Measurements of the effect of microbubbles on a zero pressure gradient turbulent boundary layer generated on the test section wall of a water tunnel are described. Microbubbles are created by injecting air through a 0.5 μm sintered stainless steel plate immediately upstream of a floating element drag balance. At the downstream edge of the balance the length Reynolds number is as high as ten million. Integrated skin friction reduction of greater than 80% is observed. The drag balance results are confirmed by measurements with a surface hot-film probe. For the case in which buoyancy tends to keep the bubbles in the boundary layer, the skin friction data are shown to collapse when plotted against the ratio of air to water volume flow rate. The effects of buoyancy on skin friction reduction are also documented.

-Differential Evolution is a simple, fast, and robust evolutionary algorithm that has proven effective in determining the global optimum for several difficult single-objective optimization problems. In this paper, the Differential Evolution algorithm is extended to multiobjective optimization problems by using a Pareto-based approach. The algorithm performs well wheh applied to several test optimization problems from the literature.

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.

Computational results of an initial attempt to model the phenomenon of skin-friction reduction by microbubble injection are presented. A well-tested boundary-layer code employing a simple mixing length model for the turbulence is used. The action of the bubbles is simulated by allowing the viscosity and density to vary locally as a function of a prescribed bubble concentration profile. Parametric studies of bubble location in the boundary layer, peak concentration and mixture viscosity model are performed. The order of magnitude and trends of the experimental skin-friction reduction are reproduced quite well by this simple model.

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