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Simulations of Fluid Mixing in Turbulent Flows

Energy systems that rely on combustion are the subject of widespread efforts aimed at increasing efficiency and controlling pollutant generation. Recent rapid advances in computational power have brought within reach the possibility of simulating accurately the performance of realistic combustion systems. Accurate computer simulations would simplify the design and development of combustion systems by reducing the need for expensive testing of physical prototypes. However, numerous challenges remain for combustion simulations. Among these challenges is the proper representation of the process by which the fuel and oxidizer species mix at the molecular level, in the presence of turbulence in the underlying fluid flow.

Professor Lester Su’s group is attempting to understand the turbulent fluid mixing process in a way that will serve efforts to simulate reacting flows, using a comprehensive approach that combines experimental and computational tools to study mixing in the well-defined turbulent jet flow system. One facet of this work involves jet simulations using the promising Large-Eddy Simulation (LES) method. In LES much of the turbulent activity is entrusted to models, rather than being computed explicitly. Complementary to these simulations, Professor Su’s group performs laser diagnostic experiments in the same flow system. Data from the experiments informs the specification of the mixing models for the simulations, and also serves as benchmarking data for the simulation results. Figure 1 shows a typical mixing field from the jet simulation, in which the jet fluid (depicted as red) issues into still surroundings (depicted as blue). Figure 2 shows how the jet fluid concentration on the jet centerline is diluted as the fluid moves downstream, for two different mixing model specifications. Comparing these simulation results with the experimental data allows a direct assessment of the validity of the models.