Water source polygon ID

[mathman]

I'm trying to have a spray of water come out of a U-shape pipe with nozzles all along the perimeter. I selected all the faces pointing the same direction as the nozzles from my emitter geometry and assigned them a unique polygon ID to use Surface Force as the emit mode. Only the uppermost polygons with this ID are emitting water particles. I want the selected faces on the sides to emit spray also. Any help would be greatly appreciated. I've tried doing a geometry liquid volume brush emitter and pushing the particles in the right direction with a plain force but it gets really chaotic and doesn't look good. I tried three emitter geometries in the same liquid source but the number of cells maxed out and it crashed.

[Svetlin.Nikolov]

Hey,

Could it be that the faces that must emit have a size similar or smaller to 1-2 voxels? If you move the grid a slight bit up/down/left/right, or increase the grid resolution a bit, and it start to work, then this was the problem.

Cheers!

[georgi.zhekov]

Hey,

Could it be that you started the sim using one of the presets? If that's the case - make sure you're not using a Discharge modifier for the source, just under the Outgoing Velocity there are Modifiers options in the three dots button, press it and choose Unlink.

[mathman]

Thank you both for your quick replies. The emitter model is 5m x 7m x 0.5m and and grid cell size is 0.069m so it wasn't an issue of emit poly's being close to cell size. I was using the waterfall preset. When I unlinked the modifier it worked properly. I would like to understand what was going on, what does the discharge modifier for waterfall do? Thanks again for helping me fix this.

[Svetlin.Nikolov]

Ah yes, so the waterfall preset has a discharge modifier that restricts the liquid to emit only from polygons whose normals face down. In this case, if polygons are pointing to the side, it would be a matter of numerical precision if they would emit or not... Here is more info on the discharge modifiers: https://docs.chaosgroup.com/display/...arge+Modifiers

[mathman]

So, the waterfall preset has a modifier something like: discharge modifier, modify discharge, Y Normal, graph: -1 to zero value=1 and zero to 1 value=0?

<edit> well, I went into the scene and examined the disconnected PHXParamMod's, it's interesting that there are six of them that all look identical: normal Z, graph -1, value=1 0, value=0.

[Gosho.Genchev]

Hey there,

can't say with certainty why you've got multiple PHXParamMods in your scene - I just tested the Waterfall preset to make sure there isn't an issue with the preset itself - works as expected on my end and only generates a single modifier. Could it be that you've deleted the Simulator and Liquid Source multiple times and used the preset multiple times as well ? That's the only possibility that comes to mind. If you find out something else is causing this, please tell us so we can fix it !

Anyway, regarding the modifiers - in the waterfall preset the Normal Z direction is used to specify a multiplier for the Outgoing Velocity. The normals of any piece of geometry are vectors that point perpendicularly to the faces. The fastest way to understand all this would be to just mess around with the options to see how they work. Try creating a box and using the Waterfall preset. By default, the Modifier is set up such that only the bottom faces of the box (the ones pointing towards the -Z direction) will emit liquid.
Try rotating the box and see what happens - because the Modifier is set up to look at the normals in World space, all faces that point towards the -Z direction (towards the ground, basically) even a little will start emitting.
Then change the Space option on the Modifier to Object and start the simulation again. This time, only the face which used to point towards the ground before you rotated the box will emit. To see why, change the Select and Move tool's space to Local (it defaults to View) and check the orientation of the manipulator.

I've attached a picture showing how you can preview the normals on a piece of geometry. Unfortunately 3ds Max doesn't provide a numerical representation of the normal vectors (as far as I know) but looking at the direction those lines are pointing can give you a basic idea of how this works.

Hope that helps!