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June 14, 2007, 15:36 
FemLab

#1 
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Hey everyone! Does anybody know wheather it is possible to similate expanding flow through a small hole (nozzle) in comsol multiphysics. Is this software relevant enough?
Thanks alot) 

June 14, 2007, 18:28 
Re: FemLab

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I have used ANSYS multiphysics and what you are describing is a straight forward analysis (There is a tutorial/verification) tutorial.
I used google to search for (comsol femlab) and the results are astonishing. Any way, if you are familiar (proficient) with the concept of Artificial viscosity, then you ought not get any problem, finite element solvers generally use that concept to stabilize the solution and get convergence. I would appreciate your feedback about that particular programme. Good luck with your analysis 

June 14, 2007, 20:47 
Re: FemLab (Correction)

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From Comsol Web Portal:
Improved CFD support Predefined interfaces for level set functions and twophase flow using level sets, including special boundary conditions for wetted walls, have been implemented in the MEMS and Chemical Engineering Modules. Furthermore, the Chemical Engineering Module along with the Heat Transfer Module have been extended with the kω turbulence model and an accompanying modeling interface. 

June 15, 2007, 03:59 
Re: FemLab (Correction)

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Hello, Ahmed. Thanks for your relpy.
Unfortunately, I am not a professional at all. I have just started with modeling. But i need to obtain the result as soon as it possible. I am trying to solve the problem without the viscosity, but FemLab fails to find a solution by statinary analysys, thus i need to use transient solvers. I have no experience in such computations, so i don't have any clue, what type of solver to use. Also i have found that initial conditions are very important, and it would be better to spesify them as close to the exect stationary solution as it possible. But i have rather complex geometry and stream pattern, so i don't know how to assess the solution. And one more problem, is the following  even when i get a solution for some test values of inlet pressure and density (say 1Pa, 1kg/m3) and the i try to scale these values to my real conditions, the same solver doesn't find anything. I woud be very happy to get any advise from you! 

June 16, 2007, 00:18 
Re: FemLab (Correction)

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Hello Daniil My sincere advice to you is to contact your support engineer, what is not clear to me is why you want to use a transient solver, Does the physics of your problem warrent such a change? I might be able to assist you but you need to fully explain your problem. For your information, transient analysis is just a steady state analysis performed at reasonably small periods of time that is determined by the physics of your problem, as I said before, contact your support engineer and give us your feedback on the programme Wish you good luck


June 17, 2007, 18:35 
Re: FemLab (Correction)

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Dear, Ahmed.
Thank you for your reply. Actually, what i need to simulate is air flow through 200 micrometer i.d. 500 mkm long rectangular nozzle. Pressure drop is from 10000 to 1000 Pa. So, according to my estimations i will have a supersonic jet, and it means that incompressible approx. is absolutely irrelevant. But at the same time viscosity is very important because of rather small Raynolds number ( approx 100). So, unfortunately, i need both incompressible and viscose gas model. And furthermore i have one more question: in order to make the first step, a have tried to solve my problem using euler compressible mode. But the solver doesn't converge. My geometry is 2D axial symmetry and looks like  ____1____________ ................ ................ ................2 ................ 8............... _________3______ . .4 . .___5__________ ................ ................ ................6 ................ ................ _____________7__ I specify inlet density, z velosity and pressure on 7 boarder outflow pressure on 1 slip/symmetry on 26 and axial symmetry on 8. Are there some common mistakes that don't allow to solve euler equations? or may be some hints, that i need to use? I would be glad to get any piece of advice. Kindly regrds Daniil 

June 17, 2007, 20:20 
Re: FemLab (Correction)

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Air at supersonic speeds is incompressible? Why is this assumption 'irrelevant'?
At the pressures you quote, are you sure the air is not in slip flow or free molecular flow? What is the mean free path? Depending on the area ratio of your nozzle, a shock wave may form between the throat and the exit plane. You mention both "rectangular nozzle" and "axisymmetric flow". How do you have both in one nozzle? Could you describe your flow in more physical terms (independent of the solver)? It sounds as if you have a highly compressible flow. 

