Analysis Of Flow Around Bluff Body Based On Modified Inlet Profiles And Rough Wall Treatments

Practical engineering applications of the Computational Fluid Dynamics (CFD) are largely based on the numerical solution of the Reynolds averaged Navier-Stokes (RANS) equations. Turbulent flow problems of engineering interest often feature high Reynolds numbers and are thus difficult to solve by means of large eddy simulation, not to mention the direct numerical simulation. The biggest advantage of using the RANS in simulating turbulence flows is that they allow to treat the turbulence as a steady phenomena, i.e. if a flow is unsteady only because of turbulence the RANS simulation will be a steady simulation, with great saving of computational means. With appliation of RANS turbulence models, there are still several problems encountered like modelling of Atmosphere Boundary Layer (ABL), rough wall treatments, eddy-viscosity modelling etc.. These deficiencies hamper the development of wind loading simulation of the building structures aerodynamics accurately. This may cause deviations in research and un-safeties in design, especially for wind-sensitive engineering structures.The most imperative and basic requirement of CFD in civil engineering is accurate modeling turbulent flow in the simulated ABL. Of the first importance, it is required to model Reynolds stress transport. Eddy viscosity modelling is still by far the most popular method of RANS turbulence modelling. The constitutive model provides the Reynolds stress tensor as a function of the turbulent scales and the mean-velocity gradient. The constitutive models range from the simple linear Boussinesq-relation up to high-order tensor-polynomial expressions which can be derived from some underlying stress-transport closure by invoking an ad-hoc basis (nonlinear eddy viscosity models). Currently, a move from standard linear eddy-viscosity modelling towards nonlinear viscosity assumptions, seems to be going on in some areas of wind-related engineering. It should also be noticed that the proper choice of inlet flows represent the influence of the upstream surroundings. However, these boundary conditions are not fully known. For steady RANS simulations, the mean velocity and turbulence quantities profiles are required. The general derivation of these profiles and the resulting formulae should be obtained from the assumption of an equilibrium boundary layer. Despite many occurrences of near wall treatments in practical engineering applications, a robust model has yet to be fully developed. Fundamentally this is due to the small scale of the roughness relative to the largest scales of the boundary layer. The predictability roughness expressions of near wall flow will provide great supplement for turbulence modeling.To response to the above circumstances, this thesis adopted theoretical and numrical methods to carry out simulation of flow around6m cube, combined with appropriate inlet condition, rough wall treatments and turbulence models. The main works are as follows:I. To implant non-linear eddy viscosity in turbulence models. These investigations suggest that a turbulence model, suitable for wind engineering applications, should be able to model the anisotropy of turbulent flow while maintaining the ease of use and computational stability of the κ-ε models. Therefore, the quadratic non-linear expansions of the Boussinesq hypotheses, have been tested in an attempt to account for anisotropic turbulence and curvature related strain effects.II. New inlet profiles of mean wind and turbulence quantities based on ABL characteristics. Aerodynamic roughness height (?) is adopted to describe rough surface characteristics and turbulence exchange of underlying earth. And the comparison of (?) functions and experiment data are made to verify the numerical models. And then (?) is imported back into mean wind and turbulence quantities profiles to represent the upstream roughness influences.III. Rough wall treatment used for near wall flow. It was known that the standard near wall treatments are insufficient to estimate the effects of roughness on the flow quantities. So several new approaches of roughness effects corresponding to the turbulence models are taken into consideration based on the near wall flow characteristics. The new models are then applied to equilibrium boundary flow tested to analyze the effects on the rough wall effects. Overall agreement between the computational predictions and analytical findings is acceptable.IV. Asymptotic analysis of new inlet profiles and near wall treatments. Asymptotic analysis method here is used to verify the boundary conditions of ABL models propoed in the thesis. Two separate length scales of ABL flows are import into N-S functions to get two individual Reynolds stress functions corresponding to the viscosity sublayer and fully developed turbulent region. The uniform expression is derived based on the matching condition. This model provides good predictions on the Reynolds stress distribution along the ABL and well agreement on the near wall flow condition.V. Flow around6m cube. Three RANS turbulence models based on the anisotropic eddy viscosity assumptions combined with inlet profiles and new near wall treatmens are tested for the flow around6m cube model using Fluent as the solver. The numerical results are compared with the experimental findings in full scale measurements available in the literature of two incident flow directions. The results are analyzed and the effects of different turbulence parameters used in the modeling are discussed. Each turbulence model is individually reviewed for the correctness of its predictions and the best model in this set is chosen.