How to calculate the average velocity over a surface through time using paraview?

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My problem is a little more complex, but here is a simplified similar model.

Let us consider a fire source inside a box and the hot gases (burnt or unburnt) from the combustion are going out of the exhaust (white surface) at a certain velocity. We have the velocity field of the entire domain and can be visualized using ParaView. We also have density and temperature fields among others.

Now, using ParaView I would like to calculate the average velocity of the gases coming out of this surface (white) over time (unsteady regime).

Note: The individual boundaries are not loaded in ParaView. That is outlet as a separate surface, the domain as a separate surface. The entire volume is treated as a single entity with specified boundary conditions.

Any help to calculate this is appreciated.

Filter -> TemporalStatistics should help you here.

You might also need to use the Slice filter to extract a plane at the opening of the exhaust (so you average only at the opening instead of everywhere). Depending on what your data look like, you might also need something to extract the exhaust hole from the box. The Threshold filter might come in handy here.