[ODE] Weird Dynamics

[John DeWeese]

Very cool simulation! I would like to hint and suggest that the molecule 
disappearances are actually an acceptable result of the simulation. This 
is what is known as the "quantum tunnelling effect" in physics, and your 
movie shows that you have dealt with it appropriately. You are welcome.

Regarding brownian motion, even though you add random forces every time 
frame, the movie shows that the atoms generally travel in a straight 
line, as Martin^2 pointed out. The second order ODE that is force 
integration may be acting as a filter to the randomness of your force 
accumulation. You can either  increase the magnitude of your forces or 
choose a period for the randomness, such that you apply the same force 
over multiple time steps. The latter is a more general way, because it 
forces you to pick a scale for the randomness of your motion. You may 
need to take smaller time steps to deal with these larger and more 
varying forces!

Alexander Klyubin wrote:

> Hello!
>
> I've been playing around with ODE for a couple of weeks now. The 
> simulations I've been running tend to produce weird dynamics and even 
> "explosions", although I don't believe I'm using any special features 
> or too large time steps. So far I've spotted and fixed a couple of 
> bugs in my code. Nevertheless, I'm still struggling with weird 
> behavior of simulated bodies.
>
>
> I've created a rectangular arena consisting of four massive walls. 
> Small spheres are placed into the arena. At each time step a small 
> force is applied in random direction to each sphere. If two spheres 
> collide, they stick to each other forever via a fixed joint.
>
> The behavior I expect is that that eventually all spheres form a big 
> fixed articulated body. Moreover, the more spheres form the body, the 
> less it should move, since random forces applied to each sphere should 
> on the average cancel each other.
>
> What I observe when running the simulation is that aggregate 
> articulated bodies indeed form. However, their behavior is really 
> weird: they start rotating too fast, or gaining highly improbable 
> impulses, or even exploding. You can take a look at a movie showing 
> one such run of the simulation.
>   http://homepages.feis.herts.ac.uk/~ka2by/tmp/aggregation.avi (1.9 
> MB, DivX 5).
>   http://homepages.feis.herts.ac.uk/~ka2by/tmp/aggregation.mpg (3.7 
> MB, MPEG).
>
>
> As I'm pretty new to ODE I believe that bugs in my code may be 
> responsible for the weird behavior. However, the code is pretty simple 
> and I can't spot any more bugs in it.
>
> Any hints and suggestions appreciated.
>
> Best Regards,
> Alexander Klyubin