Software Design for CFD Rotary-Wing Aeromechanics Modeling

April 1, 2013

Army Aeroflightdynamics Directorate (AMRDEC), Moffett Field, California

Helicopter flight involves many multidisciplinary physics problems that are difficult to predict with today’s engineering modeling and simulation tools. Rotor aerodynamic systems involve complex interactions among the rotor blades, rotor wakes, and fuselage, and they create challenges such as simultaneous modeling of rotating and nonrotating components; retreating-blade, low-speed dynamic stall; advancing-blade transonic flow; rotor “trim” requirements to balance aerodynamic and dynamic forces for particular control settings; and strong coupling between rotor-blade aerodynamics and rotor blade dynamics (both rigid and elastic blade motion).

**Helios Meshes for Simulations** of the HART-II rotor, a 40% Mach and dynamically scaled model of a hingeless rotor tested in a wind tunnel. The Helios computations used 12.6 million nodes for the nearbody grid system around the rotor and test stand.

A successful rotorcraft aeromechanics simulation must accurately represent all these physical phenomena. These software models typically require substantial engineering expertise, powerful computer systems, and must couple computational fluid dynamics (CFD), computational structural dynamics (CSD), and vehicle flight controls.

A software product called Helios has been developed for multi-disciplinary rotary-wing aeromechanics modeling. The Helios infrastructure links both new and existing software modules with little need for extensive code modifications. Data exchanges between software modules occur through a Python-based integration framework. This Python software framework is both object-oriented and scalable on large parallel computer systems.

A desirable alternative to traditional monolithic software development consists of a lightweight computational infrastructure that links together independent multidisciplinary software modules. This concept is not new, and a number of such infrastructures currently exist. They can generally be classified into two categories; high-level execution managers that coordinate the execution of standalone legacy codes, and low-level frameworks that provide a common data format and communication protocol from which the higher-level executable may be built.

Helios uses an intermediate-level software infrastructure that uses characteristics from both approaches. Like the highlevel execution managers, it links existing software modules with little need for extensive code modifications. However, instead of using file transfers for data exchanges between modules, data exchanges take place through a top-layer Python-language integration framework that is both object-oriented and scalable. The Python top layer facilitates data exchanges between individual multidisciplinary component software modules, and these data exchanges occur through direct memory access rather than file input and output (I/O). As such, the Helios Python framework provides simple data transfers among software component modules with sufficient granularity to ensure that groups of programmers can work independently on each of many multidisciplinary software component modules and then easily use Python to tie them together in order to create the final software product.

The Helios modeling approach solves the Reynolds-averaged Navier- Stokes (RANS) equations to discretize the aerodynamic flow field around a rotorcraft. These equations capture both the fluid dynamic forces on the vehicle plus the vortical rotor wake behavior beneath the rotor system. Helios uses two types of grid systems to capture these rotarywing aerodynamic effects. The “near-body” grids use body-fitted, triangular surface meshes to represent the solid surfaces on the rotor and fuselage. Such unstructured triangular grid systems are typically generated directly from digital computeraided design (CAD) models that represent the vehicle component surfaces.

Rotor structural dynamics and trim modeling is handled by the Rotorcraft Comprehensive Analysis System (RCAS). The Python-based software integration framework sends the rotor motions to the CFD solvers and then brings the corresponding rotor aerodynamics forces back to the computational structural dynamics model. At the end of a trimmed rotor computation, the aerodynamic forces on the rotor are consistent with the rotor dynamic motions and also with the pilot control inputs.

The success of the Helios software development effort is heavily dependent on the use of its lightweight Python infrastructure that connects individual component software modules using welldefined interfaces for each component software module.

This work was done by Roger C. Strawn of the Army Aeroflightdynamics Directorate. ARL-0151