Post-processing results show unexpected velocities


I have created a model for Natural ventilation of a corridor space in a building with the help of CFD simulation. The wind speed and avg. temperature of the building location was given as the inlet boundary conditions (IBC). the inlet condition being 10 mph and the 8 + 1 faces were selected as faces at the IBC. The simulation of “RUN3” has the post processing results showing 70-100 kmph, which doesnt seem reasonable.

Therefore looking forward to your support and reply in idetify the issue.


Hello @Sudheep , thanks for posting your question in the Forum.

Inlet faces have a total surface area of 29.82 meter square, and the outlet surface area is 1. 645 meter square. For now ignoring the density variations inside, the outlet velocity should be around 75 m/s with a rough computation using mass balance. Simulation results are showing an average outlet velocity of 78 m/s (the slight difference 3 m/s change is probably due to numerical errors or density variations inside), which seems reasonable to me.

Could you please check your input velocities again, and see if they are making sense with the current settings.


I am not sure if I understood it completely, why is it correlated with mass balance ? but how do you determine or identify the outlet velocity should has to be 75 m/s ?

And why do I post processing image representing the velocity magnitude to be averaging 80 km/hr ?

I have checked my input velocities and based on the location and since its a natural ventilation the wind speed at the inlet of the faces remains at 4 m/s.

Thank you .

In your situation, there are 8 inlet faces with a total area of 29.82 square meters. On the flip side, there’s only one outlet with a surface area of 1.645 square meters.

According to the law of mass conservation, the mass flow rate entering the control volume should equal the mass flow rate exiting it, implying that fluids cannot be created or destroyed within the system. Mass flow rate is computed by multiplying the density of the fluid by the cross-sectional area through which the fluid is flowing and the velocity of the fluid, expressed as the equation: Mass Flow Rate = Density × Cross-sectional Area × Velocity. Since the inlet area is very large and the outlet area is very small, it leads to a significant increase in fluid velocity as it flows from the larger cross-sectional area to the smaller one, in accordance with the principle of conservation of mass.