scholarly journals Investigating a pulsating flow in the smooth channel and at the bifurcation section with regard to the popliteal artery hemodynamics

2021 ◽  
Vol 2119 (1) ◽  
pp. 012020
Author(s):  
V M Molochnikov ◽  
N I Mikheev ◽  
A N Mikheev ◽  
A A Paereliy ◽  
A E Goltsman
Keyword(s):  
Popliteal Artery ◽  
Vortex Flow ◽  
Pulsating Flow ◽  
Experimental Setup ◽  
Wall Region ◽  
Flow Rates ◽  
Turbulent Transition ◽  
Smooth Channel ◽  
Outflow Channels ◽  
Near Wall

Abstract Experimental setup is described. Pulsating flow in a smooth channel, and steady and pulsating flows at a bifurcation section simulating the distal end of an artery anastomosis at different flow rates in the main and outflow channels are studied. Indications of laminar-turbulent transition are observed in the near-wall region of the smooth channel. Mechanisms of turbulization of the near-wall region in the pulsating flow are suggested. Vortex flow structure in the bifurcation section is analyzed.

Near-Wall Velocity and Bubble Characteristics in Microbubble Laden Flows

2006 ◽  
Author(s):  
Daniel R. Morse ◽  
Thomas G. Shepard ◽  
James A. Liburdy
Keyword(s):  
Phase Velocity ◽  
Bubble Size ◽  
Fluid Velocity ◽  
Measurement Techniques ◽  
Bubble Velocity ◽  
Wall Region ◽  
Bubble Generation ◽  
Flow Rates ◽  
Piv Measurement ◽  
Near Wall

Microbubble drag reduction has been observed in high Reynolds number turbulent flows. The interaction of microbubbles with the viscous sublayer seems to be of interest. In this study a microchannel on the order of 100 microns was used to simulate the shear rate of the near-wall region of a high Re flow. The interaction of the fluid flow with microbubbles at low void fraction in a microchannel was studied. The microbubble sizes ranged from approximately 10–50 microns. The liquid phase velocities were obtained by PIV measurement techniques. Electrolysis was used to generate bubbles within the channel and microbubble velocities in the flow were determined using separate cross correlation calculations. Simultaneous comparisons are made between the image-averaged bubble velocity and the image-averaged fluid velocity. Image processing techniques were utilized to both remove bubbles and decrease noise in the image. Results are shown comparing the fluid only velocity profile with the two-phase velocity profiles at three flow rates and two bubble generation cases. Results presented include the phase velocity differences, bubble size and bubble separation distances for three flow rates and three different bubble generation levels. It is seen that flow rates within the microchannel significantly reduce the average bubble size.

Direct numerical simulations of Rayleigh–Bénard convection in water with non-Oberbeck–Boussinesq effects

2019 ◽  
Vol 881 ◽  
pp. 1073-1096 ◽  
Author(s):  
Andreas D. Demou ◽  
Dimokratis G. E. Grigoriadis
Keyword(s):  
Numerical Simulations ◽  
Two Dimensions ◽  
Direct Numerical Simulations ◽  
Wall Region ◽  
Number Range ◽  
Bénard Convection ◽  
Benard Convection ◽  
Rayleigh Benard Convection ◽  
Rayleigh Benard ◽  
Near Wall

Rayleigh–Bénard convection in water is studied by means of direct numerical simulations, taking into account the variation of properties. The simulations considered a three-dimensional (3-D) cavity with a square cross-section and its two-dimensional (2-D) equivalent, covering a Rayleigh number range of $10^{6}\leqslant Ra\leqslant 10^{9}$ and using temperature differences up to 60 K. The main objectives of this study are (i) to investigate and report differences obtained by 2-D and 3-D simulations and (ii) to provide a first appreciation of the non-Oberbeck–Boussinesq (NOB) effects on the near-wall time-averaged and root-mean-squared (r.m.s.) temperature fields. The Nusselt number and the thermal boundary layer thickness exhibit the most pronounced differences when calculated in two dimensions and three dimensions, even though the $Ra$ scaling exponents are similar. These differences are closely related to the modification of the large-scale circulation pattern and become less pronounced when the NOB values are normalised with the respective Oberbeck–Boussinesq (OB) values. It is also demonstrated that NOB effects modify the near-wall temperature statistics, promoting the breaking of the top–bottom symmetry which characterises the OB approximation. The most prominent NOB effect in the near-wall region is the modification of the maximum r.m.s. values of temperature, which are found to increase at the top and decrease at the bottom of the cavity.

The Effects of Adverse Pressure Gradients on Momentum and Thermal Structures in Transitional Boundary Layers: Part 2—Fluctuation Quantities

1996 ◽  
Vol 118 (4) ◽  
pp. 728-736 ◽  
Author(s):  
S. P. Mislevy ◽  
T. Wang
Keyword(s):  
Boundary Layer ◽  
Shear Stress ◽  
Transition Region ◽  
Boundary Layers ◽  
Reynolds Shear Stress ◽  
Turbulent Shear ◽  
Wall Region ◽  
Pressure Gradients ◽  
Baseline Case ◽  
Near Wall

