A scalar subgrid model with flow structure for large-eddy simulations of scalar variances

A new model to simulate passive scalar fields in large-eddy simulations of turbulence is presented. The scalar field is described by clouds of tracer particles and the subgrid contribution of the tracer displacement is modell ed by a kinematic model which obeys Kolmogorov's inertial-range scaling, is incompressible and incorporates turbulent-like flow structure of the turbu lent small scales. This makes it possible to study the scalar variance fiel d with inertial-range effects explicitly resolved by the kinematic subgrid held while the LES determines the value of the Lagrangian integral time sca le T-L. In this way, the modelling approach does not rely on unknown Lagran gian input parameters which determine the absolute value of the scalar vari ance. The mean separation of particle pairs displays a well-defined Richardson sc aling in the inertial range, and we find that the Richardson constant G(Del ta) approximate to 0.07 which is small compared to the value obtained from stochastic models with the same T-L. The probability density function of th e separation of particle pairs is found to be highly non-Gaussian in the in ertial range of times and for long times becomes Gaussian. We compute the s calar variance held for an instantaneous line source and find good agreemen t with experimental data.