Friday May 5, 2023 at 8:00am
Our recent article about the use of symmetry conditions to reduce model size has a natural companion in another old technique for avoiding huge models – zoom modelling.
This technique uses two (or more) models to achieve a detailed stress field for a small feature or features in a large model where if a detailed mesh were used everywhere it would require immense RAM, maybe more than your computer has, and take a spectacular length of time to run.
So how is this done? We firstly create a system level model using appropriate mesh sizes to achieve a good deflection result. We then create a detailed mesh of a localised area and use the displacement field from the system level model to enforce displacement boundary conditions. For example this notched beam.
A reasonable deflection result can be achieved with a pretty coarse mesh, but to get the stress in the notch well resolved would take a very fine local mesh. This would cascade to a relatively fine mesh everywhere to avoid having a too abrupt transition between adjacent elements.
With the deflection result for the global model we can use the FEM GUI (in this case MSC Patran) to capture the vector field of deflection:
And then use it as an enforced displacement boundary condition for a detailed local model of the notch area by mapping it to the cut faces of the section we are meshing:
This can then be run to recover detailed stresses:
This is a simple enough technique that can be applied to multiple areas of a model if your GUI supports doing it and your results are linear static or similarly single step. The mapping of the displacement field to the boundary nodes is interpolating in space, but what if your results vary also in time?
You now need to interpolate in time as well as space to define the instantaneous displacement of each of the boundary nodes at each step of the analysis and output these as time varying fields for each node. This is a very cumbersome process using the field mapping technique, but MSC Marc can do this for you with ease.
In Marc we call this Zoom Modelling technique a “Global-Local” analysis. We run the global model and then create the detailed local models. When we define the boundary condition on the Local models we simply point Marc at the result file from the Global analysis and define the nodes to apply it to and the solver does the rest.
In this example we have a housing that is bolted to another component at a number of locations and is subject to a large external load. We can build a global shell model using beams to represent the bolts and contact between the two components.
In the subsequent Local models we can include detailed models of the bolts with pre-tensioning effects and Marc is able to map the 2D displacement results for the global shell model to local 3D meshes for some of the bolts in order to understand what’s happening in detail.
Global Shell Model
Local models of two bolts
This technique can save you time in solution and reduce the total capacity needed in hardware for the occasional large model. MSC Marc is available within the MSC One token licensing scheme. This provides access to some of the best in class simulation tools on the market as used by the biggest OEMs for simulating their products, but starting at a surprisingly affordable annual price and including access to e-learning and full hotline technical support from veteran analysts.
If you’d like to learn more, please get in touch to find out how we can help you get the most out of your company’s simulation.