EFFECTS OF FLUCTUATING PRESSURE GRADIENTS ON BOUNDARY LAYER TURBULENCE

This study investigates the effects of fluctuating pressure gradients on boundary layer turbulence. Two-dimensional (2D) time resolved particle image velocimetry (PIV) measurements have been performed in a zero pressure gradient (ZPG) boundary layer. The streamwise and wall-normal (x,y) components of the fluid material acceleration (Du/Dt and Dv/Dt, respectively) are integrated spatially to obtain the pressure distributions. Large scale pressure fluctuation gradients are found to involve three dimensional flow structures. Ejections, high wall-normal enstrophy flux, and viscous vorticity production occur mostly during periods of partial derivative p'/partial derivative x < 0 as the fluid accelerates by moving away from the wall. Conversely, fluctuating adverse pressure gradients (partial derivative p'/partial derivative x > 0) are preferentially associated with sweeps, as fluid approaching the wall is decelerating. Consequently, the outward transport of small-scale turbulence is suppressed, and the near-wall enstrophy increases. The instability of the flow associated with the adverse pressure gradients might also contribute to the increased production of near-wall turbulence. Results also indicate that the regions of fluctuating adverse and favorable pressure gradients are likely to be associated with the downwash and upwash sides, respectively, of very large scale roller structures, several boundary layer thicknesses in length.