Passive-scalar wake behind a line source in grid turbulence

The structure and development of the scalar wake produced by a single line source are studied in decaying isotropic turbulence. The incompressible Nav ier-Stokes and the passive-scalar transport equations are solved via direct numerical simulations (DNS). The velocity and the scalar fields are genera ted by simulating Warhaft's (1984) experiment. The results for mean and r.m .s. scalar statistics are in good agreement with those obtained from the ex periment. The structure of the scalar wake is examined first, At initial ti mes, most of the contribution to the scalar variance is due to the flapping of the wake around the centreline. Near the end of the turbulent convectiv e regime, the wake develops internal structure and the contribution of the flapping component to the scalar variance becomes negligible. The influence of the source size on the development of the scalar wake has been examined for source sizes ranging from the Kolmogorov microscale to the integral sc ale. After an initial development time, the half-widths of mean and scalar r.m.s. wakes grow at rates independent of the source size. The mixing in th e scalar wake is studied by analysing the evolution of the terms in the tra nsport equations for mean, scalar flux, variance, and scalar dissipation. T he DNS results are used to test two types of closures for the mean and the scalar variance equations. For the time range simulated, the gradient diffu sion model for the scalar flux and the commonly used scalar dissipation mod el are not supported by the DNS data. On the other hand, the model based on the unconditional probability density function (PDF) method predicts the s calar flux reasonably well near the end of the turbulent convective regime for the highest Reynolds number examined. The scalar source size does not s ignificantly influence the models' predictions, although it appears that th e time-scale ratio of mechanical dissipation to scalar dissipation approach es an asymptotic value earlier for larger source sizes.