help needed with Fluent

vivekkumarmishra2003 · December 21, 2009, 11:25

Hello everyone, i am stuck in my simulation, which i need to complete ASAP. Please help me out.

Below is the detail of my work.
- Turbulence model (k-e)
- Geometry: Car interior (simplified)
- 4 highly skewed elements (grid check is OK in fluent)
- boundary conditions
- Four velocity inlets (Turbulence intensity and length scale is defined)
- One Pressure outlet (turbulence intensity and length scale is defined)
- Energy equation is ON.
- k-e model selected
- Pressure based solver.
- Almost 20 lakhs (2 million) cells

My problems are:

a - Till some (150 iterations)everything seems fine but after that turbulence viscosity ratio gets limited and this keeps on happening even after 800 iterations and even after that. What could be the problem ?

b- Earlier the divergence in temperature was detected, to remove that i changed the URF to 0.9, is this method OK

c - Now the epsilon keeps on jumping like a heartbeat and takes all other ( X, Y, Z velocity and k and energy along with it), i am stuck at this situation (i have tried standard k-e, RNG and Relizable model, but of no help). How do i get convergence ?

Any help will be highly appreciated.

denizen · December 22, 2009, 01:59

it seems velocity is diverge, i recomend to use pressure coupled solver or use smoler relaxations on pressure and velocity.

SKK · December 22, 2009, 05:56

If all above mentioned fails, it could be due to some bad cells. Try and find out the maximum temperautere in your model. Possibility is that its is only a few bad cells. In which case you will need to remesh in order to get better cell quality. I am not a Fluent user, so don't know if there is any way to modify this without meshing from begining.

vivekkumarmishra2003 · December 24, 2009, 11:45

Thank you for replying, even i realized that it is happening because of the bad cells, but the problem is being a new user of Gambit, i don't really know how to check the quality of the grid. I am hoping to get some answers which show and explan the way of checking the quality of grid.

Another question was that i dont' have pressure value at outlet, but i have velocity at outlet, i applied the pressure as 0.5 X ro X velocity^2. (dynamic pressure). Is this OK or should i have to calculate it by any other method. (please guide me on the method also)

December 21, 2009, 11:25	help needed with Fluent	#1
vivekkumarmishra2003 New Member Vivek Join Date: Dec 2009 Posts: 5 Rep Power: 16	Hello everyone, i am stuck in my simulation, which i need to complete ASAP. Please help me out. Below is the detail of my work. - Turbulence model (k-e) - Geometry: Car interior (simplified) - 4 highly skewed elements (grid check is OK in fluent) - boundary conditions - Four velocity inlets (Turbulence intensity and length scale is defined) - One Pressure outlet (turbulence intensity and length scale is defined) - Energy equation is ON. - k-e model selected - Pressure based solver. - Almost 20 lakhs (2 million) cells My problems are: a - Till some (150 iterations)everything seems fine but after that turbulence viscosity ratio gets limited and this keeps on happening even after 800 iterations and even after that. What could be the problem ? b- Earlier the divergence in temperature was detected, to remove that i changed the URF to 0.9, is this method OK c - Now the epsilon keeps on jumping like a heartbeat and takes all other ( X, Y, Z velocity and k and energy along with it), i am stuck at this situation (i have tried standard k-e, RNG and Relizable model, but of no help). How do i get convergence ? Any help will be highly appreciated.

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December 22, 2009, 01:59		#2
denizen New Member Join Date: Mar 2009 Posts: 25 Rep Power: 0	it seems velocity is diverge, i recomend to use pressure coupled solver or use smoler relaxations on pressure and velocity.

December 22, 2009, 05:56		#3
SKK Member Join Date: Mar 2009 Posts: 55 Rep Power: 17	If all above mentioned fails, it could be due to some bad cells. Try and find out the maximum temperautere in your model. Possibility is that its is only a few bad cells. In which case you will need to remesh in order to get better cell quality. I am not a Fluent user, so don't know if there is any way to modify this without meshing from begining.

December 24, 2009, 11:45		#4
vivekkumarmishra2003 New Member Vivek Join Date: Dec 2009 Posts: 5 Rep Power: 16	Thank you for replying, even i realized that it is happening because of the bad cells, but the problem is being a new user of Gambit, i don't really know how to check the quality of the grid. I am hoping to get some answers which show and explan the way of checking the quality of grid. Another question was that i dont' have pressure value at outlet, but i have velocity at outlet, i applied the pressure as 0.5 X ro X velocity^2. (dynamic pressure). Is this OK or should i have to calculate it by any other method. (please guide me on the method also)