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Some topological features of the entrainment process in a heated turbulent wake

The temperature signals measured in a fully turbulent, thermally contaminated wake behind a heated cylinder at x/D = 140, are analysed using a pattern-recognition procedure. The temperature patterns detected show that the wake flow is dominated by shear-aligned structures with a strong three-dimensional character. The temperature footprints have a limited extent in the spanwise and streamwise directions, while their dimension is of the order of the wake width in the vertical coordinate. The footprints are characterized by a steep temperature gradient at their back edge, which appears as a sharp hot-to-cold transition. The topological features inferred from the temperature signals suggest that the entrainment process is accomplished mainly by engulfing external fluid. The large-scale organized structures are interpreted as the temperature footprints of the double-rollers characterizing the large-eddy motion in far wakes. The analysis of present results within the context of previous studies about intermittency in heated wakes, brings forward the idea that the intermittent bulges are the emerging double rollers at the three-dimensional turbulent/non-turbulent interface.