(Two phase)Bubble shrink in a two phase flow

October 14, 2014, 3:32 pm

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I want to simulate a bubble dynamics in the water.
I initialize a bubble in the water, and use phase-field model.
I give the water a velocity in a channel, the bubble moves with the water.
After a few time steps, the bubble become smaller and smaller, than disappear!!!
It is very odd, as I set incompressible flow and a uniform mesh.
Other parameters are default.
What is the reason for this phenomenon.

Thanks.

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Gas stratification model

March 7, 2016, 1:17 pm

≫ Next: Heat Flow in simple pipe with water

≪ Previous: (Two phase)Bubble shrink in a two phase flow

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Hi everyone,

I recently started using COMSOL, and I am having a lot of problems with a model that, in theory, should be extremely easy.

I need to model the stratification of a mixture of oxygen and helium in a closed vessel, due to gravity.

I used a Laminar Flow physic (to use a Volume Force, dependent on local density), and a Transport of Concentrated species, to get a value for the density. Boundary conditions are pretty easy - walls and no flux everywhere. Since I am interested in a proof of concept for stratification, I set the diffusion coefficient to 0 (once the model works, I will modify that, of course). I used a linear function of the height of the vessel as initial condition for the mass fraction of oxygen, so I have a different effect of gravity from the start.

With this model, I incur in the following error:

Failed to find consistent initial values.
Last time step is not converged.
- Feature: Time-Dependent Solver 1 (sol1/t1)

I even tried to switch the transport of concentrated species to the transprt of diluted species, using a variable tied to the concentration to account for the density difference . Same identical error.

I really have no idea how to solve this problem, any help will be greatly appreciated.

Heat Flow in simple pipe with water

March 8, 2016, 6:21 am

≫ Next: Water hamme problem solving - need help

≪ Previous: Gas stratification model

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Hello,
I'm really new to heat flow computation and I have no Idea how to calculate time and distance of water flowing in pipe when water temperature will stay constans.
Model:
pipe 50 m long, 0.04mm diameter, 0.036 mm inside diameter, PVC
buried in ground 2 m deep
ground temperature 10 degC
inlet water 22.7 degC, no idea of velocity (lets say 0.01m/s)
what will be the water outlet temperature afer 1h, 2h, ... 12h of flow
on what disance (from inlet) and at what time water temperature will become steady.

best regards
Jacek

Water hamme problem solving - need help

March 9, 2016, 4:27 am

≫ Next: Centrifugal Force

≪ Previous: Heat Flow in simple pipe with water

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Hi,
I am trying to simulate water hammer phenomena to validate the published results for a straight piping system. The piping system is connected to a contact head reservoir at one side and there is valve at the other end of the piping.

Now the problem is that is water hammer module provides a closed boundary condition for the piping end connected to the valve. It means it is considering sudden closure. BUT I want to simulate gradual closing of valve. Suppose the valve is linearly closed in 2.1 sec then how to provide this boundary condition in comsol 4.4. Please help.

Centrifugal Force

March 9, 2016, 5:08 am

≫ Next: Error-Automatic remeshing

≪ Previous: Water hamme problem solving - need help

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Hi,

I would like to include centrifugal force in my COMSOL (version 4.3a) model of a curved channel with laminar flow containing small particles.

Is it necessary for COMSOL to find the first and second time derivatives of the particles moving in x and y directions (to find the velocity and acceleration of these particles) at each point on the particle trajectory as the particles flow round the curve, and plug these values into the equation for the radius of curvature;

R= (x^'2+y^'2 )^(3/2)/(x^' y^''-y^' x^'' )

Since the radius of curvature is changing continuously for each particle?

Would COMSOL then use this R value to calculate the centrifugal force

F = mR(ω^2)

at each position the particle occupies?

Thanks in advance for any help that can be offered on this!

Error-Automatic remeshing

January 25, 2013, 1:09 pm

≫ Next: No change in laminar flow simulation results even after changing material properties like Dynamic viscosity and Density.

≪ Previous: Centrifugal Force

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Hi,

I encountered a problem about "Automatic remeshing" with "deformed geomerty" and Time-Dependent Solver

When I ran the program, Comsol always displayed a Error:
Unsupported operation on this type of object.
-Feature: Time-Dependent Solver 1 (sol1/t1);
-Error: Unsupported operation on this type of object.

i try all possible mesh quality expressions and resonable threshold vaules, it seems when the program reaches this value,

the above error happens immediately.

Who can tell me what's wrong?

Your information and help is deeply appreciated!

Luyao

No change in laminar flow simulation results even after changing material properties like Dynamic viscosity and Density.

