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-   -   Velocity reduces in channel (http://www.cfd-online.com/Forums/fluent/47862-velocity-reduces-channel.html)

Rayan April 9, 2008 15:28

Velocity reduces in channel
 
Hi,

I am trying to simulate open channel flow. When i run my simulation the velocity in the channel reduces significantly after time. I am not sure where am i going wrong. Can anyone help me with this.

Rayan

sachin jadhav April 9, 2008 16:01

Re: Velocity reduces in channel
 
hi Rayan, It may because of supercrical flow at upstream side. check ur upstream velocity supercritical or subcritical. I think it will help it out......

sachin.

Rayan April 9, 2008 16:19

Re: Velocity reduces in channel
 
Hi,

The flow in my case is not supercritical, the Froude number in my case is 0.32, which corresponds to subcritical Froude number. Do you have any other recommendations.

What boundary conditions are you sing while simulating open channel flow and does the flow rate stay constant after running the simulation.

Rayan.


sachin jadhav April 9, 2008 16:32

Re: Velocity reduces in channel
 
Hi,

My bc is mass flow rate and pressure outlet but my inlet velocity is supercritical and i am solving problem on hydraulic jump in open channel. I am also struggling same problem that inlet and outlet flow rate are not same as inlet mass flow rate is 5600 kg/sec and that of outlet is more than 6000 Kg/sec.

sachin


Rayan April 9, 2008 16:45

Re: Velocity reduces in channel
 
Hi,

In my case massflow rate at inlet = mass flow rate at outlet but its value reduces both at inlet and outlet. For example, intially the mass flow rate is 38 kg/s and it reduces to 27 kg/s after running the simulation for long enough time.

In your case did you try refining the mesh, may be that can help you to get massflow rate at inlet equal to mass flow rate at outlet.

Rayan.

sachin jadhav April 9, 2008 16:59

Re: Velocity reduces in channel
 
Hi,

what is your bc? If u r using mass flow rate bc, you have to give exact free surface level and bottom level. and about my problem i just refined my mesh and iteration is going on then i tell you.... can you able to suggest me turbulence model to capture high turbulence becase in hydraulic jump energy is dissipated in the form of turbulence. I uses k-epsilon model with default values....Is it help me out...

sachin.

Rayan April 9, 2008 17:07

Re: Velocity reduces in channel
 
I am using pressure inlet and outlet as my bc, there also i had to specify the free surface level, bottom level and velocity. Did you try Large eddy simulation.

What specification method are you using at inlet and outlet.

Rayan.

sachin jadhav April 9, 2008 17:23

Re: Velocity reduces in channel
 
I also got this problem by using ur bc, when i used pressure inlet and other specification as u mentioned i see that air is also coming in with that velocity so i tried for mass flow inlet i got steady mass flow throught out the simulation. so calculate mass flow rate from velocity and c/s area of channel and give mass flow inlet bc as wall as free surface level and bottom level. dont forget to give zero 0 mass flow rate to air as primary phase. surely it will help it out and reply me soon.

sachin


sachin jadhav April 9, 2008 17:34

Re: Velocity reduces in channel
 
hi Rayan

my problem is 2D so how can i use LES model for it?

give me ur mail ID and my id is sachinjadhav2004@gmail.com if possible please mail me... Thanks and regards

sachin jadhav

Rayan April 9, 2008 23:42

Re: Velocity reduces in channel
 
Hi,

Did you try outlet boundary as outflow.

Rayan.

sachin jadhav April 10, 2008 14:24

Re: Velocity reduces in channel
 
Hi, Rayan

No, i didnt use outflow bc for open channel becase as i read FLUENT mannual there is no such bc for open channel. It does not take this bc.

sachin


rahulrp December 1, 2010 01:38

Open Channel Flow boundary conditions
 
1 Attachment(s)
Quote:

Originally Posted by sachin jadhav
;149854
hi Rayan

my problem is 2D so how can i use LES model for it?

give me ur mail ID and my id is sachinjadhav2004@gmail.com if possible please mail me... Thanks and regards

sachin jadhav

Hi Sachin,

I am Rahul. I am also working on Open Channel Flow in Fluent solver. I used
Mass Flow Inlet bc
mass flow of air is 7985.6 kg/s, free surface of water is at 2m, bottom at 0m,
Pressure Outlet bc


In my case the, the channel is rectangular of cross section 4mx3m. Out of 3m height upto 2m there is water. Now I really dont understand what boundary condition should I use for remaining 1m. The geometry is attached herewith.

One more thing to be discussed is that, should I patched volume fraction of water( secondary phase) initially through out the domain. I am trying for steady state simulation.

Hoping for ur fruitful suggestions.
Thanks.


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