CFD modeling of Unmanned Aerial Systems with Cut-cell Grids and Adaptive Mesh Refinement

The demand for mission-driven Unmanned Aerial Systems (UAS) in environment, commercial, and defense fields has been constantly increasing in the recent years. A UAS’ configuration is designed on the basis of specific parameters such as flying speed, altitude, endurance, maneuvers, etc. During the design process of a UAS, aerodynamics play a significant role as the aircraft’s performance strongly depends on the forces acting on the aircraft’s body, which result from the interactions between the air and the vehicle itself. Therefore, in order to design and develop a suitable flight dynamic control system that can meet the mission requirements, accurate knowledge of the aircraft’s aerodynamic coefficients is necessary. In this paper, by means of high-performance computing, a computational fluid dynamics (CFD) methodology is proposed to perform large-scale 3-D simulations of selected NACA airfoils and a remotely piloted vehicle, the Pioneer RQ-2A. Features available in the CONVERGE code, such as cut-cell-based mesh generation and adaptive mesh refinement, allowed for fast definition of the computational grids which were dynamically refined during the simulations on the basis of local velocity gradients. This allowed to perform multiple and accurate aerodynamic analyses with reasonable time-to-science costs. An incompressible, transient, Reynolds-Averaged Navier-Stokes formulation closed by the RNG k-ε turbulence model was used to perform simulations at Reynolds numbers range from 5×104 to 2.88×106, on varying the angle of attack. The model was validated by comparing lift, drag, moment, and pressure aerodynamic coefficients against wind-tunnel data and CFD results available from the literature. The predictions were found to be in good agreement with the literature and the model was able to predict flow separation and vortex shedding for both the airfoils and the UAS. This study was performed as part of a larger project in which the goal is to map the aerodynamics coefficients of an unmanned aircraft across its whole operating range and feed this information to a system simulation tool for aircraft dynamic and performance analysis.