Hello, I am wondering if it is possible to give specific attributes in specific elements in COMSOL. In more detail I want to set some elements of the a real prostate mesh as liquid sources in an edema study. Is possible to give conditions to some elements by choice in COMSOL and if yes how?
Thank you in advance
Hi!
You can add point sources to any coordinate (point) or coordinates (curve, plane, volume) you like, but you need to specify these geometric elements when builiding the geometry (if this is not already done).
Say that you want one side of a rectangle to be a source - then this boundary is already a defined element in the geometry. If you want a source in an arbitrary point inside the rectangle, however, you need to tell COMSOL that that point "should be there" already when building the geometry (simply by adding/drawing said point).
It's almost always better to let COMSOL handle the meshing (creation of elements) automatically, but there are plenty of ways to set the parameters used for meshing to create desired effects. Adding the point on the geometry level will both make it accessible as a source and make sure COMSOL puts a mesh point there.
Hi,
as already said by Mr. Manneberg, this is done at the geometry construction. To your question, the answer is yes of course. That is why Comsol is worthy. At the boundaries level, you should be able to choose that element on which you want to address a physic. So built your geometry making sure different elements can be selected.
Hi Konstantinos,
Your question to apply properties to an element is not according to the philosophy of Comsol. Other FEM codes work with the concept of applying conditions or setting to nodes and elements. But elements and nodes are not existing features in your physics, and the solution should not be dependent of element shape, size and positions of nodes, so your conditions should rather be formulated in function of positional variables. For instance, a point source which is mentioned before, can be added in the physics as a smoothed Dirac function at any given position, independent of the presence of nodes. Of course, the smoothing is necesary to avoid that the Dirac function remains unnoticed in the nodes around. For the same reason, it should always be avoided to use boolean switching type of functions and prefer to use smoothed versions, in order to avoid spurious variations when changing mesh or time steps (if functions or atributes change as funtion of time instead of position).
The approach also makes it much easier to work with moving sources or attributes that move in time. Comsol has som predefined function for smoothed unit step (Heaviside) or smoothed Dirac. Look in the manual for functions like flsmhs, flc1hs, flc2hs (different versions of step functions) and their derivatives (=Dirac) with names that just add a "d" in their name, such as fldsmhs, etcetera.
Typically, the mootheing transition size that you must provide in the function (second parameter) should be of the order of the element size, or the time step, depending on the situation.
Hi
While constructing your geometry, you can add a work plane with the orientations that you need, using this work plane you can add any type of 2-D geometry. As a result, COMSOL will deal with the new area as a new boundary, which you can specify it for any loads or boundary condition you need.
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