Author

Shibani Bhatt

Date of Award

4-2019

Document Type

Dissertation - Open Access

Degree Name

Doctor of Philosophy in Aerospace Engineering

Department

College of Engineering

Committee Chair

Dr. Ebenezer Gnanamanickam

First Committee Member

Dr. William Engblom

Second Committee Member

Dr. J. Gordon Leishman

Third Committee Member

Dr. Reda Mankbadi

Abstract

The current work focuses on exploiting this behavior to manipulate wall turbulence by targeting the large-scales of the flow. In wall turbulence the large-scales of the flow interact with the smaller scales in a non-linear manner including through a process of amplitude and frequency modulation. A plane wall jet (PWJ) is chosen as the model flow field for this work as its unique geometry allows for the controlled introduction of large-scale perturbations through acoustic forcing. The corresponding interactions because of forcing are characterized using single hot-wire measurements. The nearwall response of the PWJ over a range of large-scale forcing showed that the friction velocity Uτ decreased for all forcing wavelengths considered. The scaling behavior of this reduction in Uτ with respect to inner, outer and global (PWJ exit conditions) variables suggested that the responsible underlying mechanism was an inner-outer interaction with a dependence on the PWJ Reynolds number. Based on this nearwall study, a more detailed study of the interactions caused by forcing was carried out focusing on three specific wavelengths. The forcing was observed to increase the spreading rate of the PWJ while reducing the maximum streamwise mean velocity. Together, this resulted in an increase in the friction Reynolds number Ret upon forcing, as well as a transfer of momentum away from the inner (wall) region to the outer free-shear region. The linear response of the flow to the forcing resulted in the introduction of distinct periodic structures into the inner and outer regions of the flow that appear to convect at different velocities. Considering the non-linear response, an increase in the turbulence intensity in the wall region of the PWJ was observed. The forcing altered the energy of the large-scales of the flow as well as its distribution across wavelengths. This redistribution of energy was seen to occur in the manner of a forward cascade as well as an inverse cascade. The direction of transfer depends on the size of the forcing scales relative to the naturally energetic large-scales of the flow. It was observed the that primary recipient scales of the flow were flow structures that matched the energetic outer free-shear layer structures. However, it was also observed that these structures are transported closer to the wall, increasing the energy in the wall region. This effect is accompanied by the reduction of small-scale energy in the wall region, which is inferred to be tied to the reduction of friction velocity upon forcing. The increase in energy of the large-scales of the flow was also accompanied by the increased amplitude modulation of the small-scales by the large-scales. It is concluded that the forcing successfully targets the large-scales of the PWJ, which changes the non-linear interaction between the scales in a manner that reduces the skin-friction.

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