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Effect of sound pressure on refractive index/density

January 21, 2013, 8:43 am
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Hello fellow Comsol users,

Most of my experience comes from structural mechanics and fracture and would like to ask a little help regarding pressure acoustics.

The presence of high and low pressure in air will change it's refractive index due to a change in the density. I would like to model this change in refractive index, but am a little lost.

Could anyone tell me how I could go about modelling/solving for a change in material property?

I am using a time dependant pressure model in air. Within the materials section I can see that the density is a function of temperature and pA [which I assume to be absolute pressure], but when I plot the change in density it says that there is no change in density even if there is a change in pressure.

I'm sorry if this question sounds a little dumb, I'm sure that I am just being stupid today.

Also, I have had a look at the forum and a little in the documentation but could not find anything to help.

Anyone have some ideas out there? I would greatly appreciate your help.

Rob
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About creation of struturegeomerty

February 20, 2013, 9:24 pm
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Hi...!
I am a student of University of Pune,working on Rf MEMS Switch. I can't get how to create a structural geometry of cantilever beam in Com sol multiphysics.
So please help me to create a geometry.

--
uhyadav
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AC Electrical Field simulation in interdigit structure

April 6, 2013, 4:22 am
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hi

I want to do AC filed simulation for interdigit structure. I am wondering what kind of physics should I select and how I can apply AC signal to my structure?

I would appreciate any help
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Temperature change from expansion of air

April 8, 2013, 6:38 am
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I am trying to model the flow fields of a fluid (air) traveling through an orifice. I have a working model that works great using only the laminar flow module. However, the viscosity of the fluid is always constant as there is no temperature change. So, I want the model to also calculate the temperature change due to the compression of the fluid in the nozzle and then the expansion of the gas following the nozzle. Is there a simple physics to add that will include this temperature change? I would like somthing similar to the high-mach-number flow physics, but without having to use this module as the Mach number does not get above ~0.3. Any help would be great.
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Coupling B/W two solid stress-strain modules

April 8, 2013, 8:00 pm
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Hi all,

I am trying to model a 3D structural mechanics problem with two separate solid stress-strain modules coupled together at the interfacial boundary through equal displacement boundary condition (i.e. Rx, Ry and Rz for one module is equal to u, v and w of other module) where Ri represents applied displacements in x, y and z directions while u, v and w represents the displacements (the solid stress-strain module always solves for u,v and w in each domain along with the stresses and strains). I think the constraint on the interfacing boundary in this way means that at the interface the displacements must be same. However I cannot acheive convergence at all and the simulations always diverges. I am using default solver settings and unstructured tetrahedral elements mesh. Can some one guide me what should I do.

Regards
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Generating continuous electrons from cathode

April 8, 2013, 10:48 pm
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Hi,
I'm trying to simulate an 13MHz electron gun with an oscillating grid voltage. I have created the necessary 13MHz fields. But I couldn't create the electrons continuously with using "Release from Grid" node. I can just set the spesific time steps with the "Release Times" parameter. But its so important to generate this particles continuously. Is there a way to to this, instead of the time steps?
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Surface Plot results not correcly moving

April 8, 2013, 11:00 pm
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Hello,

i have a "moving mesh" problem coupled with magnetic fields. I want to let the domain of a magnet move by a prescribed deformation in the ALE-Interface. It's a sinusoidal movement of a mass-spring-damper-system which i calculate by a global ODE and is used for the pescribed deformation.
If i couple the movement of the magnet with the magnetic field interface (mf) and do a time dependent study the magnet is moving by the correct displacement i calculate by the ODE but the result of the magnetic flux density is not moving or only the domain of the magnet is moving while using the spatial frame. I need the whole moving of the magnetic flux desinty in order to compute the inducted voltage of a coil.

Any advice to my problem? Thanks for your suggestions.




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Extrap

April 9, 2013, 12:23 am
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Hello Comsol gurus,

I am doing a CFD simulation in which I need to use the solution of another CFD simulation. I need to take a "slice" of the solution and use it in another geometry. As I understand, there are two ways, one is to import the data and use it in another file and the other way is to couple two models in the same file.

