Computational Aeroelasticity:A Key Enabling Technology for the Design of Next-Generation Aircraft

Sabine A. Goodwin, CD-adapco

With surging fuel prices and more people taking to the skies, aircraft companies have been forced to rethink their approach to the design of the next-generation aircraft. To reduce fuel consumption, increasingly lighter and more flexible composite structures are being introduced, and innovative, unconventional designs resulting in higher lift-to-drag ratios are on the table. With such radical changes to the structure and aerodynamic shape of aircraft, the use of high-fidelity computational aeroelasticity (CAe) early in the design process will be essential to meet tomorrow’s design challenges.

Critical Role of High-Fidelity Computational Aeroelasticity High-fidelity CAe is a key enabling technology to meet three important goals towards light-weight, innovative designs: • Take full advantage of the lighter structures by meeting

air-worthiness requirements without incurring aeroelastic weight penalties late in the development cycle.

• Develop novel active control technologies to limit the flexible vehicle’s dynamic response to atmospheric disturbances and minimize critical design loads.

• Deploy game-changing smart structures to aeroelastically shape the wing for drag reduction, improved stability and load alleviation resulting in a further reduction in weight.

Key Ingredients for SuccessMerging fluids and structures, into a high-fidelity CAe capability, demands a carefully designed implementation with: • Robust mapping and data exchange between the non-

conformal grids resulting in accurate solutions with minimal communication overhead costs.

• Accurate coupling between the solvers leading to stable solutions.

• Efficient dynamic mesh movement resulting in high quality meshes and requiring little to no user intervention.

• Intuitive user interface for seamless integration into a design process.

STAR-CCM+ Provides the Solutions

STAR-CCM+, CD-adapco’s flagship software, offers practical solutions for tackling highly non-linear fluid-structure interaction problems such as aerodynamic flutter and buffet:

Built-in mapping and co-simulation coupling are efficient

STAR-CCM+ has a direct link to Abaqus finite element analysis (FEA) through a co-simulation application programming interface (API) developed by SIMULIA, delivering a fully coupled, two-way, fluid-structure interaction. The API manages all synchronization while both codes are running in memory, reducing overhead often associated with data transfer through file exchanges and use of external middleware software. In addition, the built-in mapping implemented in STAR-CCM+ is robust and accurate with no need for writing scripts. The STAR-CCM+/Abaqus co-simulation also gives the user the flexibility to choose between explicit or implicit coupling, depending on the complexity of cross-coupling in the application.

Java automation results in flexibility and customization

In addition to direct co-simulation coupling with Abaqus, STAR-CCM+ also enables CAe computations through importing/exporting computational structural dynamics (CSD) meshes in other native formats (e.g. Nastran, Ansys) as it leverages the power of Java to give users the ability to customize every step of the simulation workflow. Although this feature requires work up front to set up and manage the simulation, the pay-off is greater flexibility to use legacy codes.

Aeroelastic wing deflection

The trend over the next 20 years is not more of the same. It is larger aircraft, cleaner aircraft, more fuel efficient aircraft.

--John Leahy, COO Airbus, Discussing Airbus Global Market Forecast 2012-2031

State-of-the-art meshing and morphing reduces turnaround time

The viability of deploying a CAe simulation in a production environment strongly depends on the quick generation of high quality computational meshes. With unrivaled polyhedral and trimmed cell meshing, STAR-CCM+ cuts meshing time on very complex geometries down from months to hours. In addition, the multi-quadratic morphing capability robustly and smoothly moves these meshes (of any topology) based on the deformation it receives from the CSD solver. STAR-CCM+ also has an overset mesh capability, a key enabler for applications in aero-servo elasticity such as gust load alleviation where it is vital to model the control surface deflections as part of the simulation.

Intuitive user simulation environment with high-fidelity physics delivers engineering solutions

STAR-CCM+ drives innovation as it seamlessly fits into any existing engineering process and gives the designer the power to handle the most complex multi-physics problems with ease. In addition to performing high-fidelity CAe simulations, with the same code, the user can easily also include temperature effects, aero-acoustics, 6 degrees-of-freedom (6DOF) and other high-fidelity physics in a fully coupled manner. The net result is more time analyzing data and less time preparing and setting up simulations. Case Study 1:

Plate Vortex Induced Vibration This simple case study investigates the strong two-way aeroelastic coupling between an elastic plate with two fundamental modes (a 4 Hz 1st bending mode and a 20 Hz 1st twisting mode) and compressible air moving normal to the plate at 10 m/s (Figure 1). The two-way coupled STAR-CCM+/Abaqus FEA co-simulation was used to perform an aeroelastic analysis and to demonstrate presence of resonance resulting from damping of the 1st bending mode and excitation of the 1st twisting mode during this experiment.

