Low Reynolds Number Instability of an Incompressible Half-Jet

The instability of the free laminar boundary layer between parallel streams is investigated for an incompressible fluid. It is found that no minimum critical Reynolds number for instability exists based on the assumption of parallel undisturbed flow. However, when the parallel flow theory is corrected for the nonparallelism that actually exists at low Reynolds numbers, the minimum critical Reynolds number for instability becomes 10.2 with the wavenumber 0.135.