Computational Analysis for the Design of NASA’s Side-By-Side Concept Vehicle
Overview
NASA has proposed several new Urban Air Mobility (UAM) concept vehicles that utilize multiple rotors, including the side-by-side (SBS) rotorcraft, which is a part of the Revolutionary Vertical Lift Technology (RVLT) Project. Aerospace engineers in the NASA Advanced Supercomputing (NAS) Division at NASA’s Ames Research Center used experimental data and computational fluid dynamic (CFD) analysis to determine how changes in the position and orientation of multiple rotors affected the aerodynamic characteristics of the SBS concept vehicle. NASA supercomputers and high-fidelity CFD methods using actual rotor blades were used to validate and quantify the errors of lower-fidelity modeling and simulation methods that replaced the rotor blades with a simplified rotor-disk model.
Project Details
The focus of this investigation is to model a two-rotor experiment without a fuselage, where the rotors are mounted side-by-side and installed in the NASA Ames 7x10-ft wind tunnel. The amount of-rotor overlap is changed to optimize cruise performance. The experiment was designed to allow adjustment of the lateral placement of each rotor, as well as to allow tilt and pitch adjustment of the whole assembly. Each rotor is equipped with a swashplate assembly, allowing for rotor blade collective and cyclic control. This CFD work seeks to capture the complex rotor blade aerodynamics, rotor performance, and rotor-rotor interactions, where the blade-tip vortices interact with each other. The CFD results are then compared to wind tunnel experimental data to quantify modeling errors.
The SBS concept vehicle flow field is very complex and requires high-fidelity Navier-Stokes simulations for an accurate representation. In general, the flow field of a conventional helicopter, as well as multirotor vehicles, whether in forward flight or hover—consists of unsteady aerodynamics with complex three-dimensional wakes dominated by the blade tip vortices. To reduce the complexity of modeling the details of the rotating blades, engineers used an option provided in the NASA’s OVERFLOW CFD code to model rotor propellers as infinitely thin disks. These disks are represented by momentum source terms in the governing Navier-Stokes equations. Both fully modeled rotor blades and disk models are loosely coupled with a rotorcraft comprehensive analysis code, CAMRADII, to ensure the aerodynamics meets flight requirements. A rotor disk model shortens the time needed to set up a new concept vehicle for CFD analysis but does eliminate some of the rotor wake complexity. This disk model has been tested on various other rotor systems.
Results and Impact
High-fidelity CFD methods are necessary for representing rotorcraft flow physics in detail. Lower-order methods, like the rotor disk model, are very important tools that help reduce the time requirements needed to explore many new vehicle concepts. It is also important to validate the computation with experiment. Detailed comparison and validation are being performed on the SBS using both rotor methods. The tunnel geometry is included in the detailed computational effort. The rotor-rotor interactions and interactions with other components of the wind tunnel test section are an important aspect of the CFD validation effort. The geometry of the SBS system is represented by boundary representation solids. Engineers in the NAS Division are developing an automatic overset surface and volume mesh generation tool that will further expedite the problem setup time. This auto-mesh process takes approximately one minute on a MacBook Pro laptop to generate the surface grid for the MTB geometry.
Why HPC Matters
High-performance computing resources are required for the large simulation effort of a new concept vehicle. This is true even for low-order methods where many simulations can be run efficiently and expediently. The unsteady simulations also require tremendous resources to manage the large dataset. In addition to the supercomputer requirement, researchers can assimilate various tools to enhance the capability of the entire analysis process. The Pleiades, Electra, and Aitkin supercomputers at the NAS facility were heavily utilized to carry out these simulations.
What's Next
The OVERFLOW CFD code and other grid-generation packages developed in the NAS Division will be used as design and simulation tools for the next generation of UAM vehicles. The rotor disk model will significantly reduce the CFD process time.
More Information
Researchers
- Jasim U. Ahmad Please enable Javascript to see email address.
