Victor M. Kulik

Turbulent drag reduction using compliant surfaces

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 in 1991 and Kulik and co–workers 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 strain–gauge 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 per cent 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 per cent, respectively. The results also indicate reductions in turbulence intensity by up to 5 per cent 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 (by Semenov and Kulik) that a compliant surface can indeed produce turbulent drag reduction in boundary–layer flows.

Experiments on the effects of aging on compliant coating drag reduction

We report the experimental results from a collaborative effort between USA, Russia, and UK on the development of compliant coatings for undersea application of reduction of drag. The focus is on “shelf-life” of coatings. The coatings are based on a linear interference theory of interaction between turbulence pressure fluctuation and the viscoelastic coating. The phase shift between boundary displacement and pressure fluctuation embodies the interference effect. The natural frequency of the coating is matched to the turbulent boundary layer region of maximum Reynolds stress production. Low-molecular weight rubber-like silicone coatings have been manufactured whose properties include slow and fast damping, slow and fast aging, and varying magnitudes of elasticity, density, and thickness as well as transparency. The dynamic modulus and loss tangent vary weakly over a range of frequencies and temperature allowing compatibility with broad spectrum of turbulence. Drag measurements have been carried out over a y...

Wave properties of coating for skin friction reduction

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 σ and the loss tangent μ. It was found that the wave parameters are strongly affected by Poisson’s ratio σ. 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 1...

The response of compliant coating to nonstationary disturbances

The amplitude and phase lag of surface deformation were determined for a compliant coating under the action of turbulent pressure fluctuations. For this purpose, pressure fluctuations were measured experimentally. The amplitude and duration of coherent wave train of pressure fluctuations were investigated using digital filtration. The transient response was calculated for stabilization of forced oscillations of the coating in approximation of local deformation. The response of coating was analyzed with considerations of its inertial properties and limited duration of coherent harmonics action of pressure fluctuations. It is shown that a compliant coating interacts only with a frequency range near the first resonance. According to the analysis, with increasing elasticity modulus of the coating material E, deformation amplitude decreases as 1∕E, and dimensionless velocity of the coating surface decreases as 1∕E. For sufficiently hard coatings, deformation amplitude becomes smaller than the thickness of the ...

Analysis of Wave Decay Characteristics of Viscoelastic Compliant Coating

Calculation was carried out for phase velocity and deformation wave decay in a layer of viscoelastic material fixed tightly on the solid substrate. Analysis has been performed regarding the inner structure of the wave, i.e., the proportions between the vertical and horizontal displacements and their profiles. The wave characteristics depend strongly on media compressibility factor. The effect of viscous losses on parameters of the main oscillation mode was studied in detail. Results were compared with the model of coating with local deformation. A new experimental approach was made in order to measure such wave properties of a compliant coating as the dependency of deformation wave velocity on frequency and decay factor was made. The method for estimation of coating parameters enabling the drag reduction in turbulent flow was then refined.

New Perspective of Turbulent Skin Friction Reduction by Compliant Coating

Drag reduction capabilities of compliant coatings has long been of a verification issue in experimental fluid mechanics. In this paper, an optimal flow velocity range leading to maximum interaction between coatings and the flow is predicted based on viscoelastic material properties coating. The prediction is performed using a semi-empirical model, which exhibit quite promising results in preliminary tests. Results on turbulent boundary layer in a wind tunnel and a water tunnel and comparison of the experimentally estimated skin friction modifications with the predictions are given.