Sources: every target reads a velocity source through one implementation. bulk_velocity applies to initial conditions as to inlets, rescaling the expressions and leaving staged data alone. sum applies to inlets as to initial conditions, including expressions added to one data term. A curvilinear component is the speed through the grid face on every target, so a uniform u_zeta needs no projection on a curved grid.
Initial conditions: channel_spectral_velocity and duct_spectral_velocity write fluctuations only; params.bulk_velocity is refused. Give the mean as a source in a sum, with constraints: {bulk_velocity: ...}.