Scalar Interface in Turbulent Plane Wall Jets*

A scalar interface (henceforth, interface) refers to a sharp and distinct boundary that separates regions marked by a passive scalar from the non-marked regions in a flow (Prasad and Sreenivasan 1989). Interfaces have been studied extensively in the literature and possess an important feature of fractal-like scaling behaviour over a certain range of scales. The present work studies interfaces in a gaseous plane wall jet experimental facility. A novel approach is implemented to identify the interface in gaseous flows. Low-speed (14 Hz) high-resolution (up to 0.6 η, where η is the mean Kolmogorov scale at the most probable location of the interface) image acquisition is performed to capture the interface. A robust method is devised that computes fractal dimensions from the box-counting algorithm with minimal subjectivity. Statistical and fractal properties of the detected interface are analysed. Results indicate that the interface in wall jets is more contorted (fractal dimension of 1.4 with standard error of 0.1%) compared to that in jets, wakes and boundary layers (fractal dimension of 1.36 with standard error of 0.2%). Possible reasons for this 'extra richness' are also discussed.