Bifurcations and bursting of streaks in the turbulent wall layer

AbstractWe investigate the scaling of the energy-containing eddies in the outer part of turbulent wall layers. Their spanwise integral length scales are extracted from a direct numerical simulation (DNS) database, which includes compressible turbulent boundary layers and incompressible turbulent Couette–Poiseuille flows. The results indicate similar behaviour for all classes of flows, with a general increasing trend in the eddy size with the wall distance. A family of scaling relationships are proposed based on simple dimensional arguments, of which the classical mixing length approximation constitutes one example. As in previous studies, we find that the mixing length is in good agreement with the size distribution of the eddies carrying wall-normal velocity, which are active in establishing the mean velocity distribution. However, we find that the eddies associated with wall-parallel motions obey a different scaling, which is controlled by the local mean shear and by an effective eddy diffusivity ${\nu }_{t} = { u}_{\tau }^{\ensuremath{\ast} } \delta $, where ${ u}_{\tau }^{\ensuremath{\ast} } $ is the compressible counterpart of the friction velocity, and $\delta $ is the thickness of the wall layer. The validity of the proposed scalings is checked against DNS data, and the potential implications for the understanding of wall turbulence are discussed.

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Instability in a viscous flow driven by streamwise vortices

Journal of Fluid Mechanics ◽

10.1017/s0022112000003256 ◽

2001 ◽

Vol 432 ◽

pp. 127-166 ◽

Cited By ~ 28

Author(s):

K. W. BRINCKMAN ◽

J. D. A. WALKER

Keyword(s):

Boundary Layer ◽

Numerical Solutions ◽

Three Dimensional ◽

Wall Layer ◽

External Flow ◽

Three Dimensional Flow ◽

Dimensional Flow ◽

Wall Flow ◽

Turbulent Wall ◽

Near Wall

Unsteady separation processes at large finite, Reynolds number, Re, are considered, as well as the possible relation to existing descriptions of boundary-layer separation in the limit Re → ∞. The model problem is a fundamental vortex-driven three-dimensional flow, believed to be relevant to bursting near the wall in a turbulent boundary layer. Bursting is known to be associated with streamwise vortex motion, but the vortex/wall interactions that drive the near-wall flow toward breakdown have not yet been fully identified. Here, a simulation of symmetric counter-rotating vortices is used to assess the influence of sustained pumping action on the development of a viscous wall layer. The calculated solutions describe a three-dimensional flow at finite Re that is independent of the streamwise coordinate and consists of a crossflow plane motion, with a developing streamwise flow. The unsteady problem is constructed to mimic a typical cycle in turbulent wall layers and numerical solutions are obtained over a range of Re. Recirculating eddies develop rapidly in the near-wall flow, but these eddies are eventually bisected by alleyways which open up from the external flow region to the wall. At sufficiently high Re, an oscillation was found to develop in the streamwise vorticity field near the alleyways with a concurrent evolution of a local spiky behaviour in the wall shear. Above a critical value of Re, the oscillation grows rapidly in amplitude and eventually penetrates the external flow field, suggesting the onset of an unstable wall-layer breakdown. Local zones of severely retarded streamwise velocity are computed which are reminiscent of the low-speed streaks commonly observed in turbulent boundary layers. A number of other features also bear a resemblance to observed coherent structure in the turbulent wall layer.

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

Journal of Fluid Mechanics ◽

10.1017/s0022112096008944 ◽

1996 ◽

Vol 329 ◽

pp. 341-371 ◽

Cited By ~ 34

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.

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Comment on "Wall Layer of Plane Turbulent Wall Jets withoutPressure Gradients"

Journal of Aircraft ◽

10.2514/3.60288 ◽

1973 ◽

Vol 10 (11) ◽

pp. 698-698

Author(s):

Peter Bradshaw

Keyword(s):

Wall Layer ◽

Wall Jets ◽

Turbulent Wall Jets ◽

Turbulent Wall

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Near-wall response in turbulent shear flows subjected to imposed unsteadiness

Journal of Fluid Mechanics ◽

10.1017/s0022112092001630 ◽

1992 ◽

Vol 238 ◽

pp. 55-71 ◽

Cited By ~ 26

Author(s):

Reda R. Mankbadi ◽

Joseph T. C. Liu

Keyword(s):

Effective Strain ◽

Unsteady Flows ◽

Wall Layer ◽

Turbulent Shear ◽

Frequency Range ◽

Closure Model ◽

Velocity Fluctuations ◽

Turbulent Shear Stress ◽

History Of ◽

Turbulent Wall

Rapid-distortion theory is adapted to introduce a truly unsteady closure into a simple phenomenological turbulence model in order to describe the unsteady response of a turbulent wall layer exposed to a temporarily oscillating pressure gradient. The closure model is built by taking the ratio of turbulent shear stress to turbulent kinetic energy to be a function of the effective strain. The latter accounts for the history of the flow. The computed unsteady velocity fluctuations and modulated turbulent stresses compare favourably in the ‘non-quasi-steady’ frequency range, where quasi-steady assumptions would fail. This suggests that the concept of rapid distortion is especially appropriate for unsteady flows. This paper forms the basis for acoustical studies of the problem to be reported elsewhere.

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

Journal of Fluids Engineering ◽

10.1115/1.4027432 ◽

2014 ◽

Vol 137 (2) ◽

Cited By ~ 1

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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Revisiting the mixing-length hypothesis in the outer part of turbulent wall layers: mean flow and wall friction

Journal of Fluid Mechanics ◽

10.1017/jfm.2014.101 ◽

2014 ◽

Vol 745 ◽

pp. 378-397 ◽

Cited By ~ 13

Author(s):

Sergio Pirozzoli

Keyword(s):

Velocity Profile ◽

Turbulent Flows ◽

Outer Part ◽

Wall Layer ◽

Wall Friction ◽

Mixing Length ◽

Mean Flow ◽

Mean Velocity ◽

The Mean ◽

Turbulent Wall

AbstractWe reconsider foundations and implications of the mixing length theory as applied to wall-bounded turbulent flows in uniform pressure gradient. Based on recent channel-flow direct numerical simulation (DNS) data at sufficiently high Reynolds number, we find that Prandtl’s hypothesis of linear variation of the mixing length with the wall distance is rather inaccurate, hence overlap arguments are stronger in justifying the formation of a logarithmic layer in the mean velocity profile. Regarding the core region of the wall layer, we find that Clauser’s hypothesis of uniform eddy viscosity is strictly connected with the observed size of the eddy structures, and it delivers surprisingly good agreement with DNS and experiments for channels, pipes, and boundary layers. We show that the analytically derived composite mean velocity profiles can be used to accurately predict skin friction in canonical wall-bounded flows with a minimal number of adjustable parameters directly related to the mean velocity profile, and to obtain some insight into transient growth phenomena.

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