Chaining Thermal and Structural Analysis with MSC Nastran & MSC Patran

WHICH ROUTE IS BEST?

The older route involves mapping the temperature results from one analysis to another as a temperature load.

The other is chained analysis where we run a two-step simulation in MSC Nastran that solves the thermal problem first and uses the temperatures predicted as a load in a second step.

It's worth us looking at the older method, because the mapping capability in MSC Patran can be extended to map results from one type of simulation to another, even accounting for a dissimilar mesh.

This might not be a simple thermal analysis. You could, for example, have used the superplastic forming capability in MSC Marc to predict the local thickness of a formed part which you then wish to map to different mesh for running a vibration analysis in MSC Nastran.

STEP 1 – THE THERMAL PROBLEM

We set up and run a thermal steady state analysis. Our model comprises a copper cylinder supported by two steel plates initially at room temperature of 296 K.

We have a convection coefficient to ambient on all surfaces and a fixed temperature of 400 K at one end. We’ll run a steady state analysis to understand what the temperature distribution will be.

We now want to assess the stress in this model due to the thermal expansion if both end plates are fixed at the bottom.

STEP 2 – THE STATICS PROBLEM

We need to map this temperature result to a field so we can apply it as a load.

In MSC Patran we use a ‘Continuous Scalar Field’ for this. This basically constructs a field that relates the XYZ co-ordinate of each node to a scalar value, in this case Temperature.

We then create a temperature load, but instead of a scalar value for the temperature change, we can select the field.

When it comes to write out the model for the solution, Patran will look at each node in turn, find its location in the field, and interpolate a temperature value for the load. This means that we could have used a completely different mesh for the thermal analysis providing it was to the same scale and within the same co-ordinate frame.

We now have a mapped thermal load and can run a static analysis to look at the stress resulting from the constrained expansion of the model.

We can see the displacement magnitude is about 0.8 mm.

With the corresponding stress result:

STEP 3 – EFFICIENCY EVALUATION

Could we do this more efficiently? Well, if a Nastran linear steady state simulation satisfies all your thermal needs, then yes, we can.

The HEATSTAT parameter in Nastran will allow you to run a two subcase SOL101 analysis that solves the thermal problem first and then the static afterwards.

This is set up quite simply in Patran using the Structural Heat Analysis option in the thermal preference by selecting the two cases in sequence.

This creates the necessary job. If you are using a more ‘traditional’ text pre-processor then the Case Control will look more like the text below...

The benefits of this technique are twofold:

1. We have eliminated a manual step where we map one result to another. For a huge model this is otherwise likely time-consuming, even though it’s automated in Patran.
2. We have our thermal and structural results in the same input and output files, eliminating potential errors where the wrong thermal results are used with the wrong structural model.

There is a limitation to be aware of. As this method forces a linear solution of the steady state thermal analysis, no temperature dependency of materials or boundary conditions can be used, as they rely on an iterative solution to update.

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