The three effects of the pressure force on turbulent boundary layers

Abstract: This study aims to isolate the three effects of the pressure force on the inner and outer layers: the local direct impact (characterized by the pressure gradient (PG) parameter, $\beta$), the local disequilibrating effect (represented here by the normalized streamwise derivative $d\beta/dX$), and the upstream cumulative effect, while accounting for the inevitable Reynolds number influence. To achieve this objective, we draw on several non-equilibrium and near-equilibrium databases from the literature, and employ a methodology based on the selection of PG parameters that capture the local direct impact and the local disequilibration effect of the PG. The pressure force impact on the inner and outer regions is represented by two parameters: the friction-viscous PG parameter, $\beta_i$, and the PG parameter based on Zagarola-Smits velocity, $\beta_{ZS}$. In the non-equilibrium flow cases, both $\beta_i$ and $\beta_{ZS}$ exhibit similar distributions, initially increasing and then decreasing. However, the rate of change of these parameters along the streamwise direction varies among the flows, indicating differing levels of pressure force disequilibration. In the outer layer, it is found that both the local and cumulative disequilibrating effects modify the mean velocity and Reynolds stress profiles at identical $\beta_{ZS}$ values. In the inner layer, which responds much faster to changes in pressure force, the local disequilibrating effect still modifies the mean velocity profile in the viscous sublayer. Notably, when the mean velocity defect is significant, the behavior of u-structures in the inner layer appears to be governed by how outer turbulence responds to pressure force effects. In contrast, the size of uv-structures in the inner layer scales with the mixed pressure-friction length. Unlike inner u-structures, they are independent of large-scale outer structures and flow history.