The spatial stability of a class of similarity solutions

The spatial stability of a class of exact similarity solutions of the Navier-Stokes equations whose longitudinal velocity is of the form xf'(y), where xis the stream wise coordinate andf'(y) is a function of the transverse, cross-stream wise, coordinate y only, is determined. These similarity solutions correspond to the flow in an infinitely long channel or tube whose surface is either uniformly porous or moves with a velocity linear in x. Small perturbations to the streamwise velocity of the form x' g'(y) are assumed, resulting in an eigenvalue problem for A, which is solved numerically. For the porous wall problem, it is shown that similarity solutions in whichf'(y) is a monotonic function of y are spatially stable, while those that are not monotonic are spatially unstable. For the accelerating-wall problem, the interpretation of the stability results is not unambiguous and two interpretations are offered. In one interpretation the conclusions are the same as for the porous problem-monotonic solutions are stable; the second interpretation is more restrictive in that some of the monotonic as well as the nonmonotonic solutions are unstable.