Honey I shrunk the Prisms

By James Clement

One of the interesting things about internet connected devices, be it a car or your washing machine is that a key benefit is free software updates.  In automobiles you get user interface improvements, or even power and range upgrades simply by applying more efficiency to the software.  Simcenter STAR-CCM+ is no exception: we have added the prism layer shrinkage model which can reduce turnaround time for your overset applications by up to 5X.  Woah woah, alright too much detail and not enough context, let me tell you the story of overset in Simcenter STAR-CCM+.  It began in back 2007, in an office in Germany…

Our goal was to produce and easy to use and scalable solution to volumetrically overlap two or more meshes.  So if you imagine I have two meshes; one of a big cube and the second a sphere subtracted from a larger sphere (like a thick walled ball).  Now, in most simulations you have to have these meshes contact at a common face in order for them to communicate. Here is where the beauty of Overset comes in, you simply overlap the two volume meshes. Then the algorithm ‘cuts’ the foreground from the background.  So the end result is that you can simulate a bouncing ball. 

Since then Simcenter STAR-CCM+ has become the market leader in Overset simulation, due to  the ease of setup, the speed of the methods and its integration with  a wide range of motions and solvers.  Now that you have the context and core understanding, let’s look at some of the key applications that are enabled by Overset.  

(view in My Videos)

First and foremost we see a large amount of usage in the Marine Industry, where we need to understand the behavior of a craft as it interacts with the water.  Typical applications here are sea keeping and performance, life boat deployment, mooring and docking.  You name it, if a ship has to experience the condition, Overset enables it.  That’s the tip of the iceberg: we can do, pumps, mixing, stores separation, helicopters, painting, vehicle wading, (*Inhale) wipers, flap dynamics, gearbox performance, valves and floating wind turbines.  I am sure I am missing about 100 more applications because if it moves, Overset can help.

Hopefully now you have some context and an appreciation of the benefits.  So now I should get back to the new feature that can speed things up. In some of the applications the walls of two overset bodies get close to each other, pumps, valves, gears and the like.  These cases present a challenge to the overset method based on one of the requirements of the data passing.  This limit is that in any given gap you must maintain 4 cells across the gap, 2 on each side. The implication here is that if you want maintain .001 inches in a gap you have to over-mesh the model for the physics to take advantage of Overset.  Not only that but that for pumps and gears the full perimeter must be refined so you are talking about a mesh that is 5X the size of the mesh needed by the physics to maintain the proper gap through the rotation. 

(view in My Videos)

The prism layer shrinkage model changes this requirement, not by changing the number of cells need but by changing mesh dynamically as it sees two boundaries coming together.  This means you can build a grid suitable for the physics and as two surfaces approach each other the prism layers on one or both sides will shrink to maintain the gap. Ultimately, this will save you a ton of run time and get you to your simulation results faster.

Another key advantage here is the ease of setup: simply turn it on, tell it how many cells to shrink, and how big of a gap to maintain.  That’s it and you are ready to go coarsen your mesh and get more from your simulations. 

As you have seen overset prism layer shrinkage is a key advantage to bring your applications to life and give your simulations an update.  Whether you are simulating your electric vehicle fording  a stream or how your washer washes, STAR-CCM+ with its overset capability will enable you to be more efficient.  

This article first appeared on the Siemens Digital Industries Software blog at