Turbulence in Favorable pressure gradient (FPG) boundary layers

Our objective is to study the effect of favorable pressure gradient on near wall structures in a sink flow turbulent boundary layer over a smooth wall. 2D PIV measurements have been performed upstream of and within the region of constant acceleration parameter, $K$=\textit{$\nu $dU}/\textit{dx/U}$^{2}$, of 0.575X10$^{-6}$. In the initial range, where $K$ increases to its asymptotic value, all the Reynolds stresses and skin friction coefficient, $c_{f}$, decay. In the region of constant $K$, the stresses continue to decay in the outer layer, but $c_{f}$ and all the Reynolds stress components increase close to the wall ($y$/\textit{$\delta $}$<$0.2). The stresses collapse when scaled with the local freestream velocity, $U_{0}$(x). TKE production and wall normal transport of turbulence also scale with $U_{0}$(x)$^{3}$/\textit{$\delta $}(x) close to the wall. PIV data obtained in wall-parallel planes show the expected low speed streaks (LSS) bounded by large structures in the zero pressure gradient range. Narrower LSS persist also in the constant K area, but the signatures of large structures diminish. In both regions, small-scale structures, with signatures suggesting inclined quasi-streamwise vortex pairs, appear predominantly in the LSS areas, suggesting that they are preferred sites of turbulence production.