# Discussion Forum

## density variable

 Topics: 4.3, 4.2a, 4.2, Structural Mechanics
RSS feed   |   Turn on email notifications   |   10 Replies   Last post: October 3, 2012 4:47pm UTC

Angela Pallone

October 2, 2012 10:14am UTC

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!

Nagi Elabbasi
Certified Consultant
Veryst Engineering

October 2, 2012 12:10pm UTC in response to Angela Pallone

Re: density variable

Specify the density to be a function of the spatial coordinates, X, Y and/or Z.

Angela Pallone

October 2, 2012 1:26pm UTC in response to Nagi Elabbasi

Re: density variable

I define a function variable analytic form? or assign material at certain points thus giving the coordinates?

Angela Pallone

October 2, 2012 3:34pm UTC in response to Angela Pallone

Re: density variable

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?

Ivar Kjelberg

October 2, 2012 6:39pm UTC in response to Angela Pallone

Re: density variable

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

Nagi Elabbasi
Certified Consultant
Veryst Engineering

October 2, 2012 8:02pm UTC in response to Ivar Kjelberg

Re: density variable

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)".

Ivar Kjelberg

October 2, 2012 9:08pm UTC in response to Nagi Elabbasi

Re: density variable

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 ...

--
Having fun COMSLing,
but only from tomorrow morning off
Ivar

Angela Pallone

October 3, 2012 10:08am UTC in response to Nagi Elabbasi

Re: density variable

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

Ivar Kjelberg

October 3, 2012 11:41am UTC in response to Angela Pallone

Re: density variable

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

Angela Pallone

October 3, 2012 1:08pm UTC in response to Ivar Kjelberg

Re: density variable

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

Ivar Kjelberg

October 3, 2012 4:47pm UTC in response to Angela Pallone

Re: density variable

Hi

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

--
Good luck
Ivar

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