Spectral measurements in transitional boundary layers on a concave wall under high and low free-stream turbulence conditions

The relationship between free-stream turbulence and boundary layer behavior has been investigated using spectral measurements. The power spectral dens ities of turbulence quantities in transitional and fully turbulent boundary layers were computed and compared to the power spectra of the same quantit ies measured in the free stream. Comparisons were made using the "transfer function." The transfer function is the ratio of two spectra at each freque ncy in the spectra. Comparisons were done in flows with low (0.6 percent) a nd high (8 percent) free-stream turbulence intensities. Evidence was gather ed that suggests that relatively low-frequency, large-scale eddies in the f ree stream buffet the boundary layer, causing boundary layer unsteadiness a ta the same low frequencies. These fluctuations are present in both transit ional and fully turbulent boundary layers. They are seen under both high an d low free-stream turbulence conditions, although they are stronger in the high-turbulence case. Examination of the turbulent shear stress suggests th at the low-frequency fluctuations enhance transport in the boundary layer b ut they are not so effective in promoting eddy transport as are turbulent e ddies reproduced and residing within the boundary layer. In the fully-turbu lent boundary layer, higher-frequency fluctuations are added to the low-fre quency unsteadiness. These higher-frequency fluctuations, not seen in the t ransitional boundary layer, are associated wit turbulence production in the boundary layer and appear not be directly related to free-stream unsteadin ess. [S0889-504X(00)00403-7].