# Propulsion, turbulence and a stick in the water (from an email I sent to a professor)

1. Jul 27, 2011

### Ntstanch

My initial email to the professor was:

" Either way... what I was wondering about were things related to propulsion. I was thinking about the effect of holding a stick in a steady flowing stream and the back and forth violent rocking like motion. Eventually it got me thinking about something people on the swim team called a 'feel for the water' on account of how the stick held in place against the stream created the two opposite and slightly spaced vortexes. Those vortexes, given which one or the other is closest to the rear of the stick, have to be 'pushing' against the stick as the main flow of the incoming water pushes back... and since the vortexes are spaced and alternating that rocking motion, in the sticks case, would be on account of the main force (incoming water) having a steady pushing force, while the vortexes at the back are alternating places and in each exchange alternating with the resistance against the main force causing a sort of 'dropping' effect of the stick into the more or less neutral sides (the spaces in between the alternating vortexes)."

His response:

"Turbulence is an extraordinarily complicated phenomenon, which no one
has solved yet. We know a lot about it, but there's much, much more that
we don't know. The situation of a fluid flowing past a cylinder smoothly
is complicated, even without the turbulence. Eddies develop when the
fluid flow exceeds a certain value.

I think the non-steady force that you perceive on the stick has to do
with the details of how vortices of water detach from the stick. There's
a complicated boundary layer next to the stick; how water interacts with
the stick and with the fluid in the main part of the flow determines the
behavior of the stick and of the water.
Brief overview here:
http://en.wikipedia.org/wiki/Vortex_shedding" [Broken]

My question for this forum is if anyone has anything else to add besides vortex shedding. Vortex shedding shows what I was wondering about, though how it is explained doesn't answer some of my questions. Mainly a clearer understanding of how the vortexes rotation applies force against the back sides of the stick (or cylinder) as well as against themselves while they are both being generated by the "main force" of fluid flowing into the cylinder.

Last edited by a moderator: May 5, 2017