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) decreases the energy in the other degrees. It is the kinetic energy of the air molecules in these other degrees, transverse to the flow, that is responsible for the pressure.
The pressure at any point along the tunnel's length can be measured using tubes linked by a common reservoir as a multiple manometer; the height of liquid in each tube indicating the relative pressures in each.
Figure 1 shows the dimensions of the tunnel; the square cross-section means that the depth dimensions are identical to the heights. It is built of 5mm Plexiglass plates glued together using methylene chloride. The pressure difference in each part of the tunnel is registered by the heights of ink in tubes (5mm bore) that are linked by vinyl hoses to outlets at the top of the tunnel (six in all, but only one is shown connected in the figure).
Setting it up
The tunnel and capillary tubes are all mounted on the same wooden base (not shown). The air source 1 hose clips to one end of the tunnel. To prevent the liquid in the manometer tubes from being sucked into the tunnel, make sure to regulate the air flow with a variac.
The tunnel looks really neat, but its appearance is not matched by its performance. Yes, the center capillaries rise higher showing lower pressure, but only just higher than those downwind of the constriction, whereas those upwind of the constriction bottom out! You may want to draw attention away from this complication by obscuring the two downwind capillaries with a wood block. Real qualitative stuff.
1 Ealing 5 gallon air blower. Ealing Corp.