Action of a turbulent flow on a hard compliant coating

Abstract

The rate and amplitude of compliant coating deformation by turbulent pressure pulsations were calculated. Complex compliance determined by a 2D model has two components: along and across the coating. Dependence of the components of dimensionless compliance on the ratio between wavelength λ and coating thickness H was determined for 0.3<λ/H<30 and dependence of these components on the ratio of flow velocity V to shear velocity Ct0 was determined for 0.1<V/Ct0<10.Deformation amplitude and velocity of surface moving for the hard compliant coatings, which can be used in practice, were calculated for the water turbulent flow from 5 to 55m/s. The effects of the loss tangent and Poisson’s ratio of the coating material were also studied.It is shown that the mean-square displacement of the surface does not exceed the thickness of a viscous sublayer in most applied cases. However, the velocity of surface motion is comparable to velocity pulsations in a boundary layer near the wall. This can be the reason for drag reduction on a compliant wall. The calculated value of ratio between energy, absorbed by the wall, and energy, dissipated within the flow because of drag, was near 10−4 for water. This estimate does not confirm the hypothesis, explaining drag reduction by energy takeoff from the flow. Drag reduction mechanism is illustrated by analogy with blowing-suction.The presented analysis allows one to explain the small drag reduction obtained in the former experiments with hard compliant coatings.