November 22, 2013, 2:05 am

≫ Next: Open Boundary

≪ Previous: Error-Automatic remeshing

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Hi,

I have been doing very straight-forward and simple laminar flow simulations in a chamber. I have observed that flow simulation results don't change if I add and use another fluid with different dynamic viscosity and density. I tried varying the viscosity values but I couldn't notice any change in velocity gradients. I don't fully trust my results because of this reason.
Also, I would not be able to run these flow simulations if I don't mention at least dynamic viscosity and density of the fluid.

Thank you for having a look.

Best,
Vishal

Open Boundary

March 10, 2016, 5:47 am

≫ Next: Unrealistic Particle Behavior

≪ Previous: No change in laminar flow simulation results even after changing material properties like Dynamic viscosity and Density.

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Hi everyone,

I have a model with two domains one of which is laminar flow and another is porous media domain (Darcy's law). The fluid stream percolates from the laminar flow domain into the Darcy law domain. I modelled the geometry as a union and then used "open boundary' condition in laminar flow but in the Darcy law domain the open boundary is still showing as wall! Any suggestions would be appreciated.

Thanks

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Unrealistic Particle Behavior

March 10, 2016, 3:58 pm

≫ Next: Rising bubble

≪ Previous: Open Boundary

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Hi,

I am simulating a 2-D Model, where I want to analyze the convection effect of a heater which is heating up the air above. I want to see the effect of particles, that are distributed in the air. I separated the problem into 2 studies.
1. Study: Analysis of laminar flow and heat transfer
2. Study: Particle tracing for fluid flow

The first study seems to give reasonable results, but in the 2. study in the first time step all particles fall down 1 cm.
- I decreased the 1. time step to different very small steps, but the result is always the same.
- I disabled the gravity force in the particle tracing study, but the result stays the same.
- When I disable the volume force in the laminar flow, the problem does not appear anymore. But obviously the results are wrong in this case

This behavior seems to be physically odd, because the effect of gravity should happen over time and not instantaneously, since the density is supposed to be a function of temperature.
For the volume force I can only use: F=-spf.rho*g_const
but if I try to use: F=-mat1.def.rho*g_const, I get the following error message:
Undefined variable.
- Variable: pA
- Geometry: geom1
- Domain: 1
Failed to evaluate variable.
- Variable: comp1.mat1.def.rho

It would be great, if somebody could help me on this!!
Any help is greatly appreciated!

Rising bubble

March 14, 2016, 9:08 am

≫ Next: How to continue particle tracing from a certain time point?

≪ Previous: Unrealistic Particle Behavior

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Hello comsol community members,

I'm trying to simulate a rising air bubble in water. I found the rising oil bubble application in the application library and tried to adjust it to use air instead of oil, but I'm running into this error:
Repeated error test failures. May have reached a singularity.

I searched for the error and found that it is because there are too little initial conditions. Could anyone assist me finding good initial conditions? Or if there is something else wrong with my model.

I've attached my model.

Kind regards,

Jurre

How to continue particle tracing from a certain time point?

March 15, 2016, 8:10 am

≫ Next: Error: "Attempt to evaluate negative power of zero."

≪ Previous: Rising bubble

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Dear all,

I am now working on the simulation of particle tracing with fluid flow.

I have finished the staionary fluid flow and time dependent particle tracing calculation and obtain the particle tracing results.

Now I want to apply a certain calculated time point particle to continue with another turbulent flow mode. Are there anyone knowing how to do this?

Thank you very much.

Error: "Attempt to evaluate negative power of zero."

March 15, 2016, 12:22 pm

≫ Next: Spiral vortex

≪ Previous: How to continue particle tracing from a certain time point?

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I'm building a model using the Reaching Flow in Porous Media in a 2D axis-symmetric component, and my goal is to remove some of the dilute species from the system as a function of concentration and fluid velocity (e.g. the reaction slows as concentration decreases, and is increased when fluid velocity is high).

I can get the model to behave as expected, but I cannot successfully implement a velocity dependent Reaction. I am able to use a constant (e.g. -0.01 [mol/m^3/s]) or concentration dependent reaction rate, but if I try to use average velocity I get the error:

Attempt to evaluate negative power of zero.
- Function: ^
Failed to evaluate temporary symbolic derivative variable.
- Variable: comp1.rfds.U@VDN$comp1.w2
- Defined as: (0.5*(((comp1.u2^2)+(comp1.w2^2))^(-0.5)))*(2*comp1.w2)
Failed to evaluate expression.
- Expression: d(comp1.rfds.U,comp1.w2)

I attempted to incorporate the velocity in the porous domain (or any domain.. just trying to get anything to work for starters) by setting up an Average of the porous domain (Component --> Definitions --> Average), then defining a variable as the velocity magnitude of that domain (comp1.aveop1(rfds.U)).

