scholarly journals Influence of Shear-Thinning Blood Rheology on the Laminar-Turbulent Transition over a Backward Facing Step

Fluids ◽  
2020 ◽  
Vol 5 (2) ◽  
pp. 57
Author(s):  
Nathaniel S. Kelly ◽  
Harinderjit S. Gill ◽  
Andrew N. Cookson ◽  
Katharine H. Fraser
Keyword(s):  
Recirculation Zone ◽  
Blood Rheology ◽  
Shear Thinning ◽  
Neurological Dysfunction ◽  
Mean Flow ◽  
Backward Facing Step ◽  
Zone Length ◽  
Turbulent Transition ◽  
The Impact ◽  
Delayed Transition

Cardiovascular diseases are the leading cause of death globally and there is an unmet need for effective, safer blood-contacting devices, including valves, stents and artificial hearts. In these, recirculation regions promote thrombosis, triggering mechanical failure, neurological dysfunction and infarctions. Transitional flow over a backward facing step is an idealised model of these flow conditions; the aim was to understand the impact of non-Newtonian blood rheology on modelling this flow. Flow simulations of shear-thinning and Newtonian fluids were compared for Reynolds numbers ( R e ) covering the comprehensive range of laminar, transitional and turbulent flow for the first time. Both unsteady Reynolds Averaged Navier–Stokes ( k − ω SST) and Smagorinsky Large Eddy Simulations (LES) were assessed; only LES correctly predicted trends in the recirculation zone length for all R e . Turbulent-transition was assessed by several criteria, revealing a complex picture. Instantaneous turbulent parameters, such as velocity, indicated delayed transition: R e = 1600 versus R e = 2000, for Newtonian and shear-thinning transitions, respectively. Conversely, when using a Re defined on spatially averaged viscosity, the shear-thinning model transitioned below the Newtonian. However, recirculation zone length, a mean flow parameter, did not indicate any difference in the transitional Re between the two. This work shows a shear-thinning rheology can explain the delayed transition for whole blood seen in published experimental data, but this delay is not the full story. The results show that, to accurately model transitional blood flow, and so enable the design of advanced cardiovascular devices, it is essential to incorporate the shear-thinning rheology, and to explicitly model the turbulent eddies.

Numerical Prediction of Particulate Flow Over a Backward-Facing Step Followed by a Filter Medium

2012 ◽  
Author(s):  
A. K. Dange ◽  
K. C. Ravi ◽  
F. W. Chambers
Keyword(s):  
Porous Medium ◽  
Recirculation Zone ◽  
Stokes Number ◽  
Velocity Profiles ◽  
Experimental Results ◽  
Phase Model ◽  
Discrete Phase Model ◽  
Backward Facing Step ◽  
Zone Length ◽  
Discrete Phase

Flow in air filter housings often is characterized by separation upstream of the filter. The effect of the separation on the motion of particles and their distribution at the filter is important to filter performance. The current research investigates these effects by applying CFD modeling to turbulent particulate flows over a backward-facing step followed by a porous medium representing a filter. The two-dimensional step flow was selected as it is an archetype for separated flow with many studies in the literature. The flow examined has a step expansion ratio of 1:2, with an entrance length of 30 step heights to the step followed by a length of 60 step heights. Computations were performed at step Reynolds numbers of 6550 and 10,000 for the step without a porous medium and with the medium placed 4.25 and 6.75 step heights downstream of the step. The mesh was developed in ICEM CFD and modeling was done using the Fluent commercial CFD package. The carrier phase turbulence was modeled using the RNG k-epsilon model. The particles were modeled using the discrete phase model with dispersion modeled using stochastic tracking. The boundary conditions are uniform velocity at the inlet, no-slip at the walls, porous jump at the porous medium, and outflow at the outlet. The particle boundary condition is “reflect” at the walls and “trap” at the filter. The numerical results for the no filter case matched experimental results for recirculation zone length and velocity profiles at 3.75 and 6.25 step heights well. The computed velocity profiles at 3.75 step heights do not match experimental profiles for the filter at 4.25 step heights so well, though the results show a profound effect on the recirculation zone length, matching the experiments. Differences are attributed to different velocity profiles at the step. With the medium 6.75 step heights downstream, the effect on the recirculation zone is negligible, again matching experimental results. The discrete phase model tracks injected particles and provides results which are qualitatively similar to the literature. It is observed that particles with lower Stokes number, and thus lower momentum, tend to follow the flow and enter the recirculation zone while particles with higher Stokes number tend to move directly to the porous medium. When the filter is moved downstream to 6.75 step heights, the increased length of the recirculation zone results in more particles entering the recirculation zone. Results for monodispersed and polydispersed particles agree.

