Experimental and numerical study of a turbulent boundary layer with pressure gradients

Abstract

The boundary layer develops along a flat plate with a Reynolds number high enough to sustain turbulence and allow accurate experimental measurements, but low enough to allow a direct numerical simulation. A favourable pressure gradient just downstream of the trip (experiment) or inflow boundary (simulation) helps the turbulence to mature without unduly increasing the Reynolds number. The pressure gradient then reverses, and the β-parameter rises from −0.3 to +2. The wall-pressure distribution and Reynolds number of the simulation are matched to those of the experiment, as are the gross characteristics of the boundary layer at the inflow. This information would be sufficient to calculate the flow by another method. Extensive automation of the experiment allows a large measurement grid with long samples and frequent calibration of the hot wires. The simulation relies on the recent ‘fringe method’ with its numerical advantages and good inflow quality. After an inflow transient good agreement is observed; the differences, of up to 13%, are discussed. Moderate deviations from the law of the wall are found in the velocity profiles of the simulation. They are fully correlated with the pressure gradient, are in fair quantitative agreement with experimental results of Nagano, Tagawa & Tsuji. and are roughly the opposite of uncorrected mixing-length-model predictions. Large deviations from wall scaling are observed for other quantities, notably for the turbulence dissipation rate. The a1 structure parameter drops mildly in the upper layer with adverse pressure gradient.