Home – Home – Jornals – Thermophysics and Aeromechanics 2011 number 4

Influence of elasticity module of coating material on the parameters of hard compliant coatings deformation has been analysed. Calculation using two-dimensional model has shown that maximum coating deformation is achieved at the ratio of flow rate U to the parameter = (E/3ρ)^0.5 approximately equal to 2.5, however, velocity of wall surface motion has first local maximum at ≈ 1. The range of coating parameters' values at which compromise between its hardness and intensity of interaction with turbulent flow is provided has been determined. For rubbery materials with Poisson coefficient of about 0.5, correlations of the flow velocity and parameter shall be in the range 1÷1.5. It is shown that at such parameters, the mean square value of the coating surface deflection/inflection is less than the viscous sublayer thickness, its correlation with the wavelength is very small and equals (1÷5)·10⁻⁴. Such form of deformed surface fundamentally differs from the parameters of the wave wall in Kendall's experiments which results are used for calculation of inverse influence of wall deformation on the flow. It was assumed that solid compliant coatings do not cause instability of interaction with the ambient flow.
Analysis of the coating surface motion rate has shown that its mean square value is 5÷6 % of the value of the dynamic flow velocity at wide change of elasticity modulus of the coating material from 0.01 to 10 MPa. This is proved by the earlier proposed mechanism of drag reduction based on the change of the picture of Reynolds stress generation in the vicinity of the compliant wall. The performed analysis allowed explaining comparatively low drag decrease in the experiments with solid compliant coatings. The valuerealized in these studies was from 0.06 to 0.3 that was much lower than the optimal value.