Overcoming the challenges of transonic aeroelasticity

Designing and building vehicles for flight is not without significant challenges, and while defying the force of gravity might be an ever-present requirement, it is probably not the most complex problem to overcome. Airflow across an aircraft is dynamic and can be highly non-linear, and the aircraft itself, while ideally rigid, will have a degree of flexibility which can lead to issues such as structural oscillation, flutter, fatigue, or potentially catastrophic failure!

The most famous and dramatic illustration of flutter is the Tacoma Narrows Bridge collapse in 1940 (see below). By imagining the bridge’s road surface as an airplane control surface, it is easy to appreciate the potential severity of this phenomenon.

via GIPHY

Aeroelasticity also causes static phenomena such as deformation of the control surface, meaning the effective angle is decreased. Elasticity is dependent on the stiffness of the aircraft structure and also its shape. Before the advent of computer simulation, aeroelasticity could only be tested in wind tunnels or in flight, which requires the structure to be built first. Simulation allows engineers to predict deformation and flutter before an aircraft is built, enabling a structure to be fully optimised for aeroelasticity at the design stage. The challenge for design engineers is to ensure that flutter occurs only at speeds well above normal operating speeds.

What has proved more difficult, until recently, is the ability of simulation software to estimate aeroelasticity at speeds within the transonic region, where airflow becomes highly non-linear and subject to shockwaves. At these speeds, the flutter boundary becomes lower, matching flight speeds, a phenomenon termed the ‘Transonic Dip’. However, traditional simulation methods, that have so far served engineers well, cannot accurately predict it.

Optimised transonic aeroelasticity solutions

Hexagon has developed solutions for virtually simulating these conditions to enable shorter turnaround and better accuracy for better aircraft with optimised designs. MSC Nastran, a multidisciplinary structural analysis application, simulation sequence for aerodynamical flutter (SOL 145) has been widely used throughout the industry for many years for flutter analysis and it is a trusted tool for aerospace design engineers. It is the go-to industry option to ensure aircraft structures have the necessary strength and stiffness to avoid failure. However, additional capabilities are needed for accurate estimations of aeroelastic behaviour at transonic speeds because MSC Nastran SOL 145 is based on a linear structure and linear aerodynamic model. Now Hexagon has a solution that enables precise simulation of non-linear fluid dynamics, such as shockwave movement and separation of flow.

scFLOW is a next-generation tool for CFD analysis that delivers high-quality, accurate representations through the use of an unstructured polyhedral mesh to accurately simulate non-linear fluid dynamics in extremely short runtimes. This software can utilise a density-based solver to analyse high speed flow and expansion/contraction of volume, and this method maintains accuracy even at a high Mach number, reaching through the supersonic and high up into the hypersonic range.

With the option to combine scFLOW and MSC Nastran, and others, through Hexagon’s dedicated MSC CoSim engine, users are able to access the benefits of true co-simulation technology for more accurate and more insightful results. Coupling through co-simulation systems removes CAE silos from the design stage and brings together separate disciplines within a single hub to reveal how design modifications impact all functional areas. Estimating aeroelasticity, with fluid structure interaction (FSI), showcases the high value of co-simulation technology, delivering the ability to depict how gusts can cause sudden changes to dynamic force, for example, but it is not always easy to do co-simulation well.

Hexagon’s MSC CoSim makes the process intuitive and reliable, implementable on any computing architecture for fast, scalable simulation, putting the data to work via direct coupling of different solvers within a multiphysics framework. Through this interface, powerful options such as scFLOW, Marc, MSC Nastran, and Adams can be connected for a holistic CAE experience accessing capabilities for multibody dynamics, computational fluid dynamics (CFD), and structural analysis. For example, MSC CoSim enables precise and unique aeroelastic analysis that takes into consideration heat transfer between CFD and FEA codes.

Pushing the boundaries of CAE

Hexagon’s offering for transonic aeroelasticity prediction represents a game-changer for the industry, enabling faster, easier aircraft design within the transonic range. With the option of utilising state-of-the-art software such as scFLOW within a true co-simulation environment, users now have a framework that reflects the close relationship between design, aerodynamics, structural dynamics and flight control.

Want to find out more? Catch up with our hypersonic-focused webinars on-demand, starting with: Multiphysics-focused CFD solutions for hypersonic vehicles. It’s only available on-demand for a limited time, so don’t miss it!

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