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Seventh International Symposium on Turbulence and Shear Flow Phenomena
July, 28-31, 2011, Ottawa Convention Centre, Ottawa, Canada

DOI: 10.1615/TSFP7

DIRECT NUMERICAL SIMULATION OF THE GROWTH OF COHERENT FLOW STRUCTURES IN A TRIGGERED TURBULENT SPOT

pages 1-6
DOI: 10.1615/TSFP7.2190
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ABSTRACT

Hairpin-like coherent flow structures have been identified as the dominant vortical structures in transitional and turbulent boundary layers and free shear layers. To fully characterize the processes occurring within such flows, it is important to study the growth mechanism of the hairpin vortices, particularly at their early stages of development, and the development of the resultant wave packets containing multiples of these flow structures. The formation of an artificially-triggered turbulent spot in isolation provides a suitable test case to study the mechanism through which a hairpin vortex forms in a shear layer and how the formation of one such flow structure may produce a local flow environment that promotes the creation of similar flow structures in sequence. The present study involves direct numerical simulations wherein an isolated turbulent spot is produced through a perturbation in the form of a pulsed jet ejected transversely through a square orifice in the test surface. The simulated spots compare favorably with spots measured experimentally under similar freestream conditions. Two levels of freestream acceleration−one that is nominally zero and another that is above the typical threshold required for relaminarization−are applied to the flow to assess the sensitivity of the hairpin-like vortical structures to freestream acceleration. Hairpin-like vortices near the spot trailing edge are observed to grow primarily through an instability of shear layers created between high- and low-velocity streaks near the spanwise edges of the spot.

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