The linear wave instability of boundary layer flow induced by a horizontal heated surface in porous media

In this paper we use the parallel-flow approximation to determine the criterion for the onset of instability in the form of travelling waves in a horizontal thermal boundary layer in porous media. We find that waves grow beyond a nondimensional distance of 28.90 from the leading edge, a result which shows, somewhat surprisingly, that waves are to be preferred over vortices, which have been found to grow beyond 33.47 from the leading edge [1]. We discuss very briefly the implications of our analysis for the use of the parallel flow approximation in the determination of stability criteria for thermal boundary layers.