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Vortex Shedding in Water
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Vortex Shedding in Water

What Happens When an Object Moves Through a Fluid?

Introduction to Vortex Shedding

  • Wolfgang introduces the concept of vortex shedding, which occurs when an object moves through a fluid, highlighting its significance in understanding fluid dynamics.
  • The experiment involves a pendulum as the object and water as the fluid, utilizing a specially designed aquarium converted into a flow tank.

Experimental Setup

  • The aquarium features a barrier that separates two ends, with pumps circulating water to create controlled flow conditions.
  • A meter stick is placed in the water to measure flow speed; initial tests with a ping pong ball reveal it floats downstream at approximately 10 cm per second.

Observations with the Pendulum

  • The pendulum is initially placed upstream where water flows fastest; it exhibits minimal motion due to chaotic vortices and eddies around it.
  • When moved downstream to slower water flow, the pendulum begins oscillating back and forth, demonstrating how shed vortices influence its movement.

Interaction of Vortices and Pendulum Motion

  • As vortices are shed alternately from either side of the pendulum, it reacts by moving in the opposite direction each time, showcasing alternating forces acting on it.
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Video description

Wolfgang uses a pendulum partially immersed in a makeshift flow tank to show us the effect of vortex shedding on a small object. When fluid flows around a cylindrical object, there is a range of flow velocities for which a von Karman vortex street is formed. The shedding of these vortices imparts a small, periodic force on the object. Here the object is a cylinder attached to a physical pendulum whose frequency of oscillation is adjustable. The end of the cylinder is submerged in flowing water. When the frequency of the pendulum is adjusted to match the frequency of vortex shedding, the cylinder swings transverse to the direction of flow with a peak-to-peak amplitude of a few centimeters. For more details on our setup as well as references for further study see http://sciencedemonstrations.fas.harvard.edu/presentations/vortex-shedding