Tips & Tricks: Simulation Modelling for Quicker Solution Times
Here at Motovated we like to integrate analysis and FEA right into the design process. One of the easier problems you will face is that FEA solutions often take a long time to solve. Thankfully we have a heap of ways to speed up this process! Here are a couple of simple tips you can use right away to lower your solve times:
Using a coarse mesh means the solve time for your FEA will be much shorter. However you must keep in mind that the results seen from these runs will likely be un-conservative! Therefore don’t forget to give yourself some room to breathe. Once your design is looking like it will work, you can then create a more refined mesh to do a final check on your results.
Remember the rule of thumb: "The time taken for these matrix solutions is about proportional to the square of the number of degrees-of-freedom in the mesh". In other words, to solve a model with 10 times the mesh size would take around 100 times as long! (Check the mesh detail dialog to see the degrees of freedom in your mesh…)
Avoid Contact Conditions if Possible!
Bogie Axle Box Analysis, Courtesy of KiwiRail Ltd
Contact conditions greatly increase solve times. In the image opposite is a simple assembly with a bogie axle box connected to steel spring bases via bolts. To set this up perfectly, this analysis would use a no-penetration contact condition between the spring base and axle box, as well as virtual bolts to clamp them together. Comparing this setup to simply bonding the faces of the components together, the solve time was over 20 times longer! It made much more sense to use the simple bonded setup while optimising the axle box shape, after which we ran a final "realistic" contact study to confirm our results.
Make sure you think about how using different contact conditions will affect the study. In some cases simplifying the contact to a simple bonded face will give vastly different results! If you must use a more complex contact condition (such as no-penetration faces), using a large mesh size on the contacting faces shows a huge improvement in the overall solve time (at the possible expense of accuracy).
Beams and Shells
Lithotripsy Bus Subframe Analysis, Courtesy of Mobile Medical Technology Ltd
Although solid element models are the most common these days, don't forget to use beam or shell elements where possible. For example if you were looking to analyse a structure made of RHS, a beam analysis is going to be the fastest way to get results. This is because the number of elements needed to mesh beams and shells is significantly less than that used in solids, and with the right setup will still give comparably accurate stress results.
It is worth noting that beam elements only provide 'global' stresses and strains, and cannot take into account any stress raisers or joint detailing etc. If this information is required the area of interest could be sub modelled using solid or shell elements.
Opposite is an example of a chassis which was perfect for beam analysis.
Bogie Frame Analysis, Courtesy of KiwiRail Ltd
As a trained FEA analyst this tip is drilled into your head from day one. If you are using a symmetrical model and load case, you can effectively ignore the unnecessary information to speed up your modelling, meshing and solve times.
By only applying FEA to 1/4 of the model opposite, we were able to save a lot of time due to the mesh having significantly less elements. Make sure that the loading, geometry and boundary conditions are all symmetrical, and adjust the load magnitude accordingly if you use this approach.
Hopefully some of these tips will help you out with your analysis solve times straight away. If all else fails, you can try a faster computer. But letting Motovated help with those challenging tasks might get you accurate results much sooner!
Note: it is important to note that while these techniques improve analysis solve times some of them may reduce the accuracy of the solution. It is important to understand the limitations of the analysis and as always to verify your results.