On the large- and small-scale motions in a separated, turbulent-boundary-layer flow

Adverse-pressure-gradient turbulent boundary layer flow was inspected at Reynolds number based on momentum thickness, (Formula presented.), using particle image velocimetry in a refractive-index-matching flume. Proper orthogonal decomposition was used to quantify the effect of large-scale motions on the Reynolds stresses at the onset of separation and within the separated flow. Results show that approximately (Formula presented.) of the Reynolds shear stress, (Formula presented.), is due to large-scale motions containing (Formula presented.) of the turbulence kinetic energy at the tested Reynolds number. The decomposed velocity field revealed that only the first (Formula presented.) of the modes is sufficient to recover (Formula presented.) of the turbulence kinetic energy. In this partition, the large-scale motion contribution to the streamwise component of the Reynolds normal stress, (Formula presented.), is about (Formula presented.) and continues to grow with flow separation. In addition, the large- and small-scale motions equally contributed to the vertical component of the Reynolds normal stress, (Formula presented.), and the contribution of the large-scale motions increased as the flow separated. Overall, results emphasise the significant impact of the large-scale motions on the Reynolds stresses in the separated flow, which may impact flow control strategies.