Low-speed streak instability in near wall turbulence with adverse pressure gradient

A low-Reynolds number k-ε model is proposed in which the anisotropic production in near-wall regions is accounted for substantially by modifying the model constants Cε(1,2) and adding cross-diffusion terms in the ε equation. Hence, it reduces the kinetic energy and length scale magnitudes to improve prediction of adverse pressure gradient flows, involving flow separation and reattachment. Unlike the conventional k-ε model, it requires no wall function/distance parameter that bridges the near-wall integration. The model coefficients/functions depend nonlinearly on both the strain-rate and vorticity invariants. The model is validated against a few flow cases, yielding predictions in good agreement with the direct numerical simulation (DNS) and experimental data.

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The effect of wall-normal gravity on particle-laden near-wall turbulence

Journal of Fluid Mechanics ◽

10.1017/jfm.2019.400 ◽

2019 ◽

Vol 873 ◽

pp. 475-507 ◽

Cited By ~ 5

Author(s):

Junghoon Lee ◽

Changhoon Lee

Keyword(s):

Turbulent Channel Flow ◽

Viscous Sublayer ◽

Stokes Number ◽

Wall Turbulence ◽

Small Scale ◽

Gravitational Acceleration ◽

Lower Wall ◽

Low Speed ◽

Near Wall Turbulence ◽

Near Wall

We performed two-way coupled direct numerical simulations of turbulent channel flow with Lagrangian tracking of small, heavy spheres at a dimensionless gravitational acceleration of 0.077 in wall units, which is based on the flow condition in the experiment by Gerashchenko et al. (J. Fluid Mech., vol. 617, 2008, pp. 255–281). We removed deposited particles after several collisions with the lower wall and then released new particles near the upper wall to observe direct interactions between particles and coherent structures of near-wall turbulence during gravitational settling through the mean shear. The results indicate that when the Stokes number is approximately 1 on the basis of the Kolmogorov time scale of the flow ($St_{K}\approx 1$), the so-called preferential sweeping occurs in association with coherent streamwise vortices, while the effect of crossing trajectories becomes significant for $St_{K}>1$. Consequently, in either case, the settling particles deposit on the wall without strong accumulation in low-speed streaks in the viscous sublayer. When particles settle through near-wall turbulence from the upper wall, more small-scale vortical structures are generated in the outer layer as low-speed fluid is pulled farther in the direction of gravity, while the opposite is true near the lower wall.

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Effect of surface roughness element on near wall turbulence with zero-pressure gradient

Science China Physics Mechanics and Astronomy ◽

10.1007/s11433-015-5677-4 ◽

2015 ◽

Vol 58 (6) ◽

pp. 1-8 ◽

Cited By ~ 4

Author(s):

Xin Zhang ◽

Chong Pan ◽

JunQi Shen ◽

JinJun Wang

Keyword(s):

Surface Roughness ◽

Pressure Gradient ◽

Roughness Element ◽

Wall Turbulence ◽

Near Wall Turbulence ◽

Effect Of Surface ◽

Near Wall

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On near-wall turbulent flow modelling

Journal of Fluid Mechanics ◽

10.1017/s0022112090003718 ◽

1990 ◽

Vol 221 ◽

pp. 641-673 ◽

Cited By ~ 102

Author(s):

Y. G. Lai ◽

R. M. C. So

Keyword(s):

Reynolds Number ◽

Pressure Gradient ◽

Reynolds Stress ◽

Rate Equation ◽

Dissipation Rate ◽

Wall Turbulence ◽

Pressure Diffusion ◽

Velocity Pressure ◽

Near Wall Turbulence ◽

Near Wall

The characteristics of near-wall turbulence are examined and the result is used to assess the behaviour of the various terms in the Reynolds-stress transport equations. It is found that all components of the velocity-pressure-gradient correlation vanish at the wall. Conventional splitting of this second-order tensor into a pressure diffusion part and a pressure redistribution part and subsequent neglect of the pressure diffusion term in the modelled Reynolds-stress equations leads to finite near-wall values for two components of the redistribution tensor. This, therefore, suggests that, in near-wall turbulent flow modelling, the velocity-pressure-gradient correlation rather than pressure redistribution should be modelled. Based on this understanding, a methodology to derive an asymptotically correct model for the velocity-pressure-gradient correlation is proposed. A model that has the property of approaching the high-Reynolds-number model for pressure redistribution far away from the wall is derived. A similar analysis is carried out on the viscous dissipation term and asymptotically correct near-wall modifications are proposed. The near-wall closure based on the Reynolds-stress equations and a conventional low-Reynolds-number dissipation-rate equation is used to calculate fully-developed turbulent channel and pipe flows at different Reynolds numbers. A careful parametric study of the model constants introduced by the near-wall closure reveals that one constant in the dissipation-rate equation is Reynolds-number dependent, and a preliminary expression is proposed for this constant. With this modification, excellent agreement with near-wall turbulence statistics, measured and simulated, is obtained, especially the anisotropic behaviour of the normal stresses. On the other hand, it is found that the dissipation-rate equation has a significant effect on the calculated Reynolds-stress budgets. Possible improvements could be obtained by using available direct simulation data to help formulate a more realistic dissipation-rate equation. When such an equation is available, the present approach can again be used to derive a near-wall closure for the Reynolds-stress equations. The resultant closure could give improved predictions of the turbulence statistics and the Reynolds-stress budgets.

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