COMSOL Multiphysics™

elpconstr
Purpose
elpconstr
Define general pointwise constraints.
Syntax
Description
el.elem = 'elpconstr'
el.g{ig} = geomnum
el.geomdim{ig}{edim}.ind{eldomgrp} = domainlist
el.geomdim{ig}{edim}.constr{eldomgrp}{ic} = constrexpr
el.geomdim{ig}{edim}.cpoints{eldomgrp}{ic} = cpind
The elpconstr element adds a set of pointwise constraints of type constrexpr=0.
The constr field has the same syntax as the fem.bnd.constr field. See further the
section “Specifying a Model” in the COMSOL Multiphysics Scripting Guide. The
constraints are enforced at the same local coordinates in all elements in one domain
group. The constraint point pattern is specified as a pattern index in the cpoints
field. Indices refer to patterns defined by an elepspec element.
Compared to the elcconstr element, the elpconstr can implement a wider range
of constraints, as a correct constraint Jacobian is always calculated on the fly. This is
in contrast to the user-specified Jacobian matrix h, used in fem.bnd.h and the
elcconstr element. Constraints implemented using elpconstr elements are
nevertheless always ideal, which essentially means that symmetric problems stay
symmetric. Note that this can have unexpected effects for nonlinear constraints.
Examples
Solve a 1D biharmonic equation (related to Euler beams) with constraints on both
value and normal derivative at the endpoints.
clear fem;
fem.geom = solid1([0,1]);
fem.mesh = meshinit(fem);
fem.shape = {shherm(1,3,'u')};
fem.form = 'weak';
fem.equ.weak = 'uxx_test*uxx';
fem.bnd.dim = {'u'};
fem.bnd.cporder = 1;
fem.elem = {};
clear el;
el.elem = 'elpconstr';
el.g = {'1'};
clear gd;
gd.ind = {{'1'},{'2'}};
gd.constr = {{'-u','1-ux'},{'-u','1+ux'}};
gd.cpoints = {{'1'},{'1'}};
el.geomdim{1} = {gd,{}};
fem.elem = [fem.elem {el}];
fem.xmesh = meshextend(fem);
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