The supersonic flow above Mach 5 is typically known as the hypersonic flow regime although there is no clear definition for the start of the hypersonic flow regimes. Therefore, this designation varies slightly in the literature between researchers. Typical hypersonic applications are flow over missiles, rockets, launch vehicles, and entry or re-entry atmospheric capsules/bodies. The flow surrounding a vehicle flying at hypersonic speed will exhibit very strong shocks and a large temperature rise behind the shock wave. The temperature increasing across shocks can be so high that the internal energy modes associated with vibration and electronic excitation are activated. At higher Mach numbers (and therefore higher total enthalpy), the temperatures are so high that the gas dissociates and eventually ionizes.The gas is no longer inert, and chemical reactions such as dissociation, recombination, exchange, and ionization are initiated. When the characteristic flow timescale is about the same order as the timescale required to complete internal energy mode relaxation or to complete chemical reactions, the flow can be in locally thermo-chemical non-equilibrium. Other physical phenomena such as radiation, radio blackout, low-density effects, wall catalysis, and wall ablation can be present, and therefore a multidisciplinary approach is necessary to address the problem.

While both Pressure-based solver (PBNS) and Density-based solver (DBNS) can be used on a wide range of flow regimes from subsonic to supersonic flows, the DBNS solver is particular very well suited for modeling the high-energy hypersonic flow due to the strong coupling between the continuity, momentum, and energy equations. For more details on DBNS solver settings and theory, see Density-Based Solver Settings in the Fluent User's Guide and Density-Based Solver in the Fluent Theory Guide. In order to model hypersonic flow, the DBNS solver is instrumented with numerics and models that will aid in solver robustness and solution prediction accuracy. This chapter will describe and highlight the reason for the additional numerics and models and when they should be activated. The last section of this chapter provides best practices for setting up and modeling hypersonic flow.