Recent Developments in Hypersonic Flow1

Our understanding of hypersonic flow over blunt-nosed bodies has progressed considerably during the last two years. Hayes showed that the lateral flow over a nearly flat-faced body is governed by a choking m e c h a n i s m at the shoulder, and recent experiments at t h e Guggenheim Aeronautical Laboratory, California Inst i tute of Technology, at Moo = 5.8 confirm the qualitative aspects of Hayes' analysis. These experiments, covering a wide range of b lunt-nose shapes, also show that the bow shock shape is determined mainly by the body geometry in t h e sonic region, while the surface pressures are influenced both by the sonic region and by the local body shape. Some of the forward integrat ion procedures that have been developed for the flow in the nose region could possibly be modified to include these essential features. At hypersonic velocity the transverse flow field produced by a b lunt nosed slender body in a fixed plane normal to the flight direction is analogous to the flow generated by an expanding blast wave. The blast wave analogy has been applied to two problems tha t exhibit flow similarity: 1. Unyawed hemisphere-cyl inder (or b lunt-nosed slab); 2. Power bodies of the form r& ^ x. Here flow s imilarity is possible only when m' < m ^ 1, where m ' = V2 for bodies of revolution, and m' — for two-dimensional bodies. Experimental results on such bodies obtained in air at MOT = 7.7 and in he l ium at M^ = 18.4 show good agreement wi th the theoretically predicted shock shapes and surface pressures. Now tha t some features of the i n viscid pressure field generated by a b lunt nose are becoming clearer, the history of the boundary layer can be properly incorporated in to the treatment of hypersonic viscous interactions downstream of the nose-dominated region. A brief discussion of the power body and the hemisphere cylinder shows that two important problems require careful a t tent ion: 1. Emergence of the boundary layer from the blanket of Mach number gas generated by the strong portion of the bow shock; 2. Effects of transverse curvature on the viscous layer in the presence of axial pressure gradients tha t may be partially selfinduced. At low Reynolds numbers the dist inction between shock layer and viscous layer in the nose region will probably have to be abandoned. B u t i t appears that sl ipflow effects are postponed to m u c h lower Reynolds numbers t h a n had been supposed at one t ime , largely because viscous interactions raise the local pressure level. This area of Reynolds number hypersonic flow is a fruitful one for future research.