Experimental analysis of the freestream length scale and turbulence intensity impact on bypass transition in boundary layer on a flat plate

An experimental investigation of the turbulent flow over a flat plate in a subsonic wind tunnel was carried out. The enhanced level of turbulence was generated by five wicker grids with square meshes, and different parameters (diameter of the grid rod 0.3–3 mm and the grid mesh size 1–30 mm). The velocity of the flow was measured by means of a 1D hot-wire probe, suitable for measurements in a boundary layer. The aim of the investigation was to explore the turbulence length scale in the flow behind grids and to study its influence on the onset of laminar-turbulent bypass transition in a boundary layer on a flat plate. To assess the isotropy of turbulence, the skewness and kurtosis factors of the flow velocity distribution were determined. Several longitudinal scales of turbulence were determined and compared (integral, dissipation, Taylor microscale, and Kolmogorov scale) for different grids and different velocities of the mean flow 4, 6, 10, 15, and 20 m/s.