URANS Prediction of Flow Instabilities of a Novel Atomizer Combustor Configuration

2005 ◽  
Author(s):  
P. Jochmann ◽  
A. Sinigersky ◽  
R. Koch ◽  
H.-J. Bauer
Keyword(s):  
Stagnation Point ◽  
Recirculation Zone ◽  
Vortex Breakdown ◽  
Experimental Studies ◽  
Expansion Ratio ◽  
Reacting Flow ◽  
Mean Flow ◽  
Design Guideline ◽  
The Impact ◽  
Spiral Type

In this paper, the flow in a gas-turbine combustor with a novel air blast atomizer configuration is studied by URANS (unsteady Reynolds-averaged Navier-Stokes) calculations and compared to experimental data. The flow is characterized by a Reynolds number of 52000, a swirl number of 0.52 and an expansion ratio of 5. It is well known that at this high swirl level, flows exhibit a vortex breakdown which is characterized by a sudden axial deceleration in combination with a radial expansion and the formation of a stagnation point followed by a recirculation zone. At high Reynolds numbers, like in the present case, the vortex breakdown is either of a quasi-axisymmetric bubble type or of a precessing spiral type. Previous experimental studies confirmed the presence of a spiral type vortex breakdown for the configuration under concern. For the non-reacting flow, the structure as well as the frequency of the precessing vortex core is captured almost perfectly by the URANS predictions which is demonstrated by a direct comparison to LDV (Laser-Doppler Velocimetry) measurements. However, it was found that a suitable discretization as well as full three-dimensional computations are crucial in order to successfully predict the precessing spiral structure. In this context also the impact of two-equation and full transport Reynolds-stress turbulence models is discussed. After validation of the URANS method, it has been applied for developing improved designs which aim to suppress the unsteady flow pattern. The investigation of different design variants revealed that, if the mean axial velocity distribution of the flow upstream of the stagnation point is jet-like, the flow is more stable and less susceptible to the spiral type vortex breakdown. This fact which is known from laminar vortex breakdown investigations seems to be also valid for the turbulent mean flow and can be used as a design guideline for achieving a stable nearly symmetric bubble-like recirculation zone.

Self-Acceleration in the Parameterization of Orographic Gravity Wave Drag

2010 ◽  
Vol 67 (8) ◽  
pp. 2537-2546 ◽  
Author(s):  
John F. Scinocca ◽  
Bruce R. Sutherland
Keyword(s):  
Gravity Wave ◽  
Middle Atmosphere ◽  
Wave Train ◽  
Leading Edge ◽  
Wave Theory ◽  
Wave Drag ◽  
Tuning Parameter ◽  
Mean Flow ◽  
Propagating Waves ◽  
The Impact

Abstract A new effect related to the evaluation of momentum deposition in conventional parameterizations of orographic gravity wave drag (GWD) is considered. The effect takes the form of an adjustment to the basic-state wind about which steady-state wave solutions are constructed. The adjustment is conservative and follows from wave–mean flow theory associated with wave transience at the leading edge of the wave train, which sets up the steady solution assumed in such parameterizations. This has been referred to as “self-acceleration” and it is shown to induce a systematic lowering of the elevation of momentum deposition, which depends quadratically on the amplitude of the wave. An expression for the leading-order impact of self-acceleration is derived in terms of a reduction of the critical inverse Froude number Fc, which determines the onset of wave breaking for upwardly propagating waves in orographic GWD schemes. In such schemes Fc is a central tuning parameter and typical values are generally smaller than anticipated from conventional wave theory. Here it is suggested that self-acceleration may provide some of the explanation for why such small values of Fc are required. The impact of Fc on present-day climate is illustrated by simulations of the Canadian Middle Atmosphere Model.