June 17, 2007, 23:53 
Re: FemLab (Correction)

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One quick observation: Have you checked the Knudson Number? in simple words, you cannot have a half million cells in your domain, 10000 or less is good (Cell Knudson Number) Could you upload a picture of your domain on any one of these free web sites Is your flow Isothermal or adiabatic (This sounds more a throtling process than a classical nozzle flow, am I right? Good Luck


June 18, 2007, 17:22 
Re: FemLab (Correction)

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Hello, Ahmed,
thank you for your reply.Here is my geometry and results of calculations http://www.spbgu.ru/albums/5579/nozzle.jpg Now, i am trying to use NonIsothermal Flow coupled with General Heat Transfer mode. Because when i finish with gas dynamics i will add gas heating term inside the hole( actually, i have a microdischarge there) Now everything goes allright, but in this mode there is no gas cooling due to the expantion. So the process seems to be isothermal ratherthen adiabatic. But, actually, when the heating is switched of it should be almost adiabatic. Do you know if it is possible to include gas cooling term due to the gas expantion in the thermal balance equation of General Heat Transfer mode? And also i would like to learn more about artificial diffusion.I have found that in my case adjustment of artificial diffusion parametres and mesh size may affect the solution. But what is the main idea of tuning these parametrs? How to assess them, and what artificial diffusion modes should be used in my case? I woul sincerely appriciate any help. Regards, Daniil. 

June 18, 2007, 17:45 
Re: FemLab (Correction)

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Dear Otd. THank you for your reply.
Here are parametrs of my problem. The scheme is here  http://www.spbgu.ru/albums/5579/nozzle.jpg the system of equations, that are silved are here  http://www.spbgu.ru/albums/5579/chem_ug_momentum431.png Inlet pressure 130 Torr, outlet  16 torr Inflowing gas temperature  1000K, 2nd camber temperature 300K. nozzle  450 mkm in lenth 200 mkm i.d I guess that this case is not a free molecular flow. My current problem is to include gas cooling due to the expantion, because my model is coupled to thermal balance eq. And also i need to learn about the concept of artificial diffusion. I would appriciate any advice. Daniil. 

June 18, 2007, 19:26 
Re: FemLab (Correction)

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Is the gas going through a hole and then expanding abruptly? Your diagram is not labeled (or the labels are too small to read). Is it a pin hole and then a large chamber? Do you in fact have a nozzle as you said in the first message? I think of a nozzle as a smooth contraction to the throat, then smooth expansion.
I suggested the possibility of free molecular flow because of the extreme pressure difference. Perhaps you can rough out the Knudsen number (as suggested by Ahmed). I wonder if you can't do a rough evaluation of the possibility of a shock wave forming downstream of the throat? You specify the momentum and continuity equations, but not an energy conservation law. In your reply to Ahmed, it appears an energy conservation law will be included. You'll of course need an equation of state as well. I recommend again that you develop a clear statement of the problem you need to solve. The code (dissipation or not) can come later. Many codes (don't know about this one) have a parameter that can be set to introduce artificial (or numerical) dissipation. Eventually, once you have the input set up to describe the phyical flow of interest, you'll be set to change the dissipation parameter and observe the changes in the flows predicted. Without being very familiar with the code, I probably have nothing else useful to say. Good luck with your problem. 

June 20, 2007, 03:27 
Re: FemLab (Correction)

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Dear otd. Thank you.
In fact I have just a pinhole in a screen and a large chamber after it. I have estimated lamda/R( where R  is a pinhole radius) to be approx 0.1. But i don't know how to estimate wheather my case is free molecular flow using a pressure drop between tho chambers. In my computations i specify the equation of state by givig density as a function of pressure and temperature. But i need one more equation to get the temperature. And in my case i am using thermal balance equation with conduction and convection terms. Because apart from gas flow there is also a heating sourse just inside my pin hole. So now my problem is to introduce gas cooling due to it's expantion in this set of equations. What would you recommend? What form of energy balance equation should i use to get the solution? Woul you please give me some literature reference. Kindly regards Daniil 

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