Recirculating Flow Region Research Articles

A Novel Design of a Noncylindric Stent With Beneficial Effects on Flow Characteristics: An Experimental and Numerical Flow Study in an Axisymmetric Arterial Model With Sequential Mild Stenoses

The objective of this study was to investigate whether a novel noncylindric "stent," implanted upstream of one or more mild arterial stenoses could be beneficial, by increasing in-stent wall shear stresses (WSS) and by limiting the flow recirculation zones in the vicinity of the stenoses. An in vitro model employing flow visualization, and computational fluid dynamics were used for the study of steady and unsteady flow fields in a model of a coronary artery with two sequential 50% stenoses. Two different tube-entrance geometries (a cylindric and a convergent nozzle), modeling an ostial stented arterial segment, were studied regarding their effect on flow patterns and WSS at various Reynolds numbers (Re = 200-800). In this preliminary study, the internal walls of these tube entrance geometries were smooth, not taking into account the roughness of the actual stent's strut. "In-stent" WSS was significantly increased at the noncylindric configuration compared to that in the regular cylindric geometry. The difference of the spatially averaged "in-stent" WSS between the two designs gradually increased with the increase in Re, reaching a peak value up to 50% in the case of the noncylindric endovascular device, for the examined Re range. For the same configuration, both the experimental and numerical analyses indicated a decrease in flow recirculation region at the vicinity distally to a sequential stenosis up to 6%, and the mean flow velocity was increased for both steady and pulsatile flow. These effects of the new "stent" configuration could be important for the delay of restenosis processes and should be further investigated in vivo.

Modulation of ATP/ADP Concentration at the Endothelial Surface by Shear Stress: Effect of Flow Recirculation

The extracellular presence of the adenine nucleotides ATP and ADP induces calcium mobilization in vascular endothelial cells (ECs). ATP/ADP concentration at the EC surface is determined by a balance of convective-diffusive transport to and from the EC surface, hydrolysis by ectonucleotidases at the cell surface, and flow-induced ATP release from ECs. Our previous numerical simulations in a parallel plate geometry had demonstrated that flow-induced ATP release has a profound effect on nucleotide concentration at the EC surface. In the present study, we have extended the modeling to probe the impact of flow separation and recirculation downstream of a backward facing step (BFS) on ATP/ADP concentration at the EC surface. The results show that for both steady and pulsatile flow over a wide range of wall shear stresses, the ATP+ADP concentration at the EC surface is considerably lower within the flow recirculation region than in areas of undisturbed flow outside the recirculation zone. Pulsatile flow also leads to sharp temporal gradients in nucleotide concentration. If confirmed experimentally, the present findings suggest that disturbed and undisturbed flow may affect EC calcium mobilization differently. Such differences might, in turn, contribute to the observed endothelial dysfunction in regions of disturbed flow.

Visualization of Mixing Flow Phenomena in Champagne Glasses under Various Glass-Shape and Engravement Conditions

For the very first time, a classical flow visualization technique was used to capture the fluid motion in traditional flutes and coupes poured with champagne. It was found that glasses engraved around their axis of symmetry produce a rising gas column along the vertical glass axis that induces, in turn, steady state recirculating flow regions. In the case of the classical engraved champagne flute, the whole domain of the liquid phase is homogeneously mixed, whereas in the case of the engraved champagne coupe, the recirculating flow region does not occupy the whole volume in the glass. In the engraved coupe, a dead-zone of very low motion was identified, which inhibits the formation of the collar at the glass edge. Our results finally strongly suggest that the glass-shape and engravement conditions should likely have a strong impact on champagne tasting by modifying the kinetics of release of carbon dioxide molecules and aromatic volatile organic compounds from the liquid medium.

