Discontinuous Galerkin Methods for inviscid low Mach number flows

In this work we present two preconditioning techniques for inviscid low Mach number flows. The space discretization used is a high-order Discontinuous Galerkin finite element method. The time discretizations analyzed are explicit and implicit schemes. The convective physical flux is replaced by a flux difference splitting scheme. Computations were performed on triangular and quadrangular grids to analyze the influence of the spatial discretization. For the preconditioning of the explicit Euler equations we propose to apply the fully preconditioning approach: a formulation that modifies both the instationary term of the governing equations and the dissipative term of the numerical flux function. For the preconditioning of the implicit Euler equations we propose to apply the flux preconditioning approach: a formulation that modifies only the dissipative term of the numerical flux function. Both these formulations permit to overcome the stiffness of the governing equations and the loss of accuracy of the solution that arise when the Mach number tends to zero. Finally, we present a splitting technique, a proper manipulation of the flow variables that permits to minimize the cancellation error that occurs as an accumulation effect of round-off errors as the Mach number tends to zero

Establishing Best Practices for X-57 Maxwell CFD Database Generation

The X-57 Maxwell is NASAs latest electric airplane concept that has been simulated for aerodynamic performance using the structured overset and unstructured grid solvers within the Launch Ascent and Vehicle Aerodynamics (LAVA) solver framework as well as the unstructured polyhedral grid solver in Star-CCM+ for code-to-code comparison. In order to validate the predictions, comparisons were made between the CFD solutions and experimental data collected in the 12-foot Low-Speed Wind Tunnel at NASA Langley Research Center. The simulations are in preparation for the development of a comprehensive aerodynamic database which will assess aircraft performance at a variety of conditions. The findings from these simulations will establish the best practices for mesh resolution, numerical discretization, and turbulence modeling to be used for this database. Preliminary database results have shown that best-practices learned from the initial validation simulations will potentially reduce error in X-57 aerodynamic loads and moments relative to experiment by up to 14%

High-Fidelity Computational Aerodynamics of Multi-Rotor Unmanned Aerial Vehicles

High-fidelity Computational Fluid Dynamics (CFD) simulations have been carried out for several multi-rotor Unmanned Aerial Vehicles (UAVs). Three vehicles have been studied: the classic quadcopter DJI Phantom 3, an unconventional quadcopter specialized for forward flight, the SUI Endurance, and an innovative concept for Urban Air Mobility (UAM), the Elytron 4S UAV. The three-dimensional unsteady Navier-Stokes equations are solved on overset grids using high-order accurate schemes, dual-time stepping, and a hybrid turbulence model. The DJI Phantom 3 is simulated with different rotors and with both a simplified airframe and the real airframe including landing gear and a camera. The effects of weather are studied for the DJI Phantom 3 quadcopter in hover. The SUI En- durance original design is compared in forward flight to a new configuration conceived by the authors, the hybrid configuration, which gives a large improvement in forward thrust. The Elytron 4S UAV is simulated in helicopter mode and in airplane mode. Understanding the complex flows in multi-rotor vehicles will help design quieter, safer, and more efficient future drones and UAM vehicles

Reacting plume inversion on urban geometries through gradient based design methodologies

An increased focus on domestic security in recent years has brought attention to several important application areas where computational fluid dynamics (CFD) has the ability to make a significant impact. In particular, disaster mitigation and post-event forensic activities are of interest. This work investigates a procedure built on gradient based design methods to allow for the solution of the so-called inverse chemistry problem in urban environments. The inverse chemistry problem consists of computing a release location based on the sensing of chemical byproducts of the release and the ability to compute an accurate flow field on the geometry of interest. In this study, Washington DC is simulated under conditions of a hazardous plume. A CFD solver is implemented which allows for the solution of the preconditioned finite-rate Navier-Stokes equations as well as the in situ computation of design gradients

Quad Tilt Rotor Simulations in Helicopter Mode using Computational Fluid Dynamics

The flow field around a simplified Quad Tilt Rotor (QTR) vehicle is simulated using computational fluid dynamics (CFD) for various low speed flight conditions in helicopter mode. A time-averaged rotor model is utilized, where the velocity field computed by CFD is coupled to blade element theory and a trim model to provide an equivalent time-averaged body force term in the compressible Navier-Stokes equations, instead of moving overset meshes; reducing the computational time while capturing the essential physics. Overset meshes are used to model the complicated geometry of the simplified aircraft fuselage and wings in order to ensure good resolution of viscous effects. The solution of the compressible Navier-Stokes equations are suitably modified using low Mach number preconditioning to properly scale the dissipation and enhance convergence. This approach is validated for the current work by comparison with experimental data for the downwash velocity underneath an isolated tilt rotor system as well as for the pressure distribution resulting on the surface of a single wing placed underneath such a tilt rotor system. A total of 8 grids with approximately 5.2 million grid points is then employed to simulate half of a simplified QTR geometry for a range of flight conditions. A high download (9% of thrust) is obtained in hover, as expected, when the QTR operates Out of Ground Effect (OGE), primarily from a strong download on the front and rear wings. A detailed analysis of the calculated flow field, along with chordwise pressure distributions and spanwise loadings on the wings, is performed to explain the observed decay in download on the vehicle with an increase in the forward flight speed. The high download obtained OGE in hover, becomes a strong upload (9% of thrust) when the vehicle operates In Ground Effect (IGE) with the wheels placed on the ground; primarily from a strong upload on the fuselage and inner portion of the rear wing. Upload observed IGE in hover gradually fades away with an increases in forward flight. An increase in forward flight speed eventually results in the flow along the ground unable to travel far upstream; the simulation shows the expected horseshoe shape of the wake near the ground. The simulations suggest that the uploads obtained IGE persist for high enough forward flight speed such that a significant increase in payload should be feasible for rolling takeoffs