An alternate viscosity-dependent scaling produces closer coincidence of the temperature and velocity disturbance extrema with the foci.Ī new definition of concentration fluctuations in turbulent flows is proposed.

In conical similarity co-ordinates there is a near coincidence of the maxima and minima in the temperature disturbance contours with the two stable foci, or points of accumulation, for particle paths found by Cantwell, Coles & Dimotakis (1978), but the position of the hot core region cannot be explained in terms of the centre-line particle paths. The observed position of the upper heating maximum appears to be related to mean streamline pictures obtained by previous investigators in which streamlines entering the rear of the spot are deflected upward into the heated region. Contours of ensemble-averaged temperature disturbance measured relative to the laminar undisturbed profile show a region of maximum heating near the upper front of the spot and a region of maximum cooling at the rear of the spot near the wall, a structure strongly anti-correlated with corresponding contours of the longitudinal velocity disturbance as measured by Zilberman, Wygnanski & Kaplan (1977). The fluid near the wall is cooled and fluid far from the wall is heated during passage of the spot, with a more complex behaviour at intermediate locations. Temperature fluctuations were measured on the centre-plane of spots artificially generated in the laminar boundary layer on a fully heated flat plate. Exploratory measurements to study transport mechanisms in turbulent spots have been made using a passive temperature-tagging technique.

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