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Measurements of local skin friction in a microbubble-modified turbulent boundary layer

Published online by Cambridge University Press:  20 April 2006

N. K. Madavan ,
S. Deutsch  and
C. L. Merkle
N. K. Madavan
Affiliation:
The Department of Mechanical Engineering and the Applied Research Laboratory, The Pennsylvania State University, University Park, PA 16802
S. Deutsch
Affiliation:
The Department of Mechanical Engineering and the Applied Research Laboratory, The Pennsylvania State University, University Park, PA 16802
C. L. Merkle
Affiliation:
The Department of Mechanical Engineering and the Applied Research Laboratory, The Pennsylvania State University, University Park, PA 16802
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Abstract

Local skin-friction reductions have been measured using an array of flush-mounted hot-film probes in a microbubble-modified, zero-pressure-gradient, turbulent bounddary layer. The results of earlier integrated skin-friction measurements, that showed the reduction to be a function of plate orientation, gas-flow rate and free-stream velocity, have been confirmed both qualitatively and quantitatively. With the measurement plate oriented so that buoyancy keeps the bubbles in boundary layer, it is shown that skin friction is reduced monotonically for all air-flow rates at each of three free-stream velocities between 4 and 17 m/s. For the opposite plate orientation it is possible for increasing gas injection to lead to smaller local skin-friction reduction at the lowest speeds. Drag reduction appears to persist for as much as 60–70 boundary-layer thicknesses downstream of the injection region. It is further shown, using a probe flush-mounted just upstream of the injection section, that there is no apparent upstream interference due to the gas injection. Spectral measurements indicate that microbubbles can cause a reduction of high-frequency shear-stress fluctuations. This suggests a destruction of some of the turbulence in the near-wall region.

Type
Research Article
Information
Journal of Fluid Mechanics , Volume 156 , July 1985 , pp. 237 - 256
Copyright
© 1985 Cambridge University Press

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