Date of Award

12-2019

Access Type

Thesis - Open Access

Degree Name

Master of Science in Aerospace Engineering

Department

Aerospace Engineering

Committee Chair

Dr. Bertrand Rollin

First Committee Member

Dr. Ebenezer Gnanamanickam

Second Committee Member

Dr. Reda Mankbadi

Abstract

Understanding wall-jet-induced turbulence and mixing is an important challenge in modern engineering, as drag reduction and mixing enhancement are attainable by modifying the flow development. Simulations are performed to investigate the effect on skin friction and flow mixing due to introducing controlled perturbations, at the initial shear layer of a planar wall-jet using jet inlet cyclic pulsing. The billow production by the Kelvin-Helmholtz instability, the instability that drives turbulence in a wall-jet, is modified by the excitation of the inlet velocity profile by a sine wave perturbation. Two types of wall-jet simulations are carried out, a two-dimensional compressible case at Rein = 5000 using the PyFR solver and a three-dimensional incompressible case at Rein = 6000 using the Nek5000 solver. The compressible wall-jet simulation indicates that the addition of a sine wave perturbation of 1% on the inlet velocity, at the initial shear layer, increases the wall-normal turbulence intensity at a Strouhal number (Sr) of 0.05 and reduces the turbulence intensity in all directions at a Sr of 0.25. The incompressible wall-jet simulations show that a perturbation of magnitude 40% of the inlet velocity at a low Sr number of 0.0048 damps turbulence and leads to skin friction reduction. The forced wall-jet experiences a repetitive re-laminarization process that delays transition as well as separation from the wall. A qualitative parametric analysis of the perturbation of the global behavior of the flow development on a plane wall-jet under forced velocity profiles is also presented. Cases at Sr = 0.0048 experience a reduction in the number of turbulent structures while becoming more stable, indicating potential drag reduction. Cases at Sr = 0.02 experience a frequent energy re-supply from the inlet that helps maintain large turbulent structures at further downstream locations, useful for mixing related applications.

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