Observed Onset of Acoustic Streaming.

Most aspects of acoustic streaming were considered understood in 1954. The time dependence of its onset, however, was controversial when Liebermann's measurements, valid for slowly developing streaming, were criticized. The criticism cited a report that streaming was present only 20 msec after turning on the acoustic source. Using a thermistor bead as flow meter, we have observed the time dependence of the onset of streaming in two fluids of widely different viscosity, each in two vessels of different geometry. The time dependence of the streaming velocity is described as follows: There is zero time derivative at the start. This is followed in succession by a positive second derivative, inflection point, and negative second derivative which persists during an approach to a steady state flow. From the start to the inflection point, the flow velocity v depends on time t according to v = btp where b and p are empirical constants. The exponent p appears to be a nearly fundamental property of streaming onset while b is sensitive to the experimental circumstances. Important to the controversy is that the development is safely slow enough to validate Liebermann's measurement technique, typically taking two seconds to reach the inflection point and five seconds to reach 90%. of steady flow. [ABSTRACT FROM AUTHOR]