[Fixed]Air friction force (-fv) wrong

[tatt61880]

Edit: (by tatt61880)
Fixed at v1.8.2(beta). Thanks.
-Tatt
----

I just noticed that -fv is dependent from the angle of the velocity that an object has got. But normally this must not happen.

This is what I made:

• I made a circle so that the cross-section is the same no matter which orientation the circle has got (radius = 1)
• After that I turned up airfrictionmult to 100
• I turned up the velocity to 10 m/s and 0°
• I displayed the air friction force -fv
• Now I made a simulation step and watched the force (-fv = 20 N)
  - - - - -
• I undid the simulation step and changed the velocity to 45°
• I made another simulation step and watched the force (-fv = 28.28 N)

I noticed:
28.28 N / 20 N = 1.414 ≈ √2

I also found out how air friction force gets calculated in Algodoo. It's
-fv =
sim.airFrictionMultiplier * airFrictionMult * cross-section * ( sim.airFrictionLinear * v + sim.airFrictionQuadratic * v² )

=>
-fv(45°) / -fv(0°) = cross-section(45°) / cross section(0°) ≈ √2

And because of that I suppose that Algodoo doesn't detect the real cross section of the circle. It detects the cross section of a rectangle around the circle. The rectangle's cross section is 2 at 0° and 2*√2 at 45°.

Unbenannt-1.png (52.93 KiB) Viewed 2145 times

Am I right?
Why did you calculate it like this? Is it easier or did someone just forget to continue working on it?

[Bronie12345]

i guess they didnt thought it was important to them...

[xplane80]

I know what's wrong however how algodoo is programmed, this cannot be fixed easily so...

[Bronie12345]

will probably be fixed when they fully re-program algodoo )will probably never happend)

[Joel]

Am I right?
Why did you calculate it like this? Is it easier or did someone just forget to continue working on it?

You are correct, although I can't say why this is the case. It is noted and should be fixed.

[tatt61880]

Not fixed yet with v1.8.0.

This issue may be difficult to be solved... //Edit (*)
In addition, compatibility issue can occur when this issue is solved.

Edit (*): This bug seems to happen only with circle, so it might be easy to solve.

[mori-d]

Suppose a circumscribed rectangle which is parallel to the velocity vector.
Then the rectangle's perpendicular side to the velocity will be cross section.

Keywords:
- minimum bounding rectangle
- minimum bounding box
- axis-aligned minimum bounding box

What's difficult?

[tatt61880]

I noticed that this issue seems to happen only with circle!
So, this issue must be easy to be solved.

[KarateBrot]

and it happens with an irregular drawn polygon too. but if it gets fixed for circles that's a step forward

[emilk]

I'll do exact diameter calculations in the next release, for circles and polygons alike.

[Kilinich]

If you do, maybe we can get simple aerodynamic? Like calculating average normal vector to surface form "windy" direction and rotate air friction force according to it?