Marginally separated flows in dilute and dense gases

The paper concentrates on flow effects which occur if a two-dimensional alm ost separated boundary layer is disturbed by a three-dimensional surface-mo unted obstacle. In addition to dilute gases which satisfy the perfect gas l aw, dense gases are also considered. These have the distinguishing feature that the fundamental gasdynamic derivative Gamma can change sign. As a resu lt, rarefaction shocks may form which, in the case of dilute gases, are rul ed out by the second law of thermodynamics. More important in the present c ontext, however, it is found that the non-monotonous Mach number variation during isotropic compression associated with the unusual Gamma -behaviour r epresents a new non-classical mechanism for the formation of marginally sep arated flows.