On conditions of modelling and choice of viscoelastic coatings for drag reduction

The experiments of various authors show a critical situation of the coating vibration properties choice for the drag reduction security. The Kramer hypothesis of the energy absorption isn’t explain a cause of facts of the friction increase. According to the interference theory,the action of viscoelastic boundary is critical. It can lead to decrease or increase of the turbulent energy generation in dependence of the wave properties of a surface. Modelling parameter is complex compliance of a boundary, which is conditioned by amplitude and phase of the boundary displacement relative to the turbulent pressure pulsation. Conditions for a choice of the vibration properties are written on the base of the interference analysis of the viscoelastic boundary action on near-wall turbulence. The first condition is the requirement of quick attenuation or absence of free vibrations of coatings. The second condition is limitation of the coating compliance from the condition of hydraulic smoothness. They follow as necessary conditions at the statement of a problem on interaction of viscoelastic boundary with viscous sublayer. The third condition of drag reduction is a choice of natural frequency according to the interference theory. The way of the material search is shown on the example of one-layer coating from material with spacious plateau of viscoelastic properties.