Analysis of Storage Effects in the Recirculation Zone Based on the Junction Angle of Channel Confluence

In this study, numerical simulations using the Environmental Fluid Dynamics Code model were conducted to elucidate the effects of flow structures in the recirculation zone on solute storage based on the junction angle. Numerical simulations were performed at a junction angle of 30° to 90° with a momentum flux ratio of 1.62. The simulation results revealed that an increase in the junction angle caused the recirculation zone length and width to increase and strengthened the development of helical motion. The helical motion increased the vertical gradient of the mixing layer and the mixing metric of the dosage curves. The recirculation zone accumulated the solute as a storage zone, which formed a long tail in the concentration curves. The interaction between the helical motion and recirculation zone affected the transverse mixing, such that the transverse dispersion had a positive relationship with the helical motion intensity and a negative relationship with the recirculation zone size. Transverse mixing exhibited an inverse relationship with the mass exchange rate of the recirculation zone. These results indicate that the transverse dispersion is replaced by mixing due to strongly developed storage zones.

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

Applied Sciences ◽

10.3390/app11041486 ◽

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.

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

AJP Heart and Circulatory Physiology ◽

10.1152/ajpheart.00428.2012 ◽

2013 ◽

Vol 304 (4) ◽

pp. H559-H566 ◽

Cited By ~ 21

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.

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

Applied Mechanics and Materials ◽

10.4028/www.scientific.net/amm.670-671.747 ◽

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.

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

Volume 1: Symposia, Parts A and B ◽

10.1115/fedsm2012-72397 ◽

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.

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

Fluids ◽

10.3390/fluids5020057 ◽

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.

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Vortex Detachment and Reverse Flow in Pulsatile Laminar Flow Through Axisymmetric Sudden Expansions

Journal of Fluids Engineering ◽

10.1115/1.2823507 ◽

1999 ◽

Vol 121 (3) ◽

pp. 574-579 ◽

Cited By ~ 2

Author(s):

S. Tavoularis ◽

R. K. Singh

Keyword(s):

Reynolds Number ◽

Recirculation Zone ◽

Reverse Flow ◽

Flow Region ◽

Vortex Rings ◽

Mean Velocity ◽

Zone Length ◽

Deceleration Phase ◽

Pulsatile Flows ◽

Flow Through

Incompressible, steady and pulsatile flows in axisymmetric sudden expansions with diameter ratios of 1:2.25 and 1:2.00 have been simulated numerically over the ranges of time-averaged bulk Reynolds number 0.1 ≤ Re ≤ 400 and Womersley number 0.1 ≤ W ≤ 50. For steady flow, the calculated recirculation zone length increased linearly with an increase in Re, in good agreement with earlier experiments. For pulsatile flows, particularly at higher values of W, the recirculation zone length correlated strongly with the acceleration of the flow and not with the instantaneous Reynolds number; it increased during the deceleration phase and decreased during the acceleration phase. The computed mean velocity and reattachment length were in general agreement with published experimental data. At relatively low W, the computed near-wall, reverse flow region extended along the full domain over part of the cycle, similarly to that in the experiments. At low values of W, the vortex rings created at the expansion remained attached and oscillated back and forth; for an intermediate range of W, they detached and moved downstream; at relatively high W, these vortices became, once more, attached.

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Double-Diffusive Recipes. Part I: Large-Scale Dynamics of Thermohaline Staircases

Journal of Physical Oceanography ◽

10.1175/jpo-d-13-0155.1 ◽

2014 ◽

Vol 44 (5) ◽

pp. 1269-1284 ◽

Cited By ~ 20

Author(s):

T. Radko ◽

A. Bulters ◽

J. D. Flanagan ◽

J.-M. Campin

Keyword(s):

Numerical Simulations ◽

Large Scale ◽

Density Ratio ◽

Three Dimensional ◽

Flux Ratio ◽

Double Diffusion ◽

Spontaneous Generation ◽

Computational Domain ◽

The North ◽

Basin Scale

Abstract Three-dimensional dynamics of thermohaline staircases are investigated using a series of basin-scale staircase-resolving numerical simulations. The computational domain and forcing fields are chosen to reflect the size and structure of the North Atlantic subtropical thermocline. Salt-finger transport is parameterized using the flux-gradient formulation based on a suite of recent direct numerical simulations. Analysis of the spontaneous generation of thermohaline staircases suggests that thermohaline layering is a product of the gamma instability, associated with the variation of the flux ratio with the density ratio . After their formation, numerical staircases undergo a series of merging events, which systematically increase the size of layers. Ultimately, the system evolves into a steady equilibrium state with pronounced layers 20–50 m thick. The size of the region occupied by thermohaline staircases is controlled by the competition between turbulent mixing and double diffusion. Assuming, in accordance with observations, that staircases form when the density ratio is less than the critical value of , the authors arrive at an indirect estimate of the characteristic turbulent diffusivity in the subtropical thermocline.

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A scaling law for the recirculation zone length behind a bluff body in reacting flows

Journal of Fluid Mechanics ◽

10.1017/jfm.2019.475 ◽

2019 ◽

Vol 875 ◽

pp. 699-724 ◽

Cited By ~ 2

Author(s):

James C. Massey ◽

Ivan Langella ◽

Nedunchezhian Swaminathan

Keyword(s):

Heat Release ◽

Turbulence Intensity ◽

Bluff Body ◽

Recirculation Zone ◽

Scaling Law ◽

Reacting Flows ◽

The Body ◽

Turbulent Shear ◽

Zone Length ◽

Isothermal Flows

The recirculation zone length behind a bluff body is influenced by the turbulence intensity at the base of the body in isothermal flows and also the heat release and its interaction with turbulence in reacting flows. This relationship is observed to be nonlinear and is controlled by the balance of forces acting on the recirculation zone, which arise from the pressure and turbulence fields. The pressure force is directly influenced by the volumetric expansion resulting from the heat release, whereas the change in the turbulent shear force depends on the nonlinear interaction between turbulence and combustion. This behaviour is elucidated through a control volume analysis. A scaling relation for the recirculation zone length is deduced to relate the turbulence intensity and the amount of heat release. This relation is verified using the large eddy simulation data from 20 computations of isothermal flows and premixed flames that are stabilised behind the bluff body. The application of this scaling to flames in an open environment and behind a backward facing step is also explored. The observations and results are explained on a physical basis.

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