Effect of adverse pressure gradient on the turbulence burst structure of low Reynolds number equilibrium boundary layers

Abstract : This study is an investigation of low Reynolds number, zero and adverse pressure gradient, equilibrium boundary layers. Two adverse pressure gradient boundary layers and one zero pressure gradient boundary layer were studied with the zero pressure gradient case serving as a comparative standard for the non-zero pressure gradient boundary layers. The non-zero cases included a mild pressure gradient and a strong (relative to the mild case) pressure gradient. The study determined the effect of pressure gradient on the turbulent burst structure for these flows. The Reynolds number (based on momentum thickness), varied from 1120 to 2500 and the pressure gradient parameter, varied from 0.0 to 1.8. Single component last Doppler velocimeter measurements were made of the streamwise velocity to determine the various boundary layer integral parameters and to create velocity records needed to calculate the period of the bursting cycle. Over the range of Reynolds numbers studied, the results indicate that the burst period (normalized with inner variables) is not a function of Reynolds number. The burst period appears to be solely a function of the pressure gradient and upstream history of the boundary layer. Although the period of the burst cycle is reduced by the adverse pressure gradient, the ensemble averaged velocity signature of the bursts do not change. This indicates that the nature of the individual burst is not significantly changed by the presence of the adverse pressure gradient.