Over the past forty years intensive investigations into the use of compliant surfaces have been undertaken, both theoretically and experimentally, in order to obtain turbulent drag reduction in boundary-layer flows. Although positive results were found in some of the studies, none of these had been successfully validated by independent researchers. In this paper the results are reported of a recent investigation carried out by the authors to verify the experimental results of Semenov and Kulik et al. in 1991, who successfully demonstrated the ability of compliant surfaces to reduce the skin-friction drag and surface-flow noise in a turbulent boundary layer. A straingauge force balance was used in the present study to directly measure the turbulent skin-friction drag of a slender body of revolution in a water tunnel. Changes in the structure of turbulent boundary layer over a compliant surface in comparison with that over a rigid surface were also examined. The results clearly demonstrate that the turbulent skin friction is reduced for one of the two compliant coatings tested, indicating a drag reduction of up to 7% within the entire speed range of the tests. The intensities of skin-friction and wall-pressure fluctuations measured immediately downstream from the compliant coating show reductions in the intensities of up to 7 and 19%, respectively. The results also indicate reductions in turbulence intensity by up to 5% across almost the entire boundary layer. Furthermore, an upwards shift of the logarithmic velocity profile is also evident indicating that the thickness of the viscous sublayer is increased as a result of turbulent drag reduction due to the compliant coating. It is considered that the results of the present experimental investigation convincingly demonstrate for the first time since the earlier work in Russia (Semenov and Kulik et al.) that a compliant surface can indeed produce turbulent drag reduction in boundary-layer flows.
Toward the understanding of the drag reduction mechanism, theoretical as well as experimental investigations have been made of the wave properties of compliant coatings such as the wave velocity and the decay parameters. The compliant coating consisted of a homogeneous layer of viscoelastic material attached to a rigid substrate. Based on two-dimensional elastic wave analysis, wave properties such as the dispersion of wave velocity, the ratio between the amplitudes of two wave components, and decay characteristics have been calculated as a function of Poisson's ratio and the loss tangent . It was found that the wave parameters are strongly affected by Poisson's ratio . A new experimental technique is devised for direct measurement of the wave parameters of the coating. The wave velocity and the rate of decay were measured based on the amplitude and phase of coating deformation with varying distances from the excitation point. Two kinds of silicon rubber were tested in the frequency range from 500 to 1200 Hz. Comparison of the present two-dimensional model with the conventional local deformation model was made. Two conditions ͑time factor and space factor͒ for drag reduction were drawn to maximize the interaction between the coating and the turbulent flow. These conditions suggest a certain optimum region in the coordinates of the flow velocity and the coating thickness.
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