Statistical Characteristics of Turbulence Quantities in Adverse-Pressure-Gradient Turbulent Boundary Layer

Abstract

Measurements in boundary layers with 'moderate'to 'strong'adverse pressure gradients are presented and discussed. With increasing adverse pressure gradients, the velocity profile in U -⁺ ∼ y⁺ coordinates lies below the standard log law, thus indicating a reduction in the thickness of the sublayer. Correspondingly, the turbulence energy components as well as the Reynolds shear stress reduce in the near - wall region of the boundary layer. Waveforms of fluctuating velocity components in the boundary layer, especially in the near-wall region, are remarkably elongated in time in comparison with those in zero-pressure-gradient flows, and thus time scales increase with an increasing pressure gradient parameter P⁺. The increase in time scales is not in proportion to the corresponding increase in the conventional viscous time scale ν/u²_τ. It is found that the Taylor time scale is most appropriate to describe the essential characteristics of non-equilibrium adverse pressure gradient flows. Even the near wall-limiting behavior of streamwise velocity fluctuations for different P⁺ is well correlated in the coordinates based on the Taylor time scale.