In this work we analyse the stability properties of the mixed convection boundary layer over an isothermal semi-infinite vertical plate, placed at zero incidence to an otherwise uniform stream at a different temperature. Using a coordinate transformation to describe the basic flow for the entire mixed convection regime, we show the coexistence of multiple stability modes and how they evolve between the forced and free convection limits. 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. The analysis of the Rayleigh equations for this problem suggests that the absolute instability has an inviscid origin.
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