drag force makes my simulation explode

[arjanM]

Hi everyone,

 

I am trying to make a simulation of a building that get destructed.

But when I add a drag force my simulation the whole building will explode in 1 frame.

I doesn't matter what I do, as soon as I add a drag force, things explode.

Even when I make a simple voronoi fractured box with a drag force it also explodes.

I am still a beginner when it comes to rigid body simulations and I think I am missing some basic knowledge.

Does somebody know how to proper use the drag force?

 

Here is a simple example scene where you see the problem in the first few frames.

When you disable the drag it will behave normally.

dragProblem.hiplc

[Skybar]

Strange. No idea why it does that. But since you are using Packed Prims, you can use the Pop Drag DOP instead:

dragProblem_dv.hiplc

[Atom]

I think you are using it ok. If you enable Ignore Mass you get a little better results. What happens is a few pieces fly off really quick. These pieces must be in close contact to a neighboring fracture piece and producing a lot of collision strength. That is what is causing everything to explode.  Also consider changing the Global Seed on the scatter. You may luck into a pattern with very little initial piece interpenetration.

 

You may want to check out the MasterClass SideFx video on the Bullet Solver. They explain in detail how to solve some of these problems.

Edited March 10, 2016 by Atom

[arjanM]

Thanks for the quick reply.

This seems to work great in my building scene.

I also really wanted to have a drag on the rotation of the objects and that seems to be the problem.

When I add a pop drag spin to the multisolver it will show the same behaviour as with the drag force.

Or when I only use the drag force and I set the scale torque to zero it will work fine.

Is there a way to get rotational drag on the packed rigid bodies?

[JohnMarston]

You can tweak your rotational velocity directly by wiring in a geometry wrangle* to either the pre-solve or post-solve inputs on your rigid body solver and messing with the @w attribute (rotational velocity). For example,

 

v@w *= 0.9;

 

will set the rotational velocity to 90% of what it would normally be on each frame. This will have the same effect as applying a rotational drag to it.

Edited March 10, 2016 by JohnMarston

[arjanM]

I didn't even know about the geometry wrangle. Thanks that works very well.

[fathom]

yeah, i've found drag force to totally explode whenever i use it.  my sim is totally fine without it so there's no crazy collision velocities making things shoot out insanely fast.  but when i put in a drag, it blows up.  it has always done this, so i've always done ghetto drag (v=v*.9 like above).

 

i suspect it might be a scene scale thing, but i dunno.  i usually use cm.  might be the default values are scaled improperly for cm... not sure.

[Skybar]

It's obviously a bug of some sort. Hopefully you all report it so it gets SESIs attention.

[Nicholas Ralabate]

Just ran into this today, thanks much for the wrangle workaround.  I wonder what it is that causes Drag DOP to make sims explode?