Experiment of the Month

Aspirators Curl

Ben Schull, MU physics, class of 2004, did his senior seminar project on entrainment of one fluid by a flowing fluid. He flowed water through a pipe with a T. The third leg of the T contained a tube which dipped into a reservoir of water. If the tube end was flush with the wall of the main pipe, water flowed from the pipe out into the reservoir. If the tube projects into the main pipe, water flows from the reservoir and is entrained in the main flow.

Mr. Schull put a pressure gauge on the tube bottom and was able to measure the "entrainment pressure" as a function of the distance that the tube penetrated into the main pipe.

To help visualize the fluid dynamics, Dr. Miziumski made a device like the one pictured below.


Blowing on the right side of the pipe produces just bubbles of air in the cup of water, as long as the red straw top is flush with the wall of the clear tygon tubing.. If the tube is further in, water is entrained with the air and dribbles out the left end of the tygon. The figures below show the two different configurations.

flush in

The air flow suggested by Dr. Miziumski was as follows, for the entrained situation:


A close up sketch of the curved part of the flow is shown below


Because the flow curves, there must be a centripetal acceleration. The force to cause that acceleration comes from a pressure gradient; high pressure on the outside of the curve and low pressure on the inside, near the mouth of the tube. Because the high pressure region (left and top) is connected to atmospheric pressure, the mouth of the red straw can experience a pressure below atmospheric.

If the water level in the cup is close enough to the top of the red straw, the atmospheric pressure on the top of the water surface is enough to push water up the straw into the tygon tubing.

Note that the Bernoulli principle has nothing to do with this, since the curling water velocity shown above changes direction but not magnitude. In short, aspirators don't suck; they curl.