[MBDyn-users] Deformable joint

masarati at aero.polimi.it masarati at aero.polimi.it
Fri Oct 30 21:49:31 CET 2009

> See comments below...
>> From: Pierangelo Masarati [mailto:masarati at aero.polimi.it]
>> Sent: 10/26/2009 9:14 AM
> <snip link to development HEAD>
>> The new element is called "viscous body".
>> The syntax is defined here
>> <http://www.aero.polimi.it/masarati/MBDyn-input/mbdyn-input-1.X-
>> Devel/node56.html#SECTION00121649000000000000000>
>> The formulas are defined here
>> <http://www.aero.polimi.it/masarati/tecman.pdf> (Section 9.3)
>> The element provides a force and a moment that depend on the absolute
>> linear and angular velocity, projected in the reference frame of the
>> node.  The angle of attack is defined as the local tan^-1(V_z/V_x), so
>> aerodynamic forces
>> 	L = 1/2 * rho * V^2 * S * C_L( tan^-1( V_z/V_x ) )
>> when linearized yield
>> 	Delta L ~= 1/2 * rho * V_x * S * C_L_/alpha Delta V_z
>> when V_z << V_x.
>> The aerodynamic moment behaves similarly.
>> You can probably produce this behavior, when the velocity does not
>> change too much, using a linear constitutive law.  Modeling the flight
>> mechanics of an aircraft would probably require an ad-hoc constitutive
>> law.  As far as I can tell, there is no general model that can directly
>> suit your needs, you'll need to develop it yourself based on your
>> specific needs.
>> In tests/joints/viscousbody/ there is an example to test the element and
>> to serve as a starting point for your model.
> Pierangelo,
> The new element you created works great for my needs.  Many thanks!
> I find that 'position' is not optional, so we will probably want to change
> this to be consistent with the documentation.

Right, the offset is not optional, but the "position" keyword is, for
consistency with other joints.  I've fixed the manual.

> I appreciate your comments regarding aerodynamic forces.  I think that in
> general I would prefer having a viscoelastic element, and use a strain
> constitutive for aerodynamic forces.    The aerodynamic angles can be
> absolute angular displacements projected in the reference frame of the
> node,
> same as the viscous constitutive.  If you agree and offer some guidance, I
> can attempt to code it.

the "elastic" part makes little sense: the angle of attack is defined with
respect to the absolute velocity, projected in a reference frame attached
to the body.  The elastic part would necessarily need to be referred to
the absolute reference frame, which is definitely not physical.

> For my immediate needs, the strain equations have the form Y = k1*roll, X
> =
> k2*pitch, and Z = (k3*roll + k4*pitch), so a collection of force elements
> might be more expedient and possibly would perform better than a general
> viscoelastic solution.  I would appreciate your thoughts on using node
> private data to implement these equations with force elements.

Please define "pitch" and "roll"; extracting that information from the
motion of the node may or may not be trivial.

Cheers, p.

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