Latest Discussions - COMSOL Forums

Specify the initial value of my time dependent model

Wed, 08 Feb 2012 13:54:28 +0000

Dear all

I have a problem with the inital values for my model. I run a time dependent simulation with a time discrete solver and I create 'sol1'. Than I want to run the model another time with the initial conditions of my first result 'sol1' at a certain time step for example the last time step. Therefore I have the following matlab code:
model.sol('sol2').feature('v1').set('initmethod', 'sol');
model.sol('sol2').feature('v1').set('initsol', 'sol1');
But how can I say that matlab should take the solution 1 at for example the time t=1e07s?

Thanks a lot for the answers
Julia

Time dependent adaptive mesh refinement output is on every mesh refinement -> large files

Wed, 08 Feb 2012 13:51:45 +0000

Hi,

I am evaluating the use of Comsol Plasma Module for the simulation of streamer propagation. The nature of the problem requires the use of adaptive mesh refinement in time. It seems that Comsol adaptive mesh refinement is doing a good job but it stores the solution in the beginning and at the end of every mesh refinement. In my case I do refinements quite often which results in huge file at the end. I do not need all these solutions (although they could be very useful for diagnostics). How can I tell Comsol not to store the solution at every mesh refinement but just in the specified by me times? The "times to store" option in the time dependent solver is set to "Specified times".

Thanks,
Stan

Identity Mapping question

Wed, 08 Feb 2012 13:33:35 +0000

Hi

In source and destination frames options of Identity mapping there is Mesh , Spatial, Material and Geometry.

What is the meaning of "Geometry" here?

thanks

David

PZT Electrical Damping.

Wed, 08 Feb 2012 12:23:43 +0000

Hi,

Can Some one help me to electrically damp a Piezoelectric crystal. the PZT Crystal is excited with an electric Potential of 100 V for 300 ns. how ever, the potential in the crystal oscillates before reaching zero. I have tried the Damping and loss feature which only serves to mechanically damp the displacement in the crystal and surprisingly, there is no effect on the oscillating electric potential. how do i suppress the oscillating potential in the crystal as it is crucial for my expt.
I m using Comsol 4.2a Peizo-acoustic structure interaction,Transient module.

Thank You

Kattoju

Time-step dependent reaction rates

Wed, 08 Feb 2012 10:51:31 +0000

Hi,

Help me how to set a "time-step" dependent "reaction-rate constant" for a time-dependent solver.

Regards,

Concentration overshoots/undershoots

Wed, 08 Feb 2012 10:45:45 +0000

Hi,

Problem Encountered:concentration over/under-shoots

I am working with Species Transport in Porous Media(chpm) interface. Inlet concentration(1169.3mol/m3) is given at bottom boundary with some fixed velocity in the z-direction(vertical). During the course of simulation concentration overshoots and undershoots are observed always.

What I have noticed is that for this interface no stabilization option appears in the interface. Because this interface is related to convection-diffusion-dispersion so chance of instability but there is no option available for stabilization of this interface.

May you like to advise about this problem how to handle it. Much better if COMSOL management make changes to let appear the Stabilization option for this interface.

Best Regards,

Nawaz Ahmad
KTH

advantages of comsol??

Wed, 08 Feb 2012 10:10:30 +0000

hi to all,

i am research scholar ... now i am doing research in heat transfer area (laser welding) with moving mesh method... previously i started with ANSYS.. but there is no moving mesh method.so now i have choose comsol..it,s very easy to handle compare to ansys...i have decided work with comsol multiphysics.. but i want to know about advantages of comsol (heat transfer module) compare to ansys or abaqus... please any one tell me about benefits of comsol multiphysics

thanking you
Parma

Hagen–Poiseuille flow

Wed, 08 Feb 2012 09:52:50 +0000

I would like to use Comsol for a microfluidics project I'm working on. As this is my first time working with Comsol I am trying to simulate a simple problem to compare the simulation with analytical results.
The Hagen-Poiseuille flow problem is a solution to a simple fluidic problem, the flow of a liquid through a circular tube. More infor here: http://en.wikipedia.org/wiki/Hagen%E2%80%93Poiseuille_equation

