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-   -   massflow outlet boundary condition ?? (http://www.cfd-online.com/Forums/main/3799-massflow-outlet-boundary-condition.html)

Charles Renard August 17, 2001 05:46

massflow outlet boundary condition ??
 
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

Sebastien Perron August 17, 2001 08:17

Re: massflow outlet boundary condition ??
 
For outflow, isn't a free exit BC ?

If it is, then you don't need any outlet massflow BC.

chiseung August 17, 2001 08:19

Re: massflow outlet boundary condition ??
 
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.

Charles Renard August 17, 2001 09:30

Re: massflow outlet boundary condition ??
 
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

Charles Renard August 17, 2001 11:08

Re: massflow outlet boundary condition ??
 
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


J. Keays August 18, 2001 06:55

Re: massflow outlet boundary condition ??
 
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.

Charles Renard August 20, 2001 03:27

Re: massflow outlet boundary condition ??
 
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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