Hi Philipp, Yes, they are shear velocity and bulk velocity. Can I say if the ffx, ffy or ffz equal to (u_t/u_b)^2, the flow is specified as a fixed pressure gradient in the channel, and parameter(54) and para(55) in rea file are fixed as 0. Otherwise, if the all ffx, ffy and ffz are 0, the flow is fixed at flow rate rather than pressure gradient? Cheers, Zhai