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Implementing a two-temperature-model (electronic/lattice) for fast laser-excitation

July 10, 2014, 11:56 pm
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Hello,

I want to model a pulsed laser that hits a solid, excites electrons which "thermalize" then transferring their energy to the lattice (solid, atoms). I think most people do not use COMSOL for this purpose, but I want to, because I started with classical heat conduction and now want to setup my model in greater detail.

The two-temperature model is basically a model where you define two temperatures for electrons and atoms in a material respectively. This is necessary if you take into account that the electrons react much faster on an external excitation (femtosecond laser pulse [1E-15 s]) than the atoms.

The question is: how to realize this in COMSOL . In principal I would expect to be able to add and couple two heat transfer physics with two different dependent variables Te (electronic) and T (atoms) and custom material parameters (thermal conductivity, source terms). But how do I couple these?

Or would it be easier to go directly for custom PDEs and without predefined physics terminals?

Please help me here.

Best regards,
Henning Hollermann
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Help !!! boundary layer meshing for natural convection

September 18, 2017, 4:18 pm
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Hi

I am trying to model four stacking fruit in a polystrene box. I know there are radiation and natural convection happen and I have done the experiemnt. I just want to creat a model to explain it. however, I cannot make the natural covnection work in my model. I don't know why I couldn't mesh the boudnary layer, and there for comsol result could not converge. could you please help me.

Great thanks

Ramsay
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Question about equation customization in viscoelastic model.

September 21, 2017, 3:54 pm
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Hi everyone,
I am now doing a simulation on a spheroid viscoelastic model. With given displacement on the boundary layer. I would like to explore the inner stress distribution in the sphere. However, the model i want to use is slightly different from the Kelvin-Voigt model which was given in COMSOL. I would like to customize the equation in order to substitute one of the spring module with a dashpot module. Is this feasible in COMSOL?
Best,
Mingshi
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Defining an inhomogeneous material as a function of the electric field in a previous simulation

September 21, 2017, 6:12 pm
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I'm simulating a structure with a 3rd order nonlinearity in the permittivity. In my first simulation, I've assumed that the nonlinear structure has a permittivity dependence on the local electric field. As a sanity check, in a second simulation, I want to define this structure to be linear and have an inhomogeneous permittivity which is a function of the local electric field in my first simulation.
Any ideas how I can define an inhomogeneous material which has a local permittivity equal to a function of my first simulation?
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Error; how to interpret.

September 21, 2017, 11:00 pm
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Hi there,

I've set up a multibody-model with contact, though when I open the model file, I get the following message:

Invalid property error:
- Property: segcomp
'Selection' is an array of strings
Variable component strings

My model runs fine as far as I can see, but I'm not comfortable with getting errors I don't know what means. Does anyone know what the error message refers to, or - in general - where / how to figure it out?

Thanks in advance!

Besr regards
David
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Frequency response of a multibody model

September 22, 2017, 4:07 am
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Hi there,

I've made a multibody model - actually it is a double pendulum, with a spring attached between the two 'free' ends These two free ends are constrained to motion in one direction only; excited by a harmonic predescribed displacement of one of the ends. I would like to get the frequency response of the opposite free end, when the other is excited with this predescribed displacement (and not a force).

I'm not familiar with the theory of multibody dynamics, and therefor not sure about if what I want to achieve actually can be achieved. Specifically I've defined a Rigid Connector to the excitation point, specifying the harmonic displacement - and right-clicking the rigid connector node, choose Harmonic Pertubation and specifying the maximum displacement. This works fine in a time-dependent analysis. When doing a frequency response analysis though, the displacement of the excited point in the structure is greater than I've specified. Also, the opposite free end of the double pendulum exhibits greater displacements than physically possible, if the pendulum arms are rigid links.

Is the approach I am applying the way to go, or should I do something else? And why do my structure displace more than physically possible?

Thanks in advance.

