Abstract
Effects of freestream turbulence on a zero-pressure-gradient turbulent boundary layer are investigated by means of laboratory experiments using a wind tunnel. The freestream turbulence is generated by grids installed upstream of the turbulent boundary layer, which generate nearly isotropic turbulence. In this experiment, two grids with the same blockage of 36% and with different mesh sizes (i.e., M = 30 and 10 mm) are used. The turbulence intensities in the freestream are approximately 1.9% and 1.1% at the measurement point (x = 1,880 mm) for M = 30 and 10 mm grids, respectively. Here, x is the streamwise distance from the entrance of the wind tunnel where the turbulent boundary layer starts to develop. For both cases, the integral length scale of freestream turbulence, L∞, is much smaller than the boundary layer thickness δ, i.e., L∞/δ<<1. The Reynolds numbers Reθ based on the momentum thickness is 3,000. Instantaneous velocities U and V are measured by hot-wire anemometry with an X-probe. The results show that the log-low profile for the time-averaged mean velocity holds for turbulent boundary layers even with freestream turbulence. However, turbulence intensities normalized by the friction velocity uτ are increased by the freestream turbulence. On the other hand, Reynolds shear stress normalized by uτ is decreased in the inner region ( y+ < 600 ) and increased in the outer region of y+ > 700 by the freestream turbulence.
