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A question about air flow in evaporative cooling process

May 3, 2017, 10:05 am
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Hello guys

I am watching on the instruction about cooling evaporation, when a cup filled with hot water are put in air. In instruction file, the air around the cup is turbulent flow and for which the Re number is 1500. I am confused here, because I think here for air flow is natural convection and we should use Ra number to identify whether is laminar or turbulent flow.

Thank you
Haowen Chen
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Unsaturated Flow/Effective Saturation/Inflow and out flow

May 3, 2017, 2:59 pm
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Hi,

I am using Richard's equation model with fracture flow model, I want to have effective saturation (dl.Se) for which i am using these settings.

Top of model Hp = 0.0005[mm]
Bottom Hp= -150mm as for (dl.Se) Hp must be in negative
Initial value Hp = -150mm
Permeability Ks = 10^-12
For evaluation of dl.Se i am using Avergae ( Component Coupling) for matrix and fractures with Global evaluation Expression ( aveop1(dl.Se).
For evaluation of inflow and out flow i am using Integration ( Component Coupling) at top and bottom of model respectively with Global evaluation Expression ( intop1( dl.U*dl.D) for flow in (m^3/s).

I want to have same inflow and outflow that means , effective saturation from dry condition to fully saturated. But when i simulate the model dl.Se rises from 0.006 at day(0) to 0.26 at day (10) and then remains constant for the day(1000) . If we change Ks to 10^-8 then at day (1) dl.Se becomes (0.97).
Any suggestions to resolve this problem will be appreciated.
copy of model is also attached.

Regards,
Shahid
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Steady particle flow simulation?

May 3, 2017, 3:56 pm
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Hi everyone!

Just wonder is that possible to simulate a continuous process for particle flow with fluid?

Thanks.
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Problem in assigning fully developed velocity profile to inlet

May 3, 2017, 7:26 pm
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I am a new user of comsol. I want to assign a velocity profile (in .txt or .csv format) at the inlet of my model (Water flow over Dune) as attached. Can anyone please help me on this matter.Thank you.
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Load distribution

May 3, 2017, 11:38 pm
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Hi guy, currently I would like to simulate the load distribution of moving vehicle on the asphalt road.

Is it possible using COMSOL to do the load distribution of moving vehicle's tyres to the asphalt road? May I know anyone have the experience on using COMSOL do this kind of simulation?

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Failed to find consistent initial values.

January 10, 2012, 2:11 am
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Hi.

I coose frekvenci transient study and get thise error.

Failed to find consistent initial values.
Last time step is not converged.

THANK
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How to solve the problem of negative concentration

June 6, 2012, 2:27 am
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I am a totally new user on comsol. Now I try to simulate the mixing in the microchannel without reactions. I use the laminar flow and Transport of dilluted species. There is a similar example from comsol model library. However, I got negative concentration and the larger value beyond physic in simulation results. Somebodies say this problem can be solved by seting up logarithmic transform of concentration. But how to do that in the model of Transport of diluted specices? Please help me. Thank you so much.
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Importing parameterized geometries?

September 23, 2015, 11:16 pm
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Hey Guys,

I'd like to import geometries used in a previous model which are partly defined with parameters (certain distances, lengths, radii). Is it possible to import an existing geometry into COMSOL without just having the "import"-bracket? I'd need to do some parametric sweeps afterwards and in that case I'd have to build a new model for that.
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how to calculate resonance frequency of double layered cantilever ?

December 15, 2015, 9:51 pm
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Hello Sir, I am using two layers Silicon and SiO2 to create a Cantilever Beam. Would you please tell me how I make it possible? The Dimension of Silicon is 300x60x5 um and SiO2 is 300x60x1 um.
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Anisotropic Transition Boundary Condition

May 4, 2017, 2:16 am
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I am trying to model an anisotropic transition boundary condition with the wave optics module, frequency domain. When I define a tensor for epsilon r, the coordinate system seems to be different than the global coordinate system. That is when I place a high imaginary number in what I thought was the zz position I get high absorption in the x direction instead of the z direction of my simulation. Is the surface boundary definition of the anisotropic tensor coordinate system different than the global coordinate system?
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Unit column disappeared from the Variable/Expression box

May 4, 2017, 4:37 am
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Hello,

While putting variables in the "Variables" box, I increased the size of the "Unit" column by dragging the bar between them. I think I pulledit too much to the right because it disappeared. Now I only have | Name | Expression | Description | instead of | Name | Expression | Unit | Description.

The same happened to "Derived Values", where the expressions are (I have attached a screenshot).

Does anyone know how to insert the Unit column again?

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Simulating expansion of bed of solid particles in reactor

May 4, 2017, 6:37 am
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Hello!

I'm trying to simulate the expansion of a bed of spherical particles using the Turbulent Mixture Model (mm) module, but i'm having trouble defining the initial values. I've tried using both HR and SN slip models without any success. I've alse tried using the laminar mixture model module (since the Rep is low), without any results as well. The inlet stream is composed solely by the continuous phase. Any ideas on how to solve this (or any BC i'm forgetting to set up)?

