SPH atom stop after touching walls

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.


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.

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.

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.