Investigation of large scale motions in zero and adverse pressure gradient turbulent boundary layers using high-spatial-resolution particle image velocimetry

Abstract

High-spatial-resolution (HSR) two-component, two-dimensional particle-image-velocimetry (2C-2D PIV) measurements of a zero-pressure-gradient (ZPG) turbulent boundary layer (TBL) and an adverse-pressure-gradient (APG)-TBL were taken in the Laboratoire de Mécanique des Fluides de Lille (LMFL) High Reynolds number Boundary Layer Wind Tunnel. The ZPG-TBL has a momentum-thickness based Reynolds number Reδ2=δ2Ue∕ν=7,750 (where δ2 is the momentum thickness and Ue is the edge velocity), while the APG-TBL has a Reδ2=16,240 and a Clauser’s pressure gradient parameter β=δ1Px∕τw=2.27 (where δ1 is the displacement thickness, Px is the pressure gradient in streamwise direction and τw is the wall shear stress). The 2C fluctuating flow field of each TBL was decomposed using proper orthogonal decomposition (POD) to investigate the large-scale motions (LSMs). The LSMs are found to be energized in the outer-layer, becoming stronger in the presence of the adverse-pressure-gradient. Profiles of the conditionally averaged Reynolds stresses show that high-momentum LSMs contribute more to the Reynolds stresses than low-momentum LSMs from the wall to the end of the log-layer while the opposite is found in the wake region. The cross-over point between the profiles of the conditionally averaged Reynolds stresses from the high- and low-momentum LSMs always has a higher value than the corresponding Reynolds stress from the unconditional ensemble average at the same wall-normal location. This difference is up to 80% in the Reynolds streamwise and shear stresses and up to 15% in the Reynolds wall-normal stresses. Furthermore, the cross-over point in the APG-TBL is found to be further from the wall than in the ZPG-TBL. The conditional Reynolds streamwise and shear stresses without the LSMs are reduced by up to 42% in the ZPG-TBL and by up to 50% in the APG-TBL, while having a minimal effect on the conditional Reynolds wall-normal stress without the LSMs in both the ZPG- and APG-TBL.