Vena Cava Filter Performance Based on Hemodynamics and Reported Thrombosis and Pulmonary Embolism Patterns

Three inferior vena cava (IVC) filters of different designs were studied to identify the potential links between published clinical results for thrombosis and recurrent pulmonary embolism (PE) rates and in vitro hemodynamics patterns in the region of the filters. The filters studied were the Greenfield over-the-wire filter (Medi-tech/Boston Scientific, Watertown, Mass), TrapEase filter (Cordis Europa, Roden, the Netherlands), and Mobin-Uddin umbrella filter (Edwards Laboratories, Santa Ana, Calif). To assess hemodynamics, velocity contour maps were generated for each filter by using the in vitro photochromic flow visualization technique. Results were obtained for both the unoccluded and partially occluded states. Steady flow (R(e) = 600) was used to model physiologic conditions. To estimate the rates of IVC occlusion and recurrent PE, the authors analyzed published clinical studies spanning more than 30 years and a U.S. Food and Drug Administration database. For both the unoccluded and partially occluded Mobin-Uddin and TrapEase filters, regions of flow stagnation and/or recirculation and turbulence developed downstream of the filter. The Greenfield filter did not produce any prothrombotic flow patterns for either the unoccluded or partially occluded states. Results of published clinical studies supported the hemodynamic findings, with the TrapEase and Mobin-Uddin filters having high rates of IVC occlusion and recurrent PE compared with those of the Greenfield filter. Flow stagnation or recirculation and turbulence have been linked to thrombosis and thrombus and/or PE formation. Thus, the hemodynamic results from this study may help explain the relatively higher rates of filter thrombosis and PE for the Mobin-Uddin and TrapEase filters versus the Greenfield filter.

Flow structure and flame stability in a micro can combustor with a baffle plate

Flow structure and flame stability to be formed inside a micro can combustor, with a baffle plate having a central fuel nozzle and multi air holes located annularly were investigated experimentally. The structures of the isothermal flow and the reacting flow behind the baffle plate are measured by using a particle image velocimetry (PIV). The result shows that generation of the flow recirculation region enhances the mixing most effectively and is useful to make the combustion chamber compact. However, for the reacting flow condition, the flow structure behind the baffle plate will be changed drastically. The flame stabilization mechanisms have to be discussed in terms of local conditions of fuel and air mixing, flame propagation speed, and so on. These local structures seem to play an important role for the lifted flame location and stability of this type of burner.

Mean Flow and Turbulence Structure in Vertical Slot Fishways

This paper presents the results of an experimental study on the mean and turbulence structures of flow in a vertical slot fishway with slopes of 5.06 and 10.52%. Two flow patterns existed in the fishway and for each one, two flow regions were formed in the pools: a jet flow region and a recirculating flow region. The mean kinetic energy decays rapidly in the jet region and the dissipation rate in most of the areas in the pool is less than 200W∕m3. For the jet flow, the nondimensional mean velocity profile across the jet agrees very well with that of a plane turbulent jet in the central part of the jet with some scatter near its boundaries. Its maximum velocity decays faster compared to a plane turbulent jet in a large stagnant ambient. The jet presents different turbulence structure for the two flow patterns and for each pattern, the turbulence characteristics appear different between the left and right halves of the jet. However, the turbulence characteristics show some similarity for each case. The normalized energy dissipation rate shows some similarity and has a maximum value on the center of the jet. The results are believed to provide useful insight on the turbulence characteristics of flow in vertical slot fishways and can be used to verify numerical models and also for guidance in the design of fishways in the future.

Three-Dimensional Mixed Convection in Plane Symmetric-Sudden Expansion: Bifurcated Flow Regime

Simulations of three-dimensional laminar mixed convection in a vertical duct with plane symmetric sudden expansion are presented to illustrate the effects of the buoyancy-assisting force and the duct’s aspect ratio on flow bifurcation and heat transfer. The stable laminar bifurcated flow regime that develops in this geometry at low buoyancy levels leads to nonsymmetric temperature and heat transfer distributions in the transverse direction, but symmetric distributions with respect to the center width of the duct in the spanwise direction. As the buoyancy force increases, due to increases in wall heat flux, flow bifurcation diminishes and both the flow and the thermal fields become symmetric at a critical wall heat flux. The size of the primary recirculation flow region adjacent to the sudden expansion increases on one of the stepped walls and decreases on the other stepped wall as the wall heat flux increases. The maximum Nusselt number that develops on one of the stepped walls in the bifurcated flow regime is significantly larger than the one that develops on the other stepped wall. The critical wall heat flux increases as the duct’s aspect ratio increases for fixed Reynolds number. The maximum Nusselt number that develops in the bifurcated flow regime increases as the duct’s aspect ratio increases for fixed wall heat flux and Reynolds number.

