Using the VBS to sink causes the vehicle to translate forwards and go into a spin.

[arjo129]

How to reproduce:

In one terminal:

ign gazebo -v4 buoyant_tethys.sdf

In another:

ign topic -t  /model/tethys/buoyancy_engine/ -m ignition.msgs.Double -p "data: 0"

Expected behavior

The vehicle pitches a little bit and then sinks mostly vertically.

Current behaviour

The vehicle sinks but keeps getting pushed backwards, this is likely to some weird numerical issue.

[arjo129]

Spinning in yaw seems to be from the Added Mass terms and Translation is from the damping terms.

[chapulina]

The longer picture is also interesting:

Another thing that could be related, these are the plots for the horizontal and vertical fins. With the vehicle's movement, they wiggle a bit, pushed by external forces:

[arjo129]

I've narrowed this down to an issue of transformations. Basically by sprinkling debug statements throughout the course of the Hydrodynamics plugin I get the following:

[Dbg] [HydrodynamicsPlugin.cc:248] Pose Rotation -0 0.105406 0
[Dbg] [HydrodynamicsPlugin.cc:249] Transformed Vel 0.062581 -0 -0.066591
[Dbg] [HydrodynamicsPlugin.cc:250] Original Vel 0.055228 -0 -0.072805
[Dbg] [HydrodynamicsPlugin.cc:321] Original force-0.015606 0 2.6208
[Dbg] [HydrodynamicsPlugin.cc:322] Transformed force0.260217 0 2.60789

The hydrodynamics should produce a force which opposes the direction of motion. This is happening in the local frame correctly, however when we transform back to the global frame we get incorrect results.

[arjo129]

This manifests itself when going up using the Buoyancy Engine:

But does not manifest itself when dropping the drop weight.

[arjo129]

Force visuallizations are very telling of a problem in hydrodynamics. The Blue arrow is hydrodynamic forces. The green arrow buoyancy engine forces.

[braanan]

HI @arjo129, almost looks like the buoyancy force is stuck in the vehicle frame and not translated into the internal frame — I'd expect the buoyancy force to always work against gravity, regardless of the vehicle's orientation. What do you think?

[arjo129]

I checked and its not that. The buoyancy force from the vbs is always pointing opposite gravity. Its not very clear in the video but when I print it I see that its only acting in the Z axis.The hydro on the other hand is all over the place.