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Difference between mass-flow inlet and velocity inlet

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Old   June 13, 2016, 06:00
Default Difference between mass-flow inlet and velocity inlet
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Hello,

I am using Simflow (an OpenFOAM gui).
I am modeling two incompressible fluids flowing in a relatively complex channel in order to see how they mix.
I run a simulation using the mass-flow inlet boundary condition with a flow rate, let's say Q. Then I run a simulation using the velocity inlet boundary condition with a velocity v calculated as Q/A where A is the section of the channel. Then I should expect the same result. However the simulations give different results.

Can anyone explain why? Which is the difference between the mass-flow inlet and velocity inlet boundary conditions?

Thank you.
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Old   June 13, 2016, 08:06
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Hi,

I mostly use a constant velocity inlet bc but when you say mass inlet, I am guessing that the density of fluid comes into play as well.

That is, you need to think in terms of kg/s rather than in m3/s.

I have no idea about the gui version of OF but if you have a check for your units that should say if what i told is true{kg/s} or not{m3/s}.

Saideep
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Old   June 13, 2016, 09:24
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When I use the mass-flow inlet boundary condition, the unit for the inlet is [m3/s]. Maybe the gui version use the density for computing the mass flow rate.

I found out on the web that for Fluent the velocity inlet bc is recommended for incompressible flows while mass flow inlet bc is recommended for compressible flows.

Do you think this is valid also in OpenFOAM? If so, can I say this is the reason why I have different simulation results?

rekap
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Old   June 13, 2016, 11:45
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For in-compressible flow density isn't varying. So, fixed flux/ fixed velocity is good enough. Even fixed mass should not be causing any sort of problem.

When you go for compressible flows, there are few parameters on which the density depends.
One example of a general case, at the inlet you fix mass/ velocity and allow pressure to vary accordingly by a Neumann condition. When your pressure is fluctuating your density also varies. (ultimately your mass flow is varying over time). So, fixing the mass flow[kg/s] is a better condition than fixing velocity.

I use in-compressible so if I am wrong any correction is welcome.

Saideep
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Old   June 14, 2016, 04:18
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Thank you for your answer, now it's clearer.

rekap
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Old   October 17, 2016, 08:07
Default boundary problem
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I want to solve a ventilated wall problem. but ı cannot describe boundary condition esspecially for air. there is no velocity or mass for air. natural convection.
how can describe boundary inlet and outle
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