AN UNSTRUCTURED HYBRID CFD APPROACH FOR COMPUTING ROTOR WAKE FLOWS Chunhua Sheng 1 and Qiuying Zhao 2 csheng@eng.utoledo.edu; qiuying.zhao@utoledo.edu; The University of Toledo Nischint Rajmohan 2 and Lakshmi Sankar 3 nischint@gatech.edu; lsankar@ae.gatech.edu; Georgia Institute of Technology Atlanta, GA John O. Bridgeman 4 and Jim C. Narramore 5 jobridgeman@bellhelicoptertextron.com; jnarramore@bellhelicoptertextron.com; Bell Helicopter Textron, Inc. Fort Worth, TX Abstract An unstructured hybrid approach is developed aimed at computing viscous flows around helicopter rotors in hover and forward flight. A single rotor blade is resolved through the Navier-Stokes solution with the consideration of the blade vortex interaction for faster and economical computations. The near wake is captured by the Navier-Stokes analysis. The influence of the other blades and of the trailing vorticity in the far field wake is accounted for by modeling them as a collection of piece-wise linear bound and trailing vortex elements. The use of such a hybrid Navier-Stokes/vortex modeling method allows for an accurate and economical simulation of viscous features near the blades, and a “non-diffusive” modeling of the trailing vortices in the far field. Two viscous rotor cases are presented for validating the hybrid methodology: a NACA 0012 rotor and a Bell Helicopter M427 main rotor in hover and forward flight. Numerical studies indicate that the current hybrid approach is capable of predicting the rotor wake flows with a good accuracy, while resulting in considerable savings in computational time compared with the full Navier-Stokes solutions. I. Introduction The prediction of rotor aerodynamic loads requires an accurate and efficient modeling of vortex-dominant flows generated by the rotor blade. This problem is highly complicated because the rotor wake/vortex is unsteady and exhibits true three-dimensional phenomena. Although high-fidelity computational fluid dynamics (CFD) tools have been developed for modeling helicopter rotor aerodynamics [1,2], it normally requires sufficient grid resolution and/or coupling with computational structure dynamics (CSD) tools in order to predict the flow physics with acceptable accuracy [3]. For rotors in forward flight, computational meshes for the full rotor configuration are required for CFD solutions due to asymmetric flow structures on the advancing and retreating sides of the rotor. For industrial users, where fast turnaround with limited computing resources is a key constraint for the routine design and analysis of rotor systems, it is prudent to develop an efficient computational tool that uses grid sizes as small as possible for modeling the rotor’s aerodynamic performance. The current effort was motivated and supported by Bell Helicopter Textron, Inc. to develop an efficient hybrid computational tool for the analysis and design of rotor blades in hover and forward flight. The general approach of this effort is to couple the high-fidelity rotorcraft CFD code U 2 NCLE [2] that is being routinely used at Bell Helicopter with a Lagrangian rotor wake model [4] developed at Georgia Tech. The U 2 NCLE code is a three-dimensional unsteady Reynolds- averaged Navier-Stokes solver based on multi-element unstructured grids for rotational machinery aerodynamic predictions. It has a unique dynamic grid capability to model the discrete rotor blades in cyclic motion. The rotor wake model adopted in the present study is the same one as used in the GT-Hybrid code [4,5], based on the Prandtl-Glauert’s inflow velocity [6] and Mello’s analytical wake strength model [7]. The hybrid coupling methodology has recently gained popularity in the  Associate Professor, Department of Mechanical Engineering, AIAA Associate Fellow 2 Ph.D. Student, AIAA Student Member  Regents Professor, School of Aerospace Engineering, AIAA Fellow 4 Staff Engineer, CFD SME, Aerodynamics Group 5 Chief, Aerodynamics and Acoustics Group 49th AIAA Aerospace Sciences Meeting including the New Horizons Forum and Aerospace Exposition 4 - 7 January 2011, Orlando, Florida AIAA 2011-1124 Copyright © 2011 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved. Downloaded by GEORGIA INST OF TECHNOLOGY on June 27, 2014 | http://arc.aiaa.org | DOI: 10.2514/6.2011-1124