I have tried the second way with satisfactory results but I want to try the other one. I exported the data as a spreadsheet in a .txt file and I used an interpolation function in the new file. I inserted the interpolation function at the inlet of my geometry (in the laminar flow moduel settings: inlet) and tried to solve.

I have an error called "Extrap not supported. Error in multiphysics compilation". So far I have not found any entry or thread about this error. Either the solution is too simple or this error is not so common, I asume is the first one. What can I do? I am sure I missing something but I can't find it

Regards and thank you in advance

Miguel
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Animation depending on distance

April 9, 2013, 12:46 am
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Hello dear Comsol gurus,

I have made a CFD simulation (stationary) which is based on the model library example of the static mixer. I have also generated the slices and surfaces to see how my fluids are being mixed (using concentration results) but I would like to make an animation with them.

Is there a way to make an animation using the slices (or surfaces)? This animation then would depend on the distance (on x for example) and not on time.

Thank you in advance

Best regards

Miguel



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Strange results when combining Induction Heating and Laminar Flow

April 9, 2013, 2:39 am
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I'm working on an inductor model in induction heating with cooling via laminar flow. I have tried exciting the inductor in two ways: via Lumped ports and Surface currents. The cooling is room-temperature air (20 degC) with an air velocity of 1 m/s. The box that confines the inductor is 0.75 x 0.75 x 0.75 m³

For the Lumped port case without cooling I get a maximum temperature of 900 degC. With cooling the result is 888 degC which I find is logical. Both cases are excited with the same voltage.

For the Surface current case without any cooling I get a max temp of 883 degC (SC_no_cooling.mph attached below). After that i implement the cooling in exactly the same way as for the lumped port case, but now the temperature RISES to 910 degC (SC_1.mph).

I also tried a different way of doing the multiphysics coupling by implementing "Heat transfer in fluids", and the results I get is a max temp of only 92 degC (SC_2.mph). All three models have the same surface current excitation.

My question now is, is any of the Surface Current models with the cooling realistic? If no, how can I improve the results? My opinion is that they aren't realistic at all but I would still like for them to work although my models with the Lumped ports are working.

Best regards
Bilal
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.mphdev for existing physics modules

April 9, 2013, 3:47 am
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Hello,

I am new to COMSOL and I am trying to build custom physics model for thermoelectric effect implementation. I am wondering whether it is possible to view the physics builder files for existing physics modules (for example, joule heating under heat transfer), so that it will be more helpful in building a custom model.

Thanks,
Aravinth.
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Simulation of an hydraulic accumulator for pulsation reducing

April 9, 2013, 4:29 am
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Hello COMSOL community,

I am trying to do a transient simulation of an accumulator in a hydraulic circuit. For simplification, the accumulator shall be placed as a bubble of a second fluid domain inside the domain of the hydraulic oil circuit. The bubble shall not mix up with the hydraulic oil domain and it shall not move. But of course, it must be deformable and the density of the bubble needs to be dependent on the surrounding pressure. Also, the E-module of the accumulator bubble needs to be considered.

Has anyone done a similar simulation or can give me an idea how to start? I'm not sure wether I have to use an additional structural mechanics physic for the deformation for the bubble and a moving mesh physic, or if I can simply use one fluid flow physic with two fluids.

Thank you for every kind of idea!

Julian
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Unusual condition between two fluids

April 9, 2013, 5:05 am
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Hello, i`m new here, then sorry if this is a silly question...

I need to set the follow condition in the interface between two fluids:

ro_v*u_v*hfg = ro_l*u_l*hfg = lambda_v*(dTv/dx) - lambda_eff*(dTl/dx)

Where:
ro = density
u = velocity
lambda = condutivity
hfg = latent heat

That is a balance of heat flux.
I must find an boundary condition that lets me set the velocities in the interface, for the two fluids, following that condition.

