[yahmed]

I am modelling 3D rectangular open channel : dimensions (2 m x 1 m x 9 m),
The channel has 192 cylindrical tubes with diameters of 25 mm and length of 1.56 m, installed in arrays modules. There is an inlet region which deliver the water
All what I know is the incoming flow rate = 0.8 m3/s
I decided to simulate half of the domain and apply symmetry to simplify the problem, since it creates a very large model with large number of elements.

The problem that it shows a reversed flow at pressure outlet and viscosity ratio exceeded limit warning messages during calculations. and I stopped the calculations to check the regions that are affected by this message. please see figures to see regions with very high velocity and viscosity ratio magnitudes.
velocityc.jpg
viscratio.jpg


I used ansys meshing tool : Cutcell assembly meshing.
In Fluent I used the following setup:

Models:
Multiphase> VOF > Open Channel Flow > Implicit
Viscous > Realizable K-e model > scalable wall functions

Phases:
primary: Air
Secondary :water

Boundaries: Boundaries.jpg
Face(1) Inlet : Velocity Inlet,
mixture : momentum > velocity ~ 0.4 m/s ( divide 0.8 m3/s by area of inlet = 2*1)
turbulence: Intensity = 1% and length scale = 0.1 (0.07* characteristic length scale I used the length of the tube which is 1.56 m)

multiphase: phase-2 - I specified free surface and bottom level such that it represents a water volume fraction of 1 at the inlet face.

Face(2) Outlet: Pressure outlet -
Normal to boundary
turbulence: Intensity = 1% and length scale = 0.1 m
-with open channel : free surface=0 and bottom level=-0.753 (the figure shows where are the origin of the coordinate system) xy.jpg

Face(3) Top : pressure outlet -without open channel selected

Face(4) Symmetry
All other faces > Walls: no slip

URF : default
Solution methods: default

I also, tried to change the inlet boundary to Mass inlet (flow rate of water = 1000 (kg/m3) *0.4(m/s) * 1 (m2)= 400 kg/s ) and for air mass flow rate=0.005 kg/s. but also I had the same problem of reversed flow and viscosity ratio.!!!

Any help please to identify causes of the problem??
Thank you..

[yahmed]

Please Fluent users I need your help ....

[yahmed]

Please I am running into this problem again now, reversed flow in the outlet and viscosity limit is exceeding. I appreciate any help please.

[devesh.baghel]

Hi,

1. Define water as primary fluid & air is secondary fluid.
2. define all the 4 boundaries as symmetry except inlet\outlet, if flow domain is core region of tube bundles.
3. If reverse flow is happening at initial stage of simulation, no need to worry about that, let the simulation run and wait for flow field stabilization.


Devesh

[yahmed]

Originally Posted by devesh.baghel

Hi,

1. Define water as primary fluid & air is secondary fluid.
2. define all the 4 boundaries as symmetry except inlet\outlet, if flow domain is core region of tube bundles.
3. If reverse flow is happening at initial stage of simulation, no need to worry about that, let the simulation run and wait for flow field stabilization.


Devesh

Thank you Devesh for your reply . Finally I am getting some help I was really hopeless.. Ok I will try to run it again using your suggestions but regarding symmetry BCs, should I set the symmetry BC even for the channel walls? or you mean only the top boundary and the symmetry face?

[devesh.baghel]

Hi,

I am sorry, I thought that you are taking core\cenral zone of tube bundles but you are considering only half of core region for simulation.

1. Do you really think top surface should be pressure outlet in actual practice, if not just make it as a wall.
2. If it is really a open channel, free surafe problem......top side should be away from the core flow & should not affect the flow parameters of mixture as density of water is much more than air.
3. Velocity is shooting upto ~300 m\sec, please check the BC's, dimension of geometry and mesh density i.e. refinement.
4. Are you considering Gravity & Surface tension also into your simulation ?


Devesh

[yahmed]

Originally Posted by devesh.baghel

Hi,

I am sorry, I thought that you are taking core\cenral zone of tube bundles but you are considering only half of core region for simulation.

1. Do you really think top surface should be pressure outlet in actual practice, if not just make it as a wall.
2. If it is really a open channel, free surafe problem......top side should be away from the core flow & should not affect the flow parameters of mixture as density of water is much more than air.
3. Velocity is shooting upto ~300 m\sec, please check the BC's, dimension of geometry and mesh density i.e. refinement.
4. Are you considering Gravity & Surface tension also into your simulation ?


Devesh

For testing purposes I tried to run the VOF model for the channel only without the tubes. I have tested all boundary conditions combinations, all of them didn't give realistic results or didn't converge. Only Mass inlet boundary condition with outflow BC for the outlet, along with pressure outlet BC for the top surface generated close results, but there were two problems that " reversed flow on pressure outlet" message was shown during all iterations, and the residuals were fluctuating as you see in the image.
Residuals_Min_Mout_Ptop (2).jpg

VOF_Min_Mout_Ptop.jpg

velocity_Min_Mout_Ptop.jpg

I appreciate your advice about this... it seems to be simple problem but it is not working..
Note: I repeated the simulations with different dimension to replace the top surface far from the free surface but same behavior occurred.. not sure how far should it be??

[Mohammad80]

Hi.
I have a problem with the reverse flow at the outlet of my geometry. I am doing a simulation to determine the water drag force on a pier inside a channel using FLUENT. I want to get the results for a Laminar flow (v=0.0001 m/sec). the simulation is going well but the drag force I got is negative, that means the reverse flow affecting on the results. I increased the elements density and I changed the geometry to be as shown in the attached picture and made the outlet far from the pier and I decreased the time step size but I still get the reverse flow warning messages. I read all the comments in this site about this problem and i did all what were suggested but unfortunately, the problem stills unsolved. Any tips please?