Hi,
I was simulating water droplets hit the ground by using SPH atoms as droplet.
There are three droplets in the data file, and the ground surface is made by stationary SPH atoms.
There is no error reported as program run, but as the first drop touch the ground, other droplets stop and float in the space.
My program can be found in the following link. I appreciate any help on how to fix this problem.
Thank you.
https://drive.google.com/drive/folders/1m7NgoKWsTHKfKaAAVsflTR0E3ayHCZWh?usp=sharing
Please keep in mind that you are using a variable timestep and your energy is going way up when the first drop hits the boundary. I suggest you keep monitoring the thermo keywords “dt” and “time” in your thermo_style custom output. If the timestep drops to near zero it will look as if everything is coming to a halt except for very fast moving particles.
Update:
I ran your input with the thermo style modification and here is (part of) the output:
Step KinEng Dt Time
0 0 0.0003 0
100 537.73364 0.0003 0.03
200 2150.9346 0.0003 0.06
300 4839.6028 0.0003 0.09
400 8603.7383 0.0003 0.12
500 13443.341 0.0003 0.15
600 19297.402 0.00028338143 0.17971613
700 25387.937 0.00024710939 0.20613482
800 31478.474 0.00022194558 0.22953266
900 37569.011 0.00020317604 0.25075669
1000 43659.549 0.00018848305 0.27031944
1100 49750.087 0.00017657675 0.28855896
1200 55840.625 0.00016667475 0.30571221
1300 61931.163 0.00015827123 0.32195284
1400 68021.701 0.00015102276 0.33741268
1500 74112.239 0.00014468688 0.35219454
1600 80202.777 0.00013908689 0.3663805
1700 86293.316 0.00013409054 0.3800373
[...]
9400 555264.78 5.2869034e-05 0.9640237
9500 561355.32 5.2581461e-05 0.96929633
9600 567445.85 5.229853e-05 0.97454044
9700 5.3448296e+12 6.106974e-10 0.97537693
9800 5.3557016e+12 7.0419205e-10 0.975377
9900 4.9407524e+12 7.0419205e-10 0.97537707
10000 5.2088395e+12 7.0419205e-10 0.97537714
10100 5.1417616e+12 7.0419205e-10 0.97537721
10200 5.2267144e+12 7.0419205e-10 0.97537728
10300 5.3336085e+12 7.0419205e-10 0.97537735
This confirms my hypothesis. Between step 9600 and 9700 your energy goes up, so atoms start to move very fast and then the length of the time step decreases by a factor of nearly 100000, i.e. to move as far as atoms were moving previously every step, now you need 100000 steps, which will make all “slow” particles look like they are suddenly frozen.
This ultimately means that your choice of parameters is not suitable for this specific system.
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hi @q207823,
Couple of comments on your question and input file:
- You are not providing the LAMMPS version you are using. If you are using an old version, maybe updating will help if any bug was corrected with new releases.
- Your input script contains command copy from the example of the pdf documentation. It is visible that it comes from the water_collapse example. However, your geometry is way different, not only because of the 3d system but also because your particles are very far away from one another (10 times more than the example.
- As such your water particles do not interact with one another when falling. The droplets do not form round shapes during the fall, and when they hit the ground some go through the walls. This is a bogus initial configuration given your parameters… Except this is what you want but I highly doubt it.
- As @akohlmey suggested, the timestep of SPH varies and depending on the energy it might be reduced significantly by the energy increase. Your droplets do not stop, the time integration is just really slow because some particles became very fast.
Please have a look at the initial configuration given in the directory examples/PACKAGES/sph/water_collapse and compare with your droplets configurations to see what I mean. I may also suggest a careful rereading of the pdf giving the implementation details of SPH.