RotCFD-A Tool for Aerodynamic Interference of Rotors : Validation and Capabilities

A novel design tool called RotCFD has been developed at Sukra Helitek, Inc. At conceptual and preliminary rotorcraft design stages, an analysis tool is successful only if problem set-up time is minimal and run times are economical. In addition, during detailed design stages, often times, additional design questions arise that need answers in a reasonable time. The above requirements led to the genesis of RotCFD which is based on its predecessor Rot3DC. RotCFD attempts to bridge the two worlds of design and Computational Fluid Dynamics (CFD) with the help of an Integrated Design Environment (IDE) specific to rotorcraft. RotCFD emphasizes user-friendliness and efficiency that streamline the design process from geometry to CFD solution. The key components of RotCFD are a geometry module, a semi-automated grid generation module, a flow-solver module, a rotor module, a flow visualization and analysis module, all integrated in one environment. The concept of rotor blades represented by momentum sources forms the basis of this aerodynamic simulation tool for rotorcraft. The rotor momentum sources are primarily a function of the local velocity of the flow and the two-dimensional airfoil characteristics of the rotor blades. The NavierStokes equations and the blade element theory are coupled implicitly to yield a self-contained method for generating performance, as well as the near and far wake including all the aerodynamic interference inherent in a situation. The method has been subjected to a set of well known cases and the results compare well with available data. The current capabilities of this unique tool, its components and preliminary validation results are presented. Introduction During initial design studies, parametric variation of vehicle geometry is routine. The paradigm of RotCFD follows the concept of Integrated Design Environment (IDE), specific to rotorcraft. Rotorcraft engineers traditionally use the wind tunnel to evaluate and finalize designs. The existence of wind tunnel walls do influence the testing environment from actual free air conditions, since the flow behavior changes near the tunnel walls. Correlation between wind tunnel results and flight tests, when not good, have been often attributed in part to uncertainty in blockage corrections. Estimation of rotor blockage is significantly more complex than bluff body corrections as the correction depends on operational characteristics such as rotor RPM and thrust produced. In addition to being a design tool, RotCFD allows simulation of a Presented at the American Helicopter Society Future Vertical Lift Aircraft Design Conference, January 18-20, 2012, San Francisco, California, Copyright c ©2012 by the American Helicopter Society International, Inc. All rights reserved. complete rotorcraft configuration with or without wind tunnel walls including all the facility effects. The key principle in the development of the architecture is to make the environment user friendly, robust and easy to learn. Often times the nature of any computational solution is that multiple tools are required that begin with geometry manipulation to post-processing, analysis and visualization of the simulated results. RotCFD provides a rudimentary set of tools for all the steps a rotorcraft design engineer needs to computationally simulate a rotorcraft and analyze the simulation. To make the tool user friendly an intuitive Graphical User Interface (GUI) has been developed and is available on popular operating systems such as Mac, Windows and Linux. The software has been designed to give the same look and feel in all the platforms supported. The robustness of the tool is enabled by semi-automating the process of grid generation with minimal user input. The learning experience is made easy by providing judicious defaults and requiring a small number of initial inputs for a simple rotor case. The underlying CFD algorithm is based on a pressure based algorithm SIMPLE[1] known for its robustness and suitability to low speed flows. The simplicity and robustness are possible because the rotor is modeled using momentum sources; individual blades are not modeled in the computational simulation. Two-dimensional airfoil tables specific to the rotor blades are required for obtaining a proper solution. Currently, the automated body-conforming grid generation does not produce grids suitable for a viscous solution. RotCFD is based on Rot3DC, a structured single-zone Cartesian grid solver. The current version of RotCFD is a mixed-element unstructured hanging-node solver. In summary, RotCFD is an Integrated Design Environment (IDE) specific to rotors which can simulate a complete rotorcraft with or without wind tunnel walls. At the heart of the innovation are: 1. A Graphical User Interface (GUI), with user friendly and intuitive input and output menus, that interacts with all the components of the software and is the front end to the capabilities of the tool. 2. A minimal CAD-like geometry engine to manipulate multiple bodies and rotors. 3. An automated hybrid grid generator that produces nearly conforming grids around the bodies and unstructured Cartesian grids everywhere else. 4. A robust and economical incompressible flow solver for the entire system of grids. 5. Momentum source based rotor model for simulating configurations with multiple rotors. 6. A flow visualization tool for CFD postprocessing. The sections that follow briefly describe the theory, the architecture, and the implementation details of the GUI; the geometry engine; the grid generator; the flow solver; and the post-processing tools natively available in RotCFD for visualizing the results. In addition, a set of validation cases have been developed and preliminary results are presented. RotCFD the Tool In this section, the various components of the RotCFD tool are described. RotCFD GUI Architecture: The RotCFD GUI acts as the front-end to all the processes of the IDE. The GUI is also designed to be used for different types of flow solvers and applications. Central to the design of the architecture of RotCFD is the commonality of most flow solvers and rotor problems. • Geometry preparation and manipulation. • The steps involved in running different processes specific to the simulation. • The data required as input to the solvers. • The tools used to visualize the results and post process the simulation. The GUI is where the particular problem or scenario is defined: the geometry is loaded, the rotor and flow properties are defined, the grid is generated, and the solver is started. The GUI is also where one visualizes the geometry, the grids, and the solution. Many of these steps and tools are common to all the different types of grid generators and solvers within RotCFD. Some of these basic commonalities are: • The ability to load, manipulate, and visualize geometry (including rotors) • The ability to load and visualize grids • The ability to load and visualize solutions Much of this common functionality is part of what is called the RotCFD Project Shell or simply the “Shell.” All projects are derived from the functionalities in the Shell. The Shell consists of two main