Dynamical Significance of Turbulent Wall Layer Streaks

1990 ◽  
Vol 43 (5S) ◽  
pp. S219-S226 ◽  
Author(s):  
P. S. Bernard ◽  
R. A. Handler
Keyword(s):  
Reynolds Stress ◽  
Turbulence Modeling ◽  
Strong Association ◽  
Turbulent Channel Flow ◽  
Wall Layer ◽  
Low Speed ◽  
Process Fluid ◽  
Low Speed Streaks ◽  
Turbulent Wall

The role of low speed streaks in the dynamical processes leading to the generation of Reynolds stress is investigated using ensembles of computed particle paths obtained from a direct numerical simulation of turbulent channel flow. Simultaneous visualization of appropriate Eulerian fields and trajectories of fluid particles which are most indicative of Reynolds stress production are given. These graphically illustrate the occurrence of ejection events at a series of discrete locations along low speed streaks. A strong association between streamwise vortices and the ejecting fluid is found. In particular, visualization of the ejecting fluid shows the presence of vortices which drive fluid from the sides up and over the low speed regions. As part of this process fluid from within the streaks appears to be entrained outward from the wall. Some of the implications of these results for turbulence modeling will be described.

scholarly journals Transfer, Segregation and Deposition of Heavy Particles in Horizontal and Vertical Turbulent Channel Flows

2003 ◽  
Author(s):  
Cristian Marchioli ◽  
Fabio Sbrizzai ◽  
Alfredo Soldati
Keyword(s):  
Coherent Structures ◽  
Particle Distribution ◽  
Outer Region ◽  
Turbulent Channel Flow ◽  
Transfer Mechanism ◽  
Wall Turbulence ◽  
Wall Region ◽  
Turbulence Structure ◽  
Low Speed ◽  
Low Speed Streaks

Particle transfer in the wall region of turbulent boundary layers is dominated by the coherent structures which control the turbulence regeneration cycle. Coherent structures bring particles toward the wall and away from the wall and favour particle segregation in the viscous region giving rise to nonuniform particle distribution profiles which peak close to the wall. In this work, we focus on the transfer mechanism of different size particles and on the influence of gravity on particles deposition. By tracking O(105) particles in Direct Numerical Simulation (DNS) of a turbulent channel flow at Reτ = 150, we find that particles may reach the wall directly or may accumulate in the wall region, under the low-speed streaks. Even though low-speed streaks are ejection-like environments, particles are not re-entrained into the outer region. Particles segregated very near the wall by the trapping mechanisms we investigated in a previous work [1] are slowly driven to the wall. We find that gravity plays a role on particle distribution but, for small particles (τp+ < 3), the controlling transfer mechanism is related to near-wall turbulence structure.

On the role of the outer region in the turbulent-boundary-layer bursting process

1994 ◽  
Vol 259 ◽  
pp. 345-373 ◽  
Author(s):  
ROY Y. Myose ◽  
Ron F. Blackwelder
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Large Scale ◽  
Outer Region ◽  
Low Speed ◽  
Moderate Reynolds Number ◽  
Eddy Structures ◽  
Low Speed Streaks ◽  
Bursting Process

The dynamics and interaction of turbulent-boundary-layer eddy structures was experimentally emulated. Counter-rotating streamwise vortices and low-speed streaks emulating turbulent-boundary-layer wall eddies were generated by a Görtler instability mechanism. Large-scale motions associated with the outer region of turbulent boundary layer were emulated with — ωzspanwise vortical eddies shed by a periodic non-sinusoidal oscillation of an airfoil. The scales of the resulting eddy structures were comparable to a moderate-Reynolds-number turbulent boundary layer. Results show that the emulated wall-eddy breakdown was triggered by streamwise acceleration associated with the outer region of turbulent boundary layer. This breakdown involved violent mixing between low-speed fluid from the wall eddy and accelerated fluid associated with the outer structure. Although wall eddies can break down autonomously, the presence of and interaction with outer-region — ωzeddies hastened their breakdown. Increasing the — ωzeddy strength resulted in further hastening of the breakdown. Conversely, + ωzeddies were found to delay wall-eddy breakdown locally, with further delays resulting from stronger + ωzeddies. This suggests that the outer region of turbulent boundary layers plays a role in the bursting process.

Modeling the pressure strain correlation in turbulent flows using deep neural networks

2021 ◽  
pp. 095440622110429
Author(s):  
Jyoti P Panda ◽  
Hari V Warrior
Keyword(s):  
Machine Learning ◽  
Neural Networks ◽  
Turbulent Flows ◽  
Reynolds Stress ◽  
Channel Flow ◽  
Turbulence Modeling ◽  
Critical Role ◽  
Turbulent Channel Flow ◽  
Correlation Models ◽  
Turbulence Data

The pressure strain correlation plays a critical role in the Reynolds stress transport modeling. Accurate modeling of the pressure strain correlation leads to the proper prediction of turbulence stresses and subsequently the other terms of engineering interest. However, classical pressure strain correlation models are often unreliable for complex turbulent flows. Machine learning–based models have shown promise in turbulence modeling, but their application has been largely restricted to eddy viscosity–based models. In this article, we outline a rationale for the preferential application of machine learning and turbulence data to develop models at the level of Reynolds stress modeling. As an illustration, we develop data-driven models for the pressure strain correlation for turbulent channel flow using neural networks. The input features of the neural networks are chosen using physics-based rationale. The networks are trained with the high-resolution DNS data of turbulent channel flow at different friction Reynolds numbers (Reλ). The testing of the models is performed for unknown flow statistics at other Reλ and also for turbulent plane Couette flows. Based on the results presented in this article, the proposed machine learning framework exhibits considerable promise and may be utilized for the development of accurate Reynolds stress models for flow prediction.

