COMSOL Multiphysics™

femsim
time-independent constraints and the eliminate constraint handling method is
supported. The Const property is not supported.
The linearization point should be an equilibrium point (stationary solution) and is
controlled by the property U, see femsolver. The inputs and the outputs are
deviations from the equilibrium values.
Model reduction can be used to approximate the linearized model with a model that
has fewer degrees of freedom, by using Reduction=on. A number of eigenmodes
(given by the property Redmodes) and static modes (if Restatic=on) will then be
computed, and the linearized model will be projected onto the corresponding
subspace. The properties Etol, Itol, Krylovdim, and Shift of the eigenvalue
solver are supported, see femeig.
When using model reduction on wave equation models that have been rewritten as
a system of first-order equations (wave extension), the algorithm needs to know the
names of the original (non time derivative) solution components. The names of the
non-time derivative solution components should be specified using the property
Redcomp. Since COMSOL Multiphysics 3.2, wave equations are usually formulated
without wave extension; then the Redcomp property should not be used.
Linearized Static Export (State=on, Static=on)
Export a static linearized model. The model is linearized about an equilibrium
solution, and a transfer matrix is computed. The COMSOL Multiphysics degrees of
freedom are not part of the Simulink state vector. To compute the outputs of the
COMSOL Multiphysics Subsystem block, the transfer matrix is used.
The linearization point should be an equilibrium point (stationary solution) and is
controlled by the property U, see femsolver. The inputs and the outputs are
deviations from the equilibrium values.
Example
Heat equation with heat source Q as input.
fem.geom = solid1([0 1]); fem.mesh = meshinit(fem);
fem.shape = 2; fem.equ.da = 1; fem.equ.c = 1; fem.equ.f = 'Q';
fem.xmesh = meshextend(fem);
% Temperature u at x = 0.5 is output
sct = femsim(fem, 'input',{'Q'}, 'outnames',{'Temp'}, ...
'output',{{'u' 0.5}});
Compatibility
For backward compatibility, the Input property can also be a vector of indices into
fem.const.
144 | CHAPTER 1: COMMAND REFERENCE