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.
|Advisor:||Wlezien, Richard W.|
|Commitee:||Choudhari, Meelan, King, Rudolph, Rogers, Chris|
|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|
Copyright in each Dissertation and Thesis is retained by the author. All Rights Reserved
The supplemental file or files you are about to download were provided to ProQuest by the author as part of a
dissertation or thesis. The supplemental files are provided "AS IS" without warranty. ProQuest is not responsible for the
content, format or impact on the supplemental file(s) on our system. in some cases, the file type may be unknown or
may be a .exe file. We recommend caution as you open such files.
Copyright of the original materials contained in the supplemental file is retained by the author and your access to the
supplemental files is subject to the ProQuest Terms and Conditions of use.
Depending on the size of the file(s) you are downloading, the system may take some time to download them. Please be