Characteristics of fluorescein dye and temperature fluctuations in a turbulent near-wake

 The Laser Induced Fluorescence (LIF) technique was used to determine a few characteristics of a high Schmidt number (Sc) passive scalar (fluorescein) in the near-wake of a circular cylinder. The mean and rms concentration indicate that increasing the Reynolds number (Re) improves the large-scale mixing and accelerates the homogenisation of the scalar. As Re increases, the intensity of segregation decreases because of turbulent diffusion but the scale of segregation increases due to dispersion. Comparison between concentration and temperature rms profiles indicates that molecular diffusion has a much smaller effect on the decay of the scalar variance than the Reynolds number. The ratio of mean-squared values of lateral and streamwise spatial derivatives of the scalar fluctuation deviates from isotropy, the departure increasing with Re. This reflects more the effect of dispersion by the turbulent velocity field rather than any molecular diffusion effect. At a given Re, the ratio decreases as the distance from the cylinder increases. This trend is more accentuated near the edge of the wake than at the centreline. Received: 16 March 1999/Accepted: 10 June 1999