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Method 2: tack some ad-hoc changes onto the existing work -- e.g. treat the DNA/jet as an extended source of spherical waves. This will be {\it inconsistent} and crude, but probably easier than method 1.\\  In any case, a model with sound might be a more accurate model (see references below). It may also 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} chapter 8 (p. 245), on sound. Specifically: how does a sound wave get generated; how much momentum does it carry / how does it affect the surrounding medium; how does it compare with vorticity?  \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.