# Fix Mass/Vel. in BC

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 January 29, 2002, 09:14 Fix Mass/Vel. in BC #1 Julie Polyakh Guest   Posts: n/a Hi! What is the difference between Fixed Mass and Fixed Velocity Flow Swithches for Inlet BC? Thanks, Julie

 January 29, 2002, 10:04 Re: Fix Mass/Vel. in BC #2 stephen Guest   Posts: n/a If you have compressible flow, the density at the inlet will vary depending on the pressure field calculated as part of the solution. If you choose a fixed velocity boundary condition then the velocity you specify will be applied at the inlet boundary. As the density at the inlet changes (in response to changes in the pressure field) then the mass flow through the boundary will change. If you choose fixed mass, the density and velocity you specify will be used to calculate a mass flux. This mass flux will be applied at the inlet boundary, so that if the density at the boundary changes, the velocity at the boundary will change also. Basically you have to choose one or the other, you obviously can't fix both the mass flow rate and the velocity if the density changes. If your flow is incompressible then it doesn't matter which you choose. Cheers Stephen

 January 29, 2002, 10:23 Re: Fix Mass/Vel. in BC #3 Julie Polyakh Guest   Posts: n/a Thank you, Stephen!

 February 14, 2002, 05:43 Re: Fix Mass/Vel. in BC #4 Vladimir Krejci Guest   Posts: n/a Hi ! I would like to know how I can enter fix volume flux. I use negative inlet i.e. fluid is exhausted from a solution domain and I do not know what density is. This situation becomes when the fluid is multicomponent. Any proposals? Thanks Vladimir

 February 15, 2002, 05:29 Re: Fix Mass/Vel. in BC #5 stephen Guest   Posts: n/a Vladimir, If you are specifying a velocity over a fixed area (using a reverse inlet) you are fixing the volume flux....Qfix=UA. The mass flow rate will vary as the density of the mixture changes (m=RHO*Qfix). If you use a fixed flow outlet, then you will be specifying a mass flow rate (in kg/s) and the volumetric flowrate will vary as the density of the mixture changes Q=mfix/RHO Make sense? Stephen

 February 15, 2002, 10:49 Re: Fix Mass/Vel. in BC #6 Vladimir Krejci Guest   Posts: n/a Thank you for your answer. If I understand, it does not matter what option I will use in this case? Vladimir

 February 18, 2002, 11:49 Re: Fix Mass/Vel. in BC #7 nicola Guest   Posts: n/a Hi Vladimir, as Stephen sais, it depends on the compressibility. If your media is incompressible, it does not matter what option you are using, but if it is compressible you should enter the volume flow = velocity.

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