Thanks in advance.
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Getting data from nongeometric parameter sweeps

May 4, 2017, 9:18 am
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Hi all,

I have a model where I am sweeping the wavelength of light that is being excited, as well as a geometrical parameter. I am coupling that model with a thermal model that uses the Qh parameter of the electromagnetic waves model to generate heat and then do a thermal model from there.

What I want to do is then select a particular domain, do a "Volume Maximum" calculation on that domain only, and output the maximum temperature in that domain to a table where both parameters are the first two columns and the temperature is the third.

However, when I try to do this, the wavelength parameter does not seem to "apply" to the sweep. It has a column, but all of its values are listed as 0, and it only sweeps through the number of values in the geometrical parameter. It has something to do with the wavelength I am fairly certain because if I switch my parametric sweep to only geometrical parameters it works fine.

It's somewhat infuriating because I can plot the solutions with all the wavelengths, so the data is there, it's just that the derived parameter function doesn't work.

Also, I'd like to attach a file I have with a trivial geometry simulation, but when I try I seem to get a "file extension error". Any ideas on how to do that as well so it's easier to see what's going on?
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Boundary conditions change during parametric sweep

May 4, 2017, 10:07 am
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I'm simulating laser spot(circle) scanning through a 2D heterostructure(triangle). Although I set up boundary conditions and PDE for each domains initially, when the laser spot moves onto the sample, the PDE for each domains change. Is there a way to link the PDEs with each domain permanently during the sweep? Thank you.
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EigenFrequency with assembly form and roller bondary condition

May 4, 2017, 12:44 pm
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Hi,

I'm trying to do an eigenfrequency computation with a model of 4 solids connected together with "identity pair" as roller or fixed.
Basically one plate supported with 2 rubbers on one beam. Both rubbers ares clamped to the beam. One rubber is clamped to the plate, the other one has a roller pair condition.

The static analysis works well. I put a lateral load on the plate to create shear in rubber. As expected, the fully clamped rubber is subjected to shear stress, and the other no.

But I tried to run and eigenfrequency analysis with the same model. It runs, but the corresponding mode does not appears, I have only mode inside rubber, as the roller identity pair was not working for eigenfrequency problem ...

I attached my file if you want to give a look.

thanks in advance for your help
Martin
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Assistance on solving heat transfer transient problem

May 4, 2017, 2:45 pm
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I was trying to solve the below mentioned differential eqn. through Finite element methods,

Its a heat transfer transient problem between two glasses in a window, were we have to find temperatures θi & θj (temperature of 2 glasses in window), θk( temperature of air in between this glasses). And they have mentioned, to compute these temperature we have to use below two differential eqn. :

k.(dθi/dt) =∑ hrij.(θj−θi)+hcij.(θk−θi)+αei.ϕ > for each gas element

k.dθk/dt=∑ hckj.(θk−θj) > for air gap between the 2 glasses

On applying above equation in our scenario i got 3 coupled System of differential equation as below:

(dθi/dt) = -.816666666666667e-2*θi(t)+.666666666666667e-3*θj(t)+.138888888888889e-2*θk(t)+2.04

(dθj/dt) = -.666666666666667e-3*θi(t)-.294444444444444e-2*θj(t)-.138888888888889e-2*θk(t)+1.59

(dθk/dt) = -(1.27/0.01060)*θj(t)+(1.27/0.01060)*θk(t)+1.59

Could you please assist me how to solve this kind of problem.
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Gravity implementation gives error

May 4, 2017, 5:35 pm
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We are modeling a simple flow of water down an inclined plane. When we add gravity, the following error shows up:

Failed to find a solution
Maximum number of newton iterations reached.
Returned solutions is not converged.

Our initial conditions are:
velocity field=0 m/s
pressure=10000 Pa

Our inlet is:
.2 m/s

Outlet:
10000 Pa

gravity:
-9.8 m/s in z direction
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MEMS Stresses and Strains

May 5, 2017, 2:00 am
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Hello,

I am relatively new to COMSOL and am trying to model a simple deformation of a MEMS bilayer. My geometry consists of a polyimide support (micro-meter dimensions) and a gold layer on top. I am constraining the polyimide (zero movement in y direction) on one side surface and applying a force on the opposite side surface (in the minus y direction). I am also applying a Roller boundary conditon on the two surfaces which are not orthogonal to the force. Due to the small dimensions of my model, I am obliged to choose a "finer" mesh.

I get the eror "Failed to find a solution. Out of memory during LU factorization. Returned solution is not converged."

I would be greatful for any suggestions that could make my model converge.

Thank you very much,
Amanda
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How to set physics and study to simulate this condition?

May 5, 2017, 5:08 am
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Hello
I'm new user in comsol. I want to simulate a cantilever beam having one face effected a constant force. And the beam is in some fluid, such as water, oil, mercury and so on. In this case, the velocity of fluid is zero, and the cantilever is vibrate and will be deformed by the force. I want to know the displacement of the face effected the force by frequency domain. So I took some physics such as solid mechanics, fluid-structure interaction, pressure acoustics and so on.
But I couldn't get the data and result I wanted.
How can I get some data and result? What physics I can use? Please let me know.
Thank you
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Catalysis

May 5, 2017, 1:09 pm
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Hello!

I would like to know how to turn the catalyst (any material present) used into a powder in order to get the right calculations. and How to turn the bulk materials in the libraries into small pieces.

Thanks in advance.
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