Discussion Forum

Posted: 1 decade ago 5 apr 2012, 12:32 GMT-4

Hi I believe that comes from the fact that COMSOL has changed a little the naming andmore the use of the different frames while going from version 3.5 to 4.2a. Today, you should talk about spatial, material, mesh and geometric frames. The geometric frame is the lowest level and exist for itself only if you use the "dg" (deformed geometry), it maps your geometry FEM Entities to the material frame entities (by defaults it's identity). The spatial frame exist for it's own (dissociates from the material frame) in "Solid" physics, if so the lower case x,y,z,r spatial frame corresponds to x = X+u etc (at east when non-linear large displacement geometry is selected) where X is the Material coordinate frame name. The mesh frame dissociates from the others in "deformed mesh" physics when you start to remesh. I beleive it 's definition and use might still change in the next version, we will see (I'm not COMSOL so I do not know all the interiousr, nor the to comes ;) as it's sometimes overlappng with the use of the spatial frame. Reference frame was used in 3.5, I understand it's to be renamed to material frame in 4.2 (but there might be exceptions). So far I have noticed too that the doc is not always fully consistent with latest 4.2a changes so I'm often confused too. I have found the recent book of E.B Tadmor: Continuum Mechanics and Thermodynamics" CUP 2012, to have an extensive discussions about tensor geometry and frames, very close to COMSOL's notation. Anyhow, when I have a doubt, I make simple models, squares ontop of each others with "soft material properties, then I sweeze them with an adequate pressure load, and select to integrate volumes over the different frames. Also to see if the frames are dissociated (by default all have lower case letters x,y,z,r (and changes names only if different from an identity mapping) by calling for a new coordinate frame, then you can select whic of the for to use and there you notice the aming convention, depending of the physics these are all lower case, or appear with the different lower, upper, _M or _G suffixes. -- Good luck Ivar

Posted: 1 decade ago 5 apr 2012, 12:59 GMT-4

Hi Ivar, As always, thank you. Jessica

Posted: 1 decade ago 5 apr 2012, 14:21 GMT-4

Ivar, As you suggested, I decided to create a simple model to check the frames and their effects. So I created a simple beam with the length of 1m, fixed one end and applied a displacement of 0.001 to the other end. Under my Solid Mechanics physics, I set the Frame Type, Material, under Discretization. The way COMSOL defines Material Coordinate Sys. is that they consider it as if it is printed on the geometry. So with this definition, I expected to get displacement (u) = 0 with Material coordinate system. Therefore, in order to see the actual deformation (u= 0.001m), I thought I need to switch to Spatial coordinate. After computing the problem, the displacement (u) of the free end of the beam was 0.001m (when using Material Frame), which is not what I expected. Next, I changed the frame type to Spatial, under Discretization, and I got the same results, I even thought maybe I have to change the frame types under Data Set\Solution, and under 2D Plot Group in order to see the potential changes from 0.001m, but No Change. So, I was wondering how we can see the difference between the effects of these two frames. I have attached the mph file. Thanks you. Jessica, [QUOTE] Hi I believe that comes from the fact that COMSOL has changed a little the naming andmore the use of the different frames while going from version 3.5 to 4.2a. Today, you should talk about spatial, material, mesh and geometric frames. The geometric frame is the lowest level and exist for itself only if you use the "dg" (deformed geometry), it maps your geometry FEM Entities to the material frame entities (by defaults it's identity). The spatial frame exist for it's own (dissociates from the material frame) in "Solid" physics, if so the lower case x,y,z,r spatial frame corresponds to x = X+u etc (at east when non-linear large displacement geometry is selected) where X is the Material coordinate frame name. The mesh frame dissociates from the others in "deformed mesh" physics when you start to remesh. I beleive it 's definition and use might still change in the next version, we will see (I'm not COMSOL so I do not know all the interiousr, nor the to comes ;) as it's sometimes overlappng with the use of the spatial frame. Reference frame was used in 3.5, I understand it's to be renamed to material frame in 4.2 (but there might be exceptions). So far I have noticed too that the doc is not always fully consistent with latest 4.2a changes so I'm often confused too. I have found the recent book of E.B Tadmor: Continuum Mechanics and Thermodynamics" CUP 2012, to have an extensive discussions about tensor geometry and frames, very close to COMSOL's notation. Anyhow, when I have a doubt, I make simple models, squares ontop of each others with "soft material properties, then I sweeze them with an adequate pressure load, and select to integrate volumes over the different frames. Also to see if the frames are dissociated (by default all have lower case letters x,y,z,r (and changes names only if different from an identity mapping) by calling for a new coordinate frame, then you can select whic of the for to use and there you notice the aming convention, depending of the physics these are all lower case, or appear with the different lower, upper, _M or _G suffixes. -- Good luck Ivar [/QUOTE]

Attachments:

• Multi-load beam.mph

Posted: 1 decade ago 7 apr 2012, 05:52 GMT-4

Hi that is not how I would have checked, for me the resulting x = X+u, where x is the spatial frame coordinate and X the material frame coordinate. u is the solve displacement along the X coordinate hence remains u=0.001 I never play with the discretisation coordnate, so I would have to look into exactly what that does. But I often integrate over either spatial or material frames depending on what I want to get out and there you do not get the same results depending on what you select. Check also the effect of the "geometric non linearity check box in the solver tab of 4.2a By the way try a search on "Frame" on the forum, this has been discussed a couple of times, and I believe I left a few example models once, but pls check again carefully, as things have changed somewhat with COMSOL between the latest versions -- Good luck Ivar