Active control in the turbulent wall layer of a minimal flow unit

1996 ◽  
Vol 329 ◽  
pp. 341-371 ◽  
Author(s):  
Henry A. Carlson ◽  
John L. Lumley
Keyword(s):  
High Speed ◽  
Wall Layer ◽  
Flow Unit ◽  
Wall Region ◽  
Minimal Flow ◽  
Test Bed ◽  
Low Speed ◽  
Turbulent Wall ◽  
Friction Drag Reduction ◽  
Smart Skin

Direct simulations of flow in a channel with complex, time-dependent wall geometries facilitate an investigation of smart skin control in a turbulent wall layer (with skin friction drag reduction as the goal). The test bed is a minimal flow unit, containing one pair of coherent structures in the near-wall region: a high- and a low-speed streak. The controlling device consists of an actuator, Gaussian in shape and approximately twelve wall units in height, that emerges from one of the channel walls. Raising the actuator underneath a low-speed streak effects an increase in drag, raising it underneath a high-speed streak effects a reduction – indicating a mechanism for control. In the high-speed region, fast-moving fluid is lifted by the actuator away from the wall, allowing the adjacent low-speed region to expand and thereby lowering the average wall shear stress. Conversely, raising an actuator underneath a low-speed streak allows the adjacent high-speed region to expand, which increases skin drag.

Review—Advances in Three-Dimensional Turbulent Boundary Layers With Emphasis on the Wall-Layer Regions

1996 ◽  
Vol 118 (2) ◽  
pp. 219-232 ◽  
Author(s):  
J. P. Johnston ◽  
K. A. Flack
Keyword(s):  
Boundary Layers ◽  
Turbulence Modeling ◽  
State Of The Art ◽  
Measurement Techniques ◽  
Three Dimensional ◽  
Turbulent Boundary Layers ◽  
Wall Layer ◽  
The Other ◽  
Pressure Gradients ◽  
Low Speed Streaks

Current information concerning three-dimensional turbulent boundary layers is discussed. Several topics are presented including (i) a detailed description of eleven experiments published since 1990. In nine cases cross flows are controlled by pressure gradients imposed from the freestream, but in two cases the cross flows are wall-shear-driven. The other topics include (ii) an examination of state of the art in measurement techniques; (iii) a look at some issues and ideas in turbulence modeling; and (iv) an introduction to new work on the visualization and description of quasicoherent structures (high/low-speed streaks and turbulent vortices) in three-dimensional turbulent boundary layers.

Characteristics of Low-Speed Streaks and Vortex Structures in Drag-Reduced Turbulent Channel Flow Investigated by Stereoscopic PIV

2004 ◽  
Author(s):  
Feng-Chen Li ◽  
Yasuo Kawaguchi ◽  
Takehiko Segawa ◽  
Koichi Hishida
Keyword(s):  
Water Flow ◽  
Channel Flow ◽  
Water System ◽  
Turbulent Channel Flow ◽  
Stereoscopic Particle Image Velocimetry ◽  
Streamwise Vorticity ◽  
Solution Flow ◽  
Low Speed ◽  
Turbulence Structures ◽  
Low Speed Streaks

In the present study, we employed stereoscopic particle image velocimetry (SPIV) to investigate the characteristics of turbulence structures in a drag-reduced turbulent channel flow with addition of surfactant. The tested drag-reducing fluid was a CTAC (cetyltrimethyl ammonium chloride)/NaSal/Water system maintained at 25°C, having a 30-ppm concentration of CTAC. SPIV measurement was performed for a water flow (Re=1.1×105) and a CTAC solution flow (RE=1.5×105 with 54% drag reduction) in both the streamwise-spanwise and wall-normal-spanwise planes, respectively. A series of wall-normal vortex cores were found to align with the low-speed streaks with opposite vorticity signals at both sides of the streaks and with the vorticity decreased averagely by about one order in CTAC solution flow compared with water flow; the spanwise spacing between the low-speed streaks in the solution flow is increased by about 42%. The streamwise vorticity of the vortex cores appearing in the wall-normal-spanwise plane was also decreased by the use of additives.

Introducing Perturbations into Turbulent Wall-Bounded Flow With Arrays of Long TiO2 Nanowires

2014 ◽  
Vol 137 (2) ◽  
Author(s):  
Henry A. Sodano ◽  
Aneesh Koka ◽  
Christopher R. Guskey ◽  
T. Michael Seigler ◽  
Sean C. C. Bailey
Keyword(s):  
Nanowire Array ◽  
Turbulent Channel Flow ◽  
Wall Layer ◽  
Tio2 Nanowires ◽  
Tio2 Nanowire ◽  
Wall Flow ◽  
Flow Manipulation ◽  
Turbulent Wall ◽  
Active Flow ◽  
Near Wall

A currently unexplored mechanical application of nanowires is near-wall active flow manipulation, with potential uses mixing and filtering chemicals, enhancing convective heat transfer, and reducing drag. Here, we present experimental evidence that it is possible to introduce persistent perturbations into turbulent flow with active nanowires. A TiO2 nanowire array was fabricated and installed in the bounding wall of a turbulent channel flow, and the array was oscillated by external actuation. Measurements indicated that the array increased turbulent kinetic energy throughout the entire wall layer. These findings suggest that dynamically actuated nanowires can potentially be used to implement near-wall flow control.

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