MIXING OF A HIGH Sc PASSIVE SCALAR IN A TURBULENT NEAR-WAKE

摘要

The characteristics of a high Schmidt number (Sc) passive scalar in the turbulent near-wake of a circular cylinder are investigated. Comparison between concentration and temperature rms profiles indicates that molecular diffusion has much less effect on the decay of the scalar variance than the Reynolds number, Re. The second-order moment of the concentration increment in the x direction, decreases as the Reynolds number increases, reflecting an improved mixing. As Re increases, the intensity of segregation decreases because of turbulent diffusion; but due to the scalar dispersion, the scale of segregation increases. Spatial derivatives of the scalar fluctuations deviate from isotropy, the departure increasing with Re. However, at a given Re, they show a tendency towards isotropy as the distance from the cylinder increases. This tendency is more evident near the wake edges than at the centreline. Scalar mixing is mainly associated with the large-scale dispersion created by the turbulent velocity field. Molecular diffusion effects, although present, are weak.