In this work we analyse the stability properties of the flow over an isothermal,
semi-infinite vertical plate, placed at zero incidence to an otherwise uniform stream
at a different temperature. Near the leading edge the boundary layer resembles
Blasius flow, but further downstream it approaches that of pure buoyancy-driven
flow. A coordinate transformation that describes in a smooth way the evolution
between these two limiting similarity states, where the viscous and buoyancy forces are
respectively dominant, is used to calculate the basic flow. The stability of this flow has
been investigated by making the parallel flow approximation, and using an accurate
spectral method on the resulting stability equations. We show how the stability modes
discussed by other authors can be followed continuously between the forced and free
convection limits; in addition, new instability modes not previously reported in the
literature have been found. A spatio–temporal stability analysis of these modes has
been carried out to distinguish between absolute and convective instabilities. It seems
that absolute instability can only occur when buoyancy forces are opposed to the free
stream and when there is a region of reverse flow. Model profiles have been used in
this latter case beyond the point of boundary layer separation to estimate the range
of reverse flows that support absolute instability. Analysis of the Rayleigh equations
for this problem suggests that the absolute instability has an inviscid origin.
Experimental study of weak shock waves influence on the supersonic boundary layer of the flat plate model