I have consulted other threads here and saw recommendations to use 'eps' when defining my variable to prevent my initial expression from being zero, but this doesn't seem to help. Any guidance is appreciated, and I can provide further details of the model as necessary.

Spiral vortex

March 15, 2016, 1:18 pm

≫ Next: Continuity Boundary in ALE + Fluid Flow + EC

≪ Previous: Error: "Attempt to evaluate negative power of zero."

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Hi,
This is Montassar Sharif, I'd like to simulate spiral vortex. I have seen two or three examples but they are not what I need. The vortex in those examples ismoving in need to simulate some thing like tornado
could you please help me with that?
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MASharif

Continuity Boundary in ALE + Fluid Flow + EC

March 16, 2016, 7:30 am

≫ Next: Counting particles by inlet; Particle tracing in laminar flow

≪ Previous: Spiral vortex

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Dear Comsolers ;

I am modeling a transient discharge through a flowing fluid in a nozzle. one of the electrode is moving so i created a sliding moving mesh for the movement by creating an assembly resulting a continuity boundary between the moving domain and the rest of the model. thus i had to add a continuity BC for the fluid flow ( hmnf ) . everything works fine but i am really disappointed by the results as in the attached images.
( In the attachment as you can see, an image of power dissipation and electric field. )

the continuity BC seems to effect the flow however i fine the meshing !! i also used automatic re-meshing but results still the same !! Extremely fine mesh for boundaries even for the near domains didn't work better !

weak constraints for the continuity gives an error in the hmnf interface !
Any help would be really appreciated .

Counting particles by inlet; Particle tracing in laminar flow

March 17, 2016, 12:24 pm

≫ Next: Gas velocity profile in laminar flow model

≪ Previous: Continuity Boundary in ALE + Fluid Flow + EC

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Hello Comsol community!

I'm simulating a simple 4-way intersection using the laminar flow module. Two streets are inflows and two are outflows. Thereafter I'm adding particles to each input flow with the particle tracing for fluid flow module to trace how many of the particles coming from inflow 1 ends up in outflow 1 and 2 respectively, and vice versa for inflow 2.

My problem is that I can only count the total number of particles in each outflow and I can't find a way to distinguish were each particle is coming form.
I would really like to do this without exporting to MatLab or similar since I plan on doing a semi-large parametric sweep over different intersection geometries and I don't want to fiddle around with MatLab scripts, counting particle IDs, for each case.

Thanks for any help!

Sincerely Linus Olofsson

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Gas velocity profile in laminar flow model

March 18, 2016, 1:31 pm

≫ Next: Liquid-Vapor interface drops very fast

≪ Previous: Counting particles by inlet; Particle tracing in laminar flow

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Hi, I am simulating gas velocity and tempeature profile in a tube using laminar flow model. But I have some problems:
1. In "Heat flux" text field, "[1/cm]", why "cm" is devided by 1, rather than "cm" directly? Is "cm" the same with the unit that I defined for ractangle?
2. At this stage (see in the model), the mesh is set to extremely course, it works. However, when I choose normal mesh, it does not work at all. What's the problem? There only should be a diffrence of calculation time.
3. In this model, both pressure and temperature profiles are right, only the velocity profile does not look like that in our quartz tube. Is there any solutions?
4. Study 2 (time dependent) still does not work at this status.
Can anybody help me solve them? You can change some parameters in the attached model! Thanks a lot!

Liquid-Vapor interface drops very fast

March 18, 2016, 3:01 pm

≫ Next: Fluid mixing in micro mixer

≪ Previous: Gas velocity profile in laminar flow model

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Hi

I have encountered a problem in COMSOL when using the “laminar two-phase flow, moving mesh” physic.
A simple model indicating what I am trying to solve is provided in the attachment.

As you can see in the attached file, the liquid evaporates at a very low pressure (300 Pa) with a uniform mass flux of 1e-3 (kg/m2s). When the contact angle is 90, everything goes well. However, as I change the contact angle even a little bit from 90 (i.e. pi/2.1), the interface drops very fast, which is physically meaningless. I have had this problem with both COMSOL 4.3b and 5.1.

I could have been able to improve it by making the contact point meshes extremely fine. However, this gives me weird values of pressure (absolute negative pressures) in these regions.

I appreciate if you could help me sort out this issue.

Amin

Fluid mixing in micro mixer

March 19, 2016, 8:01 am

≫ Next: Initial Pressure - error

≪ Previous: Liquid-Vapor interface drops very fast

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Hi,

I am working on a project and in one part I have use Comsol. This is the first time I am using Comsol and badly need some guidance.

