Influence of a 3D near-wall plate on turbulent boundary layer structure

Three dimensional numerical investigation of a velocity field in a turbulent boundary layer disturbed by near-wall thin plate were made by the LES method. The plate was located at 10% of the boundary layer thickness (\delta) apart of the water channel surface. The length of the plate chord and its spanwise size were 0.01 m and 0.024 m, respectively. The Reynolds number based on the velocity at the channel centerline and its half-height was 7750. The wall-normal profile of the mean velocity behind the plate increased in the log region up to x/\delta ≤ 3.8 (where \delta ) and fluctuations decreased within an interval 0 ≤ x/\delta ≤ 0.8 behind the plate. The longitudinal fluctuations became minimal at x/\delta = 1.8 whereas the normal and spanwise ones at this locations were higher than in the undisturbed boundary layer. The numerical data correlated with the experimental ones obtained at the similar initial and boundary conditions. The analysis of the velocity field within the buffer region revealed an impact mechanism of the plate wake and edge vortices on the near-wall structure transformation. The entrainment of a near-wall fluid into a lower shear layer of the plate wake resulted in the decrease of a velocity gradient and the wall-shear stress. The shear stress reduces locally by ~ 30% and was observed up to ~ 4\delta downstream the plate. The entrainment into the upper shear layer of the wake resulted in a downward high velocity fluid from the log region to the buffer region. The edge vortices caused nonuniform distributions of the spanwise velocity in the buffer region that creates the formation conditions for a new vorticity in the region. This vorticity (small-scale longitudinal vortices) blocks the contact of a high velocity fluid, entered from the log region, with the wall, so the shear stress rise was delayed within the interval 1.2 ≤x/\delta ≤ 2.8. The shear stresses increased downstream when the longitudinal vortices system decayed.