there is function in openfoam called ddt2 which can be used to calculate (dp/dt)^2 and after that i think temporal statistics can be used to calculated average of (dp/dt)^2 which is 1/N*sum((dp/dt)^2 ) then i think calculate can be used to get sqr

The easiest way to plot its time-series function in Paraview.

And then use Excel to open it and make the calculation.

Luofeng Huang Thanks for your answer, but in that case i have to save the data at every time step?right?