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Weak contribution to levelset interface

July 14, 2014, 1:31 am
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Dear all,

I would like to add a weak contribution (for surface tension) to a moving levelset interface for the following application:
I want to model a fluid with a free surface, on which there should be a mass source. The fluid surface is deformed by fluid flow and surface tension.

I already tried to model this with a moving mesh approach but these simulations did not run stable enough, if ever. Furthermore I thought about two-phase-laminar-flow. Here a surface tension at the interface is possible, but no mass source.

That's why I decided to track the interface between air and fluid with levelset. Here I can define: interface velocity = fluid velocity + velocity representing the mass source

Does anyone know how to add a weak contribution to a moving levelset interface? Or is there another way to implement the surface tension (for example how is it realized in the two-phase-laminar-flow)?

I would be very happy if anyone could help me!
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Problem with moving mesh

June 22, 2016, 1:11 am
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Hello

I'm trying to build a 2d-axis model of laser heating process, which will melt the powder bed.

The free-surface will deform according to the surface tension and mass loss due to vaporization. Here I use the ALE moving mesh to account the free-surface deformation. I set the boundary condition so that the mesh displacement happens on the boundary at the top. And the axis boundary on the left side only has displacement in z-direction.

But I encounter a strange problem of mesh deformation. As shown in the figure, the mesh will cross the axis boundary. A series of strange mesh deformation happened. However, I have set the mesh displacement of the axis boundary to zero at r-direction. Sometimes, this kind of mesh deformation also happens on the boundary of the right side, whose mesh displacement is also zero in r-direction.
I have no idea how this problem happens.

I'm sure all the domain, boundary conditions are propre.

Have this kind of problem happened to you in ALE moving mesh modeling.

Thank you for your response.

Chaoyue CHEN
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3d Laminar Flow

July 27, 2016, 5:49 am
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Good Afternoon

I´m trying to run a very simple 3d simulation about an air flow that flows through a cylinder (radius=1,75mm) inscribed in a cube(side=4.3mm). Now, when I put between the inlet and outlet a fixed value of pressure (2000Pa) the simulation run without any problem; But if instead of the value of inlet pressure I put value of the mass flow 0.0007 [Kg/s] the solution does not converge. I tryed to change many settings (solver, tolerance and mesh) but it seems to work just for a very small amount of flowing air, like 13e-9[kg/s], that is a very small value.
I run out of ideas, can please someone help me to figure out on what I have to focus my attention.
Thank you in advance.

Renato Baldanzini
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Multiple-size particle tracing

October 30, 2012, 4:24 am
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Hello everyone

I have a short question on the particle tracing module. Does anyone know that whether it is possible to simulat particles of different sizes in the same model. How to implement this since I could only add one unique particle property?

Many thanks

Yuexia
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How to simulate the water boiling by Level-Set method

July 19, 2012, 3:53 pm
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There is a good example simulating water boiling by phase field.
I am trying to use the Level-Set method. However, I was frustrated by many times of failure.

My problem is how to deal with the interface using weak form contribution.
Level set and Phase field can deal with the multi-phase flow well.
But, if the phase change at the interface happens, weak form equations have to be used to.

In the Phase field example,
the weak form contribution is test(psi)*phi_source+test(p)*usource.
1) I want to know why used test(psi) and test(p) as the test function?
I used test(phips) and test(u) respectively, the results change a lot. But I don't know why. I might know how to get the weak form from strong form, but I don't know how and why to choose the exact test function.

2) I derived the level set form, and tried to add correct weak form to simulate the boiling phase change.
But the results were totally wrong, So, can somebody help me find the way to understand the level set weak form or give me some examples.

Thank you!
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Continuous Casting

July 29, 2016, 3:12 pm
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Hi,
I am modeling solidification of casting process using the following model:
www.comsol.com/model/continuous-casting-382

The alloy I am using is different hence the parameters are different. The geometry is similar, except that I have a larger diameter.

I have estimated the heat transfer coefficients and they are 3-4 times that used in this problem. My solution does not converge for these higher values. However, using lower values of heat transfer coefficients, I obtained convergence.

What could be the source of this error? How do I tackle it?

Thanks in advance
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Plot data to Export - Number of data points

August 1, 2016, 11:26 am
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Hi all,
How can i determine the number of data 'points' (for example in txt) of an exported plot?
For example, the velocity profile of a fluid flowing inside a cylinder(with D diameter) , may have a curve of n points(in excel) for the D or n+m for the same distance/Diameter.. in other words, the step moving from 0 to D differs and consequently the number of data points(finally i get with txt at excel).
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internal variable porosity permeability

August 1, 2016, 8:16 pm
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I, beginner to COMSOL, am using poroelastic model in COMSOL fully saturated porous medium, all the pores are filled with fluid. Hence porosity is equal to the ratio of volume of the fluid to the total volume. The solid deformation will affect porosity an permeability.
When the volumetric strain changes occur in the model, the porosity must change accordingly. But in COMSOL poroelastic model, the variable porosity seems to be constant through out analysis.

