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circular variable dependency detected

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Hi,

The author of one paper has defined the conductivity as a function of current density and current density as a function of conductivity. In comsol, it gives a circular dependency error. How to get around it? I would appreciate any suggestion in this regard.

Regards,
Susant

Gas detonation in the pipe

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Dear colleagues! Please help me!
I need to simulate process of C2H2 and O2 mixture detonation into the cylindrical tube. But I not sure, is it possible in COMSOL?
Mixture ignition started from closed side of the pipe.
What physics I must to include to project?
I will be very glad to any support or your idea!

Imposing a rotating boundary

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Hi,

I am trying to build a laminar flow model with a rotating boundary to shear the flowing liquid. Unfortunately, when I impose the equation of this rotating boundary, the rotation equation appear correct when I check them manually; but the software does not interpret them correctly and they appear yellow. Please advise on what may be the issue at hand. The model is attached below if you may be helpful enough to assess the model itself and see if you may notice the cause of this error.

I appreciate your help!

Sincerely,
Michael

Coupling a ODE to the Boundary of a Laminar Flow Simulation

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Hello,
I'm trying to simulate blood flow in a tube using a mechanical circuit known as the Windkessel model. This model relates outlet pressure to velocity through a set of equations:
(1) P = RT*V_avg + Q/C = 0
(2) Q = C*R1*V_avg - C*R1*dQ/dt

P is Pressure at the outlet, V_avg is the average volumetric flow rate at the outlet, and Q is a variable related to tube compliance. RT, R1, and C are all fixed parameters which define the circuit.

I want to couple this equation to the outlet as a boundary condition. However since the pressure and volumetric flow rate are connected through the 2nd equation I am unsure how properly couple the ODE module to a laminar flow module.

Using the first equation (1) I created a pressure variable which I used to define the pressure at my outlet. The second equation (2) was then defined using the Boundary ODE module. I have everything fully defined but I am getting errors and I think it is cause I did not properly couple the two modules together. If you could look at my set up and tell me where I might have made a mistake I would greatly appreciate it.
Thank you,

Extraction at Arbitrary Points and NaN

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I have gone through the matlab-comsol livelink manual, and I understand that the NaN output value occurs when the extraction point is outside of the computation domain; but in this case the point is within the domain and I am still getting the NaN value. Please, I will appreciate any assistance or suggestions.

Kindly find as attached the files in case you have sufficient time to run and check.

Regards

FEMI

yield stress

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Hi


I'm trying to understand how can i define yield sterss for a bingham plastic material in COMSOL.


The stress expression for this model is:

shear stress = yield stress+m*(shear rate)


Any comments on this would be a lot of help.

Thanks!

Coaxial Borehole heat exchanger

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Dear COMSOL users,

I work on geothermal heating systems using borehole heat exchanger (see file attached for general functioning of coaxial geothermal exchanger). To that purpose, I use non-isothermal pipe flow interface (calculates fluid exchanger temperature from reservoir temperature) coupled with Darcy's law and heat transfer in porous media interfaces (pressure and reservoir temperature).

So far I was able to model complex geometry of a single layered pipe loop. Now, I was wondering if there is a way to account for coaxial heat exchanger using the non-isothermal pipe flow interface (simplify 3D geometry of the pipe to 1D element). Due to the high extent of the numerical model, I cannot afford meshing the geothermal system explicitly in 3D. I know the solution exists in other numerical codes (such as FEFLOW) and I would like to reproduce using COMSOL.

Any suggestions would be highly appreciated!

Regards,
Morgan Le Lous

heat transfer in fluid flow

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Hi, I have trouble in solving heat transfer in fluid flow with a 2 dimensional porous media, but the temperature i set in the boundaries cant take work. can you share me some case simailar to my work

thank you

welding simulation

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hi,
i am trying to process model tig welding of metal. i solved for temperature profile in my 3d comsol 4.2 model. Now how can i define the elements which are greater than melting temperature as fluid,can i export the data to matlab then identify the nodal points and then define separate physical properties for molten metal.

Earlier reply will be very useful to our progress.

Thanku in advance.

failure to find solution in newer version of COMSOL

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Hi,

I am a student with very minimal experience with COMSOL (most of what I know, I have learned just from experimenting with the program). I am modeling fluid flow through a flow chamber (see attached model). COMSOL was able to find a solution when I ran the study for this model in an older version (either 3.5 or 3.5a, I can't remember). However, now that I have updated to 4.2a, running the same model fails to find a solution (Error message is "Maximum number of Newton iterations reached.") I have tried different mesh sizes, without success.

Since I am not familiar with the commands and capabilities of 4.2a, I am hoping someone can direct me toward where I am going wrong. I have attached images of some results I obtained from running the model in the older version (Reynolds number and velocity field slices). I need these results to be verified when I run the same model in 4.2a, before I can move on to redesigning the chamber.

Thanks!

How to simulate the water boiling by Level-Set method

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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!

how to plot error vs iteration number comsol?

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All I can see is the "reciprocal of step size" vs "Time step" in the progress window of the solution but I do not know how to plot error vs iteration number in comsol. Can I do it real time or it will be shown after the solution is done? I appreciate your help. thank you.

unknown property warning

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Hi ,

I was wondering if anyone is familiar with this warning in the log window :

" Warning: Problem encountered during results while solving.
Unknown property.
- Property: usetunit "

It gives me the result but seems something is wrong !

