# [ODE] objects in water

Yordan Gyurchev yordan at gyurchev.com
Tue Jan 18 21:14:40 MST 2005

```> So, does anyone have any comments on my underwater drag and buoyancy
>approximation method?  Does anyone have any other ideas?

After I sent my initial email I tried the sampling points
approach for boxes. Not quite what you described but
similar.

It kind of works but needs few tweaks...

Few things were immediately apparent:

1. Although amount of force does not increase as you go down
in depth its good to set some interpolation to 1.0 fraction of force.
Why is that? As your sampling point enters the water surface you
get suddenly a big (comparatively) force applied to your body and
vice versa. Results in juddering. To solve that I choose some limit
distance depending on the body and apply force proportional to
the penetration distance where at the limit distance  (and deeper) the
force is fully applied.

2. When body is partially in water it is not clear how exactly the
damping (and forces) is applied as it would depend on the amount
of body that is in the water. In some cases my penetration distance
interpolation handles that. But here are some cases where the
sampling points penetrating need to be treated as special contacts...
Example: you throw a box at a good speed almost horizontally
(parallel to water surface) in reality when it touches the surface it is
likely to spin and jump up... so the forces are not always opposite to
gravity.

Your approach in particular with the averaging the sampling points
distance is not quite going to work for cylinders as they are have
no damping spinning over their length axis and quite a bit of damping
if angular velocity is along other axis.

I was thinking of may be using the normals of the sampling points
as well and the body velocity at these points to derive an approximation
damp force.

Mind you that is special contact is used at the sampling points it might
just solve the two cases together.

Thanks,
Yordan

----- Original Message -----
From: "Tyler Streeter" <tylerstreeter at gmail.com>
To: <ode at q12.org>
Sent: Tuesday, January 18, 2005 2:42 PM
Subject: Re: [ODE] objects in water

> So, does anyone have any comments on my underwater drag and buoyancy
> approximation method?  Does anyone have any other ideas?
>
> Tyler
> _______________________________________________
> ODE mailing list
> ODE at q12.org
> http://q12.org/mailman/listinfo/ode
>
>

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