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Hi

I do not really understand you there: as for me a heat "wave" cannot bounce off a boundary, because the heat equation (diffusion PDE) cannot develop standing "waves" in the sens of EM waves. For that you need a second derivative equation type phenomena.

You can add a "thin feature" to represent some properties, but I'm not aware of HT "infinite elements", perhaps theyy xst for stationry cases, anyhow how to handle that correctly in transient HT ? within the heat diffusivity length you will have transient effects happening, that a "bulk" infinite layerwill not express correctly without quite some equations.

What about adding some material or a non linear heat exchange on your boundary

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Good luck
Ivar

I am modeling in-plane thermal diffusion in a thin material. Flash heating of the material on one side and monitoring the temperature on the other end. The lateral dimension of the material is affecting the temperature rise. For the same conditions (mesh size, time step, input pulse), diffrent lateral dimension I get different temperature rise (difference of about 1-2 deg). why is this so?

Hi

are you sure you catch the thermal gradient, are aou respecting the dz<sqrt(alpha*dt) where dz is the mesh size perpendicular to the thermal gradient, and dt is the smallest time step you have the solver doing for you, while alpha=k/rho/Cp is the heat diffusivity.

These are the mesh conditions for HT diffusion equation mesh sampling in transient. In MEMS sizes it can be very demanding on mesh density. I often use boundary layer mesh elements for this.

If you are not ceraful, you will even observe "negative" temperatures, which are not really physical

Check the FORUM on HT and gradients of T Nagi has given excellent advises on several of these threads

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Good luck
Ivar

Thanks for the reply. In fact I wanted to know the condition between mesh element size and time step for a transient heat transfer model. I found a condition that is opposite to what you have mentioned. You say dz less than, is there any particular factor? I will check the other threads on this and try. I will check the bounday layer meshing also, the thickness am dealing with is about 200 micron.

Hi

I could have got the <> mixed up, but I do not think so, or was it wrong in the other tread ?

The original threead was this one:
www.comsol.com/community/forums/general/thread/16808/


but here the formula of Nagi is related to dt>... still by inverting you get dz<....

For the rest of your questions, you should find the replies in this interesting thread ;)

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Good luck
Ivar

Hi Ivar,

I am wondering if I can define an absorbing boundary condition for a transient ultrasonic wave propagation problem (let's say in a 2D plane strain transient module). The option certainly doesn't come up in the boundary conditions pulldown menu. I wonder if there is a trick.

Thanks!

Deb