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massflow outlet boundary condition ??

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Old   August 17, 2001, 05:46
Default massflow outlet boundary condition ??
  #1
Charles Renard
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hi !

i am wondering how to model a fresh air entry on a vehicle.(i am currently working on external aerodynamics) i already modeled the corresponding hot gases oulet with a mass flow inlet boundary condition.

my issue is that i only know the mass flow rate (the same as for hot gases outlet), and it seems that fluent 5 has no "outlet massflow" boundary condition.

Help needed !

thanks Charles
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Old   August 17, 2001, 08:17
Default Re: massflow outlet boundary condition ??
  #2
Sebastien Perron
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For outflow, isn't a free exit BC ?

If it is, then you don't need any outlet massflow BC.
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Old   August 17, 2001, 08:19
Default Re: massflow outlet boundary condition ??
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chiseung
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I'm afraid that I understood your question exactly. As I understood, you only know the mass flow rate at outflow in your system.

If so, give the mass-flow-inlet b.c. to your outflow boundary and assign a negative value. That means mass is flowing out of your system at constant rate.
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Old   August 17, 2001, 09:30
Default Re: massflow outlet boundary condition ??
  #4
Charles Renard
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hi !

i cannot do that because the flow is just a small part (a sort of cooling inlet/oultet) of a bigger flow around a vehicle.

i tried to input a negative value for massflow inlet but fluent does not accept it.

i think i will have to use UDF to declare a constant velocity pointing outwards the flow domain around the vehicle.

any other suggestion is welcome...

Charles
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Old   August 17, 2001, 11:08
Default Re: massflow outlet boundary condition ??
  #5
Charles Renard
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i just tried to put a negative value in inlet-velocity; fluent accept it and results don't seem too bad at first glance...

has someone already used this "not-so-pretty-method" ?

thanks

charles

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Old   August 18, 2001, 06:55
Default Re: massflow outlet boundary condition ??
  #6
J. Keays
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Hi,

Yes, I have used a negative value for velocity at the exit to obtain a velocity out of the system. It works very well. I don't think there is much of a problem with it, particularily if the boundary layer is unimportant.

J.
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Old   August 20, 2001, 03:27
Default Re: massflow outlet boundary condition ??
  #7
Charles Renard
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ok

so i guess i will use this method too. i tried to draw pathlines released from the area and i got nothing going inside my flow domain, so yes i guess lines are "going outside".

thanks Charles
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