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Recent experimental observation of supercooling in large hypersonic wind tunnels using pure nitrogen identified a broad range of non-equilibrium metastable vapour states of the flow in the test cell. To investigate this phenomenon a number of real-gas effects are analysed and compared with predictions made using the ideal-gas equation of state and equilibrium thermodynamics. The observed limit on the extent of supercooling is found to be at 60% of the temperature difference from the sublimation line to Gibbs’ absolute limit on phase stability. The mass fraction then condensing is calculated to be 12–14%. Included in the study are virial effects, quantization of rotational and vibrational energy, and the possible role of vibrational relaxation and freezing in supercooling. Results suggest that use of the supercooled region to enlarge the Mach–Reynolds number test envelope may be practical. Data from model tests in supercooled flows support this possibility.

This paper describes the results of a detailed experimental investigation of a two-dimensional turbulent boundary layer in a favorable pressure gradient where the freestream Mach number varied from 3.8 to 4.6; the ratio of wall to adiabatic wall temperature remained constant at a value of 0.82. Detailed profile measurements were made with pressure and temperature probes; skin friction was measured directly with a shear balance. The velocity and temperature profile results are compared with zero pressure gradient and incompressible results. The skin-friction data are correlated with momentum-thickness Reynolds number and the pressure gradient parameter fie -(6/T w )(,dP/dx).The skin friction increases with decreasing fie for a constant value of momentum-thickness Reynolds number. NomenclatureA = constant in Eq. (4) B = constant in Eq. (4) Cf = skin-friction coefficient H u = incompressible shape factor k = Karman's constant M = Mach number N -velocity profile exponent u/u e = (y/8) llN P = pressure Res = momentum-thickness Reynolds number T = temperature f = (T t -T v )/(T te -T w ) u = velocity U T = shear velocity = (r w /p w ) 112 U + = U/Ur x = distance along plate y = distance normal to flow 2/+ = u r y/v w $ = Clauser's equilibrium parameter = (d*/r w )(dP/dx) (3e = pressure gradient parameter = (6/r w }(dP/dx) 5 = boundary-layer thickness p = density 5* = displacement thickness B = momentum thickness T = shear stress v -kinematic viscosity Presented as Paper 69-685 at Subscripts aw -adiabatic wall e = freestream conditions o = supply conditions w = wall conditions t = total conditions

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A Mach 3 to 7 blow-down wind tunnel fac._. ty for investigating boundary layer phenomena in the transitional and turbulent range is described.