scholarly journals Experimental Study on Backflow Patterns Induced by a Bilateral Groin Pair with Different Spacing

2021 ◽  
Vol 11 (4) ◽  
pp. 1486
Author(s):  
Cuiping Kuang ◽  
Yuhua Zheng ◽  
Jie Gu ◽  
Qingping Zou ◽  
Xuejian Han
Keyword(s):  
Cross Sections ◽  
Large Scale ◽  
Recirculation Zone ◽  
Flow Direction ◽  
Wake Flow ◽  
Zone Length ◽  
Array Configuration ◽  
Contraction Ratio ◽  
River Training ◽  
Spanwise Velocity

Groins are one of the popular manmade structures to modify the hydraulic flow and sediment response in river training. The spacing between groins is a critical consideration to balance the channel-depth and the cost of construction, which is generally determined by the backflow formed downstream from groins. A series of experiments were conducted using Particle Image Velocimetry (PIV) to observe the influence of groin spacing on the backflow pattern of two bilateral groins. The spacing between groins has significant effect on the behavior of the large-scale recirculation cell behind groins. The magnitude of the wake flow induced by a groin was similar to that induced by another groin on the other side, but the flow direction is opposite. The spanwise velocity near the groin tip dictates the recirculation zone width behind the groins due to the strong links between the spanwise velocity and the contraction ratio of channel cross-sections between groins. Based on previous studies and present experimental results, quantitative empirical relationships are proposed to calculate the recirculation zone length behind groins alternately placed at different spacing along riverbanks. This study provides better understanding and a robust formula to assess the backflow extent of alternate groins and identify the optimum groins array configuration.

Flow recirculation zone length and shear rate are differentially affected by stenosis severity in human coronary arteries

2013 ◽  
Vol 304 (4) ◽  
pp. H559-H566 ◽  
Author(s):  
Ashkan Javadzadegan ◽  
Andy S. C. Yong ◽  
Michael Chang ◽  
Austin C. C. Ng ◽  
John Yiannikas ◽  
...  
Keyword(s):  
Shear Rate ◽  
Coronary Arteries ◽  
Recirculation Zone ◽  
Fluid Dynamic ◽  
Zone Length ◽  
Flow Recirculation ◽  
Stenosis Severity ◽  
Recirculation Zones ◽  
The Relationship

Flow recirculation zones and shear rate are associated with distinct pathogenic biological pathways relevant to thrombosis and atherogenesis. The interaction between stenosis severity and lesion eccentricity in determining the length of flow recirculation zones and peak shear rate in human coronary arteries in vivo is unclear. Computational fluid dynamic simulations were performed under resting and hyperemic conditions on computer-generated models and three-dimensional (3-D) reconstructions of coronary arteriograms of 25 patients. Boundary conditions for 3-D reconstructions simulations were obtained by direct measurements using a pressure-temperature sensor guidewire. In the computer-generated models, stenosis severity and lesion eccentricity were strongly associated with recirculation zone length and maximum shear rate. In the 3-D reconstructions, eccentricity increased recirculation zone length and shear rate when lesions of the same stenosis severity were compared. However, across the whole population of coronary lesions, eccentricity did not correlate with recirculation zone length or shear rate ( P = not signficant for both), whereas stenosis severity correlated strongly with both parameters ( r = 0.97, P < 0.001, and r = 0.96, P < 0.001, respectively). Nonlinear regression analyses demonstrated that the relationship between stenosis severity and peak shear was exponential, whereas the relationship between stenosis severity and recirculation zone length was sigmoidal, with an apparent threshold effect, demonstrating a steep increase in recirculation zone length between 40% and 60% diameter stenosis. Increasing stenosis severity and lesion eccentricity can both increase flow recirculation and shear rate in human coronary arteries. Flow recirculation is much more sensitive to mild changes in the severity of intermediate stenoses than is peak shear.

