Numerical and theoretical investigation of pulsatile turbulent channel flows

2016 ◽  
Vol 792 ◽  
pp. 98-133 ◽  
Author(s):  
Chenyang Weng ◽  
Susann Boij ◽  
Ardeshir Hanifi
Keyword(s):  
Shear Stress ◽  
Reynolds Shear Stress ◽  
Nonlinear Effects ◽  
Turbulent Channel Flow ◽  
Theoretical Models ◽  
Phase Lag ◽  
Reynolds Stresses ◽  
Turbulent Eddies ◽  
Turbulent Channel Flows ◽  
Physical Features

A turbulent channel flow subjected to imposed harmonic oscillations is studied by direct numerical simulation (DNS) and theoretical models. Simulations have been performed for different pulsation frequencies. The time- and phase-averaged data have been used to analyse the flow. The onset of nonlinear effects during the production of the perturbation Reynolds stresses is discussed based on the DNS data, and new physical features observed in the DNS are reported. A linear model proposed earlier by the present authors for the coherent perturbation Reynolds shear stress is reviewed and discussed in depth. The model includes the non-equilibrium effects during the response of the Reynolds stress to the imposed periodic shear straining, where a phase lag exists between the stress and the strain. To validate the model, the perturbation velocity and Reynolds shear stress from the model are compared with the DNS data. The performance of the model is found to be good in the frequency range where quasi-static assumptions are invalid. The viscoelastic characteristics of the turbulent eddies implied by the model are supported by the DNS data. Attempts to improve the model are also made by incorporating the DNS data in the model.

DNS of Drag-Reducing Turbulent Channel Flow With Coexisting Newtonian and Non-Newtonian Fluid

2005 ◽  
Vol 127 (5) ◽  
pp. 929-935 ◽  
Author(s):  
Bo Yu ◽  
Yasuo Kawaguchi
Keyword(s):  
Shear Stress ◽  
Reynolds Shear Stress ◽  
Flow Region ◽  
Turbulent Channel Flow ◽  
Turbulent Convection ◽  
Turbulent Channel Flows ◽  
Entire Flow ◽  
Shear Induced Structure ◽  
Induced Structure ◽  
And Diffusion

In the present study, we numerically investigated drag-reducing turbulent channel flows by surfactant additives. Surfactant additives were assumed to be uniformly distributed in the entire flow region by turbulent convection and diffusion, etc., but it was assumed that the shear-induced structure (SIS) (network of rod-like micelles) could form either in the region next to the walls or in the center region of the channel, making the fluid viscoelastic. In other regions surfactant additives were assumed to be incapable of building a network structure, and to exist in the form of molecules or micelles that do not affect the Newtonian properties of the fluid. With these assumptions, we studied the drag-reducing phenomenon with coexisting Newtonian and non-Newtonian fluids. From the study we identified the effectiveness of the network structures at different flow regions, and showed that the phenomenon of drag-reduction (DR) by surfactant additives is not only closely associated with the reduction of Reynolds shear stress but also related to the induced viscoelastic shear stress.

DNS of Drag-Reducing Turbulent Channel Flow With Coexisting Newtonian and Non-Newtonian Fluid

2004 ◽  
Author(s):  
Bo Yu ◽  
Yasuo Kawaguchi
Keyword(s):  
Shear Stress ◽  
Reynolds Shear Stress ◽  
Flow Region ◽  
Turbulent Channel Flow ◽  
Turbulent Convection ◽  
Turbulent Channel Flows ◽  
Entire Flow ◽  
Shear Induced Structure ◽  
Induced Structure ◽  
And Diffusion

In the present study, we numerically investigated drag-reducing turbulent channel flows by surfactant additives. Surfactant additives were assumed to be uniformly distributed in the entire flow region by turbulent convection and diffusion etc., but it was assumed that the shear-induced structure (SIS) (network of rod-like micelles) could form either in the region next to the walls or in the center region of the channel, making the fluid viscoelastic. In other regions surfactant additives were assumed to be incapable of building a network structure, and to exist in form of molecules or micelles which do not affect the Newtonian properties of the fluid. With these assumptions, we studied the drag-reducing phenomenon with coexisting Newtonian and non-Newtonian fluids. From the study we identified the effectiveness of the network structures at different flow regions, and showed that the phenomenon of drag-reduction (DR) by surfactant additives is not only closely associated with the reduction of Reynolds shear stress but also related to the induced viscoelastic shear stress.

