THE CONVECTIVE AND ABSOLUTE INSTABILITY OF FLUID FLOW OVER VISCOELASTIC COMPLIANT LAYERS

The occurrence of waves on viscoelastic compliant layers subject to fluid flow is studied from the viewpoint of their classification as convective and absolute instability of the flow-wall system. Uniform potential flow and modified potential flows representing laminar and turbulent boundary layers are considered. It is found that uniform potential flow over viscoelastic (dissipative) layer admits only absolute instability, whereas convective and absolute instability modes may be found under laminar and turbulent boundary layers. The onset velocity of absolute instability remains nearly constant with changes in wall damping, with the value of about 3Ctfor turbulent boundary layers, whereCtis the elastic shear wave speed of the compliant material. Convective instability always occurs at a lower flow velocity than absolute instability. The onset velocity for convective instability, on the other hand, increases fairly rapidly with wall damping and tends towards the onset velocity for absolute instability in the case of a highly damped layer. The results suggest that the static divergence waves observed on highly damped coatings in experiments are manifestation of absolute instability.