Driven periodic flows (constant_flux and initial_flux) now write <run.analysis.metrics>/driven_flow.csv each step: target and measured flux, bulk velocity, the controller correction, and the driving force per unit mass actually applied. Its time average gives the mean wall stress (u_tau^2 / h in a channel), so a driven run's friction velocity can be read back directly; picurv summarize --plot driven_flow.driving_acceleration draws it, and the driven-channel and driven-duct profile tools take it with --driven-flow-csv.