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Software Design for CFD Rotary-Wing Aeromechanics Modeling

Helios software solves many of the multidisciplinary physics problems associated with helicopter flight.

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

Topics:
Communication protocols Computational fluid dynamics Data acquisition and handling Data exchange Scale models Simulation and modeling Software Software Telecommunications
This Brief includes a Technical Support Package (TSP).
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Software Design for CFD Rotary-Wing Aeromechanics Modeling

(reference ARL-0151) is currently available for download from the TSP library.

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    Defense Tech Briefs Magazine

    This article first appeared in the April, 2013 issue of Defense Tech Briefs Magazine (Vol. 7 No. 2).

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    Overview

    The document presents insights from the Seventh International Conference on Computational Fluid Dynamics (ICCFD7), focusing on the software design strategies employed in the US Army's Helios software, which addresses the complexities of rotary-wing aeromechanics. The paper highlights the challenges of creating accurate, efficient, and maintainable modeling and simulation software in this multidisciplinary field.

    Helios is designed with a lightweight Python-language integration framework that facilitates the connection of various software components. This design strategy allows for the easy incorporation of alternative software components and the rapid introduction of new computational fluid dynamics (CFD) technologies. The framework promotes efficient data exchanges between modules through direct memory access rather than traditional file input/output methods, enhancing performance and scalability.

    The document contrasts high-level and low-level software coupling approaches. High-level approaches require minimal changes to existing legacy codes but often suffer from inefficiencies and limited scalability due to file-based data exchanges. In contrast, low-level approaches allow for more integrated functionality but necessitate significant rewrites of software modules, which can complicate coordination among developers. Helios adopts an intermediate-level approach, combining the benefits of both strategies while minimizing the need for extensive code modifications.

    The paper also discusses the importance of maintaining granularity in software components, enabling independent work by programming teams. This independence is crucial for managing the complexity of multidisciplinary simulations, as it allows for parallel development and integration of various software modules.

    Additionally, the document provides examples of Helios simulations that demonstrate its capabilities in solving rotor dynamics and aerodynamics, as well as achieving high-resolution modeling of rotor wake systems. These simulations illustrate the practical applications of the software in real-world scenarios, showcasing its effectiveness in addressing the intricate challenges of rotary-wing aeromechanics.

    Overall, the document emphasizes the innovative design strategies of Helios, which leverage modern programming practices to enhance the development and performance of multidisciplinary CFD software. By integrating various components seamlessly and allowing for rapid advancements in technology, Helios represents a significant step forward in the field of computational fluid dynamics.

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