[MBDyn-users] aerodynamic drag question

MBDyn users list mbdyn-users at mbdyn.org
Fri Feb 11 10:12:19 CET 2022

Quick and dirty: the damping force is actually

     D = -0.5*C_D*A*v*|v|

where "v" is a vector, as much as D is, and the force is opposite to the 
absolute velocity of the point. So you can use a regular force, and 
compute its magnitude and direction as

        force: 0, absolute,
                 position, reference, node, null,

where function "velocity" returns the norm of the absolute velocity of 
node "NODE", and "xvelocity", "yvelocity", and "zvelocity" return the 
related components.

See the attached example.

Sincerely, p.

Il 11/02/22 09:53, MBDyn users list ha scritto:
> Dear Members of MBDYN,
> I am thankful for all the help and guidance you have given me so far.
> I have a question on aerodynamic damping concerning the example of the 
> cantilever beam available on the website.
> The cantilever beam I have developed is under gravitational load 
> instead of a point node force and, due to its interaction with air, 
> air damping is occurring. The forces in the systems are:
> Fg: gravitational load, defined by the acceleration of gravity
> FD: damping forces,0.5*Drag coefficient*Area*density*(velocity)^2
> I should introduce the damping force that is not referred to the 
> global reference frame but the nodes reference frame, and it should 
> change according to the nodes' behaviour.
> How can I develop such kind of force?
> Thank you for your kind attention.
> Best Regards,
> Antonio
> _______________________________________________
> MBDyn-users mailing list
> MBDyn-users at mbdyn.org
> https://mail.mbdyn.org/cgi-bin/mailman/listinfo/mbdyn-users

Pierangelo Masarati
Professore Ordinario di Costruzioni e Strutture Aerospaziali
Dipartimento di Scienze e Tecnologie Aerospaziali
Politecnico di Milano
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# MBDyn (C) is a multibody analysis code. 
# http://www.mbdyn.org
# Copyright (C) 1996-2017
# Pierangelo Masarati	<masarati at aero.polimi.it>
# Paolo Mantegazza	<mantegazza at aero.polimi.it>
# Dipartimento di Ingegneria Aerospaziale - Politecnico di Milano
# via La Masa, 34 - 20156 Milano, Italy
# http://www.aero.polimi.it
# Changing this copyright notice is forbidden.
# This program is free software; you can redistribute it and/or modify
# it under the terms of the GNU General Public License as published by
# the Free Software Foundation (version 2 of the License).
# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# GNU General Public License for more details.
# You should have received a copy of the GNU General Public License
# along with this program; if not, write to the Free Software
# Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA

begin: data;
	problem: initial value;
end: data;

begin: initial value;
	initial time: 0.;
	final time: 1.;
	time step: 1e-2;

	tolerance: 1e-5;
	max iterations: 10;

	derivatives tolerance: 1e-5;
	derivatives max iterations: 10;

	output: iterations;
end: initial value;

begin: control data;
	structural nodes: 1;
	rigid bodies: 1;
	forces: 1;
end: control data;

set: const integer NODE = 0;
set: const real VX_0 = 10.; # m/s
set: const real C_D = 0.1; # non-dimensional
set: const real A = 20.; # m^2
set: const real M = 1.; # kg

begin: nodes;
	structural: NODE, dynamic,
		VX_0, 0., 0.,	# horizontal initial velocity
end: nodes;

begin: elements;
	body: 0, 0, M, null, diag, 1.e-6, 1.e-6, 1.e-6;
	force: 0, absolute,
		position, reference, node, null,
			string, "-model::xvelocity(NODE)*model::velocity(NODE)*0.5*A*C_D",
			string, "-model::yvelocity(NODE)*model::velocity(NODE)*0.5*A*C_D",
			string, "-model::zvelocity(NODE)*model::velocity(NODE)*0.5*A*C_D";
	gravity: uniform, 0., 0., -1., const, 9.81;
end: elements;

# vim:ft=mbd

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