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University of Southern California

1. Yang, Shanling. Boundary layer and separation control on wings at low Reynolds numbers.

Degree: PhD, Aerospace Engineering, 2013, University of Southern California

In the transitional chord-based Reynolds number regime for aeronautics, 10⁴ ≤ Re ≤ 10⁵, fluid flow over a surface is prone to separation followed by possible reattachment and transition to turbulence. The amplification of disturbances in the boundary layer promotes transition to turbulence, so boundary layer and separation control methods are especially favorable in this transitional Re regime. The use of sound to control flow separation at transitional and moderate Re for various smooth airfoils has been experimentally studied in the literature. Optimum excitation frequencies are reported to match the frequency or sub harmonics of the naturally occurring instabilities in the shear layer, and correlations between optimum frequencies for external acoustic forcing and tunnel anti-resonances have been observed. However, reported optimum frequency values based on the Strouhal number scaling St/Re^(1/2) are not in complete agreement among the different reported studies. Little attention has been given to distinguish the effects of standing waves from traveling sound waves. Mathematical and experimental studies of sound and boundary layer instability interactions have also yielded mixed results, suggesting that there still lacks a full understanding about the mechanism by which acoustic waves affect boundary layer flows. ❧ Results on boundary layer and separation control through acoustic excitation at low Re numbers are reported. The Eppler 387 profile is specifically chosen because of its pre-stall hysteresis and bi-stable state behavior in the transitional Re regime, which is a result of flow separation and reattachment. External acoustic forcing on the wing yields large improvements (more than 70%) in lift-to-drag ratio and flow reattachment at forcing frequencies that correlate with the measured anti-resonances in the wind tunnel. The optimum St/Re^(1/2) range for Re = 60,000 matches the proposed optimum range in the literature, but there is less agreement for Re = 40,000, which suggests that correct St scaling has not been determined. The correlation of aerodynamic improvements to wind tunnel resonances implies that external acoustic forcing is facility-dependent, which inhibits practical application. Therefore, internal acoustic excitation for the same wing profile is also pursued. ❧ Internal acoustic forcing is designed to be accomplished by embedding small speakers inside a custom-designed wing that contains many internal cavities and small holes in the suction surface. However, initial testing of this semi-porous wing model shows that the presence of the small holes in the suction surface completely transforms the aerodynamic performance by changing the mean chordwise separation location and causing an originally separated, low-lift state flow to reattach into a high-lift state. The aerodynamic improvements are not caused by the geometry of the small holes themselves, but rather by Helmholtz resonance that occurs in the cavities, which generate tones that closely match the intrinsic flow instabilities.… Advisors/Committee Members: Spedding, Geoffrey R. (Committee Chair), Redekopp, Larry G. (Committee Member), Eliasson, Veronica (Committee Member), Bickers, Gene (Committee Member), Radovich, Charles (Committee Member).

Subjects/Keywords: low Reynolds numbers; separation control; acoustic excitation; active separation control; passive separation control; boundary layers; fluid dynamics; aerodynamics; Helmholtz resonance; local acoustic forcing; external acoustic excitation; internal excitation; flow control

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APA (6th Edition):

Yang, S. (2013). Boundary layer and separation control on wings at low Reynolds numbers. (Doctoral Dissertation). University of Southern California. Retrieved from

Chicago Manual of Style (16th Edition):

Yang, Shanling. “Boundary layer and separation control on wings at low Reynolds numbers.” 2013. Doctoral Dissertation, University of Southern California. Accessed January 20, 2021.

MLA Handbook (7th Edition):

Yang, Shanling. “Boundary layer and separation control on wings at low Reynolds numbers.” 2013. Web. 20 Jan 2021.


Yang S. Boundary layer and separation control on wings at low Reynolds numbers. [Internet] [Doctoral dissertation]. University of Southern California; 2013. [cited 2021 Jan 20]. Available from:

Council of Science Editors:

Yang S. Boundary layer and separation control on wings at low Reynolds numbers. [Doctoral Dissertation]. University of Southern California; 2013. Available from: