Effect of Turbulence Modeling on an Excited Jet

†The flow dynamics in a high-speed jet are dominated by unsteady turbulent flow structures in the plume. Jet excitation seeks to control these flow structures through the natural instabilities present in the initial shear layer of the jet. Understanding and optimizing the excitation input, for jet noise reduction or plume mixing enhancement, requires many trials that may be done experimentally or computationally at a significant cost savings. Numerical simulations, which model various parts of the unsteady dynamics to reduce the computational expense of the simulation, must adequately capture the unsteady flow dynamics in the excited jet for the results are to be used. Four CFD methods are considered for use in an excited jet problem, including two turbulence models with an Unsteady Reynolds Averaged Navier-Stokes (URANS) solver, one Large Eddy Simulation (LES) solver, and one URANS/LES hybrid method. Each method is used to simulate a simplified excited jet and the results are evaluated based on the flow data, computation time, and numerical stability. The knowledge gained about the effect of turbulence modeling and CFD methods from these basic simulations will guide and assist future 3-D simulations that will be used to understand and optimize a realistic excited jet for a particular application. I. Introduction The flow dynamics in a high-speed jet are dominated by the unsteady turbulent flow structures that characterize the energy cascade in the jet plume. These flow structures, which range in size from largescale eddies on the order of the jet diameter that transport energy to the extremely small-scale eddies that dissipate energy through viscous forces, originate in the naturally unstable free-shear boundary layer formed between the high-speed jet flow and the surrounding fluid. As the fluids mix in the jet plume, Kelvin-Helmholtz instability waves are formed. These instability waves grow and decay as the jet develops, governing the formation and destruction of the unsteady turbulent flow structures. Jet excitation must exploit these natural instabilities to exert control on the turbulent structures or the energy of the jet will overwhelm the input perturbation. If perturbations are input at the nozzle exit impact the jet where the flow structures are smaller and less energetic, then large changes to the to the jet flow downstream can be created with a relatively small energy input. Moreover, the downstream characteristics of the jet plume can be predetermined by selecting an appropriate instability wave (mode and frequency) to amplify with the input perturbation. Jet excitation has many potential uses. It may be used to reduce the thermal signature of the jet or to mitigate the noise generated. Proper excitation, however, requires intelligently selecting the best excitation input from the infinite number of possibilities to optimize the effect. Modern experimental techniques, such as particle image velocimetry and phased array microphones, can provide detailed flow and acoustic information that could be used to optimize the excitation but these experiments are too costly for the large number of cases required for a true optimization, especially at the larger and more realistic facilities.