Flow Dynamics in Expansions Characterizing Abdominal Aorta Aneurysms

Our purpose was to identify and quantify hemodynamic factors contributing to the generation, proliferation, and rupturing of abdominal aorta aneurysms (AAAs) using a computational investigation of steady laminar and turbulent flow in AAA models. Steady laminar and turbulent flows were computed using the incompressible Navier-Stokes equations. Flow fields in symmetric shapes of different extents and degrees of expansion are presented first. Two representative cases of asymmetric AAAs are considered next: an aneurysm with an elliptic cross section and an aneurysm with preferential expansion in one direction. For symmetric aneurysms, parametric studies are presented. For asymmetric aneurysms, flow fields are computed only for high flow rates representative of systolic flow. For all cases, a recirculating flow region was found in the expanded part of the AAA. Recirculation is accompanied by a minor increase in pressure but a significant increase in wall shear stress. For cases where turbulent flow was considered, it was found that the recirculation zone diminishes but the computed wall shear stress reaches levels higher than laminar flow. The levels of wall shear stress reached in turbulent flow may cause lesions of the aneurysmal wall. The minor variation of pressure within the aneurysms with smooth expansions indicates that the structural properties of the arterial wall tissue may play a significant role for the generation and subsequent proliferation of the aneurysm. However, the high values of the wall shear stress in AAAs appear to be an important hemodynamic factor that may contribute to wall degeneration and eventual rupturing. The recirculating flow in AAAs may explain the generation of intraluminal thrombi. Furthermore, the asymmetry and complexity of the flow in asymmetric AAAs may explain the frequently observed asymmetric thrombi distribution.

Hub shape effects on the inducers performance under cavitation

In this work, experimental and three-dimensional numerical simulations are pursued with the intent of elucidating the differences that the hub shape has on three inducer performances and on other important engineering properties in cavitating and non-cavitating regimes. Two inducers have a cylindrical hub and the third one has a conical hub. They are noted as BH inducer (big hub), SH inducer (small hub), and CH inducer (conical hub). These machines were designed with the goal to reduce the amount of cavitation and vibration, on the one hand, and to improve the overall performance, on the other hand. Both sets of experimental and numerical data suggest that the cavitating and non-cavitating performances are affected by changes in the hub shape. The hub configuration and cavitation numbers considered give rise to various cavitation structures on the suction side of the blades, characterized — at some flow rates — by recirculating flow regions and backflow vortices. The results show the relative superiority of the CH inducer through the critical cavitation coefficient of 5 per cent, even if this inducer is very unstable for all flow rates. It was also found that the BH inducer is the most stable and that for a 15 per cent head drop, the three inducers have quite the same performances. The main experimental and numerical results presented here concern the overall performances, especially the hydrodynamic mechanism of head drop and the visualization of three-dimensional flow structures. We also present the vibratory behaviour versus the flow rate and the suction pressure. The computational three-dimensional flow in non-cavitating regimes enabled us to explain the unstable and cavitating operation for off-design conditions. The numerical computation in cavitating flows is carried out using the computational fluid dynamics (CFD) software CFX-TASCFlow2.12, which cavitation model was validated on our BH inducer.

