An Experimental Investigation of the Stability of the Hypersonic Laminar Boundary Layer

Abstract

An experimental investigation of the stability of the hyper sonic laminar boundary layer was carried out for the case of a flat insulated surface at zero angle of attack. The stream-wise amplitude variation of both "natural" disturbances (i.e., flow fluctuations existing naturally in the boundary layer) and of disturbances artificially excited with a "siren" mechanism was studied with the aid of a hot-wire anemometer. In both cases it was found that such small fluctuations amplify for certain ranges of the fluctuation frequency and the Reynolds number R_Θ, and damp for others. The demarcation boundaries for the amplification (instability) zone were found to resemble the corresponding boundaries 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 amplification have a wavelength estimated at about twenty times the boundary-layer thickness δ, which is appreciably longer than the corresponding wave-lengths for low- speed boundary-layer flow.