Effects of Freestream Turbulence on the Characteristics in the Boundary Layer Near the Transition Onset Location

Abstract

Experimental data is presented for a flat plate test facility with augmented levels of freestream turbulence (FST). The turbulence decay downstream of two turbulence generating grids, in addition to the integral length scales, is provided and good comparison with established correlations is presented. Boundary layer measurements using a single normal hotwire probe were obtained at FST intensities of 7%, 6%, 5.5%, 1.55%, and 1.45%, and the results presented include the fifth and 95th percentile of the velocity fluctuations and the root mean squared (RMS) velocity profiles near the transition onset region. The transition onset Reynolds number for each of the turbulence levels considered is consistent with theoretical findings. In all cases analyzed, the maximum fifth and 95th percentile far exceeded the maximum RMS values, with the location of the maximum 95th percentile closer to the wall compared to the maximum fifth percentile. Using probability density function (PDF) analysis, it is demonstrated that there is a dominating positive velocity fluctuation in the nearwall region and a dominating negative velocity fluctuation further out in the boundary layer and that the fluctuations in the boundary layer are greater compared to the freestream. The effect of the FST on the boundary layer is discussed with comparison to the Blasius solution and the influence of the fluctuations on the deviation from the Blasius profile is presented and discussed. Through investigation of the energy spectrum of the fluctuating velocity component within the boundary layer, it is shown that there is a higher energy content at lower frequency in the boundary layer when compared to that of the freestream.