Flow field investigation in a photocatalytic reactor for air treatment (Photo-CREC–air)

The aerodynamic behavior of a photocatalytic reactor for air treatment, Photo-CREC–air, with demonstrated high quantum efficiency performance, is examined using CFX-5.7.1. Photo-CREC–air consists of a venturi section that features low pressure drop and uniform illumination of the photocatalyst, resulting in high oxidation quantum efficiencies. The numerical simulations allowed the identification of several design issues in the original Photo-CREC–air unit, which include extensive boundary layer separation close to the photocatalyst support and regions of flow recirculation that render ca. 77% of the support surface area inactive. The simulations reveal that this issue could be addressed by replacing the wire-mesh basket sidewalls with perforated plates. This modification causes an increase in the pressure drop downstream of the support and achieves significant uniformization of the mass flow and air–photocatalyst contact time distributions. A modified Photo-CREC–air design is also presented and studied using CFX-5.7.1. This modified design is envisaged with the objective of improving UV-irradiation uniformity, an issue that is not completely addressed in the original design due to the shape of the windows and divergent section. CFD simulations reveal that, although the flow field is uniform, mass flow and contact time distributions are not. Nonetheless, this problem is addressed by increasing the pressure drop downstream of the support through the addition of a region modeled as a perforated plate. The simulations reveal that the mass flow and contact time distributions are significantly uniformized once this modification is implemented.

Computational investigations of a new prosthetic femoral-popliteal bypass graft design

Prosthetic femoral-popliteal bypasses are performed by using an end-to-side anastomosis, and disease can develop at the distal end; this can lead to poor long-term patency rates. Disturbed flow characteristics are hypothesized as being a major factor in promoting disease development. The objective of this study was to propose a new prosthetic femoral-popliteal bypass graft configuration specifically engineered to reduce or eliminate certain disease-influencing factors that act on the host artery. The proposed device contains a streamlined bifurcation toward its distal end that results in two end-to-end anastomoses, rather than the single end-to-side anastomosis in the traditional procedure. Comparisons are made between idealized representations of it and the traditional end-to-side anastomosis for specific femoral artery flow rates. Qualitative results in the form of velocity vector plots and wall shear stress contour plots are compared, and quantitative results examine the wall shear stress magnitudes and gradients along the bed and roof of each graft model. Velocity vector plots through each junction suggest that the proposed graft configuration promotes streamlined flow and helps to reduce the magnitude of flow recirculation and separation regions associated with the traditional end-to-side anastomosis. At peak velocity, the flow separation region distal to the toe is eliminated, as evidenced by the change in toe wall shear stress from -0.2 Pa in the traditional anastomosis to +0.5 Pa in the proposed device. Normal fully developed flow occurs sooner in the distal host artery, approximately 15 mm downstream from the toe, unlike 20 mm in the conventional case. The proposed design results in reductions of up to 58% in peak wall shear stress and 86% in peak wall shear stress gradient during the decelerative phase of the femoral pulse in the vicinity of the artery bed below the toe. In vitro tests on the proposed device suggest that the streamlined nature of blood flow through the junction does result in less disturbed hemodynamic conditions within the host artery junction. Abnormal wall shear stress magnitudes and gradients are reduced, and normal fully developed flow occurs sooner in the distal host artery. This suggests that the proposed graft may have design attributes that are relevant in the search for increased prosthetic femoral-popliteal bypass graft patency rates. A drawback of the proposed device is that significant flow recirculation and separation exist within the prosthesis itself. The search for a viable alternative to traditional end-to-side anastomosis for prosthetic femoral-popliteal bypass procedures is ongoing. Prosthetic femoral-popliteal bypass procedures have low long-term patency rates, and there is a need for methods that increase the life span of the procedure. Although research is progressing on a variety of different fronts, this study is significant in that it reports on in vitro tests on a potential device that may increase bypass patency. The device is simple, may be manufactured from clinically proven materials, does not require any additional training in its use, and combines attributes of end-to-side anastomoses with those of end-to-end anastomoses. In addition, the design concept behind the device, the natural bifurcation, may be relevant in other aspects of cardiovascular surgery.

