On the Evolution of Streamwise Vortices over Solid Surfaces: Theory, Experiments and Matched Direct Numerical Simulations

AbstractDirect numerical simulations are used to study the laminar to turbulent transition of a Mach 2.9 supersonic flat plate boundary layer flow due to distributed surface roughness. Roughness causes the near-wall fluid to slow down and generates a strong shear layer over the roughness elements. Examination of the mean wall pressure indicates that the roughness surface exerts an upward impulse on the fluid, generating counter-rotating pairs of streamwise vortices underneath the shear layer. These vortices transport near-wall low-momentum fluid away from the wall. Along the roughness region, the vortices grow stronger, longer and closer to each other, and result in periodic shedding. The vortices rise towards the shear layer as they advect downstream, and the resulting interaction causes the shear layer to break up, followed quickly by a transition to turbulence. The mean flow in the turbulent region shows a good agreement with available data for fully developed turbulent boundary layers. Simulations under varying conditions show that, where the shear is not as strong and the streamwise vortices are not as coherent, the flow remains laminar.

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A New Active Micro Mixing Strategy

ASME 5th International Conference on Nanochannels, Microchannels, and Minichannels ◽

10.1115/icnmm2007-30030 ◽

2007 ◽

Cited By ~ 1

Author(s):

Sedat Tardu ◽

Rabia Nacereddine

Keyword(s):

Numerical Simulations ◽

Temporal Resolution ◽

Direct Numerical Simulations ◽

The Other ◽

Small Scale ◽

Streamwise Vortices ◽

Wall Jets ◽

Vortical Structures ◽

Active Structures

An active micro-mixing strategy through forcing the flow by synthetic wall jets is proposed. It is based on the interaction of induced streamwise vortices in a specific way. There is a spanwise shift between two quasi-streamwise vortices in such a way that one of them compresses the wall normal vorticity layer created by the other, leading to the generation of new wall normal vortical structures. The latter are subsequently tilted by the shear to give birth to new small-scale longitudinal active structures that are efficient in mixing. The feasibility of this strategy is shown through direct numerical simulations of high spatial and temporal resolution.

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Interactions of a stationary finite-sized particle with wall turbulence

Journal of Fluid Mechanics ◽

10.1017/s0022112007009056 ◽

2007 ◽

Vol 594 ◽

pp. 271-305 ◽

Cited By ~ 51

Author(s):

LANYING ZENG ◽

S. BALACHANDAR ◽

PAUL FISCHER ◽

FADY NAJJAR

Keyword(s):

Numerical Simulations ◽

Finite Size ◽

Turbulent Channel Flow ◽

Direct Numerical Simulations ◽

Wall Turbulence ◽

Streamwise Vortices ◽

Turbulent Structures ◽

Lift Forces ◽

Drag And Lift ◽

Particle Force

Reliable information on forces on a finite-sized particle in a turbulent boundary layer is lacking, so workers continue to use standard drag and lift correlations developed for a laminar flow to predict drag and lift forces. Here we consider direct numerical simulations of a turbulent channel flow over an isolated particle of finite size. The size of the particle and its location within the turbulent channel are systematically varied. All relevant length and time scales of turbulence, attached boundary layers on the particle, and particle wake are faithfully resolved, and thus we consider fully resolved direct numerical simulations. The results from the direct numerical simulation are compared with corresponding predictions based on the standard drag relation with and without the inclusion of added-mass and shear-induced lift forces. The influence of turbulent structures, such as streaks, quasi-streamwise vortices and hairpin packets, on particle force is explored. The effect of vortex shedding is also observed to be important for larger particles, whoseReexceeds a threshold.

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