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Open Boundary

March 10, 2016, 5:47 am
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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
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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!
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Rising bubble

March 14, 2016, 9:08 am
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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
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How to continue particle tracing from a certain time point?

March 15, 2016, 8:10 am
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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.
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Error: "Attempt to evaluate negative power of zero."

March 15, 2016, 12:22 pm
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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.


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Spiral vortex

March 15, 2016, 1:18 pm
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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
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Continuity Boundary in ALE + Fluid Flow + EC

March 16, 2016, 7:30 am
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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 .

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Counting particles by inlet; Particle tracing in laminar flow

March 17, 2016, 12:24 pm
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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
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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!
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Liquid-Vapor interface drops very fast

March 18, 2016, 3:01 pm
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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
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Fluid mixing in micro mixer

March 19, 2016, 8:01 am
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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
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Initial Pressure - error

March 21, 2016, 3:00 am
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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
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Time-Dependent initial solution violates specified initial conditions

March 24, 2016, 3:19 pm
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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?
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Water jet exiting to atmosphere

March 25, 2016, 1:52 pm
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I am a 2nd semester UG Mechanical Engineering student and have no background in CFD and COMSOL. I was given a project by my fluid mechanics professor and was sort of stuck

I need to simulate a water jet exiting a nozzle and striking a plate. I need to find the pressure at the points that it strikes the plate and also the volume flow rate of the water exiting the nozzle.

If someone could provide a tutorial similar to my problem, I would really appreciate it.
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How to plot time-dependent temperature profile at a point

March 27, 2016, 6:59 am
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Hi all,

I have 1 question over here, i have been using the SAR head tutorial (Provided by comsol tutorial) to do research for my school project and i have 1 problem over here which is, i cant seems to plot temperature VS time graph. i had searched though the internet for solution and i did follow their steps but still i cant manage to get it. whatever i try to plot, the time will varies but the temperature remains the same throughout which makes my curve looks like one horizontal straight line.

I will really appreciate if someone can take a look at the attached picture (i can't seems to upload my model file here) and suggest to me what should i do next.

Thanks,
Boon
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mixer simulation, CFD

March 28, 2016, 1:31 pm
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Dear community,

Can you help me with the following:

Is there Comsol manual which has all the information about the rotating machinery which is an add-on of CFD ?
I have followed the tutorials available but in tutorials they are uploading the geometry and assigning the boundary conditions directly by selecting the predefined options in Comsol which is not possible when we create model from scratch.
Could you please tell me difference between rotating domain, rotating wall and rotating interior wall.
Why there is difference in selecting rotating domain in 2D ( by creating circle around impeller) and 3D ( directly selecting the impeller geometry).

I kindly request you to please help me in this
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PDE General for Navier-Stokes equations

October 27, 2011, 12:53 pm
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Hi I am trying to use PDE General module in COMSOL 4.2 to simulate Euler equations. I know that we have a Laminar Flow module than I can put viscosity equal to zero to have Euler equations but I would like to solve it in PDE forms.

I have a question regarding defining different variables. So this is the case if I want to solve PDE general module using conservative forms of Euler equation I will have
U=[rho, rho*u,rho*v]
gamma=[rho*u rho*v
rho*u^2+p rho*u*v
rho*u*v rho*v^2+p]

as you can see I have a pressure term that I am not sure what should I do with it since I cannot express it as my conservative variables unless I define another set of equations for energy to solve Temperature and connect "rho", "T" to "P". Even in this case I need to have a relationship like the one that we have for ideal gas to solve this. Does anyone have any suggestions for my problem?

Bottom line how should I define my variables to solve something like Euler equation or to some extent Navier-Stokes equations with the General PDE module of COMSOL?

Cheers
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Centrifugal Force

March 9, 2016, 5:08 am
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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!
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Laminar and turbulent flow in the same interface

April 2, 2016, 9:05 am
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Hello everyone,
I have a non-isothermal flow ( a melt and argon) and I would like to know if it is possible to simulate a lamnar flow (melt) and a turbulent flow (argon) on the same fluid flow interface.
Thank you.
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Two phase fluid flow adding material color

April 4, 2016, 6:28 pm
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Hello all,

I am attempting to model the combination of three fluids in an angled T-mixer (two of them are being modeled as water, and the other is a non-newtonian fluid). I think I have the physics set up correctly, but I am having a problem in postprocessing. I would like to be able to plot each fluid as a separate color in a stationary study. Any help would be greatly appreciated!

Best,
-Will
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