[MBDyn-users] How to handle air drag force?

masarati at aero.polimi.it masarati at aero.polimi.it
Mon Oct 10 18:27:05 CEST 2011

> Hi all,
> I plan to run a numerical analysis of a simple pendulum.
> The shape of bob is major factor that would affect the T (period)
> variation.
> I assume the shape of bob is a round sphere.  I have the following
> formula that explains
> how the pendulum looses movement.
> Fp - Fk - Fd
> Fp: Inital Pendulum force which comes from the height of pendulum at
> initial time t0.
> Fk: Kinetic frictional force
> Fd: Drag force of pendulum bob
> To get air drag force, I use
> Fd = 0.5 x C x density x A x (v^2)
> C: drag coefficient: 0.4 to 1.0
> A: cross sectional area
> density: air density
> v: speed of the pendulum bob
> Should I look into the relationship between C and A and calculate Fd?
> or
> MBDyn has built-in feature to handle C and A?

You need to define a force that is opposed to the velocity and whose
magnitude depends on the square of the velocity.  Something like this:

force: DRAG_LABEL, absolute,
    position, null,
    string, "-D*model::xvelocity(NODE_LABEL)*model::velocity(NODE_LABEL)",
    string, "-D*model::yvelocity(NODE_LABEL)*model::velocity(NODE_LABEL)",
    string, "-D*model::zvelocity(NODE_LABEL)*model::velocity(NODE_LABEL)";

where D is .5*density*C*A.

Cheers, p.

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