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I would have to re-derive the formulae for the submerged jet, and perhaps solve the time-dependent Navier-Stokes equation.\\  However, it would (arguably) be a better model (see references below), and may be more illuminating as to how the velocity profile is set up, and the nature of interactions between neighbouring DNA molecules.  \item {\it Which process (sound or vorticity) carries more momentum?} Read \citet{1959flme.book.....L}, \citet{1959flme.book.....L}  chapter 8, 8 (p. 245),  on sound. \item Another thing to consider: {\it does it even matter?} The time-scale they use in the paper is much larger than the sound-time, and places a more stringent test on the steady-state assumption. So my objection would only strengthen the conclusion.\\The vorticity moves slowest, and the fluid velocity profile might only assume the anticipated form once this final contribution has had time to influence the entire profile.