The convective and absolute instability of fluid flow over viscoelastic compliant layers

The occurrence of waves on viscoelastic compliant layers subject to fluid f low is studied from the viewpoint of their classification as convective and absolute instability of the flow-wall system. Uniform potential flow and m odified 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 bounda ry layers. The onset velocity of absolute instability remains nearly consta nt with changes in wall damping, with a value of about 3C(t) for turbulent boundary layers, where C-t is the elastic shear wave speed of the compliant material. Convective instability always occurs at a lower flow velocity th an absolute instability. The onset velocity for convective instability, on the other hand, increases fairly rapidly with wall damping and tends toward s the onset velocity for absolute instability in the case of a highly dampe d layer. The results suggest that the static divergence waves observed on h ighly damped coatings in experiments are manifestation of absolute instabil ity. (C) 1999 Academic Press.