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.

The minimal flow unit in near-wall turbulence

1991 ◽  
Vol 225 ◽  
pp. 213-240 ◽  
Author(s):  
Javier Jiménez ◽  
Parviz Moin
Keyword(s):  
Wall Stress ◽  
Wall Layer ◽  
Flow Unit ◽  
Wall Turbulence ◽  
Longitudinal Vortex ◽  
Spanwise Direction ◽  
Wall Region ◽  
Low Velocity ◽  
Good Agreement ◽  
Near Wall

Direct numerical simulations of unsteady channel flow were performed at low to moderate Reynolds numbers on computational boxes chosen small enough so that the flow consists of a doubly periodic (in x and z) array of identical structures. The goal is to isolate the basic flow unit, to study its morphology and dynamics, and to evaluate its contribution to turbulence in fully developed channels. For boxes wider than approximately 100 wall units in the spanwise direction, the flow is turbulent and the low-order turbulence statistics are in good agreement with experiments in the near-wall region. For a narrow range of widths below that threshold, the flow near only one wall remains turbulent, but its statistics are still in fairly good agreement with experimental data when scaled with the local wall stress. For narrower boxes only laminar solutions are found. In all cases, the elementary box contains a single low-velocity streak, consisting of a longitudinal strip on which a thin layer of spanwise vorticity is lifted away from the wall. A fundamental period of intermittency for the regeneration of turbulence is identified, and that process is observed to consist of the wrapping of the wall-layer vorticity around a single inclined longitudinal vortex.

A stereoscopic visual study of coherent structures in turbulent shear flow

1978 ◽  
Vol 89 (2) ◽  
pp. 251-272 ◽  
Author(s):  
Ananda K. Praturi ◽  
Robert S. Brodkey
Keyword(s):  
Boundary Layer ◽  
High Speed ◽  
Outer Region ◽  
Outer Edge ◽  
Turbulent Shear ◽  
Wall Region ◽  
Camera System ◽  
Low Speed ◽  
Visual Study ◽  
Axial Vortex

A visual study of a turbulent boundary-layer flow was conducted by photographing the motions of small tracer particles using a stereoscopic medium-speed camera system moving with the flow. In some experiments, dye injection at the leading edge of the flat plate helped to delineate the outer edge of the boundary layer. The technique allowed the three-dimensional aspects of the flow to be studied in some detail, and in particular allowed axial vortex motions in the wall region to be identified.The flow was found to exhibit three characteristic regions which can be roughly divided into the wall and outer regions of the boundary layer and an irrotational region, unmarked by dye, outside the instantaneous edge of the boundary layer. Briefly, the outer region of the boundary layer was dominated by transverse vortex motions that formed as a result of an interaction between low-speed and high-speed (sweep) fluid elements in that region. The present results clearly show that bulges in the edge of the boundary layer are associated with transverse vortex motions. In addition, the transverse vortex motions appear to induce massive inflows of fluid from the irrotational region deep into the outer region of the boundary layer. The outer edge of the boundary layer thus becomes further contorted, contributing to the intermittency of the region. Furthermore, the outer-region motions give rise to the conditions necessary for the dominant wall-region activity of ejections and axial vortex motions. It is not the energetic wall-region ejections that move to the outer region and give rise to the contorted edge of the boundary layer as has been suggested by others.The wall-region axial vortex motions were intense and lasted for a time short compared with the lifetime of outer-region transverse vortex motions. The present results strongly suggest that wall-region vortex motions are a result of interaction between the incoming higher-speed fluid from the outer region of the boundary layer and the outflowing low-speed wall-region fluid. This is in direct contrast to all models that suggest that axial vortex pairs in the wall region are the factor that gives rise to the outflow of low-speed fluid trapped between.Although all the elements necessary to make up a horseshoe vortex structure riding along the wall were present, such a composite was not observed. However, this could be visualized as a possible model to represent the ensemble average of the flow.Finally, the massive inflows from the irrotational region were observed to precede the appearance of low- and high-speed fluid elements in the boundary layer, thus completing the deterministic cycle of individual coherent events.

Large-eddy simulation of the turbulent flow through a heated square duct

2002 ◽  
Vol 453 ◽  
pp. 201-238 ◽  
Author(s):  
M. SALINAS VÁZQUEZ ◽  
O. MÉTAIS
Keyword(s):  
High Speed ◽  
Secondary Flows ◽  
Heated Wall ◽  
Wall Region ◽  
Central Plane ◽  
Square Duct ◽  
Low Speed ◽  
Size Limitation ◽  
Large Eddy ◽  
Near Wall