Flow of a concentrated suspension through an abrupt axisymmetric expansion measured by nuclear magnetic resonance imaging

The objective of this work is to characterize the parameters affecting the particle concentration distribution and flow field of a concentrated suspension undergoing steady flow in an abrupt axisymmetric 1:4 expansion. Of particular interest are the relationships between imposed operating conditions and the resulting spatial particle distribution and the interaction between particles and recirculating flow regions. Experiments were conducted to determine the effect of the bulk particle volume fraction, tube-particle radius ratio, and Reynolds number on observed concentration and flow patterns. Particle concentration and velocity profiles were measured by using nuclear magnetic resonance imaging. Results indicate that inlet concentration profiles formed in the upstream narrow tube greatly influenced behavior downstream, more so than direct interparticle collisions in the abrupt expansion. Also, particle depletion in recirculating flow regions was observed under all conditions studied and is consistent with the results of previous work expressed in terms of the tube-particle radius ratio. Finally, in all cases, the lengths of recirculating regions were greater for suspensions than for Newtonian fluids at equivalent Reynolds numbers, and increased with bulk particle volume fraction. The flow behavior is reminiscent of shear thinning in a single-phase material, but shear-induced particle migration is a much stronger effect.

Contribution of vortex structures and flow separation to local and overall pressure and heat transfer characteristics in an ultralightweight lattice material

Ultralightweight lattice-frame materials (LFMs) with open, periodic microstructures are attractive multifunctional systems that can perform structural, thermal, actuation, power storage and other functions [A.G. Evans, J.W. Hutchinson, M.F. Ashby, Multifunctionality of cellular metal systems, Prog. Mater. Sci. 43 (1999) 171–221]. This paper presents experimental and numerical studies of local fluid flow behaviour and its contribution to local and overall pressure and heat transfer characteristics of such a lattice material with tetrahedral unit cells. A single layer of the LFM with porosity of 0.938 is sandwiched between impermeable endwalls that receive uniform heat flux and the heat transfer is subjected to forced air convection. Experimental measurements with particle image velocity (PIV) and thermochromic liquid crystal (TLC), backed by computational fluid mechanics (CFD) simulations, revealed two dominant local flow features in the LFM. Distinctive vortex structures near the vertices where the LFM meets the endwalls and flow separation on the surface of LFM struts were observed. The vortex structures formed around the vertices include horseshoe vortices and arch-shaped vortices. The horseshoe vortex increases local heat transfer on the endwall region up to 180% more than that in regions where the least influence of the horseshoe vortex is present. The arch-shaped vortex behind the vertices creates regions of flow recirculation and reattachment, leading to relatively high heat transfer. The location of flow separation along the struts varies with the spanwise position due to the presence of vertices (or endwalls). The regions on the strut surface before flow separation contribute approximately 40% of the total heat transfer in the LFM. The delay of the flow separation leads to an increase in the overall heat transfer. Comparisons with foams and other heat dissipation media such as packed beds, louvered fins and microtruss materials suggest that the LFMs compete favourably with the best available heat dissipation media.

Bifurcated three-dimensional forced convection in plane symmetric sudden expansion

Simulations of bifurcated three-dimensional laminar forced convection in horizontal duct with plane symmetric sudden expansion are presented to illustrate the effects of flow bifurcations on temperature and heat transfer distributions. The stable bifurcated flow that develops in this symmetric geometry leads to non-symmetric temperature and heat transfer distributions in the transverse direction, but symmetric distributions with respect to the center width of the duct in the spanwise directions for the Reynolds number of 400–800. A strong downwash develops at the corner of the step and a smaller reverse flow region develops adjacent to the lower stepped wall than the one that develops adjacent to the upper stepped wall. The downwash and the “jet-like” flow that develop near the sidewall create a strong swirling spanwise flow in the primary recirculating flow regions downstream from the sudden expansion. The magnitude of maximum Nusselt number that develops on the lower stepped walls is higher than the one that develops on the upper stepped wall. The locations of these maximum Nusselt numbers on the stepped walls are near the sidewalls and are upstream of the “jet-like” flow impingement regions. Results reveal that the locations where the streamwise component of wall shear stress is zero on the stepped walls do not coincide with the outer edge of the recirculation flow region near the sidewalls. Velocity, temperature, Nusselt number, and friction coefficient distributions are presented.

