Clump Behavior: Velocity/Momentum Smoothing

After comparing the last set of simulations with moving vs. stationary clumps, I decided to investigate the role of velocity/momentum smoothing at the clump boundary. I found a small possible error in the code. Momentum gets smoothed by two factors of f where f is a tanh function. This happens because both density and velocity are smoothed by a factor of f. I think that it might be better to only smooth density, since smoothing velocity would make the code more dependent on reference frame. I took out the extra velocity smoothing, but I am not sure what affect this will have if any.

I looked at the initial frames for a few cases in velocity and momentum. There is the original smoothing, the corrected smoothing, and no smoothing. Below are the images for momentum.

Stationary (orig) Moving (orig)
Moving (correct) Moving (no smoothing)


Below are the movies showing density.

Stationary (orig) Moving (orig)
movie​ movie​
Moving (correct) Moving (no smoothing)
movie​ movie​


Some strange stuff happens without any momentum smoothing. This is most likely due to the fact that density is still smoothed, so there is a mismatch between density and momentum.

The corrected smoothing case looks a bit more like the stationary case which is good. However, there is velocity smoothing even in the original stationary model. So I should really do a new stationary run without the extra factor in velocity smoothing, and compare that to the moving clump also without that extra smoothing factor. We want these two simulations to look the same since the code should be Galilean invariant. Below are the final results with corrected smoothing:

Stationary (correct) Moving (correct)
movie​ movie​


So taking out the extra velocity smoothing didn't affect the stationary model as much as it affected the moving model. However, I would still argue that the correction is an improvement. I don't think we should expect the stationary and moving models to be exactly the same, since you will get a different amount of numerical diffusion when you have things moving on the grid differently. Stay tuned for 3D runs.

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