Thanks,
Mehrdad

error vs. time in iterative solver

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Hi,

I'm using "Transport of Dilute Solution in Porous Media" and the solver is "iterative". I expected to see "error vs. time" graph while solving the problem but still I see "reciprocal vs. time" as convergence plot. Is there any special changes I should do to get error vs. time plot ?

Thanks,
Mehrdad

Enzymatic reactions - Laminar flow + Transport of diluted species

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Hi !

I would like to stimulate a simple reaction A=>P (in presence of E) by considering A as substrate, E as enzyme and P as product. This reaction will takes place inside a rectangular glass capillary filled with water . Width = 0.08mm and Height = 2mm are the dimensions of the capillary.

A and E are two species which give rise to one species P. species E is involved in the reaction but not consumed whereas A is consumed in the reaction. For this, I have coupled two physics "Laminar Flow" and "Transport of diluted species". In Laminar flow, I selected the inlet and outlet boundary conditions. Now, I coupled the velocity from laminar flow to transport of diluted species.

I am facing the issue that, I could select only one species column in transport of diluted species setting.

Please guide me, to select multiple species in transport of diluted species setting. My overall goal is to see the concentration of product produced from given substrate concentration and also how the substrate concentration varies with time according to the product produced.

Thanks


solution as initial condition.!

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Hello all,

I have solved a model of flow over a sphere initially with stationary solver, and I have used this stationary solution, as initial condition for further time dependent study. I got the results but little over predicted.

to trouble shoot the problem, I checked that the drag value from stationary solution is not matching with drag value I got from time dependent solver at t = 0.

Here I think since I have given the stationary solution as initial condition for time dependent study. The value of all variable solved for and not solved for should be same at time t = 0 ?

can somebody please tell me about some possible mistakes I might did?

I am using solution as values of variable solved for as well as value of variable not solved for.

thanks and regards
Atul

Modeling a falling stream of liquid in air: Time-dependent vs. Stationary

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I am trying to model the shape of a stream of oil falling down from a tap to familiarize myself with laminar two-phase flow. By editing the oil drop example in the model library, I was able to solve the time-dependent model. I can use the final time step to see the shape of the oil stream, but I would like to know how to find a stationary solution, since the time-dependent behavior isn't important to me.

I have tried replacing the time-dependent study step with a stationary step, but when I do this, the solution given is no different from the initial conditions.

I have attached my working time-dependent model. Thank you for any help or suggestions!

Reynolds Number

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Hi everyone,

First off thanks for looking at my question and any feedback you could give me would be greatly appreciated!

I'm trying to run some CFD modelling but I'm not sure the flow would be laminar or turbulent as the geometry isn't just a straight tube (see the attached 2D image of the 3D design). Can COMSOL calculate the reynolds number for the geometry so I know which physics I should be used?

FYI I'm a novice at both COMSOL and fluid dynamics.

Thanks for your help.

Running Comsol Desktop Model with Matlab Script: Updating Parameters

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I am using a matlab script to update fluid parameters (viscosity and density) initially defined in comsol desktop. Based on some coupled physics, the density and viscosity of the fluid will change, once the necessary criterion is met. The flow velocity and pressure are extracted and used to determine if the criterion will be met or not, in the first place.

Now, do I need to place the "model.study('std1').run" command line after each matlab update, as schematically illustrated below:

begin code

import com.comsol.model.*
import com.comsol.model.util.*
model=mphload('XYZ');
model.study('std1').run;

% Extract Initial Pressure and Velocity based on initial prescribed parameters in comsol desktop
vf1=(mphinterp(model,'u','coord',coord1a,);
pf1=(mphinterp(model,'p','coord',coord1a);

% Given Criterion
if criterion is met
rho=rho2 (rho2 has been determined in matlab)
mu=mu2;
else if criterion is not met
rho=rho1 (rho1 is the intial fluid density)
mu=mu1;

%Updating the Parameters in Comsol
model.param.set('mu',mu)
model.param.set('rho',rho)

((model.study('std1').run; ))...xxxxxx

% Extracting the Pressure and Velocity Values
vf2=(mphinterp(model,'u','coord',coord1a,);
pf2=(mphinterp(model,'p','coord',coord1a);

% Other Calculations to be done with the second extracted pressure and velocity

end of code

Do I need to put the "xxxxx" marked line above, since I have updated the parameters, and I need to get the new pressure and velocity values? In case I erase the xxxxx line, will the extracted pressure and velocity values, (1) an (2) be different or same, considering the update on fluid properties?
In a nutshell, will the current value of the parameter in the matlab script (since the value of the parameter at latter line of the code is the current) be the current parameter in comsol desktop or do I need to rerun the comsol model with the xxxxx line command, so that I can get the corresponding flow properties based on the updated fluid properties?

Regards

COMSOL support for GPU-acceleration for CFD simulations?

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Dear COMSOL-Team,

are there plans to include GPU-acceleration into COMSOL CFD simulations (such as ANSYS FLUENT or openFOAM/RapidCFD)? I am working in microfluidics with multi-phase flows, i.e. gas-focusing of a liquid to micron-sized liquid streams that break up into microdroplets. Example here:

"Microfluidic liquid jet system with compatibility for atmospheric and high-vacuum conditions"
M. Trebbin, K. Krüger, D. DePonte, S.V. Roth, H.N. Chapman, S. Förster*, Lab Chip 2014, 14, 1733-1745. DOI: 10.1039/C3LC51363G

Looking forward to your reply.

Best wishes,
Martin Trebbin
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