Numerical Simulation for Flow over Two Circular Cylinders in Micro Channel with Lattice Boltzmann Method

2014 ◽  
Vol 670-671 ◽  
pp. 747-750
Author(s):  
Zhi Jun Gong ◽  
Jiao Yang ◽  
Wen Fei Wu
Keyword(s):  
Lattice Boltzmann Method ◽  
Lattice Boltzmann ◽  
Recirculation Zone ◽  
Flow Characteristics ◽  
Reynolds Numbers ◽  
Circular Cylinders ◽  
Micro Channel ◽  
Zone Length ◽  
Spacing Ratio ◽  
Boltzmann Method

For indepth study on flow characteristics for fluid bypass obstacles in micro-channel, the Lattice Boltzmann Method (LBM) was used to simulate fluid flow over two circular cylinders in side-by-side arrangement of a micro-channel. The velocity distribution and recirculation zone length under different Reynolds numbers (Re = 0~100) and different spacing ratio (H/D= 0~2.0) were obtained. The results show that the pattern of flow and the size of recirculation zone in the micro-channel depend on the combined effect of Re and H/D.

Strip yield model for multiple straight cracks with coalesced yield zones: A theoretical study

2021 ◽  
pp. 1-15
Author(s):  
S. Hasan ◽  
N. Akhtar ◽  
S. Shekhar
Keyword(s):  
Numerical Study ◽  
Multiple Cracks ◽  
Principle Of Superposition ◽  
Yield Model ◽  
Zone Length ◽  
Yield Zone ◽  
Strip Yield Model ◽  
Opening And Closing ◽  
The Impact ◽  
Analytical Expressions

The paper presents a complicated case of coalescence of yield zones between two internal cracks out of four collinear straight cracks weakened an infinite isotropic plate. Two solutions are presented for the case of opening and closing of multiple cracks under general yielding conditions. Using these two solutions and the principle of superposition, we found the analytical expressions for load-bearing capacity of the plate using complex variable method. A numerical study has been carried out to investigate the behavior of yield zone length concerning remotely applied stresses at the boundary of the plate and the impact of two outer cracks on the propagation of inner cracks due to coalesced yield zones. Results obtained are reported graphically.

scholarly journals Impact of SLA assimilation in the Sicily Channel Regional Model: model skills and mesoscale features

Ocean Science ◽  
2012 ◽  
Vol 8 (4) ◽  
pp. 485-496 ◽  
Author(s):  
A. Olita ◽  
S. Dobricic ◽  
A. Ribotti ◽  
L. Fazioli ◽  
A. Cucco ◽  
...  
Keyword(s):  
Sea Level ◽  
Qualitative Evaluation ◽  
Regional Model ◽  
Reynolds Stresses ◽  
Mean Flow ◽  
Qualitative Comparison ◽  
Mesoscale Structures ◽  
Sicily Channel ◽  
Along Track ◽  
The Impact

Abstract. The impact of the assimilation of MyOcean sea level anomalies along-track data on the analyses of the Sicily Channel Regional Model was studied. The numerical model has a resolution of 1/32° degrees and is capable to reproduce mesoscale and sub-mesoscale features. The impact of the SLA assimilation is studied by comparing a simulation (SIM, which does not assimilate data) with an analysis (AN) assimilating SLA along-track multi-mission data produced in the framework of MyOcean project. The quality of the analysis was evaluated by computing RMSE of the misfits between analysis background and observations (sea level) before assimilation. A qualitative evaluation of the ability of the analyses to reproduce mesoscale structures is accomplished by comparing model results with ocean colour and SST satellite data, able to detect such features on the ocean surface. CTD profiles allowed to evaluate the impact of the SLA assimilation along the water column. We found a significant improvement for AN solution in terms of SLA RMSE with respect to SIM (the averaged RMSE of AN SLA misfits over 2 years is about 0.5 cm smaller than SIM). Comparison with CTD data shows a questionable improvement produced by the assimilation process in terms of vertical features: AN is better in temperature while for salinity it gets worse than SIM at the surface. This suggests that a better a-priori description of the vertical error covariances would be desirable. The qualitative comparison of simulation and analyses with synoptic satellite independent data proves that SLA assimilation allows to correctly reproduce some dynamical features (above all the circulation in the Ionian portion of the domain) and mesoscale structures otherwise misplaced or neglected by SIM. Such mesoscale changes also infer that the eddy momentum fluxes (i.e. Reynolds stresses) show major changes in the Ionian area. Changes in Reynolds stresses reflect a different pumping of eastward momentum from the eddy to the mean flow, in turn influencing transports through the channel.

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