Best regards
David
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Screen losses in cable tutorial series

September 22, 2017, 5:50 am
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Hello everybody
I am a new to COMSOL, I came across to the cable tutorial series and found out that this series is very useful for my application. However, I tried to understand the tutorial 5 Bonding Inductive in which the author calculated the importance of single point bonding. In this example, even when the screen is connected to ground at one end to make single point bonding, the screen losses is still there (2.6kW/km). This result is counter intuitive to me as once the screen is disconnected, there is no current axially flowing in the screen and the losses are therefore zero. Where those losses come from?
Anybody have experience with this can give me an explaination?
Is that physically true or it is only a numerical error of COMSOL?
Thank you
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Using "withsol" inside of study to extract solutions

September 22, 2017, 9:56 am
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I've been struggling to implement the "withsol" operator in my study. I'm trying to model the change in eigenfrequency due to thermal diffusion.

However, since the eigenfrequency solver can't take a time series as input, I need to extract discrete solutions from my Time Dependent thermal diffusion model to feed into the Eigenfrequency solver. "Withsol" seems like the best tool for this job, but I'm unsure how to include it into my study to get the solutions at the time steps I am interested in. Has anyone had success with something like this?
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How to open models before version 3.5a, The model is posted below. Please can I get help

September 22, 2017, 10:50 am
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The below post is related to an [archived discussion](/forum/thread/34063/can-not-open-model-made-in-versions-before-comsol-3-5a-in-4-3)
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[start here]
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What Comsol module would be best for modeling the force from solar radiation pressure on a solar sail?

September 22, 2017, 11:48 am
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The equation for the force from the reflection and absorption of photons from the sail on a flat square sail is:

<math>\mathbf{F_{total}}=-PA(\mathbf{n_s}\cdot \mathbf{n})(\alpha\frac{\epsilon_bB_b-\epsilon_fB_f}{\epsilon_f+\epsilon_b}\mathbf{n}+2\rho_s(\mathbf{n_s}\cdot \mathbf{n})\mathbf{n}+\rho_d(\mathbf{n_s}-B_f\mathbf{n}))</math>

The <math>n_s</math> is the incident solar radiation direction, <math>n</math> is the normal vector from the sail, <math>\rho_d</math> is the diffuse reflection coefficient, <math>\rho_s</math> is the specular reflection coefficient, <math>\alpha</math> is the absorption coefficient, <math>P</math> is the solar radiation pressure, <math>A</math> is the area of the sail. The <math>\epsilon</math>'s come from the absorption of the solar radiation, and they are the surface emissivity, with the subscripts b and f referring to the back and front side of the sail. The <math>B</math> comes from the diffuse reflection of the solar radiation, and is the non-lambertian coefficient, with the subscripts b and f referring to the front and back side of the sail.

To start, I am just trying to find the force vectors on a flat square sail, but eventually I would also like to be able to calculate torque as well. I would also like to be able to use a non-flat sail and vary the angle of incidence.
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Time Dependent Study Piezoelectric Element

September 22, 2017, 2:48 pm
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Hello,

Im a new user to COMSOL and as a result I think I am stuck on a pretty basic part of the program.

I want to create a time dependent study for a piezoelectric element where the electric potential of each sector changes with time, see attached image. However, when I run the study, my electric fields seems not to change. Any ideas why this might be happening? Could it be because I have defined an initial voltage instead of one that varies with time ?

![](http://)
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DC Glow in a planar geometry initialisation

August 12, 2017, 7:32 am
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I am using the plasma module to model DC glow in argon using electrodes embedded in a substrate covered with a thin polymer film, it is driven by a 750V 1MHz source.

The model seems right but it (Study 2) fails to find consistant initial values even with zero voltage at the terminal suffers from a "divided by zero" error. I cannot figure out how to debug this error.

I have tried steps such as massivley increasing the extent of the argon and substrate regions to avoid significant voltage at the boundries, removing all voltage from the terminal, a variety of initial electron counts and voltages, varying film height and electrode seperation, switching solver, using a finer mesh, and initialising using a stationary solution (which is obviously inapplicable for a plasma simulation). Is there a way to step through the last few itterations of the initialisation to find where the error is occuring?

