Experimental investigation of turbulent structures in Plane Turbulent Wall Jets

Plane turbulent wall jets, formed when a high-speed jet flows along a solid surface, are traditionally considered to consist of a turbulent boundary layer (TBL) near the wall and a half-free jet above it. However, the occurrence of counter-gradient momentum transport below the mean velocity maximum, where turbulent momentum is transported against the local mean velocity gradient, raises important questions about this conventional understanding. In a two-part study published in the Journal of Fluid Mechanics, we revisit the turbulent structure of plane wall jets using long-field-of-view particle image velocimetry (PIV) measurements obtained with four side-by-side low-speed cameras at three different Reynolds numbers. Spectral analysis of the PIV velocity fields reveals two distinct structural modes: a wall mode confined to the near-wall region and a jet mode comprising large-scale motions present both above (outer jet mode) and below (inner jet mode) the mean velocity maximum. The jet mode exhibits two dominant characteristic wavelengths, approximately 5 and 2.5 times the outer jet length scale. Importantly, the coexistence of wall and jet modes below the velocity maximum challenges the conventional interpretation of this region as a classical turbulent boundary layer. Building on these findings, structural analysis using two-point correlations, linear stochastic estimation and quadrant analysis reveals the spatial organization of large-scale coherent structures and their role in turbulent momentum transport. The results demonstrate that counter-gradient momentum transport arises from the intrusion of large-scale outer jet structures into the region below the velocity maximum, where non-local turbulent motions interact with locally present inner jet structures. Together, these findings provide a new physical picture of turbulence in plane wall jets and demonstrate that the region below the mean velocity maximum differs fundamentally from a conventional turbulent boundary layer.

Speaker : Dr. Shivsai Ajit Dixit

Biography :

Dr. Shivsai Ajit Dixit is Scientist-F and Principal Investigator of the Fluid Dynamics Laboratory (FDL) at the Indian Institute of Tropical Meteorology (IITM), Pune. He holds a Ph.D. and M.E. in Aerospace Engineering from IISc Bengaluru and a B.E. in Mechanical Engineering from CoEP, Pune. His research spans turbulence, boundary-layer physics, atmospheric dynamics, and cloud microphysics. He architected the establishment of FDL, developing laboratory analogues of atmospheric flows and novel universal scaling models for turbulent skin friction, with implications for atmospheric parameterization. He contributes to the development of IITM’s next-generation cloud chamber and urban meteorological observation networks, and leads research infrastructure and upper-air observations under CAIPEEX. He has authored or co-authored more than 17 refereed journal publications, including papers in Journal of Fluid Mechanics, Physical Review Letters, Bulletin of the American Meteorological Society, Physics of Fluids etc. He is the recipient of the Prof. P. R. Pisharoty Award (2022) for outstanding scientific contribution in the field of atmosphere, weather and climate science, the Prof. A. K. Rao Gold Medal for the Best Ph.D. Thesis in the Department of Aerospace Engineering, IISc Bengaluru, and the Sabita Chaudhuri Memorial Gold Medal for the Best M.E. Student at the Department of Aerospace Engineering, IISc Bengaluru.