Fluid Dynamics

Bernoulli Wind Tunnel

What it shows

Bernoulli's Principle states that the pressure exerted by a fluid is velocity dependent; the faster the velocity the lower the pressure. This can be demonstrated by a form of Venturi tube, a pipe with a narrow constriction which forces an increase in fluid velocity.

How it works

The varying cross-section of the wind tunnel forces air to travel its length at different velocities, with the highest velocity being at its most constricted part. From equipartition of energy, the increased energy in one degree of freedom (the kinetic energy of the flow)...

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Bernoulli's Principle

What it shows

The relative velocities of two sides of a spinning ball to an oncoming wind creates a pressure difference and therefore a net force on the ball perpendicular to the air flow.

figure 1. Direction of motion of ball due to pressure difference

diagram of air flow and spinning ball
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Bernoulli Beach Ball

What it shows

Bernoulli's principle shows the velocity dependence of pressure in a fluid. Here, fast flowing air creates a zone of low pressure that holds a beach ball aloft.

beach ball held aloft by fast flowing air from a blower

How it works

Here we have a beach ball held in the air stream from a...

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Viscous Fluid

What it shows

For a body to reach terminal velocity when falling through a fluid, the drag force (given by Stoke's Law) coupled with the buoyant force (from Archimedes' principle) need to balance the falling object's weight. Leaving derivations to other great texts you end up with

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Vortex Shedding in Air Friday, April 8, 2016

A thin wire, moving through the air, is made to vibrate in the audio range at the vortex shedding frequency.

What it shows:  When air flows around an object, there is a range of flow velocities for which a von Karman vortex street is formed. The shedding of these vortices imparts a periodic force on the object. The force is quite small and not enough to accelerate the object to any significant amount, especially if the object is relatively massive. If the situation is such that the object can vibrate about a fixed position, we have the possibility of simple...

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Vortex Tube

What it shows:

James Clerk Maxwell postulated that since heat involves the movement of molecules, it might be possible to separate hot and cold air in a device with the help of a "friendly demon" who would sort out and separate the fast and slow moving molecules of air. The vortex tube is such a device and does exactly that — using compressed air as a power source, it has no mechanical moving parts and produces hot air at one end and cold air at the other.

How it works:

Room temperature compressed air is supplied to the vortex tube...

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Reversible Fluid Mixing

What it shows:

Ink is squirted into a fluid and mixed in until it disappears. By precisely undoing the motions in the reverse direction, the ink becomes unmixed! The demonstration seems to defy thermodynamics in that it appears that entropy decreases, but in actuality the reversible mixing is made possible by ensuring that the mixing/unmixing is done without turbulence.

How it works:

The space between two transparent and concentric cylinders is filled with a viscous fluid (glycerine or Karo™ syrup). One or more lines of...

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