Large-eddy simulations of a compressible turbulent square duct flow at low Mach number are described. First, we consider the isothermal case with all the walls at the same temperature: good agreement with previous incompressible DNS and LES results is obtained both for the statistical quantities and for the turbulent structures. A heated duct with a higher temperature prescribed at one wall is then considered and the intensity of the heating is varied widely. The increase of the viscosity with temperature in the vicinity of the heated wall turns out to play a major rôle. We observe an amplification of the near-wall secondary flows, a decrease of the turbulent fluctuations in the near-wall region and, conversely, their enhancement in the outer wall region. The increase of the viscous thickness with heating implies a significant augmentation of the size of the characteristic flow structures such as the low- and high-speed streaks, the ejections and the quasi-longitudinal vorticity structures. For strong enough heating, the size limitation imposed by the lateral walls leads to a single low-speed streak located near the duct central plane surrounded by two high-speed streaks on both sides. Violent ejections of slow and hot fluid from the heated wall are observed, linked with the central low-speed streak. A selective statistical sampling of the most violent ejection events reveals that the entrainment of cold fluid, originated from the duct core, at the base of the ejection and its subsequent expansion amplifies the ejection intensity.

Surface streaming on nonbreaking wind waves

2021 ◽  
Author(s):  
Wu-ting Tsai ◽  
Guan-hung Lu
Keyword(s):  
High Speed ◽  
Wind Waves ◽  
Transport Processes ◽  
Low Speed ◽  
Coherent Vortices ◽  
Aqueous Surface ◽  
Streamwise Streaks ◽  
Low Speed Streaks ◽  
Turbulent Wall ◽  
Vorticity Transport

<p>The energetic, coherent vortical motions in the aqueous surface layer beneath the wind waves dominate the liquid-phase controlled transport processes across the air-water interface. Through interacting with the interface, these coherent vortices manifest themselves by forming quasi-streamwise, high-speed streaks on the wind waves. The density of these streamwise streaks, which can be quantified by the transverse spacing of streaks, thus characterizes the interfacial transfer contributed by the coherent vortices. The formation of surface streaming on the wind waves is geometrically similar to the low-speed streaks observed in the turbulent wall layers. It is generally accepted that the mean spanwise spacing between these low-speed streaks, when scaled by the viscous length, would exhibit a universal value of 100. Observations in wind-wave flumes, however, show that the transverse scale between high-speed streaming on nonbreaking wind waves is narrower than that between low-speed streaks next to no-slip wall. Comparative numerical simulations of shear flow bounded by flat and wavy surfaces are conducted to explain the variation. Analysis of the vorticity transport in the simulated flows bounded by a wavy surface reveals that the presence of surface waves enhances the production of streamwise enstrophy and, consequently, intensifies the generation of quasi-streamwise vortices that form the elongated streaks.<br>This work is supported by the Taiwan Ministry of Science and Technology (107-2611-M-002 -014 -MY3 and 110-2923-M-002 -014 -MY3).</p>

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.

Intermittent dynamics in simple models of the turbulent wall layer

1991 ◽  
Vol 230 ◽  
pp. 75-95 ◽  
Author(s):  
Gal Berkooz ◽  
Philip Holmes ◽  
J. L. Lumley
Keyword(s):  
Wall Layer ◽  
Orthogonal Decomposition ◽  
Wall Region ◽  
Heteroclinic Cycles ◽  
Transient Behaviour ◽  
Perturbation Velocity ◽  
Characteristic Features ◽  
Turbulent Wall ◽  
Proper Orthogonal ◽  
Closure Assumption

We generalize the class of models of the wall layer of Aubry et al. (1988), based on the proper orthogonal decomposition, to permit uncoupled evolution of streamwise and cross-stream disturbances. Since the Reynolds stress is no longer constrained, in the absence of streamwise spatial variations all perturbation velocity components eventually decay to zero. However, their transient behaviour is dominated by ’ghosts’ of the non-trivial fixed points and attracting heteroclinic cycles which are characteristic features of those models based on empirical eigenfunctions whose individual velocity components are fixed. This suggests that the intermittent events observed in Aubry et al. do not arise solely because of the effective closure assumption incorporated in those models, but are rooted deeper in the dynamical phenomenon of the wall region.

Computations of equilibrium and non-equilibrium turbulent channel flows using a nested-LES approach

2016 ◽  
Vol 793 ◽  
pp. 709-748 ◽  
Author(s):  
Yifeng Tang ◽  
Rayhaneh Akhavan
Keyword(s):  
Channel Flow ◽  
Computational Cost ◽  
Turbulent Channel Flow ◽  
Flow Unit ◽  
Shear Stresses ◽  
Wall Region ◽  
Computational Domain ◽  
Minimal Flow ◽  
Turbulence Statistics ◽  
Non Equilibrium

