Boundary layer separation due to gas thermal expansion

Abstract

An analysis is carried out of the viscous-inviscid interaction due to the thermal expansion of the gas in the laminar boundary layer over a flat plate whose temperature presents a step elevation above the ambient temperature at a finite distance of its leading edge. Numerical solution of the triple-deck problem for infinitely large Reynolds numbers shows that the boundary layer separates for a critical value of the ratio of plate-to-gas temperature, dependent on the Prandtl number and the temperature sensitivity of the gas viscosity (which is assumed to increase as a power of the temperature). As the temperature ratio increases above this value, the separation point continuously shifts upstream of the temperature step whereas reattachment occurs shortly downstream of the step, leading to a triangle shaped recirculation bubble. An asymptotic description is given of the flow with incipient separation. For very large values of the temperature ratio a neatly defined interface develops between the cold and hot parts of the flow and the closure of the bubble becomes very abrupt, with the fluid recirculating near its rear section in a characteristic manner that depends on the combined effects of the large decrease of density and increase of viscosity due to the strong heating. © 1997 American Institute of Physics.