I am creating a simulation of objects popping out of a sand container using Vellum Grain.

A problem that often occurs when using grain is unexpected movement on the first frame.
(This simulation raises particles.)

I need to maintain a clean geometry this time and was wondering if there is a way to deal with this.

I tried

f@groupdensity = volumesample(1, "density", @P);
if(@groupdensity>0||@Frame<12){
i@stopped = 3;
}

groupdensity is The points of the around edge.
Even if I wrote a vex like this, it moves weirdly at 13 frames...

I think the problem is very similar to this link, but this one does not support vellum grain.
https://www.sidefx.com/docs/houdini/grains/stablepile.html [www.sidefx.com]

Is there already collision geometry inside of this pile of particles? In that case, you might just want to increase the resolution of the geometry and make sure the vellum particles do not intersect with it. If they do intersect, they will de-intersect themselves on the first frame and give this kind of pop. I cannot see exactly from the image you shared

Also here is a great video that explains a similar setup; https://www.youtube.com/watch?v=hSlHG_4lIyI&t=584s [www.youtube.com]