Prediction of Turbulent Temperature Fluctuations in Hot Jets

Large-eddy simulations were used to investigate turbulent temperature fluctuations and turbulent heat flux in hot jets. A high-resolution finite difference Navier-Stokes solver was used to compute the flow from a 2in. round nozzle. Three different flow conditions of varying jet Mach numbers and temperature ratios were examined. The large-eddy simulation results showed that the temperature field behaved similarly to the velocity field but with a more rapidly spreading mixing layer. Predictions of the mean u>i and fluctuating velocities were compared to particle image velocimetry data. Predictions of the mean T and fluctuating T temperatures were compared to data obtained using Rayleigh scattering and Raman spectroscopy. Very good agreement with experimental data was demonstrated for the mean and fluctuating velocities. The large-eddy simulation correctly predicted the behavior of the turbulent temperature field but overpredicted the levels of the fluctuations. The turbulent heat flux was examined and compared to the Reynolds-averaged Navier-Stokes results. The large-eddy simulation and Reynolds-averaged Navier-Stokes simulations produced very similar results for the radial heat flux. However, the axial heat flux obtained from the large-eddy simulation differed significantly from the Reynolds-averaged Navier-Stokes result in both structure and magnitude, indicating that the Reynolds-averaged Navier-Stokes model was inadequate. Finally, the large-eddy simulation data were used to compute the turbulent Prandtl number and verify that a constant value of 0.7, which is typically used in the Reynolds-averaged Navier-Stokes models, was a reasonable assumption.