Experimental measurements of a self-similar adverse pressure gradient turbulent boundary layer

The structure and dynamics of adverse pressure gradient turbulent boundary layers (APG-TBL) have a significant role in the efficiency and performance of a range of transportation and energy generation platforms. This is particularly true in regions of strong adverse streamwise pressure gradient, such as near the trailing edges of wings and turbine blades, where separation of the boundary layer can significantly reduce performance with the potential for catastrophic consequences. The optimal design of such systems remains suboptimal due to a lack of fundamental understanding of how the pressure gradient influences the complex structure of the turbulent boundary layer, as illustrated by our inability to adequately scale the statistical flow quantities with varying pressure gradient. To enable an investigation of both the spatially and temporally coherent structure of APG-TBLs, a series of high-speed particle image velocimetry measurements were performed in a large water tunnel with a flexible and fully adjustable roof in order to allow for the adjustment of streamwise pressure gradient. Results show a significant departure from the more widely studied zero pressure gradient flow as a significant proportion of the turbulent activity moves out to a position roughly one displacement thickness above the wall, forming an outer turbulent fluctuation peak. The results show good agreement with DNS of a self-similar APG-TBL.