Investigation and validation of a cubic turbulence model in isothermal and combusting flows

Computational Fluid Dynamics relies upon turbulence models for predicting most engineering flows. Relatively accurate models exist but are computationally intensive; simpler, more practical models, however, often return poor predictions. The new cubic, eddy-viscosity turbulence model is a compromise of these extremes, employing a nonlinear (cubic) stress-strainr elationship. The primary objective of the current research is to compare the cubic model against a range of other two-equation turbulence models, for a variety of isothermal and combusting flows. The TEACH research code is the main platform for the investigations of the new turbulence model. Other, industry-standard models (standard k-c, ReNormalisation Group k-c and Launder & Sharma low Reynolds-number models) are also implemented for comparative purposes. The nonlinear model is found to be numerically unstable and several remedies are required before any converged solutions can be obtained for the complex flows investigated. The turbulent, isothermal test cases are: fully-developed pipe flow, axisymmetric pipe expansion (three different flows) and strongly-swirling pipe flow (for which a Reynolds Stress Model, 'available in a commercial CFD code, is also utilised). In most cases, the nonlinear model provides the best results relative to the other two-equation models. A detailed anälysis is carried out to account for the different ways in which the physics of the flows are represented by the various turbulence models. A challenging, reacting flow is the bluff-body stabilised, nonpremixed flame. Initial simulations, utilising the flame sheet combustion model, reveal that the accuracy of the computed temperature and mixture fraction distributions depends largely upon the predictions of the flow field. The nonlinear turbulence model gives slightly improved results relative to the other, standard models. However, detailed velocity distributions are required for further analysis of the cubic model. Since no flow-field data for confined, bluff-body burners exists in the public domain, an experimental combustor is designed and built on-site. An optical technique, Particle Image Velocimetry (PIV), is utilised to obtain detailed profiles of the flow, temperatures are measured using a standard thermcouple probe. After extensive processing, the experimental results are compared with the simulation predictions. The nonlinear turbulence model captures all the flow features and is seen to significantly improve results compared to the other models. Reasons for its relative success are presented.