[cfdiscool]

Hello All

I am quite new to the forum & FLUENT and try to present my problem setup here:

My Mesh has 4.9 million hexa cells

Fluent----3d --- parallel---k-epsilon model--- residuals 10^-4

I am simulating a cylindrical domain( Symmetry:BC) where another cylindircal domain starts almost closely to the pressure inlet, at the opening of the outer cylinder.

problem: In the Fluent cortex I receive the message :

turbulent viscosity limited to viscosity ratio of 1.000000e+05 in 285 cells
and
reversed flow in 3380 faces on pressure-inlet 101

My URF of Turb. Viscosity is set to 1 and
i tried to balance by setting URF sum of Pressure and velocity as 1 (for example like 0.1-0.9 0.2- 0.8 )

I used the Turbulence specification method as Intensity and Hydraulic Diameter

I calculated the intensity of entering air at presuure inlet using a velocity of 1 m/s.

I get both reversed flow and turbulent viscosity warning repeatedly but. solution converges atleast first 100 iterations

Questions:
1.does setting turbulent viscosity ratio by FLUENT affect my Results ?
2. how to find the exact turbulent intensity at pressure inlet ( I dont have readily the air velocity,i should take it randomly... how to set this air velocity)?

More inputs and suggestions are greatly appriciated.....i have little time to get around this....please help

thanks a lot

[-mAx-]

This is typical initialization issues (reverse flow & turbulence).
I assume your are working with pressure difference between inlet and outlet. For the pressure initialization, try to set it in order with 1/2 of your pressure loss. (it should correct your reversed flow issue, if not, try to enforce the flow direction with the initialization of velocity computed from velocity inlet, see bottom)
You may have also an idea of the massflow which should be generated through this pressure loss (empiric equation, etc...).
It gives an idea of your the velocity inlet. From here you can set the turbulence quantities at your inlet (Even if your massfow assumption isn't 100% exact, your initialization will be more accurate)
Hope it helps

[cfdiscool]

Firstly thank u max

Yeah! you are right...after trying hard....i set my inflow BC as "velocity inlet" and NOT as Pressure Inlet

At velocity inlet I assigned the exact values of Turb. Intensity and hydraulic diameter of the inlet.....Then it initialized the flow well with out any reversed flow and rest iterations went well......

I think in case of reversed flow at inlets its better to switch "velocity inlets" , if one has the exact values there....

nice day

[-mAx-]

You could let the pressure Inlet as prior, but set the turbulent BC as you did for the velocity inlet BC

[Tanjina]

Hello,

I am trying to model a 3D object. My model has 3 inlet and I don't know the velocity. So I had to assign pressure inlet. And two of these inlet have hydrostatic pressure. I get reverse flow in every inlet. I know the mass flow rate in outlet, but don't know the individual mass flow rate for each inlet. I used Turbulence intensity 5% and calculated the hydraulic diameter from the dimension of inlets. How can I solve this issue, and how this affect my final result ? Any suggestion will be highly appreciated.

Regards,
Tanjina

[Vidyanand Kesti]

I face the same problem when I am giving mass flow inlet this occurs it can be avoid proper setting of turbulence intensity as ratio,2.It can be avoid by increase the length of the inlet and outlet (3-4 *diameter of inlet/outlet)

[Tanjina]

Originally Posted by Vidyanand Kesti

I face the same problem when I am giving mass flow inlet this occurs it can be avoid proper setting of turbulence intensity as ratio,2.It can be avoid by increase the length of the inlet and outlet (3-4 *diameter of inlet/outlet)

Hello Vidyanand,

Thanks for your reply. I ran one model with turbulence intensity 2%, but there is still reverse flow in inlet.

And would you please explain what do you mean by increase the length of inlet/outlet? I am replicating an experimental setup, so I can't change the length of inlet/outlet.

Regards,
Tanjina

[Vidyanand Kesti]

Good point. I Am also working with experimental setup, when i increases the length of the outlet 3-4 times diameter, I will created the extra length as separate fluid domain, or i will get the plain at the experimental set up and find the pressure drop

[Tanjina]

Originally Posted by Vidyanand Kesti

Good point. I Am also working with experimental setup, when i increases the length of the outlet 3-4 times diameter, I will created the extra length as separate fluid domain, or i will get the plain at the experimental set up and find the pressure drop

Hello,

I don't understand what you are saying. I am sorry. Can you elaborate a bit ?

Regards,
Tanjina

[kabilan]

respected sir/ mam.
what is physical significance of Turb. Intensity and hydraulic diameter?
when i was trying cd nozzle problem reversed flow cause is there any relation?
how can i rectify this problem?
thank you

[Ashke]

I have reverse flow problem that is crashing my solution. I use VOF model to create a water flow on a plate. inlet velocity is 0.1 m/s, water volume fraction of 0.5 (used UDF to define bottom half as filled with water), pressure inlet. I get a "reveresd flow on pressure_outlet" and a huge backward velocity (50 m/s!) at the bottom of the inlet, that is pushing the fluid towards the inlet. The initial velocity exists and is making a very nice looking boundary layer, but everything gets pushed back when the backwards velocity catches up. I don't know what causes this... never seen simmilar problem...