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# A Simple Approach to Modelling Fluid-Filled Structures with MSC Marc

Monday January 29, 2024 at 8:00am

Simulating the static loading of closed containers can lead to issues of approximating the effects on the structural behaviour by the fluid contained within.

With air, we’ve commonly seen it ignored with an assumption that the internal volume doesn’t change ‘much’ so the pressure doesn’t either.

But what if you have a liquid-filled container? Do you need to move to couple physics to model the internal fluid with a CFD solution?

It depends on what you’re doing, but if you’re not interested in the movement of the fluid and just the stiffness effect it has on the structure, then MSC Marc can help.

As an example, here are two axisymmetric simulations of a ball being compressed between two plates.

The first one shows the behaviour assuming there’s no internal fluid of any kind, and the second shows the change when we fill the ball with a liquid and constrain the total mass as fixed.

The Cavity feature has been in Marc for many years, introduced just for gases originally, around a demand from the tyre industry.

It allowed you to define a closed volume and the properties of the ideal gas within it - this could be a 3D volume, but it also worked for 2D solids like axisymmetric conditions.

The cavity could be constrained to have a fixed mass, the mass could change over time (to simulate a leak for example), or the cavity pressure could be controlled to simulate inflation/deflation. This was simply done using Boyle’s Law with Marc calculating the volume enclosed by the cavity at each load increment.

The ability to support liquids was introduced in 2014 which enhanced the functionality greatly.

Setting this up in the GUI, Mentat, is very simple. For a 3D entity you need to have a volume enclosed by the elements selected as the cavity.

In the example below the ‘sausage’ of thin rubber shell elements is selected. All the element normals need to be aligned to point inside or outside of course.

In this case we are going to squeeze the tube between the rigid roller and the ground plate, then move the roller axis along the major axis of the tube to see how the fluid internally affects the behaviour.

We do this with one simple load - a Cavity Mass Load - where we tell Marc that the mass of the cavity is fixed, and then run the simulation.

The plot below is showing contact status, a result to indicate where contact is taking place.

We get results output for the cavity as a whole rather than local face pressure results – this is not a transient looking at the flow of fluid but a static assuming the cavity pressure is uniform.

The example below shows an axisymmetric seal model with three cavities, two air-filled and one liquid-filled.

We can see the collapse of the two air-filled ones and the corresponding pressure rises, while the liquid filled (rightmost) appears to retain its volume.

The graph of Volume vs Time shows the true story: the liquid filled cavities are treated as ‘almost incompressible’.

The mass of the fluid is not included in the mass matrix for a dynamic solution, nor can you apply gravity or centrifugal loading to the fluid mass, nor can you model partially full containers, but it is useful nonetheless for many applications.

MSC Marc is available within the MSC.One token licensing system which comes in a Start Edition flavour that is very affordable for smaller customers.