Influence of Environment in Laminar Boundary Layer Control

Transition Reynolds number is not a unique function of stream turbulence percentage, but is influenced by the frequency spectra, spatial scales, and other physical properties of the disturbance environment of the particular test facility and by the physical characteristics of the test plate. Hot-wire measurements of boundary layer disturbances show that the response to stream turbulence, to downstream traveling acoustic waves, and to test plate vibration is primarily in the Tollmien-Schlichting (T-S) fundamental eigenmode. Growth rates of T-S waves are initially greater under two-dimensional acoustic wave excitation than under three-dimensional stream turbulence excitation in accordance with predictions from spatial stability theory. Conversion of environmental disturbances to T-S waves is found to occur primarily in the vicinity of the test plate leading edge. Analytical, numerical and experimental results suggest strongly that the efficiency of conversion increases with increased bluntness of the leading edge.