Marginally separated flows in dilute and dense gases

The paper concentrates on flow effects which occur if a two–dimensional almost separated boundary layer is disturbed by a three–dimensional surface–mounted obstacle. In addition to dilute gases which satisfy the perfect gas law, dense gases are also considered. These have the distinguishing feature that the fundamental gasdynamic derivative Γ can change sign. As a result, rarefaction shocks may form which, in the case of dilute gases, are ruled out by the second law of thermodynamics. More important in the present context, however, it is found that the non–monotonous Mach number variation during isotropic compression associated with the unusual Γ–behaviour represents a new non–classical mechanism for the formation of marginally separated flows.