To validate the use of the STAR-CCM+ SST Κ−ω turbulence model for this application, rigid unsteady Reynolds-Averaged Navier-Stokes (RANS) computations were initially performed. Both the computed time-averaged drag coefficient and Strouhal Number (a non-dimensional number describing the frequency of vortex shedding) matched well with previously documented experimental results, confirming that the turbulence model accurately predicts the complex turbulent flow patterns in the wake behind the flat plate.

The shedding frequency of the vortices in the wake was predicted at 19.5 Hz, nearly identical to the natural frequency of the twisting mode. These results suggest that resonance will likely occur when aeroelastic effects are included in the simulation.

During the two-way aeroelastic computation with STAR-CCM+/Abaqus FEA, the elastic deformation of plate initially was as expected: bending in the direction of the wind. As the solution progressed in time, the bending mode of the structure was damped out by the air flow and the plate began to twist as the periodic driving force of the vortex shedding excited the 1st twisting mode. The simulation was successful in predicting the resonance expected to occur during this experiment.

Figure 1: Experimental set-up

State-of-the-art meshing with STAR-CCM+

Figure 2: Resonance on plate

Case Study 2: Flutter of the AGARD Wing 445.6The AGARD Wing 445.6 is a standard aeroelastic configuration specifically designed for dynamic aeroelastic response. This geometry was extensively tested in the 16-ft Transonic Dynamics Tunnel at the NASA Langley Research Center and resulting data has been widely used for verification of various CAe codes for the past 20 years [1, 2]. This study was performed to validate the two-way coupled STAR-CCM+/Abaqus FEA co-simulation for flutter prediction.

Using the polyhedral meshing capability in STAR-CCM+, a fine viscous mesh with prisms in the boundary layer was generated, sufficiently refined to ensure accurate capture of the shock locations at transonic conditions. The surface mesh is depicted in Figure 3.

To compute the flutter boundary, the motion of the wing was initiated by applying an impulse load to the structure, and unsteady RANS time marching calculations using the two-way coupled STAR-CCM+/Abacus FEA co-simulation were performed.

The computed flutter characteristics, represented in terms of the flutter velocity index (FVI), are depicted in Figure 4. Results demonstrate that the method nicely captures the experimental flutter boundary including the significant drop in the flutter speed at transonic conditions (also called the flutter dip). Similar results were observed when analyzing the flutter frequencies. When comparing these results to published results of other codes, the located boundary is inside the range of the published data spread with an error of less than 15%. This is considered good from an engineering point of view and validates the ability of the CAe capability to accurately predict flutter.

Case Study 3: Static and Dynamic Computations on the HIRENASD Wing For this validation study, static and dynamic aeroelastic computations were performed on the High Reynolds Number Aerostructural Dynamics (HIRENASD) Wing. The wing was originally tested in the European Transonic Wind Tunnel [3] and offers both static and dynamic measurements at transonic conditions with realistic flight Reynolds Numbers. This work was part of the first Aeroelastic Prediction Workshop (April 2012).

To ensure CFD solution accuracy, a grid density study was performed on the rigid wing at transonic conditions. As the mesh was refined, lift and drag were compared with previously published rigid body computational data and a drag converged mesh was identified for this validation study [4].

To validate the FEA model, an eigenfrequency extraction analysis was performed in Abaqus resulting in good comparisons with experimental data [4].

The wing aeroelastic equilibrium shape and pressure coefficients (Cp) were computed for various angles of attack (α) using the two-way coupled STAR-CCM+/Abaqus FEA co-simulation. Figure 6 depicts chord-wise Cp distributions on an outboard section of the wing (M=0.8, α=2⁰), dramatically showing the effect the deformation of the structure has on the loads and indicating a good comparison with experiments. Similar results were obtained when looking at the span-wise displacements and lift/drag distributions vs. α. [5]

Figure 3: Surface mesh on Wing 445.6

Figure 4: Flutter boundary (Wing 445.6)

Figure 5: HIRENASD Wing surface mesh

Figure 6: Static aeroelastic solution

CD-adapco is the world’s largest independent CFD-focused provider of engineering simulation software, support and services. With over 30 years of experience in delivering industrial strength engineering simulations, the scope of CD-adapco’s activities extends well beyond software development to encompass a wide range of CAE engineering services in both CFD and FEA. CD-adapco’s flagship software, STAR-CCM+, provides a comprehensive engineering physics simulation inside a single integrated package and is unrivalled in its ability to tackle problems involving multi-physics and complex geometries. CD-adapco’s ongoing mission is to “Inspire innovation and reduce costs through the application of engineering simulation software and services.”

