| 1. |
- Andersson, Magnus, et al.
(författare)
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Model Verification and Error Sensitivity of Turbulence-Related Tensor Characteristics in Pulsatile Blood Flow Simulations
- 2021
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Ingår i: Fluids. - : MDPI. - 2311-5521. ; 6:1
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Tidskriftsartikel (refereegranskat)abstract
- Model verification, validation, and uncertainty quantification are essential procedures to estimate errors within cardiovascular flow modeling, where acceptable confidence levels are needed for clinical reliability. While more turbulent-like studies are frequently observed within the biofluid community, practical modeling guidelines are scarce. Verification procedures determine the agreement between the conceptual model and its numerical solution by comparing for example, discretization and phase-averaging-related errors of specific output parameters. This computational fluid dynamics (CFD) study presents a comprehensive and practical verification approach for pulsatile turbulent-like blood flow predictions by considering the amplitude and shape of the turbulence-related tensor field using anisotropic invariant mapping. These procedures were demonstrated by investigating the Reynolds stress tensor characteristics in a patient-specific aortic coarctation model, focusing on modeling-related errors associated with the spatiotemporal resolution and phase-averaging sampling size. Findings in this work suggest that attention should also be put on reducing phase-averaging related errors, as these could easily outweigh the errors associated with the spatiotemporal resolution when including too few cardiac cycles. Also, substantially more cycles are likely needed than typically reported for these flow regimes to sufficiently converge the phase-instant tensor characteristics. Here, higher degrees of active fluctuating directions, especially of lower amplitudes, appeared to be the most sensitive turbulence characteristics.
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| 2. |
- Asnaghi, Abolfazl, 1984, et al.
(författare)
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Impact of Leading Edge Roughness in Cavitation Simulations around a Twisted Foil
- 2020
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Ingår i: Fluids. - : MDPI AG. - 2311-5521. ; 5:4
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Tidskriftsartikel (refereegranskat)abstract
- The simulation of fully turbulent, three-dimensional, cavitating flow over Delft twisted foil is conducted by an implicit large eddy simulation (LES) approach in both smooth and tripped conditions, the latter by including leading-edge roughness. The analysis investigates the importance of representing the roughness elements on the flow structures in the cavitation prediction. The results include detailed comparisons of cavitation pattern, vorticity distribution, and force predictions with the experimental measurements. It is noted that the presence of roughness generates very small cavitating vortical structures which interact with the main sheet cavity developing over the foil to later form a cloud cavity. Very similar to the experimental observation, these interactions create a streaky sheet cavity interface which cannot be captured in the smooth condition, influencing both the richness of structures in the detached cloudy cavitation as well as the extent and transport of vapour. It is further found to have a direct impact on the pressure distribution, especially in the mid-chord region where the shed cloud cavity collapses.
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| 3. |
- Bordois, Lucie, et al.
(författare)
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Computation of Density Perturbation and Energy Flux of Internal Waves from Experimental Data
- 2020
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Ingår i: Fluids. - : MDPI AG. - 2311-5521. ; 5:3
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Tidskriftsartikel (refereegranskat)abstract
- We hereby present two different spectral methods for calculating the density anomaly and the vertical energy flux from synthetic Schlieren data, for a periodic field of linear internal waves (IW) in a density-stratified fluid with a uniform buoyancy frequency. The two approaches operate under different assumptions. The first method (hereafter Mxzt) relies on the assumption of a perfectly periodic IW field in the three dimensions (x, z, t), whereas the second method (hereafter MxtUp) assumes that the IW field is periodic in x and t and composed solely of wave components with downward phase velocity. The two methods have been applied to synthetic Schlieren data collected in the CNRM large stratified water flume. Both methods succeed in reconstructing the density anomaly field. We identify and quantify the source of errors of both methods. A new method mixing the two approaches and combining their respective advantages is then proposed for the upward energy flux. The work presented in this article opens new perspectives for density and energy flux estimates from laboratory experiments data.
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| 4. |
- Corrochano, Adrian, et al.
