Coarse-grained simulation approach for turbulent nonequlibrium plasma flow

Non-transferred arc plasma torches are at the core of diverse applications such as plasma spray and waste treatment. Computational simulations of these torches can provide important understanding to aid equipment design and process optimization. These plasma flows are often turbulent and present significant differences in the temperature of electrons compared to that of heavy-species. The multiphysics nature of plasmas, large variation of material properties, together with their propensity to turbulent behavior, make their computational simulation exceedingly challenging. No approach is particularly established for modeling of turbulent plasmas. This research addresses the pressing need for a consistent and complete, yet computationally feasible, turbulent plasma flow model for the exploration of industrial turbulent plasma phenomena with the fidelity and predictive capabilities achieved in other disciplines. To that end, a nonlinear type of Variational Multiscale (VMS) method, denoted as VMSn is developed. In contrast to classical VMS that neglects the effect of small scales on the transport operator, VMS n addresses the inter-dependence between large- and small-scales upfront. Moreover, a novel algebraic approximation of the small-scales, suitable for generic transport problems, denoted as Transport-Equivalent Scaling (TES) is investigated. The evaluation of the TES is done through the simulation of benchmark incompressible, compressible, and magnetohydrodynamic flow problems. Results demonstrated that TES seamlessly handles different type of flows in a unified manner. Furthermore, the new VMSn approach is evaluated with benchmark two- and three-dimensional incompressible, compressible, and magnetohydrodynamic laminar flow problems, the incompressible Taylor-Green vortex flow, and the turbulent free jet. Simulation results show that VMS n leads to minor improvements in accuracy with respect to classical VMS for laminar flow problems, but to significantly greater accuracy for the turbulent incompressible flow cases. Finally, based on the successful performance of VMSn in resolving the small scales in turbulent incompressible regimes, VMSn is used for simulation of turbulent plasma flows in arc plasma torches. Results show that the nonequilibrium plasma flow simulation using VMSn reproduces the main experimentally-observed flow dynamics of the arc inside the torch together with the evolution of turbulence in the produced plasma jet in a cohesive manner. The obtained results depict that the high temperature of the jet core have a more pronounced effect on the development of turbulence than the forcing caused by the dynamics of the arc.