A new nested-LES approach for computation of high Reynolds number, equilibrium, and non-equilibrium, wall-bounded turbulent flows is presented. The method couples coarse-resolution LES in the full computational domain with fine-resolution LES in a minimal flow unit to retain the accuracy of well-resolved LES throughout the computational domain, including in the near-wall region, while significantly reducing the computational cost. The two domains are coupled by renormalizing the instantaneous velocity fields in each domain dynamically during the course of the simulation to match the wall-normal profiles of single-time, ensemble-averaged kinetic energies of the components of ‘mean’ and fluctuating velocities in the inner layer to those of the minimal flow unit, and in the outer layer to those of the full domain. This simple renormalization procedure is shown to correct the energy spectra and wall shear stresses in both domains, thus leading to accurate turbulence statistics. The nested-LES approach has been applied to computation of equilibrium turbulent channel flow at $Re_{{\it\tau}}\approx 1000$, 2000, 5000, 10 000, and non-equilibrium, strained turbulent channel flow at $Re_{{\it\tau}}\approx 2000$. In both flows, nested-LES predicts the skin friction coefficient, first- and higher-order turbulence statistics, spectra and structure of the flow in agreement with available DNS and experimental data. Nested-LES can be applied to any flow with at least one direction of local or global homogeneity, while reducing the required number of grid points from $O(Re_{{\it\tau}}^{2})$ of conventional LES to $O(\log Re_{{\it\tau}})$ or $O(Re_{{\it\tau}})$ in flows with two or one locally or globally homogeneous directions, respectively.

Streamwise vortices associated with the bursting phenomenon

1979 ◽  
Vol 94 (3) ◽  
pp. 577-594 ◽  
Author(s):  
Ron F. Blackwelder ◽  
Helmut Eckelmann
Keyword(s):  
High Speed ◽  
Vortex Pair ◽  
Turbulent Shear ◽  
Sampling Techniques ◽  
Wall Region ◽  
Turbulent Shear Flow ◽  
Streamwise Vortices ◽  
Defect Region ◽  
Low Speed ◽  
Hot Film

The streamwise and spanwise velocity components and the gradients of these components normal to the wall were examined by using hot-film sensors and flush-mounted wall elements to study the vortex structures associated with the bursting phenomenon. Quadrant probability analysis and conditional sampling techniques indicated that pairs of counter-rotating streamwise vortices occur frequently in the wall region of a bounded turbulent shear flow. A streamwise momentum defect occurred between the vortices as low-speed fluid was ‘pumped’ away from the wall by the vortex pair. The defect region was long and narrow and possibly forms the low-speed streak as observed in visualization studies. The velocity defect was terminated by a strong acceleration followed by a high speed region.

Do You Name Speedy Objects Faster Than Slow Objects: SPEEDED NAMING OR NAMING SPEED? THE AUTOMATIC EFFECT OF OBJECT SPEED ON PERFORMANCE

2018 ◽  
Author(s):  
Moshe Shay Ben-Haim ◽  
Eran Chajut ◽  
Ran Hassin ◽  
Daniel Algom
Keyword(s):  
High Speed ◽  
Mental Representations ◽  
Reading Aloud ◽  
Naming Task ◽  
Naming Speed ◽  
Low Speed ◽  
Everyday Objects ◽  
Pictures And Words

we test the hypothesis that naming an object depicted in a picture, and reading aloud an object’s name, are affected by the object’s speed. We contend that the mental representations of everyday objects and situations include their speed, and that the latter influences behavior in instantaneous and systematic ways. An important corollary is that high-speed objects are named faster than low-speed objects despite the fact that object speed is irrelevant to the naming task at hand. The results of a series of 7 studies with pictures and words support these predictions.

scholarly journals Splatters and Aerosols Contamination in Dental Aerosol Generating Procedures

2021 ◽  
Vol 11 (4) ◽  
pp. 1914
Author(s):  
Pingping Han ◽  
Honghui Li ◽  
Laurence J. Walsh ◽  
Sašo Ivanovski
Keyword(s):  
Particle Size ◽  
Disease Transmission ◽  
High Speed ◽  
Low Speed ◽  
Dental Procedures ◽  
Dental Equipment ◽  
Produce Contamination ◽  
Fluorescein Dye ◽  
Filter Papers ◽  
Airborne Disease

Dental aerosol-generating procedures produce a large amount of splatters and aerosols that create a major concern for airborne disease transmission, such as COVID-19. This study established a method to visualise splatter and aerosol contamination by common dental instrumentation, namely ultrasonic scaling, air-water spray, high-speed and low-speed handpieces. Mock dental procedures were performed on a mannequin model, containing teeth in a typodont and a phantom head, using irrigation water containing fluorescein dye as a tracer. Filter papers were placed in 10 different locations to collect splatters and aerosols, at distances ranging from 20 to 120 cm from the source. All four types of dental equipment produced contamination from splatters and aerosols. At 120 cm away from the source, the high-speed handpiece generated the greatest amount and size (656 ± 551 μm) of splatter particles, while the triplex syringe generated the largest amount of aerosols (particle size: 1.73 ± 2.23 μm). Of note, the low-speed handpiece produced the least amount and size (260 ± 142 μm) of splatter particles and the least amount of aerosols (particle size: 4.47 ± 5.92 μm) at 120 cm. All four dental AGPs produce contamination from droplets and aerosols, with different patterns of distribution. This simple model provides a method to test various preventive strategies to reduce risks from splatter and aerosols.

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