Dissertation/Thesis Abstract

The interaction of crossflow instabilities and a backward facing step in swept boundary layer transition
by Eppink, Jenna, Ph.D., Tufts University, 2014, 259; 3612779
Abstract (Summary)

A low-speed wind tunnel experiment was performed to study the effect of a backward-facing step on transition in a swept-wing flow. Detailed hot-wire measurements were used to assess the flow field characteristics on a swept flat plate with and without a backward-facing step. A pressure body was installed on the ceiling to induce a pressure field simulating that of an infinite swept wing. The step height was approximately 50% of the boundary-layer thickness at the step. Measurements without the step confirmed the dominance of the stationary crossflow instabilities leading to a high-frequency secondary-instability breakdown. The backward-facing step had a local destabilizing effect on the growth of the dominant stationary crossflow mode and the harmonic of the dominant mode. The stationary crossflow disturbances reached small amplitudes (3 to 5% Ue) before breakdown occurred. The transition front moved forward as the initial amplitude of the stationary crossflow disturbance was increased. The step introduced a flow field rich with unsteady disturbances. Three different families of unsteady disturbances were identified corresponding to three distinct frequency bands in the 80 to 1500 Hz range. Wave angles and phase speeds were measured for each type of disturbance. The disturbances are believed to correspond to a traveling crossflow-type disturbance, a TS-type disturbance, and a free shear layer instability. Each of the disturbances were modulated through interaction with the stationary crossflow modes. The spanwise modulation was different for each family and was seen in the distortion of the amplitude and phase. Larger stationary crossflow vortices resulted in larger peak amplitudes of the unsteady disturbances at similar streamwise locations. The mean-flow modulation appears to affect the local stability of the unsteady disturbances even at low stationary-crossflow amplitudes. The local destabilization of the unsteady disturbances is believed to be responsible for the sensitivity of transition location to stationary crossflow amplitude. Breakdown was initiated despite the low amplitude of the unsteady disturbances (2 to 4% Ue). Nonlinear interactions were observed between the different unsteady disturbances and may be ultimately responsible for breakdown to turbulence.

Indexing (document details)
Advisor: Wlezien, Richard W.
Commitee: Choudhari, Meelan, King, Rudolph, Rogers, Chris
School: Tufts University
Department: Mechanical Engineering
School Location: United States -- Massachusetts
Source: DAI-B 75/06(E), Dissertation Abstracts International
Subjects: Aerospace engineering, Mechanical engineering
Keywords: Backward facing step, Boundary layers, Stationary crossflow, Swept wing, Transition
Publication Number: 3612779
ISBN: 978-1-303-74651-2
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