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Old   January 5, 2009, 02:03
Default Steady or Unsteady
  #1
Amir
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Hi,

I want to run a simulation on Savonius like turbine. for using the MRF or Moving mesh method I should run steady or unsteady simulation?

The flow is steady on x-direction and the turbine is turning on z-direction.

Please give me some information...

Best regards.

Amir.
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Old   January 5, 2009, 23:27
Default Re: Steady or Unsteady
  #2
Davy
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depends on your needs. MRF can give you an approximate solution immediately but not as accurate as moving mesh method
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Old   January 6, 2009, 00:02
Default Re: Steady or Unsteady
  #3
Amir
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Dear Davy,

Should I use steady or unsteady simulation? As I know my case is unsteady, however the free stream velocity is constant. But I'm thinking if the MRF or Moving mesh method use the time average so no need to run the unsteady simulation!

I appreciate your concentration,

Amir.
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Old   January 13, 2009, 01:50
Default Re: Steady or Unsteady
  #4
Anastasios
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i think you should go for steady since your initial condidions are constant. Further more i suppose that this depends on what you want to examine in the results.
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Old   January 13, 2009, 02:27
Default Re: Steady or Unsteady
  #5
AMIR
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Dear Anastasios,

I tried to run steady and unsteady simulations. while I'm running the steady one there is no problem, but when I try the unsteady simulation after clicking on start iteration there is error "non-positive volume exists".

I use the same mesh as I used for steady, while on steady-moving mesh there is no problem.

when i import the mesh into fluent after grid check also there is no problem, till i start iteration.

What is the problem and how I can resolve it.

regards,

Amir.
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Old   January 13, 2009, 03:41
Default Re: Steady or Unsteady
  #6
Anastasios
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after you do grid check in fluent, see the value of "minimum volume" is it possitive or negative? please check it and reply to me in order to advise you what to do.
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Old   January 13, 2009, 06:54
Default Re: Steady or Unsteady
  #7
AMIR
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Dear Anastasios,

I checked and its positive. As I mentioned before I can run simulation with this mesh for steady-moving mesh case. when I change it to unsteady after clicking the iteration it gives error "non-positive volume exist".

I don't understand how one mesh can give once error, once not.

best regards,

Amir.
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Old   January 14, 2009, 02:26
Default Re: Steady or Unsteady
  #8
Anastasios
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Since this value is positive it means that your mesh is OK. Hence this error is not mesh related. When you switching to unsteady mode what is the time-step that you use for the simulation?
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Old   January 14, 2009, 05:36
Default Re: Steady or Unsteady
  #9
AMIR
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Dear Anastasios,

When I use Unsteady the time step is 0.001145 second and 10 time steps.

the free stream velocity is 20 m/s and the rotation speed of turbine is 655 rpm.

Thank you so much,

Amir.
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Old   January 14, 2009, 06:11
Default Re: Steady or Unsteady
  #10
Anastasios
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If you are using Gambit for the meshing of the geometry do a mech check in Gambit and see the value of skewness! A general rule is for this value to be smaller than 1. The ideal value should be close to 0.7. If your mesh fails this skewness check this means that you have to consider meshing the geometry with a different scheme. Please do this check and let me know.
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Old   January 14, 2009, 06:26
Default Re: Steady or Unsteady
  #11
AMIR
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How I can check the value of the skewness in Gambit? and what I must do to resolve it?

Thank you so much,

Amir.
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Old   January 15, 2009, 01:20
Default Re: Steady or Unsteady
  #12
Anastasios Georgoulas
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Is your geometry 2d or 3d?
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Old   January 15, 2009, 07:23
Default Re: Steady or Unsteady
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AMIR
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my geometry is 3D. I checked and the highest value of skewness is 0.83.

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Old   January 16, 2009, 01:42
Default Re: Steady or Unsteady
  #14
Anastasios
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then its ok. i cant understand whats hapenning! And its very strange that it works for steady and not for unsteady!

You could probably ckeck by visual inspection if everything in your geometry is meshed or some parts are not meshed. I cant think of anything else. Sorry.....
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Old   January 16, 2009, 07:04
Default Re: Steady or Unsteady
  #15
AMIR
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Dear Anastasios,

thank you for your interest, I will remesh it with this hope that it will work..

Regards,

Amir.
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Old   January 18, 2009, 23:55
Default Re: Steady or Unsteady
  #16
AMIR
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Dear Anastasios,

I meshed again my geometry, this time with the maximum value of skewness "0.76", but again same problem while I click to iterate.

in the mesh in some parts I'm using tetrahedral and in some parts hex/wedge cooper. can this cause problem?

or I should do anything in fluent for this mesh..? Really I'm going crazy.

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Old   January 19, 2009, 01:35
Default Re: Steady or Unsteady
  #17
Anastasios Georgoulas
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try using oly one kind of mesh elements i would go for tetrahedral. If you like send the .dbs file of the geometry in my email in order for me to take a better look of what might happens wrong. My mail is ageorg@civil.duth.gr
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