Project 21 – The Finale!

Well… what a summer! If you’d have told me a year ago that I’d be using a supercomputer to run simulations on submarines, I’d have thought you were crazy – and then probably asked where I can sign up! It’s been a fantastic journey the whole way through, now all that’s left is to share my final results with you.

Since my last post we’ve had one or two issues to overcome. Our initial goal of comparing our results to experimental ones fell through, as we only had access to graphical data and not exact numerical values. This meant that we needed to improvise.

In the end, we settled on comparing the effects of different turbulence models based on their drag coefficient. We tested several models, including k-epsilonk-omegaSSTBSL, and SSG. The drag coefficient, as you may be able to guess, gives us an idea of how streamlined an object is (with a lower value implying a more streamlined object). Along with this, compared how the drag coefficient changed with drift angle, and for different configurations of the submarine.

A visual example of what a drift angle is (only for an aeroplane!)
http://www.aviation-history.com/theory/wind_drift.htm

We also took a look at the distribution of the turbulent kinetic energy around the submarine when we considered some other turbulence models. In short, we ran a lot of simulations!

The first 5 seconds of one of the simulations from three separate angles!

Depending on who you are, you’ll either find the next sets of results much more interesting or infinitely more boring. As much as the above video does look cool, the following graphs show us exactly what is happening with the drag coefficient! If you’re in the latter camp, I’m hoping I might be able to sway you slightly.

We were very pleased to see the behaviour that we expected for the different configurations of the submarine, as well as the change with drift angle. As expected, the submarine configuration with all parts attached (configuration 2) had the highest drag coefficient by far for all drift angles, the configuration just containing the hull (configuration 3) had the lowest drag coefficient for all drift angles, and the rest were somewhere inbetween. This did end up showing us something interesting, where for lower drift angles (less than ~6 degrees), configuration 6 has a higher drag coefficient than configuration 5. However, this relation swaps around at angles higher than this.

Somewhat unsurprisingly, we found that the only turbulence model to significantly differ from the rest was the k-epsilon model. Although only two lines are visible in the above graph, the red, yellow, and green lines are all underneath the orange line. In fact, the results used to plot the overlapping lines were all identical until 8 degrees, and even here only SST began to differ very slowly. We believe that the reason the k-epsilon model gave such different results is the way it handles no-slip walls (walls where fluid against it will feel frictional force). In our model, the entire submarine is modelled as a no-slip wall. Thus, since k-epsilon is not accurate for no-slip walls, it gave a vastly different result for every angle. Although, it can still be seen that the shape of the curve as the drift angle increases is very similar for all models.

So there it is! My online internship with PRACE has come to an end. I have loved learning all about CFD and HPC, alongside meeting a bunch of great new people! I sincerely hope you’ve enjoyed reading my posts along the way, and maybe sparked an interest in CFD. Any questions, let me know in the comments below. Thanks for reading!

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6 comments on “Project 21 – The Finale!
  1. Ben says:

    Great work Matt!

  2. James Asker says:

    Top work Matt, kept you out of trouble over the summer

  3. Jack McGinn says:

    Interesting!

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