Second-Moment Modelling of Turbulent Scalar Transport
Available from UMI in association with The British Library. Requires signed TDF. This thesis is concerned with the modelling of turbulent scalar transport. A second-moment closure approach is adopted so that the problem is that of modelling transport equations for the Reynolds stresses, scalar fluxes and dissipation rates. Part of the present contribution is the development of a new model of the pressure-scalar gradient correlation appearing in the scalar flux transport equations. This model is intended to complement that obtained by Fu (1988) for the corresponding pressure-strain term appearing in the Reynolds stress equations, and incorporates a 2-component turbulence limit. It has additionally been found necessary to include an explicit dependence of the model on the mean scalar gradient, as suggested by Jones & Musonge (1983). Results of applying the new model to simple homogeneous flows are reported and show encouraging agreement with experiments. The corresponding contributions to the pressure-correlation models arising from buoyancy forces are also derived. Further applications of the model to inhomogeneous, free, plane and axisymmetric jets are reported. Within this framework the question of modelling the dissipation rates of both turbulent kinetic energy and scalar variance is addressed. A refinement of the standard varepsilon equation is proposed, whereby the conventional production term is partially replaced by a mean-strain dependent source term. This is found to bring about a significant improvement in the spreading rate of the axisymmetric jet. A new model for the scalar variance dissipation rate varepsilon_theta is also proposed and is found to perform adequately over the range of flows considered in this thesis. Finally, the application of these models to the impinging jet is considered. In computing this flow it is found that the most widely used wall-reflection models actually have the wrong effect in an impinging flow by increasing the Reynolds stress normal to the wall. A new wall-reflection term is therefore proposed which does exhibit the correct behaviour in flows both parallel and perpendicular to a wall. With the addition of this term, significant improvements are obtained in the predictions of mean velocity, Reynolds stresses and Nusselt number distribution.
- Pub Date:
- REYNOLDS STRESS;
- SCALAR FLUX;
- DISSIPATION RATE;
- Engineering: Mechanical; Physics: Fluid and Plasma