By employing the Nanoparticle-based Planar Laser Scattering method and the particle image velocimetry, structural responses of a Mach 2.95 turbulent boundary layer to two concave curvatures (radiuses are 113 mm and 350 mm, respectively) are experimentally investigated. Large scale vortices formed in the flat plate region are observed to break up into smaller ones immediately after they flow into the concave region. Larger curvature tends to introduce more immediate breakup of the vortices. Fractal analysis reveals that after the turning angle and impulse of bulk dilatation reach 11° and 0.6, respectively, the breakup of the vortices slows down evidently.
By employing particle image velocimetry, the response of a Mach 2.95 turbulent boundary layer to the concave curvature is experimentally investigated. The radius of the concave wall is 350 mm, and the turning angle is 20∘. Logarithmic law is well preserved in the profile of streamwise velocity at all streamwise positions despite the impact of curvature. The varying trend of principal strain rate is found to be different at different heights within the boundary layer, which cannot be explained by the suggestion given by former researchers. Based on the three-layer model proposed in this paper, distribution of the principal strain rate is carefully analyzed. The streamwise increase of wall friction is suggested to be brought by the increase of velocity gradient in the thin subsonic layer. Increases of the static temperature and the related sound speed are responsible for that. Larger correlated turbulent motions could be introduced by the concave curvature. The probability density histograms of streamwise velocity reveal that the large scale hairpin packets are statistically well organized. The concave curvature is found to have the potential of reinforcing the organization, which explains the increase of turbulent level in the supersonic concave boundary layer.
With an inflow Mach number of 2.95, the isolated impacts of the convex curvature and the streamwise favorable pressure gradient on the supersonic turbulent boundary layer are experimentally investigated by employing particle image velocimetry. An experiment on the convex turbulent boundary layer with a zero streamwise pressure gradient is carefully arranged to investigate the impact of the streamline curvature. The results are compared with the favorable-pressure-gradient case (with pressure gradient parameter β = −0.60). For both of the zero-pressure-gradient and the favorable-pressure-gradient convex boundary layers, the log law is found to be well preserved in the streamwise velocity profile with inner scaling. The streamline convex curvature and the streamwise favorable pressure gradient are found to have similar impacts on the distribution of mean streamwise velocity. They both weaken the boundary layer’s wake strength, as well as the principal strain rate in the near wall region. While both of the convex streamline curvature and the favorable pressure gradient are observed to dampen the streamwise turbulent fluctuation, their effects on the normal fluctuation are opposite. The convex curvature reinforces the normal fluctuation, and its strengthening effect is much stronger than the relaxation effect of the favorable pressure gradient. Retrograde vortices are notably weakened by the favorable pressure gradient, which contributes to the relaxation of turbulence. The impacts of the streamline convex curvature and the streamwise favorable pressure gradient are more notable in the outer layer, while in the near wall region their contributions are weakened.
Structural responses of the supersonic turbulent boundary layer to the expansions induced by a convex wall and a ramp are experimentally investigated. Relaminarization of part of the turbulent boundary layer in the near wall region is clearly visualized, which has been seldom presented before. The relaminarized layers formed over two test models are different. While a thicker relaminarized layer is observed for the ramp, a longer lasting layer is noticed for the convex wall. The structure angle is found to be increased by the expansions. Increases of turbulence scale and boundary layer thickness are observed. The contribution of the bulk dilatation to the boundary layer growth is stronger than that of the centrifugal force.
The oblique shock/vortex interaction (OSVI) is numerically investigated based on the large-eddy simulation method. A Mach interaction between separated shock and incident shock can be found when the pressure at recirculation region reaches a certain level. Based on the idea of spatial-temporal correlation, which considers the three-dimensional steady interaction as a two-dimensional unsteady problem, a qualitive analysis is conducted to explain complicated three-dimensional shock structures. The interaction can be regarded as a combination of the following events: the interaction between circular shock and normal shock, the reflection of shock wave on subsonic interface, and the interaction between secondary circular shock and other shock structures. Though the original vortex has broken down, a pair of the streamwise vortices can be observed in the downstream flow field, the formation of which is associated with the split of recirculation region. Moreover, the recirculation region is found to act as a solid body, which means that the flow angle along splitting curve can reflect the splitting speed. Three stages can be identified according to the change process of flow angle along splitting curve, which are rapid growth, linear growth and decrease stages. Inspired by the studies on shock-induced boundary layer separation, the flow filed of strong OSVI with regular interaction is modeled to predict the initial flow angle of splitting point which is the foundation of the study on other stages. The interaction type between separated shock and incident shock can also be judged according to this approach.
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