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Exploiting the Non-linear Interactions within Wall Turbulence for Flow Control

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Technical Report,01 Jul 2016,31 Dec 2019

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The flow interactions and mechanism within complex wall-bounded flows were studied using the plane wall jet PWJ as the model flow field. The PWJ is forced using large-amplitude, large-scale, acoustic forcing. In this manner a controlled flow scale is introduced into the base flow and the resulting flow mechanism are studied. Complimentary hot-wire anemometry and particle image velocimetry measurements were carried out on a forced and unforced PWJ. A range of forcing scales were studied and all the forcing scales resulted in a reduction in friction velocity at all streamwise locations considered. Three different flow scales were chosen to examine in detail the energy transfer mechanisms within the forced PWJ. The forcing resulted in a large increase in the streamwise turbulence intensity in the wall region, specifically in the large-scale intensities. The linear response of the PWJ resulted in flow structures that resembled the naturally occurring flow structures in the unforced flow. These are the boundary layer like structures in the wall region and jet like structures in the outer free shear layer. The excess energy from forcing was transferred to scales of the wavelength of the outer jet like structures, though this occurred in the wall region. This energy transfer occurred irrespective of the direction of the transfer. At the upstream locations, the direction if energy transfer was in the manner of a forward cascade while at the downstream locations it was in the manner of an inverse cascade. It was then concluded that the natural energy transfer pathway in the PWJ was to transfer energy away from the wall region structures into the outer jet like structures. The forcing then isolates a single energy transfer pathway where the input energy is at the forcing scales.

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  • Fluid Mechanics

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