LARGE AND VERY LARGE SCALE STRUCTURES IN THE OUTER REGION OF AN ADVERSE PRESSURE GRADIENT TURBULENT BOUNDARY LAYER

Elongated regions of low and high momentum have been studied in the outer region of a turbulent boundary layer subjected to a strong adverse pressure gradient. Large sets of spanwise-streamwise instantaneous velocity fields are acquired by PIV (Particle Image Velocimetry) at three wall-normal positions (0.2δ, 0.5δ, 0.8δ) at three different streamwise locations in the adverse-pressure-gradient zone, from downstream of the strong suction peak up to detachment. Low- and high-speed regions are defined based on the fluctuating streamwise component of velocity. A pattern recognition method and a classification scheme are employed in order to obtain certain characteristics of the low- and high-speed u-structures. Like in the case of zero-pressure-gradient turbulent boundary layers, long meandering regions of low and high speed are observed in the outer region of the present flow. These structures are often longer than the 3δ streamwise length of our measurement planes and have dimensions that scale on boundary layer thickness. High- and low-speed u-structures are evenly spaced and their width is generally comparable to those previously reported for such structures in the overlap region of zero-pressure-gradient turbulent boundary layers. The results also show that the adverse pressure gradient not only reduces the frequency of appearance of streaky structures but also shortens them in the lower part of the outer region (at 0.2δ) while in the upper part (at 0.5δ and 0.8δ) their dimensions and arrangement seem to be unaffected by the pressure gradient.