Porosity seems appear only in storage coefficient but not inside any differential term.


How could i input this into model ?? Would anyone has any idea ?? I am sure this is already implemented in Abaqus, but i am wondering how to do it in COMSOL ?

I desperately need to model porosity as vol. strain dependent as permeability in soft tissue is porosity/strain dependent.

Thank you in advance !
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two phase laminar flow in 3d

June 9, 2016, 9:02 pm
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hi I m supposed to simulate two-phase laminar flow problem (moving mesh).

the size of a cyl is ,
r=5 h=5
The bottom of the cylinder is fixed and the top of the cylinder is moving with a certain speed in normal direction.
the side face of the cyl. is exposed to the air.

geometry
cyl : r=5 h=5
material water
physics
wall at h=0 : no silp and predescribed mesh velocity =0
wall at h=5 : moving wall u=0.01 m/s and predescribed mesh velocity=0.01m/s
r=5 external fluid interface predescribed mesh velocity inactive

the error mesg. :
remeshing fulfilled for initial time. or last time step of newton method is not converged.
singularity problem..etc


help.





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Particle trajectory. it seems to be same velocity regardless of size of particle.

August 8, 2016, 10:52 pm
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Hi. I'm trying to simulate particle tracking in the fluid(water)
and my situation is laminar flow.
And I applied Drag force, but the particle is moving to same direction.
I think the particle shows different movement as diameter, because Drag force depends on diameter of particle.
Even though comsol calculated Drag force which is related with diameter of particle, the velocity is the same.
why?

For example, there are 1um particle and 10um particle.
Drag force was calculated as 1N and 10N. it is obviously as equation.
However the velocity is the same. so two particles are moving the same.

is anyone why it is occurred?
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Trouble coupling heat transfer physics for both fluids and solids

April 24, 2012, 9:19 pm
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Hey all,

So I'm getting stuck with my model where I try to couple three different physics. The three physics are: Electric Currents, Heat transfer in Solids, and Heat transfer in Fluids. Basically what I'm trying to model is a silicon microbridge which is doped, which makes it so that it has a temperature gradient accross it when heated. I am able to couple and run the electric currents and heat transfer in solids physics, where the power dissipated from the ec physics is inputed into the ht physics as the heat source, and then the temperature that is calculated is used to calculate a new resistivity value for the silicon, until the model converges to a value for the temperature at the given voltage.

The problem arises when I try to incorporate the ht2 physics to mimic the effects of natural convection from the sorrounding gases. I set a temperature boundary condition within the ht2 physics which tells the program that the temperature of the microbridge is given by T, which is calculated in the ht physics. However, when I run this, I get the following error:

"Constraing found for variables in different Segregated groups. Try to merge these groups.
-Feature: Stationary Solver 1 (sol1/s1)
-Error: Constraint found for variables in different Segragated groups. Try to merge these groups."

Any ideas? Attached you will find my model. Thanks.
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Which interface choose for fluid flow with heat transfer

August 5, 2013, 7:05 am
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Good day!

I would like to ask for an advice.
I wish to solve the following problem: I have a plate of a solid material with low thermal conductivity. There is a pipe with cold water inside the plate. The water flows throuth the pipe with a fixed rate and goes outside. I need to simulate the temperature field in this system.
I'm not sure which interface I should use: "Conjugate Heat Transfer" or "HT -> HT in fluids".

Thanks!

With best regars,
Irina
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Pulsating liquid surrounding the human brain

August 10, 2016, 7:24 am
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Dear COMSOL community,

after reading the COMSOL documentation and this forum for a couple of weeks I am reaching out to you. I am using COMSOL V4.4.

I have been desperately trying to run my simulation model and either failing or receiving a very slow convergence (time step in the order of 1e-6). So I am hoping for some fresh ideas and help. Or just one or the other comments would be really helpful as most of you had a whole lot more experience and expertise as myself who is a master’s student of biomedical engineering.

It is a 2D FSI model and I am trying to model the pulsatile movement of the liquid that surrounds the brain and its. The pulsatile motion comes from the pulsation of the expansion of the arteries that are all over the human brain. So the model consists of the brain and the spinal cord. Both of them are surrounded by the liquid. Within the brain there is a wormlike structure which also contains some of that liquid. In the ongoing research a parameter dependency study shall be conducted.

Inlets: I do have 14 inlets , “holes” all around the brain which are supposed to model the arteries. Fluid comes at a given time dependent velocity and goes back, netto volume flow is equal to zero. Together with the area it is supposed to be a kind of Volume flow as Volume flow=velocity*area. The same idea is behind the the artery which is in the brain itself. “Volume” is added to the area and the expansion together with the “volume” from the 14 arteries shall drive the pulsation. I can’t define a mass flow because I don’t have the CFD license.

