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,t,Wasinto'B. Under Contract F44620-75-C-0016 AIR FORCE OFrICZ OF SCIENTIFIC RESEARCH (AJSC) NOTICE OF TRANS ITTAL TO DDCTh S teohnioal roA:,rt k" t-e:, reviewed and 18 tproved for pubil: releoaso IAN AM 190-12 (7b). appvars to be generated by the first and second mode instabilities but no theoretical guidance exists to help identify the contributing modes for the other regions. The second-mode instability has the highest amplification rates and generates the optically detectable "laminar waves"; a "second harmonic" to these waves is generated by the modes populating the next most prominent unstable region. The stability diagram extends smoothly into the non-linear and transitional regimes, which are marked by the attainment of a miximum amplitude and then a decay, of the laminar waves. Wall cooling causes a substantial increase of the amplification rates with a concurrent upstream movement of the transition zone. UNCLASSIFIED SICURITY C .ASIFCATION OF '.PAGE11'hert rat* ntere) ABSTRACTAn experiment was performed to verify and extend earlier observations of instabilities in the hypersonic laminar boundary layer of cones. The present tests were performed at edge Mach number 7 in a continuous wind tunnel, at edge Reynolds numbers from one to three million and wall-to-stagnation temperature ratios of 0.41 and 0.8. Stability data were obtained for nondimensional frequencies as high as 5 x 10 -4 with high data density provided by computerized data reduction techniques, which were also used to re-examine earlier data. The results are consistent with all earlier data and reveal a complex stability diagram with at least three unstable regions. The lower of these regions appears to be generated by the first and second mode instabilities but no theoretical guidance exists to help identify the contributing modes for the other regions. The second-mode instability has the highest amplification rates and generates the optically detectable "laminar waves"; a 'second harmonic" to these waves is generated by the modes populating the next most prominent unstable region. The stability diagram extends smoothly into the non-linear and transitional regimes, which are marked by the attainment of a maximum amplitude and then a decay, of the laminar waves. Wall cooling causes a substantial increase of the amplification rates with a concurrent upstream movement of the transition zone.

Measurements of the mean flow, intermittent structure and turbulent fluctuations were made in a cold-wall boundary layer at a stream Mach number of 9·4 and Reynolds number based on momentum thickness of 36 800. For these conditions, the r.m.s. sublayer thickness was 32 times smaller than that of the boundary layer proper, and the interfacial standard deviation of the latter was about three times proportionately smaller than has been found at low speeds. The mean flow data, which extended well into the sublayer, revealed a large increase in static pressure from the layer edge to the wall and a quadratic law relation between the total temperature and velocity. While the transformed velocity profile was in good agreement with the incompressible law of the wake, no indication of a linear variation of velocity in the sublayer was detected.Hot-wire fluctuation data, interpreted with the use of appropriate assumptions concerning the nature of the sound field, indicated that the turbulence is dominated by high-frequency pressure fluctuations whose magnitude at the wall and beyond the layer edge agree with extrapolation of data acquired at supersonic speeds. The static temperature fluctuations agreed with expectations from adiabatic, supersonicdata apparently because they were suppressed by the cooled-wall condition. The fluctuations in the longitudinal velocity component were generally small and differed little from lower Mach number results. The high turbulence Reynolds numbers found generated an inertial-subrange spectral decay, while the longitudinal integral scales were found independent of turbulence mode and about one-fifth the boundary-layer thickness.

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An experimental investigation of the hydrodynamic stability of the laminar hypersonic boundary layer was carried out with the aid of a hot-wire anemometer. The case investigated was that of a flat surface at zero angle of attack and no heat transfer.The streamwise amplitude variation of both natural disturbances and of disturbances artifically excited with a siren mechanism was studied. In both cases it was found that such small fluctuations amplify for certain ranges of frequency and Reynolds number Rθ, and damp for others. The demarcation boundaries for the amplification (instability) zone were found to resemble the corresponding limits of boundary-layer instability at lower speeds. A ‘line of maximum amplification’ of disturbances was also found. The amplification rates and hence the degree of selectivity of the hypersonic layer were found, however, to be considerably lower than those at the lower speeds. The disturbances selected by the layer for maximum amplifications have a wavelength which was estimated to be about twenty times the boundary-layer thickness δ.