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Navier-Stokes Characteristic Boundary Conditions (NSCBC) for Direct Simulations of Turbulent Compressible Flows in Athena-RFX


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The Athena-RFX massively parallel solver has been demonstrated over the years for various complex studies involving chemically reacting flows, compressible turbulence, and more recently multiphase turbulent transport. However, one major challenge that remains is the handling of outflow boundary conditions to mitigate the spurious propagation of artificial instabilities. This problem arises due to the application of zeroth or first-order extrapolation at the boundary ghost cells that fail in the presence of large gradients. To address this problem, this report describes the implementation of the Ghost Cell Navier Stokes Characteristic Boundary Conditions (GC-NSCBC) method to Athena-RFX. The theoretical formulation of the GC-NSCBC method is described in detail along with benchmark cases for 1) a spherical pressure wave and 2) vortex flows with weak and strong circulation intensities. The method is further demonstrated in the turbulent flow regime involving a spatially developing jet that is a canonical model of the US Army Combat Capabilities Development Command Army Research Laboratory Hot Particulate Ingestion Rig. The solutions presented demonstrate that this method is able to successfully mitigate artificial wave instabilities without altering the transport properties. This enables the application of Athena-RFX to a new class of problems involving interactions of turbulent structures near boundaries or time-varying inflow dynamics.



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