Unsteady RANS-based Computational Aeroelastic Simulations of X-56A Flutter for the Third Aeroelastic Prediction Workshop

Abstract

This paper presents a high-fidelity aeroelastic study of the the Multi-Utility Technology Testbed (MUTT) X-56A, designed to exhibit aeroelastic instabilities such as body free flutter (BFF). The primary objective of this work is to assess the use of high-fidelity CFD-based aeroelastic simulations for flutter prediction. This research was originally conducted as part of the Third Aeroelastic Prediction Workshop (AePW3) aiming to enhance the knowledge in aeroelastic predictions using mid to high-fidelity computational aerodynamics. This particular study details the contribution from the Flight Physics & Loads group at the Netherlands Aerospace Centre (NLR), exploring two computation methods for generating the Generalised Aerodynamic Forces (GAFs), namely, ZAERO solver (ZONA Technology) using higher-order panel code ZONA6, and an unsteady RANS-based Computational Fluid Dynamics (CFD) and Computational AeroElastic (CAE) simulations implemented within NLR's ENFLOW simulation system for multi-block flow domains. The high-fidelity CFD and CAE analyses were performed using the flow solver ENSOLV with unsteady Reynolds-Averaged Navier-Stokes (RANS) flow formulation implemented with Explicit Algebraic Reynolds Stress Model (EARSM) turbulence modelling based on the TNT $k-\omega$. The CAE computational procedure consists of four tool chains, involving structural dynamics (modal) analyses; grid interpolation procedure; steady CFD computations on the undeformed shape; unsteady CFD computations on a deforming grid under prescribed, small-amplitude sinusoidal excitations based on the structural mode shapes; and the transformation of the time-domain unsteady solutions to frequency domain in order to construct the GAF matrices. The X-56A configuration used for this study is the 10lb fuel state model released within the AePW-3 group. The resulting GAFs were compared to the ZAERO results, showing good agreement for both the rigid body and elastic modes. Earlier work on X-56A within AePW-3 conveyed the need for further refinement of the high-fidelity aeroelastic methodology. Improvement efforts in this regard, included alternative structural dynamics methods for modal model computations, CFD grid refinements, and adjustments to the (un)steady CFD/CAE simulation procedures and methods.