
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
URL: http://digitallibrary.usc.edu/cdm/compoundobject/collection/p15799coll3/id/337675/rec/1157
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 http://digitallibrary.usc.edu/cdm/compoundobject/collection/p15799coll3/id/337675/rec/1157
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.
http://digitallibrary.usc.edu/cdm/compoundobject/collection/p15799coll3/id/337675/rec/1157.
MLA Handbook (7th Edition):
Yang, Shanling. “Boundary layer and separation control on wings at low
Reynolds numbers.” 2013. Web. 20 Jan 2021.
Vancouver:
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: http://digitallibrary.usc.edu/cdm/compoundobject/collection/p15799coll3/id/337675/rec/1157.
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: http://digitallibrary.usc.edu/cdm/compoundobject/collection/p15799coll3/id/337675/rec/1157