capabilities have previously only been applied to a very limited extent on the analysis of LSPs aiming for going greenerPractical Impact: Based on this case, other LSPs may get inspiration to how they can develop their business to become more sustainable. The analysis points to that greening efforts cannot be seen in isolation, rather it is the combination of efforts, uniquely orchestrated for each case, that can make a difference. Therefore, the results point to relationships between various efforts, as well as how the different greening efforts fit the context of the LSP.
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| 42. |
- Jansson, Marcus, et al.
(författare)
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Water Hammer Induced Cavitation - A Numerical and Experimental Study
- 2017
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Ingår i: Fluid Power in the Digital Age.
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Cavitation erosion is one of the main concerns in hydraulic rock drills and can reduce both performance as well as life span. Current simulation tools can detect a potential risk of cavitation, however, the equations do not include cavitation physics and therefore cannot estimate the severity nor erosion locations. In order to evaluate the cavitation damage, long term tests are performed which are both costly and time consuming. With better computational capacity and more accurate numerical flow models, the possibilities to simulate the course of cavitation have increased. So far, most numerical studies on cavitation focus on steady-state problems while studies on hydraulic transients and water hammer effects have received less attention. This paper is a step towards simulation of water hammer induced cavitation and cavitation erosion in pipe flow using Computational Fluid Dynamics (CFD). In order to validate the results, experimental measurements are performed with a test equipment that creates hydraulic transients in a pipe and records these using piezoelectric pressure sensors. The results from CFD are compared to both the experimental data and to numerical results from a software called Hopsan, a one-dimensional multi-domain system simulation tool that uses wave characteristics to calculate pressures and flows. For smaller transients where no cavitation occur, all results show good agreement. For larger transients with cavitation, the results from Hopsan do not longer agree with the measurements, while the CFD model still performs well and is able to predict both formation and collapse of cavitation.
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| 43. |
- Johannes, Erik, et al.
(författare)
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Sustainable Timber Transport : Economic Aspects of Aerodynamic Reconfiguration
- 2018
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Ingår i: Sustainability. - : MDPI. - 2071-1050. ; 10:6, s. 1-18
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Tidskriftsartikel (refereegranskat)abstract
- There is a need to reduce fuel consumption, and thereby reduce CO2-emissions in all parts of the transport sector. It is also well known that aerodynamic resistance affects the fuel consumption in a major way. By improving the aerodynamics of the vehicles, the fuel consumption will also decrease. A special type of transportation is that of timber, which is performed by specialized trucks with few alternative uses. This paper follows up on earlier papers concerning Swedish timber trucks where aerodynamic improvements for timber trucks were tested. By mapping the entire fleet of timber trucks in Sweden and investigating reduced fuel consumption of 2–10%, financial calculations were performed on how these improvements would affect the transport costs. Certain parameters are investigated, such as investment cost, extra changeover time and weight of installments. By combining these results with the mapping of the fleet, it can be seen under which circumstances these improvements would be sustainable. The results show that it is possible through aerodynamics to lower the transportation costs and make an investment plausible, with changeover time being the most important parameter. They also show that certain criteria for a reduced transportation cost already exist within the vehicle fleet today.
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| 44. |
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| 45. |
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| 46. |
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| 47. |
- Karlsson, Matts, 1965-
(författare)
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On modelling and simulation of the human cardiovascular system : with special reference to aortic coarctation and mitral regurgitation
- 1992
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In this thesis the method of identification has been proposed for interpreting data obatined from pressure measurement on patients with aortic coarctation. In order to investigate its charcteristics and applicability the algortithm for identification was initially tested using a simple model of the human cardiovascular system.A more accurate description of an aortic segment was thus developed. The model is based on an existing model for an elastic tube, which was generalized into a description for a tube with visco-elastic walls. The visco-elastic behaviour of the wall is then modelled in the same sence as the viscosity in the fluid itself.The PISA-method for quantifying mitral regurgitation has been theoretically analysed. The purpose was to analyse the foundations of the PISA method: The flow through an orifice may be estimated by multiplying the center line velocity with the assumed hemispherical area. The calculations were also compared with experimental measurements. By using computer simulations, experimental measurements and clinical observations a better insight into the hemodynamics is obtained as well as an increased understanding of the shortcomings of the measurement technique under consideration.
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| 48. |
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| 49. |
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