Use of advanced modeling techniques and ultra-high resolution grids in numerical simulations have predicted rotorcraft rotor performance within experimental error for the first time. Navier-Stokes detached eddy simulation of a flexible UH-60 Blackhawk rotor in forward flight. Magenta and blue colors correspond
to high and low values of vorticity, respectively. Image credit: NASA Ames / Tim Sandstrom
Advances in Rotorcraft Computational Fluid Dynamics Using Detached Eddy Simulation (ARMD Fundamental Aeronautics Program, Rotary Wing Project)
Helicopters and tiltrotors provide many useful civil and military functions without the need for airports and runways. Modern rotorcraft designs continue to push the technology to improve vehicle performance, safety, and reduce its impact on the environment. However, the accurate simulation of rotorcraft flow fields with computational fluid dynamics (CFD) continues to be a challenging problem. Rotor blades generate blade-tip vortices as they spin around and encounter the vortices of other blades as well. This can result in very complex blade vortex interaction (BVI) and vortex wake interactions. Moreover, rotorcraft simulation is multidisciplinary and must take into account the interaction between rotor blade aerodynamics, blade flexibility, and blade motions to achieve trimmed steady flight.
The figure of merit (FM), the chief hover performance parameter, is under-predicted by 2-6% depending on the numerical approach. To bring this into perspective, every ½% error means there is one less passenger the vehicle can carry. Moreover, under-resolved rotor wakes result in inaccurate interactions between the rotor vortices and the rest of the vehicle. One goal of NASA's Rotary Wing (RW) Project is to develop physics-based computational tools to improve the predictive accuracy of these flow fields. The OVERFLOW CFD code is used to solve the time-dependent Navier-Stokes equations for the V22 Osprey tiltrotor and UH-60 Blackhawk helicopter rotors in hover and forward flight.
Results and Impact
The FM has been predicted within experimental error for the first time for the V22 Osprey and Blackhawk helicopter rotors in hover over a wide range of flow conditions. This dramatic improvement is due to the use of algorithms with high-order spatial accuracy, refined grid resolution on the rotor blades, and an improved detached eddy simulation (DES) hybrid turbulence model. High resolution accuracy on the rotor blades was important for all cases studied, and the DES hybrid turbulence model was crucial in BVI cases. Dynamic adaptive mesh refinement (AMR) is used to efficiently resolve the rotor wakes. This is accomplished by detecting vortices and locally added blocks of Cartesian grids of higher resolution. This allows the vortices and rotor wake to be resolved by grids four times finer and at half the computational cost than a uniform (non AMR) grid refinement approach. The AMR resolution revealed tremendous detail of the blade vortices and turbulent flow that had never before been observed computationally or experimentally. The flow simulations have been quantitatively validated with experimental measurements. The use of AMR greatly improved the prediction of vortex core size and growth with wake age (distance along the tip vortex). The error in vortex core growth rate has been reduced from 60% to 4% error. These groundbreaking results will impact the next generation of rotorcraft analysis and design.
Role of High-End Computing Resources and Services
These flow simulations were made possible by NASA's Pleiades supercomputer. Solutions typically required 1,500-4,600 cores for 1-2 weeks. Grid sizes varied from 60-750 million grid points, and required 5-24 hours of wall time per rotor revolution.
Researcher: Neal M. Chaderjian, NASA Ames Research Center