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What exactly is a point probe (3D) measuring?

Posted Apr 19, 2015, 3:41 p.m. EDT Chemical Reaction Engineering, Geometry, Mesh 2 Replies

Gerardo Felix
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I'm solving a reaction-diffusion model in a 3D geometry consisting in a simple sphere (radius 10 mm) with some point sources in its surface. I've configured a point probe right in front of one of those point sources, exactly 0.05 mm (50 um) away from it. However, the mesh I'm using consists of elements with a minimum size of 0.1 mm. When I plot the location of the point probe it seems it is located where I wanted (0.05 mm away from the point source) and I'm getting some results (the concentration) in real time as well.

The question is: Are the results that I'm getting an estimation of the concentration where I put the point probe? or COMSOL is giving me the concentration at 0.1 mm (the minimum element size) or how does COMSOL "measures" the concentration at the point probes??

Thanks a lot....
2 Replies Last Post Apr 20, 2015, 2:20 a.m. EDT
Daniel Connelly
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Posted: 10 years ago Apr 19, 2015, 4:01 p.m. EDT
Assuming concentration is defined at the nodes, and assuming linear discritization, it SHOULD be interpolating the values of the concentration linearly onto the point in question. If there's tetrahedral elements than there's only one way to do this (4 points bound the element, the values at these 4 points defining a mean value and the gradients along each of three dimensions: in 1D you need 2 points, in 2D you need 3 points, and in 3D you need 4 points). If there's more than 4 points, for example a prismatic element, then the problem may be overconstrained, and so the interpolation must proceed with certain assumptions.
Assuming concentration is defined at the nodes, and assuming linear discritization, it SHOULD be interpolating the values of the concentration linearly onto the point in question. If there's tetrahedral elements than there's only one way to do this (4 points bound the element, the values at these 4 points defining a mean value and the gradients along each of three dimensions: in 1D you need 2 points, in 2D you need 3 points, and in 3D you need 4 points). If there's more than 4 points, for example a prismatic element, then the problem may be overconstrained, and so the interpolation must proceed with certain assumptions.
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 10 years ago Apr 20, 2015, 2:20 a.m. EDT
Hi

I'm not sure the interpolation is linear, as most operations in COMSOL uses higher order integrations/interpolations, see the operator and plot definition nodes.
I'm not behind my WS, but I seem to remember you can define the order for the "probe" point integration/averaging

--
Good luck
Ivar
Hi I'm not sure the interpolation is linear, as most operations in COMSOL uses higher order integrations/interpolations, see the operator and plot definition nodes. I'm not behind my WS, but I seem to remember you can define the order for the "probe" point integration/averaging -- Good luck Ivar

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