Assessment of turbulence models and corrections with application to the Orion Launch Abort System

The aim of this research is to assess turbulence modeling issues for rocket plumes with application to the Orion Launch Abort System (LAS). Turbulence models, used with the Reynolds-averaged Navier-Stokes (RANS) equations, are critically important for an accurate flow field prediction. NASA’s OVERFLOW code is used for computations of supersonic jets. First, baseline turbulence performance is assessed. Then, corrections for compressibility, temperature, and rotation/curvature effects are evaluated based on comparisons with experimental data. Test cases progressively increase in complexity and include axisymmetric jets, 3-D jets with crossflow, and a full ”LAS-like” configuration. For the axisymmetric jets, the test cases are isothermal and heated, perfectly expanded and underexpanded axisymmetric jets with design Mach numbers around two. The 3-D jet in crossflow is based on a NASA Glenn experiment and includes elevated conditions (higher Mach numbers and temperatures) more similar to the actual LAS. The SST turbulence model shows the best performance and is chosen for the flows examined in this study. For the axisymmetric jet, results demonstrate that using a compressibility correction is important for a good prediction of the jet development. The temperature correction shows improved results for the heated cases, but its effect is not as significant. For the 3-D jet in crossflow, conclusions from using corrections are not as decisive. While use of the compressibility correction improves the vorticity contours shape, the velocity decay is slightly underpredicted. The temperature and rotation/curvature corrections show little improvement. The overall recommendation is that the corrections must be used with caution, especially when approaching high temperature and high Mach number regimes. Finally, two modified compressibility correction are introduced based on the notion that the growth rate in compressible free shear flows levels off at a certain turbulence Mach number. The modified corrections do not disturb the beneficial behavior of the standard corrections observed for the axisymmetric jets at the given conditions. For the 3-D jet in crossflow, the modified corrections show significant improvement over the Sarkar and uncorrected models. The compressible boundary layer test case shows that there is still an underprediction of the wall skin friction coefficient, but this can be solved by using a simple modification in the turbulence model. Finally, analysis of the compressible mixing layer shows that the modified corrections exhibit the correct trend in limiting the growth rate, but do not match exactly with the experimentally-obtained results