On the impact of adverse pressure gradient on the supersonic turbulent boundary layer

By employing the particle image velocimetry, the mean andturbulent characteristics of aMach 2.95turbulent boundary layer are experimentally investigated without the impact of curvature. The physical mechanism with which the streamwise adversepressure gradient affects the supersonic boundary layer is revealed. The data are compared to that of the concave boundary layer with similar streamwise distributions of wall staticpressure to clarify the separate impacts of the adversepressure gradient and the concave curvature. The logarithmic law is observed to be well preserved for both of the cases. The dip below the logarithmic law is not observed in present investigation. Theoretical analysis indicates that it could be the result of compromise between the opposite impacts of the compression wave and the increasedturbulent intensity. Compared to the zeropressure gradient boundary layer, the principal strain rate and theturbulent intensities are increased by the adversepressure gradient. The shear layer formed due the hairpin packets could be sharpened by the compression wave, which leads to higher principal strain rate and the associatedturbulent level. Due to the additional impact of thecentrifugal instability brought by the concave wall, even higherturbulent intensities than that of the adversepressure gradient case are introduced. The existence of velocity modes within the zeropressure gradient boundary layer suggests that the large scalemotions are statistically well organized. ...

Source: Physics of Fluids - November 29, 2016 Category: Physics Authors: Qian-Cheng Wang, Zhen-Guo Wang and Yu-Xin Zhao Source Type: research