Dear Dr. Huang,

     The complete file concerning the above manuscript has been
reviewed by a Divisional Associate Editor.  The enclosed comments
advise against publication in Physical Review Letters.  The
Editors accept this advice.

     Your appeal has been considered, and our decision to reject
is maintained.

Sincerely,

Saad E. Hebboul
Assistant to the Editor
Physical Review Letters
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Report of Divisional Associate Editor

Report on LN7307  Huang

The Trajectory of a Curveball in 2D Navier-Stokes Flow

The trajectory of a moving, spinning sphere in a three-dimensional
Navier-Stokes flow is an interesting problem in a number of contexts,
including baseball, which is the application motivating this work. A
prediction for the trajectory of a spinning sphere could be readily
tested in a laboratory experiment.  However, this manuscript treats
the two-dimensional problem of a spinning disc, which is an
interesting exercise in computational fluid dynamics but not one of
broad interest. Also, it would be difficult to test the results in a
laboratory experiment.  Perhaps a long spinning cylinder could be
ejected into a rapidly rotating fluid with the cylinder axis parallel
to the axis of rotation of the fluid; the fluid would be constrained
to be two-dimensional by the Taylor-Proudman theorem.  Even if such a
two-dimensional experiment were performed, it would not help much in
understanding the problem that motivates the work, because
three-dimensional flows (with stretching and bending of vortex lines)
are quite different from two-dimensional flows.

The author is to be commended for his careful response to the comments
of the referees. The presentation in the present version of the
manuscript is logical and clear.  While the manuscript is not
appropriate for PRL, publication is warranted. I suggest that
the author compare his analysis with other techniques used to study
moving particles in incompressible fluids (see comments of Referee C)
and submit the manuscript for publication in another journal.

Harry Swinney
Divisional Associate Editor
Physical Review Letters