For a thorough understanding of the aerodynamic properties of aircraft, high fidelity viscous simulations are necessary as a complement to wind tunnel investigations. RotCFD, a CFD code developed by Sukra Helitek, Inc., offers a design tool that allows the user to manipulate the geometry, generate body-fitted viscous grids, run the flow solver and visualize results, all combined in an easy-to-use, seamless Integrated Design Environment (IDE). RotCFD is widely accepted in the rotorcraft industry as an effective design tool that can carry out aerodynamic simulations economically before model scale testing is considered, thereby saving cost and time and permitting a larger design space than is otherwise possible. Some key features of the software are:

- A CAD-like pre-processing tool for parametric variation and manipulation of geometry
- A semi-automatic grid generator, with body-fitted tetrahedral grid near the body and cartesian unstructured grid in the farfield
- A RANS-based, high accuracy general purpose unstructured flow solver, capable of solving laminar as well as turbulent flows
- A fast and efficient discrete blade unsteady rotor model with maneuvering capabilities, along with rotor trim calculations
- Flow visualization along with rotor performance, trim and body force calculations
- Packaged in a user friendly single Graphic User Interface (GUI), compatible with Linux, Windows and Mac operating systems

A wide variety of manipulations can be performed to a given geometry through the embedded application, called ShapeGen. A user friendly GUI (Graphical User Interface) allows the user to perform the following operations efficiently and to visualize the resulting geometry.

- Create 2D and 3D geometries (sphere, cube, 3D wing, duct, 2D airfoils, etc)
- Create geometries by revolving 2D contours
- Translate, rotate and scale geometries
- Change the attitude and the position of the body
- Save the geometry in plot3D definition.

1. Grid Generation

In any CFD computation, proper geometry resolution is essential to obtain reliable numerical solutions. The ease of grid generation for complex geometries encountered in real-life engineering problems is also an important factor that determines the overall simulation time. RotUNS offers the capability of generating high quality viscous grids with minimal user intervention. In RotUNS, the region close to a body is meshed using body-fitted tetrahedral grids, which transition into cartesian unstructured grids in the farfield. Through an interactive interface, the grid quality and the required refinement can be controlled using features such as grid refinement boxes. The semi-automated mesh generation process allows the user to identify each component separately and generate viable grids even for surfaces with zero thickness.

2. Flow SolverAn incompressible, unsteady, unstructured Navier Stokes flow solver is implemented in RotUNS to obtain accurate and economic CFD solutions for flows over rotorcrafts and general purpose aircraft. Conservation of mass and momentum comprise the governing equations of the flow. The salient features of the flow solver are:

- A finite volume based spatial discretization method for collocated grids with implicit time integration
- Patankar’s SIMPLE solution algorithm for pressure-velocity coupling
- An H-type data structure that can handle mixed grid cell types
- Realizable k-epsilon turbulence model for correct modeling of turbulence often encountered in rotorcrafts flows
- A second order gradient limiter to improve solution stability and accuracy

3. Rotor Model

The rotor model incorporated in RotUNS is based on the popular momentum source approach that models the effect of the rotor in terms of the momentum that it imparts to the flow. The magnitude and direction of the imparted momentum depends on the rotor geometry and local flowfield characteristics. This eliminates the need for a body-fitted, rotating grid for the rotor blades, thus significantly reducing the computational requirements for complete rotorcraft simulations. The rotor model accounts for the effect of discrete rotor blades to furnish a time-accurate rotor flowfield along with a good prediction of the rotorcraft performance characteristics. The versatility of this rotor model makes it suitable to be used for a wide variety of problems like propeller blade modeling, horizontal and vertical axis wind turbines, general helicopter rotors including tandem, tilt and coaxial rotors. Some salient features of this rotor model are :

- Obtain thrust, torque, figure of merit, power required and other performance and design characteristics of a rotor
- Obtain time-accurate rotor wake solution and its effect on bodies present in the flowfield (i.e., the force on a terminal building due to nearby helicopter operations)
- Model maneuvering rotors, including general translation and rotation relative to a fixed reference
- Model helicopters in formation flight
- Perform rotor trim simulations to ascertain handling qualities and design requirements of a rotorcraft

Rot3DC is Sukra Helitek’s first generation flow solver module that has been widely used in the rotorcraft industry by the U.S. Army, Navy, and NASA, as well as companies such as Boeing and Sikorsky, among others. Rot3DC is capable of performing rotorcraft simulations like formation flights and interactional rotor-body simulations. Rot3DC has now been incorporated into the RotCFD GUI architecture, so that the user can access both the UNS and Rot3DC structured solver through one easy-to-use, seamless GUI architecture. Rot3DC provides a good platform for validating unstructured simulations and can be used as an independent test bed for modeling of fluid dynamics problems.

1. Geometry Representation and Grid GenerationRot3DC uses a Cartesian structured framework, in which bodies are represented by blanked grid cells. The Cartesian structured grid is generated in the form of layers, defined with respect to a characteristic length like the rotor radius. The interactive GUI environment allows the user to vary the parametric position of rotors and bodies in the flow domain and visualize changes being made to the grid in real time. Notwithstanding the shortcomings of structured grids, Rot3DC has proven to be a powerful CFD tool for modeling fluid flow problems even with complex bodies and wind tunnel walls.

2. Flow SolverRot3DC utilizes an incompressible, unsteady Navier-Stokes solver with a staggered grid approach for primitive variables. The salient features of the flow solver are:

- A finite volume based spaital discretization with implicit time integration.
- Discretization schemes available include Power Law, Flux Corrected Method, and Runge Kutta
- Patankar’s SIMPLE solution algorithm for pressure-velocity coupling

In Rot3DC, the rotor is treated as a distribution of momentum sources in the flow. Such a treatment of the rotor eliminates the need for a body-fitted rotor grid. The momentum imparted by the rotor depends on the geometry of the rotor and the flow characteristics. The rotor model accounts for the effect of discrete rotor blades to furnish a time-accurate rotor flow field. The rotor model also allows for good prediction of the rotorcraft performance characteristics. The versatility of this rotor model makes it suitable for a wide variety of problems from wind turbines to complex rotorcraft modeling. A steady rotor model is also available, which treats the rotor as a time-averaged source of momentum without taking into account the instantaneous position of rotor blades, thereby simplifying the rotor modeling approach. The rotor model has the following capabilities :

- Obtain rotor performance such as thrust, torque, and figure of merit
- Time-accurate solution of the rotor flowfield
- A trim package soon to be incorporated

Along with running CFD simulations with RotCFD, a user can visualize the solution in the same GUI. RotCFD comes with its own flow visualization package, embedded as an integral part of the Unstructured Flow Solver. Some capabilities of the flow visualization tool are:

- Visualize 3D volume as well as 2D cut plane solutions as vectors, contours, or iso-surfaces
- Visualize surface pressures, body forces, and moments on a component-by-component basis
- Extract data on user-specified 2D cut planes and export as contour or line plots
- Visualize pressure, velocities and rotor performance contours on the rotor plane