Streamwise evolution of a high-Schmidt-number passive scalar in a turbulent plane wake

The concentration fluctuation c of diluted fluorescein dye, a high-Schmidt- number passive scalar (Sc = nu /D approximate to 2000, nu and D are the flu id momentum and dye diffusivities, respectively), is measured in the wake o f a circular cylinder using a single-point laser-induced fluorescence (SPLI F) technique. The streamwise decay rate of the mean and rms values of c is slow in comparison to that of theta, the temperature fluctuation for which the molecular Prandtl number Pr = nu/kappa is about 0.7 (kappa is the therm al diffusivity). The comparison between mean and rms distributions of c and theta highlights the combined role the Reynolds and Schmidt numbers play i n terms of dispersing the scalar. The streamwise evolution of the probabili ty density functions (pdfs) of c and theta suggest that while p(theta) is a pproximately Gaussian in the intermediate wake (x/d approximate to 80), p(c ) is strongly non-Gaussian, and depends on both x/d and Re. The skewness of c is larger than that of theta along the wake centreline. Arguably, the as ymmetry of p(c) reflects the relatively strong organisation of the large-sc ale motion in the far-wake.