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Interaction of a Backward-Facing Step and Crossflow Instabilities in Boundary-Layer Transition
Jenna L Eppink1, Richard W Wlezien2, Rudolph A King1
1NASA Langley Research Center, Hampton, Virginia 23681.
AIAA Journal. American Institute of Aeronautics and Astronautics
|July 31, 2019
Summary
A backward-facing step advances flow transition in crossflow-dominated conditions. Increasing stationary crossflow amplitude amplifies this effect by modulating unsteady disturbances, leading to earlier transition.
Area of Science:
- Fluid Dynamics
- Aerodynamics
- Boundary Layer Transition
Background:
- Stationary crossflow instabilities are critical in swept wing aerodynamics.
- Flow separation, such as that induced by a backward-facing step, can significantly alter boundary layer behavior.
Purpose of the Study:
- To experimentally investigate the impact of a backward-facing step on flow transition in a stationary crossflow-dominated boundary layer.
- To determine how step height and initial crossflow amplitude influence the transition process.
Main Methods:
- Utilized a swept flat plate model in a low-speed wind tunnel.
- Employed hotwire anemometry for detailed boundary-layer flow measurements.
- Imposed a pressure gradient and introduced a backward-facing step (49% of boundary layer thickness).
Main Results:
- Observed an upstream shift in the transition front due to the backward-facing step.
- Increasing stationary crossflow amplitude promoted earlier transition.
- Identified three distinct unsteady disturbance families downstream of the step: highly oblique, Tollmien-Schlichting-wave-like, and shear-layer instability.
Conclusions:
- The backward-facing step locally amplified stationary crossflow instabilities but did not solely cause transition.
- Modulation of unsteady disturbances by stationary crossflow was identified as the mechanism for the upstream transition shift.
- The findings are crucial for understanding and predicting flow transition in complex aerodynamic configurations.
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