Foam disappears when water level drops

[shawkins]

I'm working on a sewer pump animation (attached). The idea is that the well water is at a certain level at the beginning, then drops as the pump is turned on to lower the water level. I've created a cascading simulation that seems to work pretty well, however, when the water level drops the foam disappears for a while then comes back when the water level stops dropping.

I drop the water level by using an initial fill in the simulation, then animate that simulation grid dropping. Any ideas why the foam would disappear then reappear? Also, the grid seems to show periodically as the water is dropping. Is this a product of too low of a resolution of the grid or spf or ???

Any help is greatly appreciated.

[Ivaylo Katev]

i think there are two independent reasons - first try to decrease the falling limit of the foam. when the level drops the foam is for a moment without support and starts to fall down. however, because the simulation uses some time step, the foam appears under the water. the second reason is the rendering. it seems like the underwater foam is totally invisible. the underwater foam is a bit tricky. first it is shadowed by the water, disable the cast shadows option of the water, or just render the foam in separate pass and add it in post. beside the liquid shadow, the fog color also may be a problem. ensure that the liquid is set in the foam shader liquid slot, this will make the needed correction of the fog color. in few words - make sure that the underwater foam is also visible.

[shawkins]

Thank you. I'll give that a try. Any thoughts on how I could remove the appearance of the grid lines showing in the water and foam?

[Ivaylo Katev]

this is the same issue, when you solve the foam disappearing, the grid lines will gone too

[shawkins]

Your method does look much better, but I seem to lose the above water foam completely until the water settles down once it's done dropping. I reduced my foam falling speed from 2'-0" down to 6". Is there anything I can do to still have the above water foam still visible during the dropping period? Should I maybe have the water drop at a much slower pace?

[Ivaylo Katev]

hm, it should be the opposite, the foam water must remain visible always. can you tell me the sizes of the simulator grid (the physical size, not the cells)

[shawkins]

Sure. The size is 140 in. X 140 in. X 68 in. Weird thing I just noticed. As I was simulating the file yesterday I periodically rendered frames to make sure the foam was there. It all seemed good. Today, we lost power so I had to load an autoback max file. I made sure the input file was pointing to the latest .aur files. When I rendered, my foam was gone. Strange.

Also, the foam was extremely chaotic until frame 136, but then went normal from 137 on. Files attached.

Thank you for your help.

[Ivaylo Katev]

can't say why the foam is gone.
however i just made a fast test, foam over water with decreasing level, there is no problem at all, even if the falling speed is big.