(författare)
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Flow Structures on a Planar Food and Drug Administration (FDA) Nozzle at Low and Intermediate Reynolds Number
- 2021
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Ingår i: Fluids. - : MDPI. - 2311-5521. ; 6:1
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Tidskriftsartikel (refereegranskat)abstract
- In this paper, we present a general description of the flow structures inside a two-dimensional Food and Drug Administration (FDA) nozzle. To this aim, we have performed numerical simulations using the numerical code Nek5000. The topology patters of the solution obtained, identify four different flow regimes when the flow is steady, where the symmetry of the flow breaks down. An additional case has been studied at higher Reynolds number, when the flow is unsteady, finding a vortex street distributed along the expansion pipe of the geometry. Linear stability analysis identifies the evolution of two steady and two unsteady modes. The results obtained have been connected with the changes in the topology of the flow. Finally, higher-order dynamic mode decomposition has been applied to identify the main flow structures in the unsteady flow inside the FDA nozzle. The highest-amplitude dynamic mode decomposition (DMD) modes identified by the method model the vortex street in the expansion of the geometry.
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| 5. |
- Criscuolo, Gennaro, et al.
(författare)
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Experimental Characterization of the Heat Transfer in Multi-Microchannel Heat Sinks for Two-Phase Cooling of Power Electronics
- 2021
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Ingår i: Fluids. - : MDPI. - 2311-5521. ; 6:2
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Tidskriftsartikel (refereegranskat)abstract
- This study aims to characterize experimentally the heat transfer in micro-milled multi-microchannels copper heat sinks operating with flow boiling, in the attempt to contribute to the development of novel and high heat flux thermal management systems for power electronics. The working fluid was R-134a and the investigation was conducted for a nominal outlet saturation temperature of 30 degrees C. The microchannels were 1 cm long and covered a square footprint area of 1 cm(2). Boiling curves starting at low vapor quality and average heat transfer coefficients were obtained for nominal channel mass fluxes from 250 kg/m(2)s to 1100 kg/m(2)s. The measurements were conducted by gradually increasing the power dissipation over a serpentine heater soldered at the bottom of the multi-microchannels, until a maximum heater temperature of 150 degrees C was reached. Infrared thermography was used for the heater temperature measurements, while high-speed imaging through a transparent top cover provided visual access over the entire length of the channels. The average heat transfer coefficient increased with the dissipated heat flux until a decrease dependent on hydrodynamic effects occurred, possibly due to incomplete wall wetting. Depending on the channel geometry, a peak value of 200 kW/m(2)K for the footprint heat transfer coefficient and a maximum dissipation of 620 W/cm(2) at the footprint with a limit temperature of 150 degrees C could be obtained, showing the suitability of the investigated geometries in high heat flux cooling of power electronics. The experimental dataset was used to assess the prediction capability of selected literature correlations. The prediction method by Bertsch et al. gave the best agreement with a mean absolute percent error of 24.5%, resulting to be a good design tool for flow boiling in high aspect ratio multi-microchannels as considered in this study.
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| 6. |
- Fan, Wenyuan, et al.
(författare)
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Progress in Phenomenological Modeling of Turbulence Damping around a Two-Phase Interface
- 2019
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Ingår i: Fluids. - : MDPI. - 2311-5521. ; 4:3
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Tidskriftsartikel (refereegranskat)abstract
- The presence of a moving interface in two-phase flows challenges the accurate computational fluid dynamics (CFD) modeling, especially when the flow is turbulent. For such flows, single-phase-based turbulence models are usually used for the turbulence modeling together with certain modifications including the turbulence damping around the interface. Due to the insufficient understanding of the damping mechanism, the phenomenological modeling approach is always used. Egorov's model is the most widely-used turbulence damping model due to its simple formulation and implementation. However, the original Egorov model suffers from the mesh size dependency issue and uses a questionable symmetric treatment for both liquid and gas phases. By introducing more physics, this paper introduces a new length scale for Egorov's model, making it independent of mesh sizes in the tangential direction of the interface. An asymmetric treatment is also developed, which leads to more physical predictions for both the turbulent kinetic energy and the velocity field.
