Set Your Fluids Free

Wednesday October 4, 2017 at 7:54am

A look at the new Free Surface feature coming in SOLIDWORKS Flow Simulation 2018.

2018 is almost here and I have been eagerly exploring the big new feature for SOLIDWORKS Flow Simulation. That is Free Surfaces. We can now have two immiscible fluids. We can have a liquid and a gas in the same fluid domain. However, any phase transitions (including Humidity, Condensation, and Cavitation), Rotation, surface tension and boundary layer on an interface between immiscible fluids are not allowed in this version. So what does this mean? Well we used to be able to examine flow over our private jets and personal submarines, but now we can look at our yachts as well! So that’s exactly what I did. First thing was to acquire a luxury yacht.

A quick look through GrabCAD and I found a tasty looking number. I did need to sort out some surfacing issues to ensure that the boat would not fill with water, but with that done it was time to set up the test.

I thought that speed should in knots, so we can easily set that up. Next we need to set the free surface tag. Doing this does mean that we have to make the analysis time dependent. Gravity is optional, but was a must for this study. Fluids of Air and Water were selected. The rest was fairly standard until the final page of the wizard.

In the Initial and Ambient Conditions we need to set up a dependency for the main flow and the initial fluid. The Initial Fluid can be set as a function of height in the simulation to give an initial surface level. Next the velocity was set up. Again this was set as varying with time as follows:

Through a number of tests I determined that there was a limit to how fast I could get the boat to accelerate. Starting at high speed was out of the question for this as the surface is initially flat, which would not be the case in a real system. With that I set the global mesh to level 5, I was looking for reasonable results, but did not want refinement. I was also saving every two seconds and had turned on the transient explorer too. I then hit run. After only three hours and twenty two minutes I had my results. I always start with a cut plot as a sanity check to look at velocity here and things were looking promising. A great thing about the cut plot is that you can also use this in the transient explorer to see ultra-detailed results from the complete study.

We can even get a great animation from that too

But what was the water levels? For this we need to use the isosurface plot. This allows us to show the point where the fluid volume fraction was at a particular value. Here we see the result of that.

The shape of the surface is the result. For a colour I set that to look at the height of the surface. The zero point was at the base of the keel. We can also change the colour bar to get a more pleasing output too. I can almost feel the spray! We can also get a great animation from this. We cannot use the transient explorer for this one so we have to ensure that we set our save increment high enough. If we have then the results have me steaming off to the yacht club for a G&T with the Commadore.

You can even get a great render from this. Just export the plot as a VRML file and use ScanTo3D to make the surface. Then extrude a block up to that surface to create the body of water. Import into visualize and set the materials and off you go. The render at the start was created using this method. This looks like a fantastic addition to SOLIDWORKS Flow simulation and I can’t wait to see what we can do with it. Tank filling, wave like behaviour, overflow. Could I get a vortex from a plug hole? Note the Coriolis Effect is not the only thing that causes this. Geometry is a far more likely cause. Maybe I need to have a visit to the bathroom store. SOLIDWORKS 2018 is almost here, so start planning how Free Surfaces can help unlock more solutions to your design questions.
Gordon Stewart - Elite Applications Engineer

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