I am working with a micro mixer where I have mix two fluids. In brief, first I want to fill all the channels of the mixer with DI water at a rate of 5 µl/min through inlet one and when all the channels are filled the flow is halted. Then I want to flow a higher index liquid (highly concentrated CaCl2 solution) again at a rate of 5µl/min through the second inlet. So water will be replaced by the solution of cacl2 but it will not be replaced abruptly. There will be a period of transition and the mixer will maintain a homogeneous mix of the two liquids during this period. As a result with time, change of concentration will occur. I want to monitor this change of concentration on a period of 50 second.

Being a new user I am a bit confused which physics should I use and how to couple them in time dependent condition. Any suggestion or advice in order to express my logic properly in Comsol will be great.

Thanks in advance
Haider

Initial Pressure - error

March 21, 2016, 3:00 am

≫ Next: Time-Dependent initial solution violates specified initial conditions

≪ Previous: Fluid mixing in micro mixer

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Hi everyone,

I try to model the influence of a heated boundary on surrounding air in a 3-d model. When I try to set the initial pressure in the laminar flow to:
-spf.rho*g_const*(0.05[m]-z)

I always get the following error message:

Error in user-defined function.
- Function: dcomp1.mat1.def.rho_drho__T__internalArgument
Failed to evaluate temporary symbolic derivative variable.
- Variable: comp1.ht.rho@VDN$comp1.T
- Defined as: (unit_kg_cf*(dcomp1.mat1.def.rho_drho__T__internalArgument(comp1.ht.fluid1.minput_pressure/unit_Pa_cf,comp1.ht.fluid1.minput_temperature/unit_K_cf)*(1/unit_K_cf)))*(1/(unit_m_cf^3))
Failed to evaluate temporary symbolic derivative variable.
- Variable: comp1.ht.C_eff@VDN$comp1.T
- Defined as: (((unit_kg_cf*(dcomp1.mat1.def.rho_drho__T__internalArgument(comp1.ht.fluid1.minput_pressure/unit_Pa_cf,comp1.ht.fluid1.minput_temperature/unit_K_cf)*(1/unit_K_cf)))*(1/(unit_m_cf^3)))*comp1.ht.Cp)+(comp1.ht.rho*((unit_J_cf*(dcomp1.mat1.def.Cp(comp1.ht.fluid1.minput_temperature/unit_K_cf)*(1/unit_K_cf)))*(1/(unit_K_cf*unit_kg_cf))))
Failed to evaluate expression.
- Expression: dvol*(comp1.ht.d*(comp1.Tt*(-((((unit_kg_cf*(dcomp1.mat1.def.rho_drho__T__internalArgument(comp1.ht.fluid1.minput_pressure/unit_Pa_cf,comp1.ht.fluid1.minput_temperature/unit_K_cf)*(1/unit_K_cf)))*(1/(unit_m_cf^3)))*comp1.ht.Cp)+(comp1.ht.rho*((unit_J_cf*(dcomp1.mat1.def.Cp(comp1.ht.fluid1.minput_temperature/unit_K_cf)*(1/unit_K_cf)))*(1/(unit_K_cf*unit_kg_cf))))))))
Failed to evaluate Jacobian of expression.
- Expression: dvol*(((-comp1.ht.C_eff)*comp1.Tt)*comp1.ht.d)
Failed to evaluate Jacobian of expression.
- Expression: (-comp1.ht.C_eff*comp1.Tt*test(comp1.T)*comp1.ht.d)*(dvol)

It would be great, if somebody could help me!!

Thanks a lot in advance! :D

Time-Dependent initial solution violates specified initial conditions

March 24, 2016, 3:19 pm

≫ Next: Water jet exiting to atmosphere

≪ Previous: Initial Pressure - error

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I am trying to build a dye-washout model (using Reacting Flow in Porous Media physics to predict the time-dependent concentration of a dilute species). I'm able to run simulations for varying lengths of time, but I'm having issues with my initial conditions.

I specify what the initial concentration of the dilute species should be with an Initial Values node, but when I run the time-dependent study the t0 concentration does not match what I specify. For example, I specify concentration should uniformly be 0.1 mol/m^3 at t=0, but instead the solution will show a non-uniform concentration distribution with an average ~5-50% lower than the Initial Values node indicates. The discrepancy from the specified initial concentration also changes with study length (i.e. ~5% error for study from t=0-0.1sec, ~50% error for study from t=0-10sec).

Am I specifying initial conditions incorrectly? Is there a way to strictly enforce initial conditions?