The effects of unsteadiness and the stability of boundary-layer flows as governed by the Orr-Sommerfeld equation are discussed. The condition required for validity of the quasi-steady approximation of governing flow equations is that the ratio of diffusion time to flow time should be small. It is also shown that criteria for validity of the quasi-steady approximation of the Orr-Sommerfeld equation are based on modifications of this ratio, and are not nearly as stringent. Examples of heated wedge flows in water that are presented and discussed show the profound effect of even slowly varying unsteadiness on both laminar boundary-layer flow and its stability.