Flow Visualization of Micro Air Vehicle Scaled Insect-Based Flapping Wings.

Afl ow-visualization experiment was conducted on an insect-based flapping-wing mechanism. This enabled greater understanding and insight to be gained on the unsteady aerodynamic phenomena that are responsible for the enhanced lift of wings operating at low Reynolds numbers in hovering flapping flight. Flow-visualization images were acquired with a strobbed laser sheet to illuminate the flow, which was seeded with a mineral oil fog. The general flowfield structure was found to consist of a folded wake, with a relatively large starting vortex at the end of each half-stroke. A large flow recirculation region was generated in the plane of flapping, which was centered around the two extreme flapping displacements. These general flowfield features were enhanced by detailed observations of the local flowfield around the wing section. One observation was the presence of multiple vortices on the top surface of the wing as it underwent translation. Furthermore, the local flowfield images clearly showed the growth of the leading-edge vortex as a function of span and identified the presence of separated flow on the outboard regions of the wing. These experimental results were supported by a free vortex modeling of the wake developments. The model was found to predict similar wake flowfield dynamics to that found in the experiments. This research has contributed to a better understanding of the unsteady aerodynamic mechanisms that are responsible for the enhanced lift of insect-based flapping wings in hover.

Implications on flame stabilization in a precessing jet flame from near field velocity measurements

The near field of the three-dimensional velocity field in a turbulent precessing jet diffusion flame is investigated. In the present case the precessing jet flame is generated by mechanically rotating a round jet of propane inclined at 45° relative to the axis of rotation. Three-dimensional laser Doppler anemometry enables the fully turbulent, three-dimensional velocity field of a precessing jet flame to be measured. This provides insight into the mechanisms by which the flame is stabilized and supports previous explanations of why NOx emissions are reduced and fuel efficiency is increased in industrial precessing jet gas burners. Velocity data are presented, time-averaged, and phase-averaged at the frequency of precession. The conditional phase-averaging technique enables phase-averaged velocity contours and vectors to be obtained that reveal flow patterns and structures within the flame. Time-averaging of the velocity data also reveals the presence of a reverse flow (recirculation) region between the emerging fuel jet and its spinning axis. The recirculation and high shear is associated with a rapid decay of the mean velocity, which together act to stabilize the flame in the high velocity and high shear region near to the nozzle exit. The characteristics of the velocity field of a precessing jet flame found here are compared with previous cold-flow investigations of the same flow and with other investigations of turbulent jets.

Three-Dimensional Forced Convection in Plane Symmetric Sudden Expansion

Simulations of three-dimensional laminar forced convection in a plane symmetric sudden expansion are presented for Reynolds numbers where the flow is steady and symmetric. A swirling “jetlike” flow develops near the sidewalls in the separating shear layer, and its impingement on the stepped wall is responsible for the maximum that develops in the Nusselt number adjacent to the sidewalls and for the reverse flow that develops in that region. The maximum Nusselt number on the stepped wall is located inside the primary recirculation flow region and its location does not coincide with the jetlike flow impingement region. The results reveal that the location where the streamwise component of wall shear stress is zero on the stepped walls does not coincide with the outer edge of the primary recirculation flow region near the sidewalls.

