Unraveling the structural dynamics of plane turbulent wall jets

The plane turbulent wall jet is traditionally viewed as a turbulent boundary layer (TBL) below the velocity maximum, topped by a half-free jet. However, recent studies reveal that the region below the velocity maximum is far more complex than a conventional TBL, featuring a meso-layer overlap region and a counter-gradient momentum diffusion (CG) region (Gupta et al. 2020, JFM; Choudhary et al. 2024, JFM). To further investigate this complexity, the present study employs a long-field-of-view two-dimensional particle image velocimetry (2D-PIV) technique with four side-by-side cameras. Analysis of the obtained PIV fields using two independent approaches confirms the presence of an inner jet mode below the velocity maximum, which corresponds to jet-scaled structures. First, two-point correlation of binned PIV fields indicates that both outer and inner jet modes are coherent, with the inner jet mode exhibiting a different structural angle compared to typical TBLs. Second, conditional averages based on prograde swirl strength in the outer region show the presence of an inner jet mode attributed to clockwise eddies of the full-free jet, which move wall-ward with increasing Reynolds number. Finally, quadrant analysis reveals that the CG region exists due to the intrusion of outer jet-scaled structures into the region below the velocity maximum, which act as non-local events, leading to the production of negative Reynolds shear stress in the CG region.