Experimental evidence of jet-mode motions below velocity maximum in plane turbulent wall jets*

Plane turbulent wall jets (PWJs) are complex wall-bounded flows formed when a plane jet is blown along a solid surface. A PWJ in a quiescent ambience is characterised by a non-monotonic mean velocity profile with maximum velocity located at a distance from the wall and velocity decreasing on its either side attaining zero value at the wall as well as far way from the wall. The part of the PWJ between the velocity maximum and the wall is traditionally considered to be akin to a turbulent boundary layer whereas the part above velocity maximum is treated as a half free jet. Our earlier work (Gupta et al. 2020, JFM) demonstrates that the mean velocity scaling does not support this framework and suggests two different scaling regions, namely full free jet region and wall region, where the former extends below the velocity maximum as well. In this work (see also Choudhary et al. 2024, JFM), we provide strong experimental evidence for the structure of a PWJ comprising of two modes - the full free jet mode and the wall mode. Novel long field-of-view PIV measurements reveal that the long-wavelength jet-mode motions are dominantly present in the region below velocity maximum and contribute significantly to the dynamics of that region. Directly computed spatial spectra (without Taylor's hypothesis) of various turbulence quantities strongly suggest this structure of PWJs. The region between the wall and the velocity maximum of a PWJ is thus far more complex than a turbulent boundary layer.