The results of an experimental study of stability, receptivity and transition of the
flat-plate laminar boundary layer at Mach 3 are discussed. With a relatively low free-stream
disturbance level (∼0.1%), spectra, growth rates and amplitude distributions
of naturally occurring boundary layer waves were measured using hot wires. Physical
(mass-flux) amplitudes in the boundary layer and free stream are reported and provide
stability and receptivity results against which predictions can be directly compared.
Comparisons are made between measurements of growth rates of unstable high-frequency
waves and theoretical predictions based on a non-parallel, mode-averaging
stability theory and receptivity assumptions; good agreement is found. In contrast,
it was found that linear stability theory does not account for the measured growth
of low-frequency disturbances. A detailed investigation of the disturbance fields in
the free stream and on the nozzle walls provides the basis for a discussion of the
source and the development of the measured boundary layer waves. Attention is
drawn to the close matching in streamwise wavelengths for instability waves and the
free-stream acoustic disturbances. It was also found that a calibration of the hot
wire in the free stream yields a double-peak boundary layer disturbance amplitude
distribution, as has been found by previous investigators, which is not consistent
with the predictions of linear stability theory. This double peak was found to be
an experimental anomaly which resulted from assumptions that are frequently made
in the free-stream calibration procedure. A single-peak amplitude distribution across
the boundary layer was established only when the hot-wire voltage was calibrated
against the mean boundary layer profile. Finally, the late stages of transition, at a
higher Reynolds number with a higher free-stream disturbance level, were explored.
Calibrated amplitude levels are provided at locations where nonlinearities are first
detected and where the mean boundary layer profile is first observed to depart from
the laminar similarity solution. A qualitative discussion of the character of ensuing
nonlinearities is also included.