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air flow in a tunnel

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Old   October 18, 2013, 06:30
Default air flow in a tunnel
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Tariq
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Hi all
I am a new user of fluent, i am trying to simulate air flow in a closed tunnel about 60 m long, the air is coming through a duct (50 m long 0.6m dia), i have modeled and meshed the scene in gambit.
the velocity at the exit of the duct should be 2m/s. the air will come out of the duct hit the end of the tunnel ventilate it and go out.
what boundary conditions should i use at the inlet of the duct, outlet of the duct, the end of the tunnel (if required) and the outlet of the tunnel.
when using velocity inlet boundary condition, how should i decide about the velocity specification method (component, magnitude and direction, and magnitude normal to boundary)
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Old   October 19, 2013, 15:48
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Ashwani
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Hi,
Do you really need a duct to be modelled. If you need a constant velocity profile you can give that at inlet of tunnel (i.e. in your case 2m/s) using velocity inlet. See help file for knowing the specification method. It is given. At outlet of tunnel you can use mostly Pressure outlet. May be with gauge pressure = 0, i.e. open to atmosphere.
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Old   October 19, 2013, 18:30
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The air has to be delivered by the duct at a certain distance from the closed end of the tunnel, where it will strike the wall and change direction, ventilating the tunnel and go out from the opening of the tunnel. the opening of the tunnel and the inlet of the duct are on the same plane.
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Old   October 21, 2013, 13:45
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At the end of the duct would you like to have a constant velocity profile or a fully developed profile. From the duct you would get a constant velocity profile. Also, if you do not change the cross-section at the inlet and outlet of the duct, by continuity the velocity will be same (since, your case is incompressible one). So, You can give pressure outlet with pressure =0. While at inlet also, you should also keep pressure zero.
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Old   October 21, 2013, 15:52
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Thanks for the response, u r really giving me company and help:-)

i got your point regarding pressure inlet and out, but what difference will it make if i do not use any velocity profile and just give any required input velocity at the inlet, the velocity at the exit will than automatically be fully developed. what difference will i get with constant velocity, ( i want the actual scenario, in actual the velocity should be fully developed).

the tunnel is very rough, i am not understanding how to cater for the roughness factor of the tunnel or even the duct, i think i am just using the no slip condition. do we have any option of specifying the material or roughness of the duct and tunnel.
what i have understood, this option is not available for all turbulent models, please guide, and where ever it is available the input required is the roughness factor, what value should i use for my tunnel (i mean how should i decide).
regards
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Old   October 21, 2013, 18:17
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Andrew Kokemoor
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You have two options to control the inlet velocity profile:
First, extend your domain farther upstream so that a uniform velocity inlet will have developed by the time it reaches your real inlet location.
Alternatively, look up profiles (Section 6.6 in the Fluent User's Guide) to define your fully developed profile at the real inlet.

Regarding wall roughness: if you don't know anything about your surface characteristics, the default is probably okay. Unless your case is very sensitive to turbulence, it won't have a large effect.
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Old   October 22, 2013, 09:15
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I am sorry, I typed tunnel instead of duct. At inlet of duct you will give constant velocity profile. And outlet of duct you can give Pressure outlet. Then you will have at exit of the duct, a fully developed flow, with avg. velocity 2m/s.
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Old   January 3, 2014, 18:32
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for pressure outlet turbulence specification:- which method should be used and why (intensity and viscosity ratio, k and epsilon, intensity and backflow hydraulic diameter,intensity and length scale)? is the backflow hydraulic diameter the same as hydraulic diameter?
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Old   January 3, 2014, 18:44
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If you expect backflow, then your turbulence specification will depend on the source of that backflow; probably the hydraulic diameter of the tunnel. If you don't expect backflow, then this won't have an effect on your final solution.
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Old   January 4, 2014, 04:16
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so I should keep the intensity and backflow hydraulic diameter to zero, or use the intensity as per the velocity and use the hydraulic diameter of the exit. how should I choose amongst the available turbulence specification methods?
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