Abstract
The effects of an accelerating freestream from a nonzero velocity on the transitional separation bubble characteristics were investigated quantitatively. Hot wire anemometry was used to determine the boundary layer velocity profile repsonse to the acceleration at selected chordwise locations on a Wortmann FX 63-137 airfoil at 7 ° angle of attack. Both positive and negative accelerations were studied from base chord Reynolds number of 100,000 and 150,000, respectively. The purpose of this experiment was to verify the trends witnessed in previous research concerning a sinusoidally oscillating freestream velocity by uncoupling the accelerating and decelerating boundary layer effects. The experimental results indicate that as a result of a freestream acceleration, the separation bubble position shifts in the direction opposite to the chordwise direction it would move for a quasi-steady velocity change. The transition location was more responsive to the acceleration than was the separation position. This supports the oscillating freestream experiment conclusions.
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Abbreviations
- a :
-
freestream acceleration, m/s2
- A c :
-
nondimensional acceleration parameter, ac/U b 2
- c :
-
airfoil chord, mm
- H 12 :
-
boundary layer shape factor, δ 1/δ 2
- R c :
-
chord Reynolds number, U c/v
- R cb :
-
initial chord Reynolds, number, U b c/v
- t :
-
time, milliseconds, ms
- U :
-
mean velocity, m/s
- u :
-
mean local velocity, m/s
- x :
-
distance along chord, mm
- y :
-
distance normal from the airfoil surface, mm
- α:
-
angle of attack
- δ :
-
boundary layer thickness
- δ 1 :
-
displacement thickness, \(\int\limits_0^\delta {\left( {1 - \frac{u}{{U_2 }}} \right)} {\text{ }}dy\)
- δ 2 :
-
momentum thickness, \(\int\limits_0 {\frac{u}{{U_e }}\left( {1 - \frac{u}{{U_e }}} \right){\text{ }}dy} \)
- Δ :
-
nondimensional integrated change in boundary layer velocity
- v :
-
kinematic viscosity
- b :
-
initial flow condition
- e :
-
external to boundary layer
- ref:
-
reference condition
- s :
-
separation
- t :
-
transition
References
Bloch, D. R.; Mueller, T. J. 1986: Effects of distributed grit roughness on separation and transition on an airfoil at low reynolds numbers. AIAA Paper No. 86-1788-CP
Brendel, M, 1986: Experimental study of the boundary layer on a low Reynolds number airfoil in a steady and unsteady flow. Doctoral dissertation, University of Nôtre Dame, June, 1986
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Ellsworth, R. H. 1988: Boundary layer measurements on an airfoil at low reynolds numbers in an accelerating flow from a nonzero velocity. Master's Thesis, University of Nôtre Dame, July, 1988
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Russell, J. 1979: Length and bursting of separation bubbles: a physical interpretation. Science and Technology of Low Speed and Motorless Flight, NASA Conference Publication 2085, part I
Schmidt, G. 1986: The prediction of transitional separation bubbles at low Reynolds numbers. Doctoral Dissertation, University of Nôtre Dame, December 1986, 59–61
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This research was supported by the U.S. Navy Office of Naval Research under contract N00014-83-K-0239
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Ellsworth, R.H., Mueller, T.J. Airfoil boundary layer measurements at low Re in an accelerating flow from a nonzero velocity. Experiments in Fluids 11, 368–374 (1991). https://doi.org/10.1007/BF00211791
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DOI: https://doi.org/10.1007/BF00211791