A privately owned company, CD-adapco has maintained 17% organic year-on-year growth over the last 5 years and employs 700 talented individuals, working at 30 different offices across the globe.

For more information: visit www.cd-adapco.com or email us at info@cd-adapco.com

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The accuracy of STAR-CCM+ was further validated using the one-way coupled prescribed motion technique. In this approach, the second eigenmode was extracted from Abaqus and used to prescribe a harmonically-varying grid motion about the wing aeroelastic equilibrium. The geometry was moved using mesh morphing in STAR-CCM+. Several periods of prescribed vibration were computed aimed at verifying STAR-CCM+ code accuracy compared to experiments and other simulation codes. Figures 7 and 8 depict the magnitude and phase of the Fourier transforms of Cp on the upper surface on an outboard station of the wing (M=0.8, α=-1.34⁰), showing good comparison to experimental values. Similar results were obtained on the lower surface and on span stations further inboard on the wing. Although not shown, these results also compare well to results of other CAe codes [6].

Free and forced wind-on vibrations using 2-way coupling with STAR-CCM+/Abaqus FEA co-simulation were also performed and results compared well to the published experimental results [7].

This investigation confirms that the STAR-CCM+/Abaqus FEA capability accurately predicts both static and dynamic aeroelastic effects at transonic conditions and realistic flight Reynolds Numbers.

Conclusion In today’s competitive climate, driven by climbing fuel costs and increasing demand for air travel, high-fidelity CAe is a key enabling technology for aerospace companies to develop innovative minimum structural weight designs while meeting the tight schedule and cost constraints of a typical production environment. It is imperative that the CAe capability is accurate, robust and efficient and that it easily fits into the current engineering design processes, producing high-quality results with minimum user efforts. With its unrivaled meshing technology, high-fidelity physics, intuitive user environment and direct link to Abaqus FEA for co-simulation, STAR-CCM+ seamlessly integrates CAe into the design process, driving innovation and resulting in engineering success.

References [1] Yates, E.C., “AGARD Standard Aeroelastic Configurations for Dynamic Response. Candidate Configuration I. – Wing 445.6”, NASA TM 100492, Aug. 1987 [2] Yates, E.C., Land, N.S., and Foughner, J.T., “Measured and Calculated Subsonic and Transonic Flutter Characteristics of a 45 Degree Sweptback Wing Platform

in Air and Freaon-12 in the Langley Transonic Dynamics Tunnel”, NASA TN D-1616, March 1963 [3] Ballmann,J. et al. “Aero-Structural Wind Tunnel Experiments with Elastic Wing Models at High Reynolds Numbers (HIRENASD-ASDMAD)”, AIAA -2011-0882,

January 20110 [4] Florance, J. Chwalowski, P. and Wieseman, C., “Aeroelasticity Benchmark Assessment”, Aeroelasticy Branch, NASA Langley Research Center, Subsonic Fixed

Wing Program, Interim Report, March 2010 [5] Heeg, J., Florance, J., Chwalowski,P., Perry, B. and Wieseman, C., “Information Package: Workshop on Aeroelastic Prediction”, Aeroelasticity Branch, NASA

Hampton, Virginia. October 2010 [6] Schuster,D., Chwalowski, P., Heeg,J., and Wieseman, C. “Summary of Data and Findings from the First Aeroelastic Prediction Workshop”, ICCFD7-3101 [7] Ballmann, J. et al. “Experimental Analysis of High Reynolds Number Aero-Structural Dynamics in ETW”, AIAA 2008-841. Presented at the 46th AIAA

Aerospace Sciences Meeting and Exhibit, Reno, NV, January 7-10, 2008

Figure 8: Phase from prescribed motion

Goal for Sustaining our Future: To develop and operate an aviation system that reduces aviation’s environmental and energy impacts to a level that does not constrain growth and is a model for sustainability

--FAA Strategic Plan - Destination 2025

Figure 7: Magnitude from prescribed motion