Exploring Force Dynamics of Vortex Shedding on a Cylinder

[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"

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.

 

[eljose79]

Thank you for raising this interesting question about fluid dynamics and propulsion. As the professor mentioned, turbulence is a complex and ongoing area of research in fluid dynamics, and there is still much to be understood about it. Vortex shedding, as you have discovered, is one phenomenon that contributes to the forces acting on an object in a fluid flow. However, there are other factors at play as well.

One aspect to consider is the shape of the object itself. In the case of a cylinder, the angle at which it is placed in the flow can greatly affect the forces acting on it. For example, if the cylinder is perpendicular to the flow, the alternating vortices will be symmetrical and create a steady force on the cylinder. However, if the cylinder is angled, the vortices will be asymmetrical and create a fluctuating force known as lift. This lift force can potentially be harnessed for propulsion in certain applications, such as in the design of airplane wings.

Another factor to consider is the viscosity of the fluid. Viscosity is a measure of a fluid's resistance to flow, and it plays a significant role in the generation and behavior of vortices. In a high viscosity fluid, such as honey, the vortices will be larger and slower-moving compared to a low viscosity fluid, such as water. This can affect the forces acting on an object, as well as the overall flow patterns in the surrounding fluid.

Additionally, the size and speed of the object in the flow can also impact the forces acting on it. In your example of a stick in a stream, the size and shape of the stick can influence the size and behavior of the vortices that are shed. The speed of the stick in the flow can also affect the frequency and intensity of the vortex shedding.

Overall, the forces acting on an object in a fluid flow are a result of a complex interplay between various factors, including the shape and size of the object, the viscosity of the fluid, and the speed and direction of the flow. While vortex shedding is one important aspect to consider, it is just one piece of the puzzle in understanding the forces of propulsion in fluid dynamics.

I hope this helps to provide a deeper understanding of the topic. Keep asking questions and exploring the fascinating world of fluid dynamics!