IMPACT OF A 3D PLATE ON THE STRUCTURE OF A TURBULENT BOUNDARY LAYER

The authors have performed 3D numerical investigations into the velocity field behind a 3D thin plate by the LES method. The plate was located at a height of 0.002 m in a turbulent boundary layer formed in a water channel. The chord length of the plate and its spanwise size were equal to 0.01 m and 0.024 m respectively. The Reynolds number calculated from the half-width of the channel and the velocity on its axis was equal to 7500. It has been shown that under the impact of the wake of the plate, the longitudinal velocity normalized to the stagnation velocity grew in the logarithmic region at a distance to x/delta = 3.8, and pulsations of all the velocity components decreased in the buffer region to the distance x/delta = 0.8. As the lower shear layer of the wake approached the surface, longitudinal pulsations reached their minima at the wall at x/delta = 1.8, whereas vertical and transverse pulsations became higher than those in an unperturbed boundary layer. The calculated characteristics of the velocity field satisfactorily correlated with the relevant characteristics obtained in experimental investigation with similar initial and boundary conditions. An analysis of the velocity field has revealed the mechanism of impact of the wake on the structural change of wall flow. The involvement of the wall medium in the lower shear layer generated the outflow of the medium from the channel wall, and the involvement in the upper shear layer formed the inflow of a high-speed medium to the buffer region. The medium's outflow from the wall led to a reduction in the velocity gradient at the surface. Shear stresses decreased at a distance of four thicknesses of the boundary layer, and their local reduction amounted to as much as similar to 30%. Trailing vortices descending from the plates side edges created a nonuniform transverse-velocity distribution. This nonuniformity caused the vorticity to form in the buffer region. The arising system of small-size vortices blocked the feed of the high-speed medium to the wall, retarding the growth in the shear stresses on the surface in the interval 1.4 <= x/delta <= 2.6. The vortex system degenerated upon the stabilization of the transverse velocity and shear flow on the surface was restored.