Turbulence Measurements in Channel Flows With Rough and Hydrophobic Walls

2007 ◽  
Author(s):  
N. Ahmad ◽  
R. N. Parthasarathy
Keyword(s):  
Shear Stress ◽  
Rough Surface ◽  
Test Section ◽  
Reynolds Shear Stress ◽  
Turbulent Channel Flow ◽  
Surface Flow ◽  
Wall Region ◽  
Mean Velocity ◽  
Image Velocimetry ◽  
Coated Surface

Particle Image Velocimetry (PIV) measurements were made in a fully-developed turbulent channel flow. The channel test section was 1 ft wide and 1 inch in height and was constructed out of plexiglass. One wall of the test section was made removable. Four walls were used: a plexiglass smooth wall, and three hydrophobic walls: (i) a lotus paint coated plexiglass wall, (ii) a treated aluminum sheet attached to the plexiglass wall and (iii) a treated rough surface attached to the plexiglass wall. The bulk velocity was held constant to yield a Reynolds number (based on the channel half-height) of 5,500. Several images were averaged to obtain mean velocity and Reynolds shear stress and turbulence kinetic energy measurements. It was found that the mean velocities in the near-wall region were higher for the lotus-paint coated surface flow and the treated rough surface flow than the flows with the other two surfaces. The friction velocity estimated from the Reynolds shear stress measurements was significantly lower for these two flows as well. The reduction in the wall shear stress in these flows is attributed to the finite slip that occurs at the hydrophobic surfaces.

Logarithmic Expansions for Reynolds Shear Stress and Reynolds Heat Flux in a Turbulent Channel Flow

2008 ◽  
Vol 130 (9) ◽  
Author(s):  
Abu Seena ◽  
A. Bushra ◽  
Noor Afzal
Keyword(s):  
Heat Flux ◽  
Shear Stress ◽  
Channel Flow ◽  
Intermediate Layer ◽  
Reynolds Shear Stress ◽  
Turbulent Channel Flow ◽  
Closure Models ◽  
Fully Developed Turbulent Flow ◽  
Logarithmic Functions ◽  
Heat And Fluid Flow

The heat and fluid flow in a fully developed turbulent channel flow have been investigated. The closure model of Reynolds shear stress and Reynolds heat flux as a function of a series of logarithmic functions in the mesolayer variable have been adopted. The interaction between inner and outer layers in the mesolayer (intermediate layer) arising from the balance of viscous effect, pressure gradient and Reynolds shear stress (containing the maxima of Reynolds shear stress) was first proposed by Afzal (1982, “Fully Developed Turbulent Flow in a Pipe: An Intermediate Layer,” Arch. Appl. Mech., 53, 355–377). The unknown constants in the closure models for Reynolds shear stress and Reynolds heat flux have been estimated from the prescribed boundary conditions near the axis and surface of channel. The predictions are compared with the DNS data Iwamoto et al. and Abe et al. for Reynolds shear stress and velocity profile and Abe et al. data of Reynolds heat flux and temperature profile. The limitations of the closure models are presented.

Errors Characterization of a RANS Simulation

2021 ◽  
Author(s):  
Hugo D. Pasinato ◽  
Ezequiel Arthur Krumrick
Keyword(s):  
Shear Stress ◽  
Turbulent Flows ◽  
Reynolds Shear Stress ◽  
Strain Tensor ◽  
Numerical Code ◽  
Reynolds Stresses ◽  
Mean Velocity ◽  
Rans Simulation ◽  
Eddy Viscosity Model ◽  
The Mean

Abstract This research uses data from direct numerical simulation (DNS) to characterize the different errors associated with a Reynolds-averaged Navier-Stokes (RANS) simulation. The statistics from DNS (Reynolds stresses, kinetic energy of turbulence, $\kappa$, and dissipation of turbulence, $\epsilon$), are fed into a RANS simulation with the same Reynolds number, geometry, and numerical code used for DNS. Three integral metrics error based on the mean velocity, the moduli of the mean rate-of-strain tensor, and the wall shear stress are used to characterize the errors associated with the RANS technique, with the RANS model, and with the linear eddy viscosity model (LEVM). For developed and perturbed flow, it is found that the mean velocity of the RANS simulations with the DNS statistics is almost the same as the mean velocity from DNS data. This procedure enables the study of the relative importance of the different Reynolds stresses in a particular flow. It is shown that for the bounded perturbed turbulent flows studied here, almost all the necessary effects of turbulence are contained in the Reynolds shear stress.

scholarly journals Near-field flow structure of a confined wall jet on flat and concave rough walls

2008 ◽  
Vol 606 ◽  
pp. 27-49 ◽  
Author(s):  
I. ALBAYRAK ◽  
E. J. HOPFINGER ◽  
U. LEMMIN
Keyword(s):  
Shear Stress ◽  
Shear Layer ◽  
Outer Layer ◽  
Reynolds Shear Stress ◽  
Near Field ◽  
Wall Jet ◽  
Reynolds Stresses ◽  
Mean Flow ◽  
Free Jet ◽  
The Mean

