[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,
0,
position, reference, node, null,
component,
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";
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
https://www.dona.polimi.it/
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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
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
# 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;
gravity;
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,
null,
eye,
VX_0, 0., 0., # horizontal initial velocity
null;
end: nodes;
begin: elements;
body: 0, 0, M, null, diag, 1.e-6, 1.e-6, 1.e-6;
force: 0, absolute,
0,
position, reference, node, null,
component,
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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