I am working an a stationary, laminar flow model and would like to calculate the drag of that model.

Now the Comsol forum guys suggest to use the operator reacf(u). It is also used in the "Flow over a circular cylinder" tutorial. Does anyone know where to get it from or how to use the reacf() operator?

I have been looking everywhere for this tutorial, asked for help in the comsol forum, read the Comsol guide but I haven't found any useful information.

I am really desperate for help, please.

Appreciate it