Initial conditions (experimental): properties.initial_conditions.source may be a sum of velocity sources and at most one generator that writes a Ucat, such as a mean profile plus the fluctuations of channel_spectral_velocity. A project constraint, on by default for a sum, makes the initial field divergence-free before the first step with the time step's own pressure projection. channel_spectral_velocity and duct_spectral_velocity accept a streamwise axis running from an inlet to an outlet, and bulk_velocity: 0 for the fluctuations alone.