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| 7. |
- 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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| 8. |
- Gallino, Giacomo, et al.
(författare)
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The Hydrodynamics of a Micro-Rocket Propelled by a Deformable Bubble
- 2019
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Ingår i: Fluids. - : MDPI. - 2311-5521. ; 4:1
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Tidskriftsartikel (refereegranskat)abstract
- We perform simulations to study the hydrodynamics of a conical-shaped swimming micro-robot that ejects catalytically produced bubbles from its inside. We underline the nontrivial dependency of the swimming velocity on the bubble deformability and on the geometry of the swimmer. We identify three distinct phases during the bubble evolution: immediately after nucleation the bubble is spherical and its inflation barely affects the swimming speed; then the bubble starts to deform due to the confinement gradient generating a force that propels the swimmer; while in the last phase, the bubble exits the cone, resulting in an increase in the swimmer velocity. Our results shed light on the fundamental hydrodynamics of the propulsion of catalytic conical swimmers and may help to improve the efficiency of these micro-machines.
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| 9. |
- Grönstedt, Tomas, 1970, et al.
(författare)
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Design Considerations of Low Bypass Ratio Mixed Flow Turbofan Engines with Large Power Extraction
- 2022
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Ingår i: Fluids. - : MDPI AG. - 2311-5521. ; 7:1
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Tidskriftsartikel (refereegranskat)abstract
- The possibility of extracting large amounts of electrical power from turbofan engines is becoming increasingly desirable from an aircraft perspective. The power consumption of a future fighter aircraft is expected to be much higher than today's fighter aircraft. Previous work in this area has concentrated on the study of power extraction for high bypass ratio engines. This motivates a thorough investigation of the potential and limitations with regards to performance of a low bypass ratio mixed flow turbofan engine. A low bypass ratio mixed flow turbofan engine was modeled, and key parts of a fighter mission were simulated. The investigation shows how power extraction from the high-pressure turbine affects performance of a military engine in different parts of a mission within the flight envelope. An important conclusion from the analysis is that large amounts of power can be extracted from the turbofan engine at high power settings without causing too much penalty on thrust and specific fuel consumption, if specific operating conditions are fulfilled. If the engine is operating (i) at, or near its maximum overall pressure ratio but (ii) further away from its maximum turbine inlet temperature limit, the detrimental effect of power extraction on engine thrust and thrust specific fuel consumption will be limited. On the other hand, if the engine is already operating at its maximum turbine inlet temperature, power extraction from the high-pressure shaft will result in a considerable thrust reduction. The results presented will support the analysis and interpretation of fighter mission optimization and cycle design for future fighter engines aimed for large power extraction. The results are also important with regards to aircraft design, or more specifically, in deciding on the best energy source for power consumers of the aircraft.
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| 10. |
- Hamedi, Naser, et al.
(författare)
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On the Interaction of Side-By-Side Circular Cylinders in Viscoplastic Fluids
- 2019
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Ingår i: Fluids. - : MDPI. - 2311-5521. ; 4:2
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Tidskriftsartikel (refereegranskat)abstract
- In this paper, the static interaction of a train of three cylinders in a Bingham fluid is studiednumerically using Computational Fluid Dynamics. The variation of drag forces for the cylinders inseveral configurations is investigated. Positions of the particles in relation to the reference particleare recognized by the separation distance between the cylinders. A steady state field is considered,with Bingham numbers between 5 and 150. Several separation distances (d) were considered, such that2.0D d 6.0D where D is the cylinder diameter. The Reynolds number was chosen in the range of5 Re 40. In particular, the eect of the separation distance, Reynolds number and Bingham numberon the shape and size of the unyielded regions was investigated. The functional dependence of thisregion and the drag coecient is explored. The present results reveal the significant influence of thegap between the cylinders on the drag force and the shape of the unyielded regions surrounding thecylinders. It was found that there are several configurations in which the drag forces over the first andthe third cylinders are almost equal depending on variation of the Bi, Re and the separation distance.
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