The effects of adverse pressure gradients on the thermal and momentum characteristics of a heated transitional boundary layer were investigated with free-stream turbulence ranging from 0.3 to 0.6 percent. Boundary layer measurements were conducted for two constant-K cases, K1 = −0.51 × 10−6 and K2 = −1.05 × 10−6. The fluctuation quantities, u′, ν′, t′, the Reynolds shear stress (uν), and the Reynolds heat fluxes (νt and ut) were measured. In general, u′/U∞, ν′/U∞, and νt have higher values across the boundary layer for the adverse pressure-gradient cases than they do for the baseline case (K = 0). The development of ν′ for the adverse pressure gradients was more actively involved than that of the baseline. In the early transition region, the Reynolds shear stress distribution for the K2 case showed a near-wall region of high-turbulent shear generated at Y+ = 7. At stations farther downstream, this near-wall shear reduced in magnitude, while a second region of high-turbulent shear developed at Y+ = 70. For the baseline case, however, the maximum turbulent shear in the transition region was generated at Y+ = 70, and no near-wall high-shear region was seen. Stronger adverse pressure gradients appear to produce more uniform and higher t′ in the near-wall region (Y+ < 20) in both transitional and turbulent boundary layers. The instantaneous velocity signals did not show any clear turbulent/nonturbulent demarcations in the transition region. Increasingly stronger adverse pressure gradients seemed to produce large non turbulent unsteadiness (or instability waves) at a similar magnitude as the turbulent fluctuations such that the production of turbulent spots was obscured. The turbulent spots could not be identified visually or through conventional conditional-sampling schemes. In addition, the streamwise evolution of eddy viscosity, turbulent thermal diffusivity, and Prt, are also presented.

Turbulent Mass Transfer Simulations of Binary Electrolyte in Parallel-Plate Electrode Channel

2012 ◽  
Vol 550-553 ◽  
pp. 2014-2018
Author(s):  
Xiao Lan Zhou ◽  
Cai Xi Liu ◽  
Yu Hong Dong
Keyword(s):  
Mass Transfer ◽  
Turbulent Flows ◽  
Primary Objective ◽  
Wall Turbulence ◽  
Limiting Current ◽  
Solid Boundary ◽  
Wall Region ◽  
Schmidt Numbers ◽  
Near Wall ◽  
Turbulent Mass Transfer

Electrochemical mass transfer in turbulent flows and binary electrolytes is investigated. The primary objective is to provide information about mass transfer in the near-wall region between a solid boundary and a turbulent fluid flow at different Schmidt numbers. Based on the computational fluid dynamics and electrochemistry theories, a model for turbulent electrodes channel flow is established. The turbulent mass transfer in electrolytic processes has been predicted by the direct numerical simulation method under limiting current and galvanostatic conditions, we investigate mean concentration and the structure of the concentration fluctuating filed for different Schmidt numbers from 0.1 to 100 .The effect of different concentration boundary conditions at the electrodes on the near-wall turbulence statistics is also discussed.

Large-eddy simulations of tip leakage and secondary flows in an axial compressor cascade using a near-wall turbulence model

2009 ◽  
Vol 223 (6) ◽  
pp. 645-655 ◽  
Author(s):  
D Borello ◽  
G Delibra ◽  
K Hanjalić ◽  
F Rispoli
Keyword(s):  
Secondary Flows ◽  
Tip Clearance ◽  
Navier Stokes ◽  
Wall Region ◽  
Compressor Cascade ◽  
Tip Leakage ◽  
Computational Mesh ◽  
Large Eddy ◽  
Reynolds Averaged Navier Stokes ◽  
Near Wall

This paper reports on the application of unsteady Reynolds averaged Navier—Stokes (U-RANS) and hybrid large-eddy simulation (LES)/Reynolds averaged Navier—Stokes (RANS) methods to predict flows in compressor cascades using an affordable computational mesh. Both approaches use the ζ— f elliptic relaxation eddy-viscosity model, which for U-RANS prevails throughout the flow, whereas for the hybrid the U-RANS is active only in the near-wall region, coupled with the dynamic LES in the rest of the flow. In this ‘seamless’ coupling the dissipation rate in the k-equation is multiplied by a grid-detection function in terms of the ratio of the RANS and LES length scales. The potential of both approaches was tested in several benchmark flows showing satisfactory agreement with the available experimental results. The flow pattern through the tip clearance in a low-speed linear cascade shows close similarity with experimental evidence, indicating that both approaches can reproduce qualitatively the tip leakage and tip separation vortices with a relatively coarse computational mesh. The hybrid method, however, showed to be superior in capturing the evolution of vortical structures and related unsteadiness in the hub and wake regions.

Correction of Hot-Wire Measurements in the Near Non-Conducting Wall Region

2000 ◽  
Author(s):  
Li Wenzhong ◽  
B. C. Khoo ◽  
Xu Diao
Keyword(s):  
Numerical Simulation ◽  
Shear Flows ◽  
Control Volume ◽  
Domain Boundary ◽  
Wall Region ◽  
Computational Domain ◽  
Hot Wire ◽  
Conducting Wall ◽  
Simulation Results ◽  
Near Wall

Abstract The present paper is to determine the correction of hot-wire measurements when it is used to measure the shear flows region very close to the non-conducting wall. By numerical simulation of the Navier-Stokes and energy equations using the control volume method, we found that reasonably deployed grid distribution could largely reduce the computational domain size (for a typical Reynolds number for hot-wire near-wall measurements 4.0×10−3∼1.2, the domain boundary placing 650 diameters from the cylinder in front, rear and top is fair enough for accurate simulation, other than the domain boundary which places the 2000 diameters from the cylinder in front and top, and 3000 diameters from the cylinder in rear), and obtain the similar accuracy results for the correction of hot-wire measurements in the near-wall region. Numerical simulation results also show that, only taking the εf,εw (the maximum difference between the respective values of stream function and vorticity on successive iterations) as the criterion for convergence without judge to convergence of the temperature field seems not enough to obtain a convergent simulation result. This may be the possible reason which caused the discrepancy between the simulation results for hot-wire correction when using hot wire to measure the shear flows very close to the non-conducting wall.

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