Evolution of jets emanating from short holes into crossflow

The evolution of a short injection-hole jet issuing into a crossflow at low blowing ratios is presented. Particle image velocimetry (PIV) is used to determine structural features of the jet/crossflow interaction throughout its development from within the jet supply channel (which feeds the holes), through the injection hole, and into the crossflow. The effect of supply channel feed orientations, i.e. counter to, or in the same direction as the crossflow is emphasized. Feed orientation profoundly affects such jet characteristics as trajectory and lateral spreading, as well as its structural features. Fluid within the high-speed supply channel exhibits swirling motions similar to the flow induced by a pair of counter-rotating vortices. The sense of rotation of the swirling fluid depends upon the orientation of the supply channel flow with respect to the crossflow, and in turn impacts the in-hole velocity fields. In the coflow supply channel geometry (channel flow is in the same direction as the free stream), a pair of vortices exists within the hole with the same sense of rotation as the primary jet counter-rotating vortex pair (CRVP). In contrast, the counterflow supply channel configuration has in-hole vortices of opposite rotational sense to that of the CRVP. The in-hole vortices interact constructively or destructively with the CRVP, thus affecting the strength and coherence of the CRVP. The counterflow configuration has a weakened CRVP because of destructive interference with the in-hole vortices. The weaker CRVP has a lower trajectory and increased spanwise spreading. External to the injection hole, a pair of vortices exists immediately downstream of the jet. They are initially perpendicular to the boundary-layer plate near the wall and are outboard of the streamwise hole centreline. These vortices, denoted ‘downstream spiral separation node’ (DSSN) vortices, are affected by both the supply channel feed direction and the blowing ratio. They appear to form by free-stream fluid wrapping around the jet and interacting with the CRVP. The coflow supply channel geometry is associated with the largest and most well-defined DSSN vortices, and their size is inversely proportional to the blowing ratio. At low blowing ratio, these vortices induce a large recirculating flow region downstream of the injection hole at the wall.

Measurements in three-dimensional laminar separated flow

Velocity measurements are reported for three-dimensional laminar separated airflow adjacent to a backward-facing step using two-component laser Doppler velocimeter. The backward-facing step, with a height of S=1.0 cm, is mounted in a rectangular duct that has an upstream height of h=0.98 cm, downstream height of H=2 cm, and a width of W=8 cm. This geometry provides an aspect ratio of AR=8 and an expansion ratio of ER=2.02. The flow measurements covered a Reynolds number range between 98.5⩽ Re⩽525. Measurements of velocity distributions reveal that a swirling “jet-like” flow develops near the sidewall in the separating shear layer, and the impingement of that flow on the stepped wall causes a minimum to develop in the spanwise distribution of the reattachment region. Reverse and recirculation flow regions develop adjacent to both the sidewall and the step, and these regions increase in size as the Reynolds number increases. Velocity distributions that were measured at various planes downstream from the step are presented, and predictions compare favorably with these measurements. The results show some interesting flow behaviors that could not be deduced from two-dimensional studies.

Comparison of blood particle deposition models for non-parallel flow domains

Adhesions of monocytes and platelets to a vascular surface, particularly in regions of flow stagnation, recirculation, and reattachment, are a significant initial event in a broad spectrum of particle–wall interactions that significantly influence the formation of stenotic lesions and mural thrombi. A number of approximations are available for the simulation of both monocyte and platelet interactions with the vascular surface. For the simulation of blood particle adhesion, this study hypothesizes that: (a) the discrete element approach, which accounts for finite particle size and inertia, is advantageous in the context of non-parallel flow domains including stagnation, recirculation, and reattachment; and (b) the likelihood for particle deposition may be effectively approximated as being non-linearly proportional to local particle concentration, residence time, and wall proximity. Models such as wall shear stress correlations, the multicomponent mixture approach, and Lagrangian particle tracking with and without hydrodynamic particle–wall interactions were evaluated. Quantitative performance of the selected models was established by comparisons to available experimental data sets for non-parallel axisymmetric suspension flows of monocytes and platelets. Factors including the convective-diffusive transport of particles, finite particle size and inertia, as well as near-wall hydrodynamic interactions were found to significantly influence blood particle deposition. Of the models studied, the near-wall residence time approach was found to be a particularly effective indicator for the deposition of monocytes ( r 2=0.74) and platelets ( r 2=0.57), given that nano-scale physical and biochemical effects must be greatly approximated in computational simulations involving relatively large-scale geometries and complex flow fields.