# Difference between mass-flow inlet and velocity inlet

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 June 13, 2016, 06:00 Difference between mass-flow inlet and velocity inlet #1 New Member   Join Date: Apr 2016 Posts: 6 Rep Power: 7 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.

 June 13, 2016, 08:06 #2 Senior Member   Saideep Join Date: Apr 2015 Location: INDIA Posts: 203 Rep Power: 9 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

 June 13, 2016, 09:24 #3 New Member   Join Date: Apr 2016 Posts: 6 Rep Power: 7 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

 June 13, 2016, 11:45 #4 Senior Member   Saideep Join Date: Apr 2015 Location: INDIA Posts: 203 Rep Power: 9 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 soheil_r7 and ViniSpark like this.

 June 14, 2016, 04:18 #5 New Member   Join Date: Apr 2016 Posts: 6 Rep Power: 7 Thank you for your answer, now it's clearer. rekap

October 17, 2016, 08:07
boundary problem
#6
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rihen full
Join Date: May 2012
Posts: 10
Rep Power: 11
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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 Tags boundary conditions, inlet bc, massflow bc, simflow, velocity bc

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