JOURNAL OF THE AMERICAN HELICOPTER SOCIETY 56, 042007 (2011) Improved Two-Dimensional Dynamic Stall Prediction with Structured and Hybrid Numerical Methods K. Richter ∗ A. Le Pape T. Knopp Research Scientist Research Scientist Research Scientist Institute of Aerodynamics and Flow Technology Applied Aerodynamics Department Institute of Aerodynamics and Flow Technology German Aerospace Center (DLR) ONERA German Aerospace Center (DLR) G¨ ottingen, Germany Meudon Cedex, France G¨ ottingen, Germany M. Costes V. Gleize A. D. Gardner Research Scientist Research Scientist Research Scientist Applied Aerodynamics Department Numerical Simulation and Aeroacoustics Institute of Aerodynamics and Flow Technology ONERA Department, ONERA German Aerospace Center (DLR) Meudon Cedex, France Chˆ atillon Cedex, France G¨ ottingen, Germany A joint comprehensive validation activity on the structured numerical method elsA and the hybrid numerical method TAU was conducted with respect to dynamic stall applications. To improve two-dimensional prediction, the influence of several factors on the dynamic stall prediction was investigated. The validation was performed for three deep dynamic stall test cases of the rotor blade airfoil OA209 against experimental data from two-dimensional pitching airfoil experiments, covering low-speed and high-speed conditions. The requirements for spatial discretization and for temporal resolution in elsA and TAU are shown. The impact of turbulence modeling is discussed for a variety of turbulence models ranging from one-equation Spalart–Allmaras-type models to state-of-the-art, seven-equation Reynolds stress models. The influence of the prediction of laminar/turbulent boundary layer transition on the numerical dynamic stall simulation is described. Results of both numerical methods are compared to allow conclusions to be drawn with respect to an improved prediction of dynamic stall. Nomenclature b span, m c chord, m c d , c d drag coefficient, difference in drag coefficient c l , c l lift coefficient, difference in lift coefficient c m , c m pitching moment coefficient, difference in pitching moment coefficient f frequency, Hz M Mach number r radius, m Re Reynolds number s,s LE ,s max cell size, cell size at airfoil leading-edge, maximum cell size, m T period, s v ∞ freestream velocity, m/s x,y coordinates, m y + normalized wall distance α, α angle of attack, difference in angle of attack, deg t timestep, s ω ∗ reduced frequency, ω ∗ = 2π fc/v ∞ Introduction Dynamic stall is one of the most challenging flow phenomena exist- ing in the field of helicopter aerodynamics. During forward flight and ∗ Corresponding author; email: kai.richter@dlr.de. Presented at the American Helicopter Society 65th Annual Forum, Grapevine, TX, May 27–29, 2009. Manuscript received July 2010; accepted August 2011. maneuvers, dynamic stall appears on the helicopter main rotor blades due to high blade loading and low freestream velocities on the retreating blade side. The occurrence of dynamic stall on large portions of the rotor blade can constitute a critical flight condition and therefore limits the flight envelope. During dynamic stall, the blade encounters high peak loads in lift, drag, and pitching moment. This leads to an undesirable increase in the overall drag of the helicopter and thus to increased fuel consumption in high-speed forward flight. In addition, the flight envelope is constrained for maneuvers to avoid structural damage to the rotor due to excessive pitch link loads. The dynamic stall flow phenomenon is a complex fluid mechanical problem. Unsteady laminar/turbulent boundary layer transition, massive flow separation, vortex formation, vortex shedding, and flow reattach- ment interact and make detailed understanding of the physical flow pro- cesses difficult. Dynamic stall has been experimentally investigated by many authors (e.g., Refs. 1–3), but few numerical studies are available since accurate numerical simulation of dynamic stall is challenging and expensive. Despite this, numerical prediction is an important part of the aerodynamic assessment of rotor blade airfoils and will be part of the aerodynamic design process in the future. As the prediction capabilities depend on several factors, such as spatial and temporal discretization, turbulence modeling, and boundary layer transition prediction, a careful validation of the numerical tools has to be performed to ensure dynamic stall simulations of high quality. This paper discusses the results of a comprehensive validation of the numerical methods elsA and TAU, conducted with respect to dy- namic stall applications for the improvement of the two-dimensional dynamic stall prediction with structured and hybrid numerical methods. The work was performed in the joint German/French project SIMCOS (Advanced Sim ulation and Co ntrol of Dynamic S tall) in the frame of the DLR/ONERA cooperation on helicopter technologies. DOI: 10.4050/JAHS.56.042007 C  2011 The American Helicopter Society 042007-1