# Discussion Forum

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## density variable

how to define a variable density in the same piece? I give 100% at the center of the piece and the edges 70, 80!

10 Replies Last Post Oct 3, 2012, 12:47 PM EDT
Posted: 5 years ago
Specify the density to be a function of the spatial coordinates, X, Y and/or Z.

Posted: 5 years ago
I define a function variable analytic form? or assign material at certain points thus giving the coordinates?

Posted: 5 years ago
if I had a cylinder, assuming that I know the radius and the radius max min and the density to 100% and the density to 80! how should I define this variability?

Posted: 5 years ago
Hi

what about a density of something like (assuming rho is a scalar)

"rho*(0.8+0.2*2*(r-rmin)*(rmax-r) / (rmax-rmin))"

this should make a nice parabola, non ?

--
Good luck
Ivar

Posted: 5 years ago
Thank Ivar, that gives 100% at the middle and 80% at the extremes. I want to fix a type though: "rho*(0.8+0.2*4*(r-rmin)*(rmax-r) / (rmax-rmin)^2)".

Posted: 5 years ago
Hi Nagi

Très perspicace I see ;) As often I read through the question a bit too quickly ;)
Thanks for the correction, with COMSOL I check the units, but I'm not by my WS, and obviously getting lazy ...

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Having fun COMSLing,
but only from tomorrow morning off
Ivar

Posted: 5 years ago
can I define a vector in which the distance from the center varies between 0 and R? cause the problem is to vary the r small!
Thanks

Posted: 5 years ago
Hi

I'm not sure we understand us all correctly, but if you select an Edge in 2D then you have access to a variable "s" that goes from 0 to 1 along the edge in the direction of the "edge" arrow, you can use this to define a variable along an edge.

If it is for a domain in 2D (surface) then by selecting this entity, you implicitly define an "(x,y)" coordinate and if you define a variable on this entity it can be seen as a field i.e "rho(x,y)", with the trivial case "rho" is a scalar hence independent of (x,y)

This is why and how material properties are dependent in T and pA ...

--
Good luck
Ivar

Posted: 5 years ago
I applied from the center to the outside of all the geometry,a material with a density that varies linearly with a decreasing from the center towards the outside of it!So my geometry is in 3D! I can, however, define this edge in 2D? I can not understand under which entry you can do this!
I can not find a tutorial to provide guidance

thanks

Posted: 5 years ago
Hi

you can then "define" a cylindrical coordinate and refer to sys2.r or whatever your radial vector name is ;)

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Good luck
Ivar

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