Control of the Streaky Structure in Turbulent Boundary Layer by means of Streamwise Vortex Pair

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Interaction process between turbulent boundary layer and longitudinal vortex pair was investigated experimentally. Two-types of longitudinal vortex pair, that is, common-flow up and common-flow down, were generated by delta wing located in free stream and introduced into developed turbulent boundary layer. Radius of vortex is comparable with the boundary layer thickness at the start of the interaction. Circulation of the longitudinal vortex is much less than circulation of whole boundary layer, but, of the same order of circulation in the outer layer. In the present study, spanwise oscillation was applied to vortex generator. Amplitude of spanwise oscillation is a half of wing span of the delta wing. Frequencies of sinusoidal oscillation were chosen as 0.5Hz and 1.0Hz.Effect of spanwise oscillation was investigated with behavior of statistical quantities in free stream. Contours of streamwise vorticity becomes to flattened shape by applying spanwise oscillation. Some strong peaks appea … More r in contours of Reynolds stresses. It is expected that Reynolds stresses measured at a given cross-stream position contain apparent components depending on probability distribution of vortex cantor. Circulation generated by the wing was measured by two different ways, that is, integral of longitudinal vorticity and estimation from lift force obtained pressure measurement. The two results prove that spanwise oscillation has no significant effect on the vorticity generating process.In the interaction process, deformation and relaxation of the mean velocity and Reynolds stresses profile were measured in the boundary layer. The longitudinal vortex pair moves away from the wall in faster rate in case of common-flow up. Spanwise spreading rate of two longitudinal vortex path is larger in case of common-flow down. Decay of maximum longitudinal vorticity is slower in case of common-flow down. At symmetrical plane between two vortex, the interaction between longitudinal vortex makes closely distributed mean velocity contour lines and higher skin-friction coefficient in case of common-flow down. Otherwise, in case of common-flow up the interaction makes widely distributed mean velocity contour lines and low skin-friction coefficient. Effect of extra rate of strain involved in momentum integral equation is applied to explain boundary layer behaviors in the two cases.Effect of the extra rate of strain, ∂W/∂z and ∂V/∂y, was examined in the profiles of three turbulent intensity components and Reynolds shear stress profiles. The boundary layer along symmetrical plane is classified into "cross flow" in case of common-flow down and "identifiable streamwise vortices" in case of common-flow up. The extra rates of strain modify magnitude of turbulent intensity and Reynolds shear stress in the inner layer in case of common-flow down. Otherwise, close to the wall in the case of common-flow up, local adverse pressure gradient is deduced from the fact that magnitude of transverse divergence is grater than that of spanwise convergence.Effect of the interaction on the streaky structure was investigated by simultaneous multi point hot-wire measurement and conditional detection technique. The multi point hot-wire sensor has 12 single hot-wire sensors with 2mm spanwise separation that corresponds to 40 times viscous wall length. The Variable Interval Time Averaging method (VITA) was employed as a detection scheme for low speed streaks. Averaged spanwise spacing of the low speed streaks detected by the scheme is about 150 times viscous wall length. In the common-flow down case, the average spacing is larger at wall-ward secondary current and smaller at up-ward secondary current. The comparison shows that the spanwise oscillation makes the effect somewhat weaker. Less