Fluid Flow at a Small Hole due to Vibration

Abstract

A REPETITION of the damonstration described by Prof. G. D. West in Nature1 showed that it works particularly well with a wide test-tube of length suitable for a 256-fork (even the small laboratory pattern), and having at the closed end a central hole of diameter less than 1 mm., shaped rather like the small aperture of a Helmholtz resonator. By performing the experiment with the aid of a projection lantern, both this effect and a reverse effect not mentioned by Prof. West can readily be shown to a large class. On bringing the tuning fork near the open end of the tube, a puff of smoke is seen to issue from the hole in the closed end, followed by a steady stream of smoke so long as the vibrating tuning fork is held in position. Under the same conditions, a puff of clear fresh air can be seen to enter the tube from outside, followed by a steady stream of fresh air. This is precisely what one would expect, for the closed end of the tube is a pressure antinode, and the so-called 'steady pressure' of sound is in reality a differential effect in which the compressional phase preponderates over the rarefactional phase, the alternating emergence and intake of gas observed at the orifice taking place so rapidly that the eye sees what appears to be a steady stream of gas flowing simultaneously in both directions at the orifice.