Large Scale Adverse Pressure Gradient Turbulent Boundary Investigation by means of PIV and PTV

Abstract

subjected to a streamwise adverse pressure gradient Δp+ =ν/(ρu3 τ) dp/dx. However the statistically averaged behaviour of turbulent boundary layers subjected to adverse pressure gradients (i.e.Δp+ >0) is still an poorly understood. For zero-pressure gradient boundary layers the log-law u+ = 1/Ki log(y+) + Bi gives a proper description of the mean velocity at sufficiently large Reτ [7]. However, for adverse pressure gradient flows the log-law breaks down and a socalled half-power law or modified log-law with a functional depends on Δp+ may exist [4, 6]. Another open point is the spatial (and temporal) organisation of the coherent large and small-scale structures. While these aspects are far better understood for zero-pressure gradient boundary layers, there is little information for wall-bounded flows with streamwise adverse pressure gradients. In order to study the coherent flow structures, higher-order turbulent statistics and wall shear stress characteristics in a turbulent boundary layer subjected to adverse pressure gradients an experiment that builds on the recent progress in Particle Image Velocimetry (PIV) and Particle Tracking Velocimetry (PTV) is carried out. The experiment is designed such that the flow starts with a fully developed zeropressure gradient turbulent boundary layer and subsequently the flow experiences a streamwise adverse pressure gradient. The imposed pressure gradient is such that the streamwise flow development is slow with no history effects, but sufficiently large to cause changes from the universal log-law (e.g. Δp+ ≫0.015).