Detachment of Turbulent Boundary Layers With Varying Free-Stream Turbulence and Lower Reynolds Numbers
Experiments were conducted to determine if free-stream turbulence scale affects detachment of turbulent boundary layers. In consideration of possible interrelation between scale and intensity of turbulence, the latter characteristic also was varied and its role was evaluated. Flow over a 2dimensional airfoil in a subsonic wind tunnel was studied with the aid of hot-wire anemometry, liquid-film visualization, a Preston tube, and static pressure measurements. Profiles of velocity, relative turbulence intensity, and integral scale in the boundary layer were measured. Detachment boundary was determined for various angles of attack and free-stream turbulence.
The free-stream turbulence intensity and scale were found to spread into the entire turbulent boundary layer, but the effect decreased as the airfoil surface was approached. When the changes in stream turbulence were such that the boundary layer velocity profiles were only slightly changed, detachment location was not significantly affected by the variations of intensity and scale. Pressure distribution and laminar or turbulent state remained the key factors in determining detachment location.
New data on the detachment of turbulent boundary layers were obtained. The range of flow conditions made it possible to evaluate the best-known rapid methods for predicting turbulent detachment at lower Reynolds numbers than previously reported. This revealed that approximations and empirical constants based on data for high Reynolds numbers and strongly adverse pressure gradients should not be uncritically accepted for predicting detachment of flows at low Reynolds numbers.
KeywordsTurbulence Intensity Turbulent Boundary Layer Pressure Coefficient Integral Scale Adverse Pressure Gradient
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