Hypersonic flows have recently re-acquired an important role in fluids research, with many nations interested in their applications. Due to their complex multidisciplinary aspects, the limitations of recreating them in laboratory experiments and the high cost and uncertainties of flight-testing research vehicles, CFD has assumed a prominent role as a design and analysis tool in this fast-advancing field.
This Special Issue aims to capture the strengths that numerical methods can bring to the exploration of this highly challenging multidisciplinary subject, highlight recent progress, prompt discussions on open questions and become a springboard for future developments. Its scope is novel developments and applications of numerical methods for hypersonic flows, including, but not limited to, rarefied gases, Eulerian and Lagrangian approaches, reacting flows, magneto-hydrodynamics (MHD), finite volume and finite element methods, high-order numerical schemes, numerical stability, and mesh optimisation.
Unnikrishnan and Gaitonde present an analytical technique to study the effect of secondary boundary layer instabilities in high-speed flows and a comparison is made to Direct Numerical Simulation (DNS) results.
Liu, Cao, Chen, Agarwal, and Zhao describe the coupling of the gas-kinetic scheme (GKS) and two turbulence models, k-ω SST and Langtry-Menter γ-Reθ, to investigate transitional and turbulent flows at hypersonic speeds. The code validation showed a clear advantage over the uncoupled formulation in better capturing aerothermodynamic loads.
Parent and Hanquist present advances in the simulation of hypersonic ionised flows by accounting for plasma sheath effects in MHD simulations. An accurate and time-efficient coupling mechanism is laid out between the collisional plasma sheath equations and the non-equilibrium MHD equations – that notably include the electron energy equation – for the purpose of analysing Electron Transpiration Cooling (ETC).
3D MHD simulations with imposed magnetic field are also showcased in the article by Zhang, Habashi, Baruzzi, and Ben Salah to demonstrate the increased accuracy of an edge-based finite element multidisciplinary solver for hypersonic flows combined with anisotropic solution-driven mesh optimisation. The low-magnetic Reynolds number approximation of the current-continuity equation is applied and various adaptation criteria are compared for a re-entry capsule flow field.
Lo, Pan, and Wu develop a model for axisymmetric DSMC computations where radial weighting factor and local time-stepping are intertwined to conserve mass, momentum and energy as particles cross the cell faces. It alleviates the need for particle cloning or deletion, and as a result, it eliminates random walk error.
The DSMC method is also employed by Hsieh, Pan, and Lo to determine the aerothermodynamic coefficients of a CubeSat in Very Low Earth Orbit (VLEO) when subject to various flow conditions and for different satellite configurations, and the authors assert that such parametric analysis could be carried out to evaluate their service life and support mission planning.
Collectively, these six papers provide a unique insight on current challenges in the field of hypersonics, both theoretical and practical, and it is the hope of the Guest Editors that they will prove useful to the scientific community.