Browsing by Author "Worthy, Jude"
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Item Open Access A comparison of modified K-ε turbulence models for buoyant plumes(Taylor and Francis, 2001-02-01) Worthy, Jude; Sanderson, V. E.; Rubini, Philip A.The effect of buoyancy on the production and dissipation of turbulent kinetic energy is investigated in variants of the popular k- ε turbulence model. The standard gradient diffusion model is considered for the scalar flux as well as a generalised gradient diffusion model. Also, the addition of the non-isotropic component of an algebraic stress model for the Reynolds stresses is assessed. The relative significance of the various models and terms are demonstrated using different combinations of the models, including the important flux Richardson correction term. The generalised gradient diffusion and algebraic stress models are shown to give a strong increase in turbulence production, although the effect on the flow can be largely controlled by the coefficient of the flux Richardson term. Recommendations are made regarding optimum models and coefficients.Item Open Access Large eddy simulation of buoyant plumes(Cranfield University, 2003-05) Worthy, Jude; Rubini, Philip A.A 3d parallel CFD code is written to investigate the characteristics of and differences between Large Eddy Simulation (LES) models in the context of simulating a thermal buoyant plume. An efficient multigrid scheme is incorporated to solve the Poisson equation, resulting from the fractional step, projection method used to solve the Low Mach Number (LMN) Navier-Stokes equations. A wide range of LES models are implemented, including a variety of eddy models, structure models, mixed models and dynamic models, for both the momentum stresses and the temperature fluxes. Generalised gradient flux models are adapted from their RANS counterparts, and also tested. A number of characteristics are observed in the LES models relating to the thermal plume simulation in particular and turbulence in general. Effects on transition, dissipation, backscatter, equation balances, intermittency and energy spectra are all considered, as are the impact of the governing equations, the discretisation scheme, and the effect of grid coarsening. Also characteristics to particular models are considered, including the subgrid kinetic energy for the one-equation models, and constant histories for dynamic models. The argument that choice of LES model is unimportant is shown to be incorrect as a general statement, and a recommendation for when the models are best used is given.Item Open Access A Study of LES Stress and Flux Models Applied to a Buoyant Jet.(Taylor and Francis Ltd, 2005-09-03) Worthy, Jude; Rubini, Philip A.Large eddy simulation (LES) stress and scalar flux subgrid scale models are evaluated in the context of buoyant jets. Eddy viscosity, eddy diffusivity (including formulations of the generalized gradient diffusion hypothesis), “structure” (Bardina and Leonard), mixed, and dynamic models are scrutinized. The performance of the models is examined in terms of the main flow variables and also with respect to the “internal” behavior of the models in terms of the relative contributions to the turbulent kinetic energy budget.