Date of Award

Spring 1-1-2015

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Aerospace Engineering Sciences

First Advisor

Sedat Biringen

Second Advisor

Mahmoud Hussein

Abstract

Efficient methods of drag reduction in wall-bounded shear flows remains an important, yet elusive problem in fluid mechanics. There are two potential avenues for drag reduction. One involves the delay of laminar-to-turbulent transition in the boundary layer, where in general the skin friction drag is lower for laminar flows. The other involves reducing the skin friction in a fully-developed turbulent flow, where finding an efficient method of transition delay remains a challenging problem. Steady/unsteady disturbances in the freestream such as sound or vorticity are introduced into the boundary layer through receptive processes and are catalysts for transition, whereby these disturbances provide the initial conditions of disturbance amplitude, frequency and phase for the breakdown of laminar flow. In low disturbance environments (such as cruise-flight conditions), the 2D Tollmien-Schlichting (TS) wave may amplify in the boundary layer and cause the flow to transition. Attempts have been made for laminar flow control using suction/blowing slots, wall heating and cooling, Lorentz forcing, passive compliant surfaces, or prescribed wall movement among others.

In the present computational study, we first utilize a suction/blowing slot to delay transition in a channel flow with three-dimensional instabilities, where the phase and amplitude of the slot is tuned in a specific way such that a substantial reduction in the amplitude of the primary TS wave is achieved. Problems involving transition delay in channel flows using passive phononic subsurfaces are then presented, where the elastodynamic response of the subsurface is precisely tuned to control the phase relationship between the pressure and velocity at the fluid/structure interface in such a way to attenuate the growth of perturbation energy in the fluid. To do this, the three-dimensional, time dependent, non-linear Navier-Stokes equation is coupled to a linear elastodynamics model using direct numerical simulation. We demonstrate the effectiveness of the concept of a phononic subsurface to control primary (TS) instability in channel flows, flows with multiple excitation frequencies, and flows containing high-amplitude, weakly non-linear secondary instabilities.

Bursting events responsible for the production of a majority of the turbulent kinetic energy in turbulent flows are detected in both periodic and spatial turbulent channel flows using a second quadrant technique. Flow visualization is also carried out utilizing the method of Lagrangian Coherent Structures (LCS) to identify the flow structures associated with the production of turbulent kinetic energy. Measurements of the bursting period, time between bursts, and the convection velocity of the structures associated with the bursts are made, whereby excellent correlation is found between the LCS visualizations and burst detection techniques employed in this study. From this, an estimation of the bursting frequency is made which can be utilized in the design of a phononic subsurface to alter this bursting mechanism in such a way to lower the production of turbulent energy in wall-bounded flows, thus potentially re-laminarizing the flow field near the wall and reducing skin friction.

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