The following are best practices when modeling hypersonic flow:
Setting up suitable gas properties is important for getting accurate results at a reasonable computational cost.
If the maximum temperature is below 1000K, the flow can be modeled with the one-temperature model, and chemical reactions can be ignored. You can select air as the material and select nasa-9-piecewise-polynomial for specific-heat, and kinetic-theory for both viscosity and thermal conductivity.
If the maximum temperature is between 1000K and 10,000K, the flow can be in thermo-chemical non-equilibrium. You can enable the two-temperature model, select air-5species-park93, mars-5species-mckenzie, or mars-8species-park93 mixture, and enable volumetric reactions.
If the maximum temperature is above 10,000K, the flow can be partially ionized. You can enable the two-temperature model, select air-11species-park93 or air-11species-gupta mixture, and enable volumetric reactions.
The following solver settings are recommended for hypersonic flow:
Choose the density-based solver and select the implicit formulation. For details, see Implicit Formulation.
Enable High-Speed Numerics. For details, see High Speed Numerics.
If the mesh is unstructured, distorted, or skewed, select the Green-Gauss Node-Based gradient evaluation. For details, see Green-Gauss Node-Based Gradient Evaluation.
If the mesh contains stretched cells to resolve boundary-layers, use the enhanced Convergence Acceleration for Stretched Meshes (e-CASM). For details, see Convergence Acceleration for Stretched Meshes (CASM).
Review the prescribed solution limits (Setting Solution Limits). The default maximum temperature may be too low for the extreme heating encountered in the hypersonic regime, resulting in an undesirable clipping of the solution and stagnation of the residuals.
If the flow is reacting, select None – Direct Source chemistry solver. For details, see Enabling Species Transport and Reactions and Choosing the Mixture Material.
If convergence difficulties are encountered, you can impact the performance of the solver and stabilize the solution by:
Lowering the CFL value from 5 to 1
Reducing the positivity rate limit (Adjusting the Positivity Rate Limit) from 0.2 to 0.1 or 0.05.
Engaging the High Order Term Relaxation. For details, see High Order Term Relaxation.
Starting the calculation with the first-order scheme and switching to the second-order discretization once the flow is developed. For details, see First-Order Accuracy vs. Second-Order Accuracy.
Providing a good initial solution is very important for getting fast results and improving the solver robustness. The Full Multi Grid (FMG) initialization (Full Multigrid (FMG) Initialization) provides best-in-class solution initialization for hypersonic flows. Applied after the Standard Initialization (Initializing the Entire Flow Field Using Standard Initialization), the FMG produces an improved initial solution allowing Fluent to start solving with the second-order scheme directly. Some FMG settings can be adjusted to suit specific scenarios:
The number of coarse-grid levels, set to 5 by default. For small cases with fewer than 100 000 cells, it is recommended to reduce the number of coarse-grid levels to 3 or 4.
The CFL value, set to 0.75 by default. For hypersonic cases, this may be lowered to 0.5 or 0.25 to enhance the robustness of the initialization procedure.
The number of iterations performed on each level. These may be doubled or quadrupled to compensate for a lowered CFL.
The viscous FMG option (Additional FMG Initialization Options with the Density-Based Solver), for developing boundary-layers and accounting for heat-transfer effects along isothermal walls.
The species-reaction FMG option (Additional FMG Initialization Options with the Density-Based Solver), accounts for finite-rate reactions used by air-dissociation or high-speed combustion models.
The verbosity option, monitoring the convergence of the FMG iterations.
It is a good practice to visually inspect the initial solution produced by the FMG initialization. Discontinuities, although heavily smeared, should be present at their expected location and the pressure and temperature fields should have realistic value ranges. If divergence is encountered during the FMG iterations, verify that the initial values set by the Standard Initialization (Initializing the Entire Flow Field Using Standard Initialization) are adequate, and apply the settings recommended above.
As is the case for low-speed flows, the convergence of hypersonic calculations is assessed by monitoring the evolution of the residuals (Monitoring Solution Convergence). Due to strong discontinuities, complex physical models, and highly coupled thermochemical effects, deep and rapid convergence of the residuals may be hard to achieve. For that reason, it is useful to monitor additional quantities to judge the evolution of the solution. It is recommended to:
Monitor aerodynamic loads, such as lift and drag.
Monitor mass-flux balance at the inlet and outlet of the domain.
Monitor total heat-flux at isothermal walls.
Monitor the maximum values of pressure, temperature, and Mach-number in the domain.
For the two-temperature model, monitor the maximum vibrational-electronic temperature.
For turbulent flows, monitor the maximum turbulent viscosity ratio.
For reactive flows, monitor the volume-average of reacting and produced species.
During the solution process, these monitors are expected to stabilize to their asymptotic steady-state values, and solutions exhibiting oscillatory monitors should be iterated further. Once a steady-state solution is obtained, it is recommended to analyze the solution:
Examine all shock waves by visualizing the pressure field, especially along walls and symmetry boundaries, or by creating additional visualization planes.
Examine post-shock quantities to assess the rise in temperature, the excitation of the vibrational-electronic energy mode, and the production of new species.
Examine surface quantities such as total heat-flux in the case of isothermal walls, and species mixture in the case of catalytic walls.
Examine wall-normal resolution y+ in the case of turbulent calculations.
Examine the Schlieren contours with the gray-black colormap. If necessary, adjust the range to see the shock. For Schlieren field function definitions, see Field Function Definitions.
All shock fronts should be smooth and regular, without obvious signs of under/over-shoots across the discontinuity. Likewise, the total heat-flux contour along boundaries should not present significant oscillations. The knowledge gained from the above post-processing may inform additional modifications to the mesh, for example, to cluster grid-points near strong gradients or add refinement zones around areas of interest. Mesh Adaption (Using Adaption), and in particular Aerodynamic Adaption (Aerodynamics Adaption), may be used to automate that task.