Measurements Of Local Skin Friction In A Microbubble Modified Turbulent Boundary Layer

Measurements of Local Skin Friction in a Microbubble Modified Turbulent Boundary Layer

Measurements of Local Skin Friction in a Microbubble Modified Turbulent Boundary Layer

Local skin friction reductions have been measured using an array of flush-mounted hot-film probes in a microbubble-modified, zero pressure gradient, turbulent boundary layer. The results of earlier integrated skin friction measurements, that showed the reduction to be a function of plate orientation, gas flow rate and freestream velocity, have been confirmed both qualitatively and quantitatively. With the measurement plate above the boundary layer, it is shown that skin friction is reduced monotonically for all air flow rates at each of three freestream velocities between 4 and 17 m/sec. For the plate below the boundary layer, however, 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. Skin friction, microbubble, turbulent boundary layer.

Frontiers in Experimental Fluid Mechanics

Dynamical systems theory and flow control are two research areas of great current interest. These and other special situations are among the topics covered in this volume. Each article emphasizes the use of experiments to achieve better physical understanding of a particular class of flow problems. The topics covered were chosen because of their importance to the field, recent appeal, and potential for future development. The articles are comprehensive and coverage is pedagogical with a bias towards recent developments.