Materials: The brain and spinal cord shall be modeled as an linear viscoelastic material model, Generalized Maxwell. The elasticity of the spinal canal shall be modeled as an membrane at the bottom of the spinal canal. I also have been thinking of removing that mebrane and replacing it by an outlet with a time dependent pressure which is known, as I do have But then mass seems to be out- or inflowing into the system which is supposed to be closed.

My ideas what one should check:
- the way the reality is modelled (are further simplifications modeled)
- initial values
- boundary conditions (especially inlets/outlets,)
- fixed constraints
- solver settings
- do I have too many degrees of freedom
- material model (especially the viscoelasticity because by disabling it the DOF are decreased, from 400k to 200k)
- is it really necessary to model the movement of the structure as I receive really small movement, movements is seen with a scaling of the deformation by 1e10)?
- is the mesh I came up with (different element types) okay? Is it too coarse for the first try? Shall I add a boundary layer as I deal with CFD?
The .mph file, some photos and the geometry are attached to this post. I highly appreciate all your efforts and look forward to your comments and ideas. Feel free to ask in case of any questions or uncertainties!
Many regards
Artem


I am sorry but when clicking the "Attach File" button nothing happens so here are some links to the files in my Dropbox.
www.dropbox.com/s/b3fj2fo38gsc...rain_liquid_pulsating.mph?dl=0
www.dropbox.com/s/nud3zebchjm74hj/brain_geometry.mphbin?dl=0
www.dropbox.com/s/a4119buq3tie375/membrane.png?dl=0
www.dropbox.com/s/fynuw24pkrclyln/brain.png?dl=0
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Mixing of Water in a Flat Bottom Mixer

August 11, 2016, 5:33 am
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Hi,

I am doing the tutorial for the Mixing of Water in a Flat Bottom Mixer model according the instructions available on the COMSOL site, and after I insert the geometry sequence and click the Build All button, I keep getting this error: You need to provide input entities, -Property: input. I cannot procede from there, because I cannot select any domains in the Physics tool bar. Any help and advice would be appreciated.

Thank you.

Alen
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serval questions of heat transfer in my project

May 2, 2016, 6:52 pm
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Hello
As the figure shows, I am using the fllowing models
(Heat Transfer>Conjugate Heat Transfer>Laminar Flow
Stationary situation
Surface-to-Surface Radiation), and try to find the temperature distribution of the air and the spheres.
I simplified the original object into two spheres,since its shape was very complex. The latter sphere has a boundary heat source.
I have some questions to solve:
1.Since the object was simplified, so the real convective area was changed, i need to modify the convective heat transfer coefficient, but I can't find the modify interface in comsol.
2.The Surface-to-Surface Radiation was considered, and the model is periodic, so I need to define the periodic boundaries as perfect mirrors (the default insulated conditions are just fine to the convective heat transfer and conduction heat transfer)， but I can't find the interface in comsol,too.
3. Since the original object was simplified into two spheres, so the heat conduction was incorrect，because the spheres are not contacted.I want to find the ht.gardT between the spheres and use the Fourier's law of heat conduction to define extra heat source on spheres, is it right?
thank you
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laminr vs turbelent

August 14, 2016, 8:21 pm
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hello !
my model is PV cell with duct embedded below in order to cool it down . I used laminar conjugate heat transfer ,so the result show good .But depend on Reylond number the flow is turbulent so I try to solve it using k_epsilon turbulent but the result give unexpected high temperature for PV panel reaches to 90 c.As I know the turbelent flow rises the heat exchange coefficient .
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Staggered grids

August 14, 2016, 9:07 pm
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Hi,

How can one apply staggered grid in comsol 5.2?

Thanks,
Mahvash
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meshing

August 15, 2016, 11:03 pm
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hello !
my model is rectangular PV panel(6 mm thickness) with embedded duct of height 30cm .

I used conjugate heat transfer to compute the temperature of PV

how can I mesh this component in right way with PV of small thickness , and can I used to 2d it is time consuming using 3d, or 2d just used with specific case .

best regard ,
hussein
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Non-Convergence

August 17, 2016, 10:27 pm
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Hi,

For a particular simulation, a time dependent problem converges for a lesser time period, say 4 hours. But as we increase the time say 12 hours the same problem doesn't converge. What could the problem and possibility of convergence of such a problem.

Warm Regrards,
Gurpreet Singh Sodhi
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Is there any possibility to model rotating porous media in comsol?

August 18, 2016, 1:07 am
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We are to purchase COMSOL; however, we will be having a system with a rotating porous media. Can the free and porous media flow be coupled with rotating machinery module? On the advanced stages of our modeling, the porous media also has some surface reactions as well while rotating.
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