Experimental results are presented of the mean flow and turbulence characteristics in the near field of a plane wall jet issuing from a nozzle onto flat and concave walls consisting of fixed sand beds. This is a flow configuration of interest for sediment erosion, also referred to as scouring. The measurements were made with an acoustic profiler that gives access to the three components of the instantaneous velocities. For the flat-wall flow, it is shown that the outer-layer spatial growth rate and the maxima of the Reynolds stresses approach the values accepted for the far field of a wall jet at a downstream distance x/b0 ≈ 8. These maxima are only about half the values of a plane free jet. This reduction in Reynolds stresses is also observed in the shear-layer region, x/b0 < 6, where the Reynolds shear stress is about half the value of a free shear layer. At distances x/b0 > 11, the maximum Reynolds shear stress approaches the value of a plane free jet. This change in Reynolds stresses is related to the mean vertical velocity that is negative for x/b0 < 8 and positive further downstream. The evolution of the inner region of the wall jet is found to be in good agreement with a previous model that explicitly includes the roughness length.On the concave wall, the mean flow and the Reynolds stresses are drastically changed by the adverse pressure gradient and especially by the development of Görtler vortices. On the downslope side of the scour hole, the flow is nearly separating with the wall shear stress tending to zero, whereas on the upslope side, the wall-friction coefficient is increased by a factor of about two by Görtler vortices. These vortices extend well into the outer layer and, just above the wall, cause a substantial increase in Reynolds shear stress.

Drag Reduction Due to Streamwise Grooves in Turbulent Channel Flow

2016 ◽  
Vol 138 (12) ◽  
Author(s):  
C. T. DeGroot ◽  
C. Wang ◽  
J. M. Floryan
Keyword(s):  
Shear Stress ◽  
Drag Reduction ◽  
Turbulent Channel Flow ◽  
Surface Modifications ◽  
Channel Flows ◽  
Wave Number ◽  
Stress Profile ◽  
Bulk Fluid ◽  
Geometry Model ◽  
Turbulent Channel Flows

Drag reduction in turbulent channel flows has significant practical relevance for energy savings. Various methods have been proposed to reduce turbulent skin friction, including microscale surface modifications such as riblets or superhydrophobic surfaces. More recently, macroscale surface modifications in the form of longitudinal grooves have been shown to reduce drag in laminar channel flows. The purpose of this study is to show that these grooves also reduce drag in turbulent channel flows and to quantify the drag reduction as a function of the groove parameters. Results are obtained using computational fluid dynamics (CFD) simulations with turbulence modeled by the k–ω shear-stress transport (SST) model, which is first validated with direct numerical simulations (DNS). Based on the CFD results, a reduced geometry model is proposed which shows that the approximate drag reduction can be quantified by evaluating the drag reduction of the geometry given by the first Fourier mode of an arbitrary groove geometry. Results are presented to show the drag reducing potential of grooves as a function of Reynolds number as well as groove wave number, amplitude, and shape. The mechanism of drag reduction is discussed, which is found to be due to a rearrangement of the bulk fluid motion into high-velocity streamtubes in the widest portion of the channel opening, resulting in a change in the wall shear stress profile.

Turbulent Stresses in the Region of a Hancock Porcine Bioprosthetic Aortic Valve

1985 ◽  
Vol 107 (3) ◽  
pp. 200-205 ◽  
Author(s):  
F. J. Walburn ◽  
H. N. Sabbah ◽  
P. D. Stein
Keyword(s):  
Shear Stress ◽  
Aortic Valve ◽  
Stroke Volume ◽  
Normal Stress ◽  
Reynolds Shear Stress ◽  
Laser Doppler Anemometer ◽  
Bioprosthetic Valve ◽  
Laser Doppler ◽  
Reynolds Stresses

The purpose of this study was to measure stresses associated with turbulence (Reynolds stresses), in the region of a 29-mm-dia porcine bioprosthetic valve (Hancock, Model 242). Studies were performed in an in vitro pulse duplicating system with the valve mounted in the aortic position. The Reynolds stresses were calculated from velocities obtained with a two channel laser Doppler anemometer. The largest Reynolds shear stress and normal stress occurred at the highest stroke volume used (80 mL). Averaged over ejection they were 38 dynes/cm2 and 380 dynes/cm2, respectively. The maximal instantaneous Reynolds shear stress was 2500 dynes/cm2 and the maximal instantaneous Reynolds normal stress was 6800 dynes/cm2. Stresses of these magnitudes are in the range reported to damage platelets.

Eddy Viscosity and Reynolds Stress Models of Entropy Generation in Turbulent Channel Flows

2017 ◽  
Vol 139 (3) ◽  
Author(s):  
J. Sun ◽  
D. Kuhn ◽  
G. Naterer
Keyword(s):  
Shear Stress ◽  
Entropy Production ◽  
Eddy Viscosity ◽  
Reynolds Shear Stress ◽  
Channel Flows ◽  
Alternative Formulation ◽  
Mean Flow ◽  
Turbulent Channel Flows ◽  
Entropy Transport ◽  
New Models

This paper presents new models of entropy production for incompressible turbulent channel flows. A turbulence model is formulated and analyzed with direct numerical simulation (DNS) data. A Reynolds-averaged Navier–Stokes (RANS) approach is used and applied to the turbulence closure of mean and fluctuating variables and entropy production. The expression of the mean entropy production in terms of other mean flow quantities is developed. This paper presents new models of entropy production by incorporating the eddy viscosity into the total shear stress. Also, the Reynolds shear stress is used as an alternative formulation. Solutions of the entropy transport equations are presented and discussed for both laminar and turbulent channel flows.

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