Aeronautics

Simulations of the Complex Multirotor Test Bed Experiment

Overview

As part of NASA’s Revolutionary Vertical Lift Technology (RVLT) Project, the agency has developed the Multirotor Test Bed (MTB) system to facilitate wind tunnel testing for new and existing multirotor aircraft configurations such as Urban Air Mobility vehicles. The MTB structure, which undergoes testing in the 7-by-10-foot wind tunnel at NASA Ames Research Center, is currently capable of testing up to six rotors, with the potential to expand to even more. It is designed to allow engineers to adjust the vertical, lateral, and longitudinal placement of each rotor, as well as rotor tilt and the pitch of the whole aircraft assembly. The system is currently undergoing design changes to improve the quality of the data measurements, and researchers in the NASA Advanced Supercomputing (NAS) Division are using high-fidelity and reduced-order computational fluid dynamics (CFD) methods to validate and analyze the system as a whole.

a picture of the Multirotor Test Bed structure installed in the U.S. Army’s wind tunnel

The OVERFLOW code, coupled with comprehensive code CAMRADII, is used for both full unsteady flows with moving grids for the rotor as well as the steady-state algorithm of rotor disk model where blade effects are modeled using source terms added to the Navier-Stokes equations. The rotor and structural properties are closely approximated, and the rotor and vehicle are properly trimmed. Use of high-fidelity and a reduced-order method capture rotor blade aerodynamics, blade and rotor performance, rotor-rotor, and rotor-body interactions.

Project Details

a figure comparing both rotor disk and full rotor configurations for a six-rotor Multirotor Test Bed experiment

Detailed comparison and validation were performed for both higher-fidelity full rotor unsteady flows and reduced-order rotor-disk methods, using the high-performance computing resources at the NAS facility at NASA Ames. To reduce the complexity of simulating unsteady flows of the rotating blades with moving overset grids, the team used an option in the NASA-developed OVERFLOW CFD flow solver to model rotors and propellers as infinitely thin disks. Both full rotor and disk models were loosely coupled with the CAMRAD II comprehensive rotorcraft analysis software and tested on various rotor systems, with recent work adding a rotor trim algorithm. With this CAMRAD-coupled OVERFLOW CFD solution, the MTB engineers will be able to adjust the rotor parameters more precisely and in new ways, especially the rotor pitch. The trim algorithm can eventually help prepare a more detailed test matrix as well.

High-performance computing resources are necessary for the large simulation efforts at this scale. This is even true for low-order methods where multiple simulations could be run efficiently and expediently on NAS systems.

Jasim U. Ahmad, NASA Ames Research Center

Results and Impact

High-fidelity CFD simulations are necessary for representing the complex flow physics of multirotor vehicles in detail. Comparison to physical experiments and validation were performed for both the full rotor and disk model of the MTB. The wind tunnel geometry was included in the model, allowing for rotor-rotor interactions and interactions with the other components of the tunnel to be simulated, to ensure parity with experimental data. The CFD simulation maintain rotor lateral symmetry in the rotor flowfield, as opposed to the measured asymmetry. Hopefully, the MTB system can eventually explain why lateral symmetry is not maintained.

Why HPC Matters

a picture of a flow visualization of a hover experiment on the Multirotor Test Bed with only one rotor

High-performance computing resources are necessary for the large simulation efforts at this scale. This is even true for low-order methods where multiple simulations could be run efficiently and expediently on NAS systems. The unsteady MTB simulations required a large amount of computational and storage resources to manage the large dataset. Additionally, researchers needed the ability to assimilate various tools to enhance the capability of the whole analysis process. For this work, simulations were carried out using the Pleiades, Electra, and Aitken supercomputers at NAS.

What's Next

a video of a flow visualization of a forward flight experiment of the Multirotor Test Bed system with six rotors

The CFD tools at the NAS facility will be used as design and simulation tool for next the generation of rotor-based Unmanned Aerial Systems and other concept vehicles. The rotor disk model used in the MTB work will be utilized to accelerate this process.

Researchers

  • Jasim U. Ahmad, NASA Ames Research Center