A note on the absolute instability of wakes

The family of wake profiles formed from the Falkner-Skan boundary-layer sol utions with negative pressure gradient, lit, was shown in (Woodley and Peak e, 1997) to be inviscidly absolutely unstable. However, due to numerical di fficulties, the limit m --> 0 was not considered there, although the result s that were found appeared to indicate that the double Blasius profile (m = 0) is absolutely unstable. In this note we repeat those calculations using an alternative formulation, and show that in fact the absolute instability present for m <0 becomes neutral as m --> 0, so that the flat-plate wake i s only convectively, and not absolutely, unstable. In addition, the effects of viscosity are included here using the Orr-Sommerfeld equation, and two sets of saddle points are now found, one set corresponding to the inviscid family, which can be either pinching or non-pinching depending on the value s of in and the Reynolds number, and another set which is always pinching, but which is stable for in close to zero. (C) Elsevier, Paris.