Flat Fliptank Wave Issue (Moving Narrowband)

[m-egger]

Hello!

So at the moment I'm doing a sim for penguins jumping in a pool, and since the camera pans along with them, I decided to roll with a moving narrow-band container, to reduce the cost of the sim.

In an earlier shot the narrowband worked nicely for jumping in, however, in this shot, I'm having problems with the container producing a sort of wave. It's going in -Z (To right of image).

So for testing purposes, the simbox is super tiny, but no amount of extending the narrowband towards the direction of movement had any impact.

Does anyone know what's causing this? I'm quite stuck at the moment.

 

Greetings from 127 Wardour Street London,
Martin

Edited September 6, 2019 by tortoise

[m-egger]

Alright, after spending almost two workdays on trying to fix this I have decided to ask SideFX for support - but I still would like to post my latest findings.

Here is a file where I tried to recreate it in the simplest way possible: A sphere (in boat size/shape) travelling in +Z with the vanilla "flat tank" shelf tool applied to it.
This image clearly shows the issue:

Here is the scene: Skipping_Basesetup_Flattankissues_v01.hip

With speeds of 1m/Frame it is completely fine, but as soon as I go below 0.1m/Frame this wave starts occuring really badly. And no amount of tweaking that I could think of would reduce it.
Apparently, the particles created at the boundary layer have a weird offset and velocity upwards. In certain testing cases, this offset upwards was the partice seperation.
Eg.: When water level is Y=0, the initial particles rest at Y=0-particleseperation, however, sourced at the boundary layer the particles had Y=0 leading to an uneven surface and thus a wave.

I'm thankful for any advice at all,
Martin

Edited August 31, 2019 by tortoise

[m-egger]

This image illustrates quite well what is happening on the first sim frame. At the bounding regions, the sourced fluid is at the surface level when the initial source (By shelf tool default) Lies at Y=-0.15.

When I try to raise this initial source however, the extension is by that amount higher as well. It's a bit mind-boggling.