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# Fluid Structure Interaction in CAE

Tuesday January 10, 2023 at 11:00am

The relationship between fluid flow and mechanical performance of a system can be complex. The shape of the structure affects the flow pattern around it, while the pressures from the flow can deform the structure thereby affecting the flow.

Think of an aircraft wing. If we model the fluid flow around the wing using the geometry straight from CAD we are not getting an accurate picture – the wing is a very flexible structure and undergoes a lot of bending.

Aircraft wings bending during takeoff

The flow around this will not be the same as the nominal condition and neither will the loads into the structure from that flow. The degree of flex will vary with altitude and speed of the aircraft too which vary considerably through the operating envelope of the aircraft.

So as a structural analyst, what can we do to address this and get accurate loads to design our structure? MSC Software has a couple of solutions for this problem at different phases of the design through the use of fluid-structure interaction techniques. The process is an iterative one. We start by running a fluid step where we simulate the flow around the structure. This generates forces which are passed to the structural FEA, deforming the mesh. These deformations are passed back to the fluid solution which is re-run to get updated forces to pass back to the structural solution and so on until an acceptable convergence is reached.

In the early stages of the design, we need to understand the range of loads that the aircraft structure will see from potentially hundreds of manoeuvre. Performing this on a very high-fidelity model with full resolution of the flow for hundreds of cases is impractical if not impossible.

For this scenario, MSC Nastran makes use of a built-in panel code method. A panel code, or aerodynamic potential flow code, uses a simple 2D representation of the aerodynamic surfaces to predict the flow rate and hence pressure distribution. Coupled with a lower fidelity, stiffness only, FEA representation of the aircraft allows convergence to a steady state result in a few minutes for each manoeuvre.

The MSC Patran Flightloads interface gives us easy to use tools to set up and verify the coupling between the aero and structural representation of the lift surfaces as well as setting up the analysis in terms of speed, angle of attack etc. for each load case.

The ascii nature of the Nastran input format allows for relatively easy automation of the set up of a large number of configurations which can then be batch solved.

Further into the design progresses engineers will come up against flow generated issues that can’t be represented using the method described above. Examples might be simulating the deployment of flaps or landing gear into the flow, opening a stores bay in flight or even ejecting the pilot in their seat. This is where co-simulation between mechanical simulation and CFD offers a solution. The methodology is essentially the same, pressures and forces passed from the fluid model to the structural and deformations passed back to the CFD simulation to update the flow. This is less likely to be a transient simulation rather than a steady state, so instead of iterating to convergence, a time-stepping method is used.

MSC Software has a very capable pair of CFD solvers from their Cradle division, scFLOW and scSTREAM. The Cradle team have developed an application, called MSC Cosim, that acts as a ‘brokering’ service between two or more applications to enable this type of Fluid-Structure co-simulation as well as other mechanical coupling methods.

Other 3rd party tools have been around for many years to do this, but they tend to require some very high end skills in model set up and the coding of user subroutines. MSC Cosim works with the native model definition capabilities to allow you to quickly and easily pair entities between the different solvers making this technique accessible to most simulation engineers.

These techniques may have been developed in the aerospace industry, but they have applications anywhere that fluid flow impacts or is part of the product design. Replacing guesswork, rules of thumb, and large factors of safety with a real understanding of the loading effects on a design by the complex action of fluids will lead to cost savings, either through reducing the conservativism in the design, there to account for the unknown, or by allowing you to account for unexpected effects long before they become an expensive warranty problem.

A ship propellor modelled with Marc non-linear FEA and Cradle

An example of the extreme deflection possible co-simulating between Marc and Cradle

All the tools discussed are available within the flexible MSC One token licensing scheme. This gives engineers access to a wide variety of simulation tools from simple FEA through to CFD, Motion simulation, Acoustics, Fatigue and many more.

If any of this is of interest, please get in touch for a chat with our experienced application engineers about how this technology could help you.