Can anyone suggest anything which may solve this issue?
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plasma module discharge application

August 20, 2017, 7:03 pm
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hello all,

got following issues when use this module.
1. can this module use in 1 atm pressure to calculate discharge? due to the 1 atm low-temperature plasma discharge phenomenon is exist.
2. adding Ballast resistor influence the discharge electron density or not? compare with ballast resistor is not there.
3. solver can not convergence when use AC(50Hz) load, is there any setting for AC?
4. do we should change some setting when the voltage load is high, like 460000V?
5. do you have some example for "The local field approximation" application, or the equation for mean electron energy vs electrical field for the setting area.

thanks a lot.
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Droplet shrinkage / creation for phase field model

September 23, 2017, 7:27 am
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Hi folks,
I'm modeling a two phase (laminar) flow in COMSOL 5.2a. When I am using non-conservative form for the phase field, small droplets disappear, i.e. droplet shrinkage. When I turn the conservative form on some droplets are being created at one of the inlets!
Does anybody have any idea about the source of this problem?

Thanks in advance for your help.

Hamed
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How to rectify the uploaded screenshots?

September 24, 2017, 5:55 am
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The below post is related to an [archived discussion](/forum/thread/15233/cannot-evaluate-the-expression)
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[start here]
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invert legend order

September 24, 2017, 8:48 am
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Hi all,

does anyone know how to invert the legend in comsol 4.3b ? I have a time dependant study during 1 hour with 1min time step. In the figure the plot for the first time step is in the top, while in the legend, the plot for the first time step is in the bottom . I couldn't find a way to make the both in the same order. can anyone help me.

best regards
Ahmed
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Microwave Doppler Flow meter

September 18, 2017, 1:51 am
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I'm interested in modelling a microwave plane wave interacting with a moving laminar fluid in order to monitor the doppler shift induced. (Essentially I'm looking at a microwave flowmeter which isn't magnetic based)

I can find plenty of examples based on acuoustics however, this is different physics so I was wondering if anyone was able to help with regards to the microwave regime.
I have managed to get a simple flow and microwave plan wave interacting, however, I am unsure how to actually define the doppler shift in the physics (using the RF module).

Regards.
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Mass flow rate in heat transfer model

September 20, 2017, 1:16 am
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Hello,

I'm currently busy with a model for scalp cooling. This means the head is cooled with a cold fluid that flows through a cap wich is placed over the head. I know this cooling fluid gets warmer as a result of the biological heat transfer. But this heat will be taken away because of the mass flow system of the cooling cap.

My problem is that I don't know how to model this mass flow. If someone could explain me how to make a mass flow inlet with a certain temperature and volume and a mass flow outlet with the same volume but warmer because of the biological tissue heat transfer I would be very gratefull.

Sincerly,

Max
Graduate student applied physics
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Principal stress 2D vs 3D. Stationary vs time dependent solid mechanics

September 25, 2017, 4:50 am
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Hi,

I have created a 3D model with the same boundary and initial condition as my 2D one in solid mechanics.
when I solve the model time dependent, the principal stresses=0 in the 3D model (solid.sp1, 2,& 3), while the values in 2D model are logical. I checked all the steps and it looks exactly the same as my 2D model. This is while when I solve it stationary, the values are correct.
However I dont want it to solve it stationary because it just solves for time 0 while I wanna couple it with my hydrology at different time steps and my hydrology condition is time dependent.
I would greatly appreciate if someone could help me with this problem.

bests,
Shirin
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Include function as a variable

September 25, 2017, 6:26 am
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Hello,

I have a problem to define a variable. This is how my variable look like: y^(n-1). y is my shear rate and can also be written as “spf.sr” and n is a variable I defined already.
For the function I tipped different solution in the variable expression:
spf.sr*e(n-1)
or
spf.sr\*e(n-1)
or
exp(log(spf.sr)*(n-1)

All this solutions doesn’t worked and there is written "unexpected unit of input". Has somebody an idea how I can solve this problem.

Best wishes
Feli
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