Remotely Operated Vehicle | Drag

[jprobst]

Hi @Maciek

@Maciek

I did and… instead of expected 300-400 MB I got nearly 25GB

this is indeed a lot… If you did all the simulations (you wrote 40 different ones) in one project it might have grown up to this size. If not and you don’t mind sharing your project, I can check your setup to see where all that data went. Out of curiousity: how many cells do you have? (you find the info when you click on the Mesh Operation and scroll all the way down in the middle panel to “Mesh Operation Event Log”).

I’d like to join the discussion about the consistent underestimation of drag you’re seeing. First of all: I would also call the accuracy you achieved (-0.2% and -4.7%) “very good”. Some people might think this is a large tolerance (e.g. in aeronautic applications, F1, or regarding the precision needed for a steam turbine in a power plant) but as you descibed, the kind of application and the maximum accuracy in experiments count as well in my opinion. Especially for an ROV, you will probably have changes in water temperature, mud, algae, currents and so forth, which have big impact on its operation, too.

You’re mentioning 2 potential sources:

1. solver tolerances and convergence criteria
2. mesh

Let’s start with 1, solver tolerances and convergence criteria.

@Maciek

Moreover, it sounds a bit strange to me: why is it being solved no longer? I thought the convergence criteria are to trigger solver stop and a case is being solved as long as all the criteria have not been met. Does it work with turbulence only? If we look at the velocity it goes down even if all three reached e-5 threshold.

Convergence criteria (called “residual control” on SimScale), depicted by @dheiny in his post Remotely Operated Vehicle | Drag - #2 by dheiny tell the solver when to stop completely. They are normally used with steady-state solvers and terminate the entire computation if all residuals have fallen below the threshold and the solution is assumed converged.

There is another criterion, applied at each time step (or pseudo time step in steady-state simulations). These are the equation system solver tolerances which apply to each equation. If the initial residual for an equation (e.g. omega) is already below the threshold (in this case 1e-5) then this solver will not do any iterations. The other solvers (e.g. p) will still iterate until the final residual reaches the absolute tolerance or if the quotient between final and initial residual has reached the relative tolerance.

@gholami what do you think?

Earlier, you stated:

@Maciek

3. Solver seems to be really solid (!). I would even say that default convergence level set at e-5 is rather strict and e-4 is more than enough. I tested it on simpler cases and I saw no (real) differences.

This indicates that the levels of residuals are fine…

This brings us to point 2, the mesh. Since you say that the drag is consistently predicted too low, I think that the problem could come from the boundary layers. If the boundary layers are meshed too coarse, the velocity shear gradient (and hence the wall shear stress) cannot be predicted correctly. You can use the y+ result control item to check if the mesh is adequately resolved near the wall. Y+ on the ROV’s outer surface should be between 30 and 100 if wall functions are used. If no wall functions are used, you should have a y+ < 1.

But your results are impressive anyways so I think we shouldn’t really speak of an “error” in this case. It looks like you’re not doing anything wrong.