Velocity Field behind a Plate Installed in the Inner Region of a Turbulent Boundary Layer
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Journal of Engineering Physics and Thermophysics, Vol. 93, No. 5, September, 2020
VELOCITY FIELD BEHIND A PLATE INSTALLED IN THE INNER REGION OF A TURBULENT BOUNDARY LAYER V. L. Zhdanov,a I. G. Kukharchuk,a and V. I. Terekhovb
UDC 532.526.4
The authors have presented results of an experimental investigation into the velocity field in a turbulent boundary layer behind a thin (0.00045 m) three-dimensional plate. The chord of the plate (streamwise length) was equal to 0.55δ (δ is the boundary-layer thickness), and its width, to 1.0δ. The plate was installed at a zero angle of attack at the center of a water channel at a distance of 0.09δ from the surface. Velocity-field measurements have been performed by the Particle Image Velocimetry method at the Reynolds number Reh = 7750 calculated from the channel half-width and the velocity at the center of the channel. It has been shown that the average velocity increased in a logarithmic region of the boundary layer at a distance of its three thicknesses behind the plate. Longitudinal-velocity pulsations decreased in the buffer region of the boundary layer, but grew in the logarithmic region. Vertical pulsations only decreased to a distance of 0.8δ behind the plate, but downstream they were higher than in an unperturbed boundary layer. The high resolution of the velocity field (50·10–6 m) has made it possible to determine shear stresses on the wall from the velocity gradient in a laminar sublayer. Shear stresses on the surface behind the plate decreased in the interval where a growth in the average velocity in the logarithmic region was noted. Maximum reduction in the shear stresses occurred at a distance of 1.8δ and amounted to ~33%. The influence of edge effects was manifested in the less intense reduction on shear stresses in the shorter interval behind the plate. Keywords: water channel, turbulent boundary layer, shear stresses, PIV. Introduction. A turbulent boundary layer (BL) is conditionally divided into the outer region adjacent to an unperturbed flow and the inner one contacting the surface directly. In the inner region, we single out a laminar sublayer where the velocity on the surface decreases to zero, a buffer region where maximum pulsations are generated, and a logarithmic region where the rate of change is described by a logarithmic dependence. Investigations of recent decades have established than an instability appears on the boundary of the laminar sublayer and the buffer region, which grows into a Ω-shaped vortex structure [1–6]. These structures in their development cause the high-velocity medium to move to the surface, which leads to growth in shear stresses on the surface. On the basis of this mechanism of occurrence of shear stresses, a method to reduce them by placing a thin plate across the boundary layer at a height of 0.5–0.8δ (δ is the boundary-layer thickness) was proposed. The method was named LEBU (Large Eddy Break Up Device). The plate "cuts" the incident eddy into two parts, and its upper part becomes incapable of forcing the high-velocity
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