I've set up the simulation as follows:
The geometry is 2D axisymmetric (radius is 15, length is 800)
The material is water
The physics is laminar flow (spf)
I use incompressible flow as my physical model
And define the inlet and outlet
The inlet pressure is 100 Pa, the outlet pressure is 0 Pa
The mesh is 'physics controlled' and normal

I run the simulation and to obtain the flow rate I apply a surface integration spf.U with Dataset 'solution 1'.
The result for this particular simulation is: flow rate = 5.593e-11 M3/s

The problem is that the analytical result is different. I've put it in Matlab to make sure it isn't me. The result is a factor of 22 too high.

r = 15e-6; % radius
dP = 100; % Pressure
Nu = 0.001; % viscosity
L = 800e-6; % Length
Fr = (pi*r^4*dP)/(8*Nu*L) % Hagen-Poseiulle equation for flow rate

Fr = 2.4850e-012 M3/s

I've triple checked these results but I cannot find why they don't match up. A factor 22 indicates that something's wrong, either on the theory side, or on the simulation side. Because I've just started with Comsol I would like to ask anyone to look over this model and see if something is evidently wrong. If anyone can give me suggestions why the analytical model is not correct I would like to hear it as well.

Thanks in advance

In the Transport of Diluted Species, how to handle with the neutrality problem?

Wed, 08 Feb 2012 04:58:42 +0000

When I only used the Transport of Diluted Species module, H ion and Cl ion had the different concentration within the domain, since I only added flux boundary condition of H ion.
Then I tried to couple this module with Poisson Equation. I used Electric Currents to govern Poisson Equation. But the result was not reasonable. In theory, the concentration of H and Cl should be different, but maybe it's the neutrality that made the H and Cl had the exactly same distribution in the domain. The Neutrality condition should be satisfied globally, but it seemed that it was satisfied locally. I'm a new bird of COMSOL, and I have no idea how to fix this problem. I hope somebody can help me.
Many thanks in advance.

How to calcualte capacitance between two parallel plates

Tue, 07 Feb 2012 23:37:19 +0000

I tried to calculate the capacitance between two parallel plates. I set one plate as floating and another as electrode with 5V. When I used global evaluation to calculate capacitance. It calculated the self capcaitance for the electrode. How can I caluclate the capacitance between this two plate.

Thanks

where can I find the user guide for "Heat transfer module" and "joule heating module"?

Tue, 07 Feb 2012 23:15:53 +0000

Hi guys, just wondering where can I get the user guide of "Heat transfer module" and "joule heating module"? Electronic files will be better.

I do have it for v 3.5, but as the software upgraded to 4.2, I would like to see the explanations corresponding to conditions and equations.

Thanks for letting me know.

How to integrate fluxes across a line

Tue, 07 Feb 2012 15:27:04 +0000

I know that it is very easy to integrate a quantity over a boundary. My question is: it is possible to integrate a quantity over a line between two points belonging to a boundary ? I dont need to integrate the quantity over the entire boundary, but only over a part of that boundary

Modifying Physics Equation

Tue, 07 Feb 2012 14:35:14 +0000

Hello ,

Basically, I am doing a simulation for a squeeze film damping in perforated srtucture based on Veijola Numerical Model (VNM). This model is based on extended reynold's equation that is the same as Reynolds equation except that a lekage term is added to count for the pressure drop through perforations
Reynlod's equation is implemented in thin film damping shell in COMSOL 4.2, but how shuld Imodify it to include this lekage term.

Should I start form the begining looking for the required mathematical equation. OR can I modify the existing Reynold's equation in Thin Film Damping?