Someone have any idea, please??
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Geometric Nonlinearity problem

February 2, 2012, 10:49 am
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I have an odd problem. I am trying to recreate the Large Deformation Beam model (www.comsol.com/showroom/documentation/model/204/) before I start my own project to investigate buckling. I was following along with the model, but when I go to Study 1 > Step 1: Stationary and open Study Settings, I do not see any buttons that say "Include Geometric Nonlinearity". Indeed, I do not see any buttons at all.

When I download the support file for the Large Deformation Beam model, I do see this button.

However, if I delete the stationary solver and add a new one, I do not see the button any more.

It seems like I must be doing something dumb here, but I can't seem to figure out what. Any help would be appreciated.

Thanks,
-jfb
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materials of a surface mesh element

February 6, 2012, 9:05 am
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Hi everyone ,
I’m looking for a possibility to distinguish the different materials that belong to a single surface-mash-element (for example: a triangle) in matlab? For better understanding - my problem in a second way. Starting from a model containing a single meshed surface. Each mesh-element is now between two materials. So how do I get knowledge about the two materials/groups/… the element is between?
Thanks for help!
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Colors messed up when exporting GIF

April 4, 2012, 8:30 am
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Hello,

When I export a GIF from COMSOL, often times the color scheme will be all messed up.

Instead of a fluid change from blue to red, I will see about 16 distinct colors. This looks terrible and is not what is shown when I export a still image.

Also, sometimes the colors will be replaced as black on large domains, completely blocking out the domain.

This is not an issue on just my computer. It is happening to my entire lab. I am running version 4.2.

Has anyone else seen this before?

~Azer
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Non-isothermal flow problem

April 18, 2012, 4:49 am
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Hi there,

I am trying to create a model that is similar to the cold water glass model found within the comsol model library. However, I want to have a temperature governed by a step function at the bottom (which I have) and a cold temperature at the top that I just define in the initial values.

However, when I try to compute the model, an error appears that says that the time-dep solution is not converging, could I get some help with this please!

Thanks

Ross

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2D electrostatic Force

February 5, 2013, 4:32 am
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Hi,

I am modeling a slice of a simple comb capacitor in 2D. I would like to determine the force acting on the fingers. I can calculate the electric field, but when I try to determine the force through Fy=dE/dy along the line it always returns zero. Please find the attached model.
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Modeling a water turbine using actuator disc theory

April 3, 2013, 12:16 am
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Hello, I'm trying to model the flow in a (simplified) tidal channel. For now there is one turbine placed in the channels (this might be extended to two or more). With this model I hope to study the flow trough the channel under different load conditions of the turbine(s).

As many documents suggest a simple and reliable model is the use of an actuator disc. This disc imposed a force on the flow dependent on the flow speed (the force can be computed using other tools). I'm trying to perform a steady state simulation where the force of the turbine on the flow is constant.

My question is what is the best way to model this force on the flow? There are different method's these are the one's I've come up with but other suggestions are welcome.

My first idea would be to create a line and add a force condition. Unfortunately the line (in the form of Bezier Poligon) does not allow me to create a boundary condition. Is there another way to model this force?

Second option would be to create a turbine rectangle and a difference (the inside of the rectangle is empty). Creating a turbine 'inlet' and 'outlet' boundary. This allows boundary conditions to be set (either in the form of pressure or a stress force). The problem is however that the velocity that exits the domain via the turbine 'outlet' must re-enter via the turbine 'inlet' (or vise versa). How do i correctly impose this constraint?

For the simulation i'm using laminar, single phase, incompressible flow in 2D.

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Issues with Full Coupling of Physics

April 6, 2013, 5:26 pm
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Good day,

I am having issues with fully coupling two physics.

My system contains the solid mechanics physics coupled with a general form PDE to model concentration change and the diffusive properties of a spherical particle.

the solid mechanics stress results depend on the concentration results (solved for by the general form PDE) and the concentration results depend on the stress results.

Before solving anything I am defining my stresses ( that will be solved in the solid mechanics physics) as
sigx = solid.sx
sigy = solid.sy
sigz = solid.sz

so that I can write my PDE that involves these stresses.

My question is: Am I naming these variable wrong in regards to suitable convention? I ask because it does not allow me to run my problem as it is saying that the variables cannot be determined.
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