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-   -   Steady or Unsteady (http://www.cfd-online.com/Forums/fluent/50191-steady-unsteady.html)

Amir January 5, 2009 03:03

Steady or Unsteady
 
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.

Davy January 6, 2009 00:27

Re: Steady or Unsteady
 
depends on your needs. MRF can give you an approximate solution immediately but not as accurate as moving mesh method

Amir January 6, 2009 01:02

Re: Steady or Unsteady
 
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.

Anastasios January 13, 2009 02:50

Re: Steady or Unsteady
 
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.

AMIR January 13, 2009 03:27

Re: Steady or Unsteady
 
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.

Anastasios January 13, 2009 04:41

Re: Steady or Unsteady
 
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.

AMIR January 13, 2009 07:54

Re: Steady or Unsteady
 
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.

Anastasios January 14, 2009 03:26

Re: Steady or Unsteady
 
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?

AMIR January 14, 2009 06:36

Re: Steady or Unsteady
 
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.

Anastasios January 14, 2009 07:11

Re: Steady or Unsteady
 
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.

AMIR January 14, 2009 07:26

Re: Steady or Unsteady
 
How I can check the value of the skewness in Gambit? and what I must do to resolve it?

Thank you so much,

Amir.

Anastasios Georgoulas January 15, 2009 02:20

Re: Steady or Unsteady
 
Is your geometry 2d or 3d?

AMIR January 15, 2009 08:23

Re: Steady or Unsteady
 
my geometry is 3D. I checked and the highest value of skewness is 0.83.


Anastasios January 16, 2009 02:42

Re: Steady or Unsteady
 
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.....

AMIR January 16, 2009 08:04

Re: Steady or Unsteady
 
Dear Anastasios,

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

Regards,

Amir.

AMIR January 19, 2009 00:55

Re: Steady or Unsteady
 
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.


Anastasios Georgoulas January 19, 2009 02:35

Re: Steady or Unsteady
 
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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