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 ind icate that increasing the Reynolds number (Re) improves the large-scale mix ing and accelerates the homogenisation of the scalar. As Re increases, the intensity of segregation decreases because of turbulent diffusion but the s cale of segregation increases due to dispersion. Comparison between concent ration 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 departur e increasing with Re. This reflects more the effect of dispersion by the tu rbulent velocity field rather than any molecular diffusion effect. At a giv en Re, the ratio decreases as the distance from the cylinder increases. Thi s trend is more accentuated near the edge of the wake than at the centrelin e.