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Hot-wire anemometry is used to study the origin and growth of natural fluctuations in zero pressure-gradient boundary layers for several Mach numbers between 1.6 and 8.5. The importance to transition of certain physical mechanisms is examined through comparison of the fluctuation growth with the sound-forcing and stability theories of Mack. Flow fluctuations of substantial amplitude were observed within the laminar layer ahead of stations where instability amplification is expected to be important. These fluctuations were found to be cross-correlated with the sound field for the higher supersonic speeds, but not for the lower ones. The fluctuation growth rates in the unstable Reynolds number range ahead of the nonlinearity region were in reasonably close agreement with the theory for Mach 4.5; the agreement for Mach 2.2 and 8.5 was qualitative. The second mode of instability was predominant at Mach 8.5.
J. M. Kendall (Sat,) studied this question.