Models for Coherent Structures in the Wall Layer

The focus of this paper is on physics of coherent structures in boundary layer flow in near-wall region of a vibrating plate. A dynamical model is developed based on Galerkin projection of the governing equation of the wall layer flow onto a subspace spanned by the orthogonal divergence-free Fourier basis functions. The interactive physics of the coherent structures with the wall vibration is studied with the established model truncated at any order. The compared results show that the prevailing coherent structures in the layer flow near a vibrating wall region are captured.

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Coherent Structures in Open-Channel Flows Over a Fixed Dune

Volume 2, Parts A and B ◽

10.1115/ht-fed2004-56284 ◽

2004 ◽

Author(s):

Wusi Yue ◽

Ching-Long Lin ◽

Virendra C. Patel

Keyword(s):

Coherent Structures ◽

Open Channel ◽

Wall Layer ◽

Mean Flow ◽

Near Surface ◽

Vortical Structures ◽

Eddy Simulation ◽

Numerical Predictions ◽

Scale Models ◽

Strong Shear

Turbulent open-channel flow over a two-dimensional laboratory-scale dune is studied using large eddy simulation. Free surface motion is simulated using a level set method. Two subgrid scale models, namely dynamic Smagorinsky and dynamic two-parameter models, are employed. The present numerical predictions of mean flow field and turbulence statistics are in good agreement with experimental data. Streaky structures are observed in the wall layer after flow reattachment. Quadrant two events dominate near-wall and near-surface motions. Coherent structures are produced behind the dune crest by strong shear layer riding over the recirculation zone. These tube-like vortical structures are transported downstream with the mean flow and most are destroyed before arriving at the next crest.

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Dilute Polymers and Coherent Structures in Wall Turbulence

10.1115/imece1999-1169 ◽

1999 ◽

Author(s):

Elisabetta De Angelis ◽

Carlo M. Casciola ◽

Renzo Piva

Keyword(s):

Coherent Structures ◽

Wall Layer ◽

Wall Turbulence ◽

Viscoelastic Flows ◽

Mean Velocity ◽

Newtonian Flows ◽

Level Of Activity ◽

Main Effect ◽

The Mean ◽

Low Speed Streaks

Abstract Both numerical and experimental data confirm that the introduction of dilute polymers in a turbulent flow changes some mean characteristics of the flow: the wall drag can decrease up to 40% while the average spacing among streaks can even double. Actually a link exists between streaks spacing and mean velocity gradient at the wall. In such viscoelastic flows the wall turbulence regeneration is still influenced by the mean shear and by the interaction of the coherent structures even if they appear more ordered and larger with respect to Newtonian flows. These phenomena, which have been repeatedly observed since the findings of Toms, would require a better understanding of the mechanisms that locally induce this behaviour. The analysis of data from direct numerical simulation with FENE-P model for the polymers, seems to suggest, as the main effect of the viscoelastic reaction, a stabilizing action on the low speed streaks and a related decrease in the population of the wall layer coherent structures. The decreased level of activity induces relevant changes in the scales of the turbulence and originates the observed drag reduction.

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Experimental investigation of coherent structures in turbulent boundary layers

Journal of Fluid Mechanics ◽

10.1017/s0022112091000757 ◽

1991 ◽

Vol 230 ◽

pp. 183-208 ◽

Cited By ~ 58

Author(s):

C. E. Wark ◽

H. M. Nagib

Keyword(s):

Reynolds Stress ◽

Coherent Structures ◽

Three Dimensional ◽

Wall Layer ◽

Space Time ◽

Turbulent Shear ◽

Wall Region ◽

Turbulent Shear Flow ◽

Outer Flow ◽

Probability Density Distributions

The events which are responsible for strong Reynolds-stress production in the near-wall region of a bounded turbulent shear flow have been investigated in a turbulent boundary layer at a Reynolds number based on momentum thickness of Reθ = 4650. The coherent structures associated with the production process have been studied using the quadrant detection technique. All three velocity components were measured in a three-dimensional sampling volume about the point of detection. The conditional ensemble-averaged velocity field associated with the detection of a sweep or an ejection is presented and compared with non-conditioned space–time correlations. Conditional space–time probability density distributions were calculated at all measurement locations based on the occurrence of a Reynolds-stress-producing event at the detection point. The resulting three-dimensional representation of the conditional probability demonstrates that a significant fraction of the events are relatively large in scale, that a hierarchy of sizes exists and that there is a link between the outer flow and the ’bursting’ process. However, many investigators have shown that the ’bursting’ frequency scales with wall variables. Therefore all indications suggest that the scales are generated by a wall-layer mechanism but grow to sizes and convect with velocities scaling with the outer layer.

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