Thanks a lot
Waiting for your reply

Regards,
Mohamed

semi infinte

Tue, 07 Feb 2012 04:38:48 +0000

Dear Ivar

I have infinte cable (3 m) and i want to study a sector (0.165 m) to get the temperture distribution

how can i define the boundary at the infinte end?

i.e how can i tell comsol that this end of cable is too long and infinte

thank you very much

two phase level set mehod, droplet coalescence

Mon, 06 Feb 2012 11:03:31 +0000

Dear Sir,
I am solving droplet coalescence using tho phase level set method. for 2-D rectangular domain is selected,and the wall on which droplet is kept i gave 80 degree contact angle and the distance between the droplet of radius 1.5mm is 3.5 mm center to center, while other 3 walls i selected no slip condition, and on the top of the droplets i took pressure constraints as 101325 pa. while solving the simulation, when the composite droplet formed due to coalescence is about come in equilibrium condition, the whole composite droplet shift towards right and then come to left. plz suggest me, why this is happening.

Thanks
Rehan

Distributed Heat Source and model coupling

Fri, 03 Feb 2012 18:27:06 +0000

Hello everybody,

I have to convert a model for a simulation of SLM-process from Comsol 3.5a to version 4.2. The whole simulation consists of two models. One model for the material, which will be molten and one modell for the heat source, which calculates the intensity distribution of the laser. The dependet variable for the intensity is I. The 3.5a version model uses a extrusion variable from the powder domain to the laser heat source model domain, which is named T. In the material domain, T is the temperature of the model. The intensity I and the temperature in the material model are linked via a weak form PDE in the heat source model, but this did not work for me in the version 4.2.

So I tried to couple both models the other way round. The transmitted heat in W/m³ is a function of the intensity I. I defined a general extrusion operator named 'laser_extru' from the heat source model to the powder domain. I added a heat source node to the heat transfer module. For Q in W/m³ I added my function 'some_coefficents*mod2.laser_extru(Intens)' with Intens=mod2.I. When I try to solve the problem, I get an error, that the interpolation of the extrusion operator did not work, because no origin could be found and the operator Jacobian could not be computed. I am new to Comsol so I'm not familiar with the extrusion operator concept. Does anybody have a hint for me?

I also had a look at this model (http://www.comsol.com/showroom/gallery/699/). They used almost exactly the same weak PDE as it is used in the version 3.5a model, but my port to the version 4.2 just does not work.

Eigenmode analysis

Fri, 03 Feb 2012 09:02:12 +0000

Hello,
I am a new user in comsol multiphysics. I want to find out the eigenmode in the Titanium.
Can anybody help me?

Fuel droplets in a injection pipe - Problems with the "drag" node

Tue, 10 Jan 2012 17:58:22 +0000

Hello every one,

I'm a student and I need to study a fuel injector as a project. I need some advices to begin with, for instance which physics should I use to model the transport of the fuel droplets from the exit of an injector nozzle, and along a tube with flowing air. My goal is to see at the end, how the fuel reach the combustion chamber a little farther down the pipe.

I use 4.2a with a turbulent physic node, solved with a stationnary solver to have the velocity field and turbulence of the injected air. I have also the particule for fluid physic enabled and solved with a time dependant solver. I have the fuel injection inlet with a initial velocity defined, and I use the drag node to see the trajectories of the particles when they reach the turbulent air flow. But the particles stop right after the inlet, since, I think, I specified in the drag options to use the results of study1 (turbulent air flow) as velocity field en turbulent field. And as the velocity field after the inlet is zero, I think that is why the particles stop.

Eventually, I would like the particles to interact with the fluid flow, but maybe not with the drag option which forces me to inject my particles in the flow. You wil find enclosed my model for further information.

Thank you for your help, regards.

Mesh sensitivity Test

Sun, 25 Dec 2011 09:46:01 +0000

Hi all,

I had carried out mesh sensitivity test on my 2-D asymmetrical model, using stress-strain axial symmetry module to calculate the maximum von Mises stress. However, the more mesh I refine, the result seem does not convergence.The stress value getting bigger and bigger.

Appreciate for expert comment.

Merry Christmas and happy new year.

Phase animation in COMSOL 4.1

Wed, 30 Mar 2011 11:39:19 +0000

Is it possible to get a phase animated plot of a RF solution in COMSOL 4.1?

Thanks.