Experimental study of TRPIV for turbulent boundary layer of longitudinal concave curvature wall

Turbulent boundary layers with curvature and pressure gradient are widely used in various engineering applications. A wall model with concave curvature in streamwise was used as the research object, and the instantaneous velocity fields at two different streamwise positions upstream and downstream were measured by TRPIV experiment with dual camera size field of view. Using experimental data from a smooth flat plate as a benchmark, the variation pattern of the turbulent boundary layer on a concave wall is investigated. By comparing the mean velocity profile and the Reynolds stress profile with those of the smooth flat plate, it was found that the mean velocity profile in the concave wall case gradually deviated from the conventional logarithmic law and that the turbulence intensity was weaker than that in the flat plate case. Conditional averaging is carried out under the condition of the prograde vortex identified by ${\varLambda _{ci}}$ criterion. It is found that the peak of the positive fluctuations above the vortices in the concave wall varies oppositely to the scale range, while the negative fluctuations below the vortices is stronger than that of flat plate. The coherent structures were further extracted using a spatial two-point correlation method and the inclination of the coherent structures in the near-wall region to the wall was calculated using an ellipse fitting method. It was found that the scale range of the coherent structure on the concave wall gradually increases along the streamwise. The results show that when the turbulent boundary layer is subjected to a combination of concave curvature and favorable pressure gradient, the difference in turbulence intensity near the near-wall region and within the wake region increases, along with an increase in the intensity of vortices in the coherent structures within the boundary layer. With development downstream, the scale range of positive fluctuations above the prograde vortex on the buffer layer increases, but the peak value decreases, while the opposite phenomenon occurs on the upper side of logarithmic region. The trend of coherent structure migration to higher normal height in the curved wall turbulent boundary layer is weakened, and scale range of the coherent structures grows mainly towards the downstream side.