Validation of Rotorwash Simulation using Blade-resolved and Blade-modeled CFD

  • Caprace, Denis-Gabriel (NASA/Oak Ridge Associated Universities)
  • Ventura Diaz, Patricia (NASA/Science and Technology Corporation)
  • Yoon, Seokkwan (NASA)

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Assessing how rotorwash affects the operation of urban air mobility vehicles in ground proximity is crucial to guaranteeing their safety. As complex unsteady flows are involved, high-fidelity CFD is preferred to capture the details of the rotor-airframe-ground interference. However, mesh-based numerical simulation of rotors in ground effect is very computationally intensive due to extensive fine-resolution regions and long simulation times. Medium-fidelity methods —with less stringent requirements on mesh resolution and faster simulation turnarounds— are therefore attractive, provided they can be validated. We present the results of an ongoing validation effort. We consider, on the one hand, high-fidelity delayed detached eddy simulation (where the blades are resolved) and, on the other hand, medium-fidelity vortex particle-mesh large eddy simulation (where the blades are modeled). As can be seen in Fig. 1, we find that satisfactory agreement is obtained between the two levels of fidelity in the region close to the rotor. The region extending from 3 radii and out is not well-resolved in the high-fidelity simulation to reduce the computational cost. We demonstrate that such reasonably-expensive high-fidelity simulations (350M grid points) can be used to validate the medium-fidelity simulation (18M particles). This argument will be backed by additional comparisons to experimental data. Subsequently, the medium-fidelity method can be used for case studies that would be intractable with the high-fidelity code, considering for example the wake at large distances from the rotor, or dynamic approaches. To conclude, we formulate guidelines pertaining to rotorcraft simulation in ground effect with both frameworks.