Acoustic Generation of Circulation Currents and Jet Effects

Abstract

In his theory of streaming caused by sound waves, Eckart shows that time independent streams necessarily follow as part of the solution of the complete wave equation, taking into account viscosity and second-order terms. His treatment is mainly valid for liquids and it proves that the driving force of the streams is proportional to frequency squared. The effect, therefore, is especially important in the ultrasonic region (crystal winds). However, he suggests that slow streams might also be carried in air at audiofrequencies. Studies of acoustical streaming phenomena around orifices have been made by the use of smoke particles in a three-inch diameter circular tube. Within the tube is placed transversely a thin metal plate with a diameter the same as that of the tube interior. In the center of the plate there is an orifice which may be observed through a window in the tube. These studies covered a range of orifices from thicknesses of 0.5 mm to 19 mm and diameters of 3.5 mm to 20 mm. The frequency lay between 150 to 1000 c.p.s. Velocities in the orifice cover the range of 0 to 700 cm/sec. Close studies of the flow patterns have disclosed that there exist three definite regions of flow as the particle velocity in the orifice is increased: (1) a low velocity region with laminar flow which reverses its direction above a certain level; (2) a medium velocity region in which pulsations are superposed upon the d.c. circulation; (3) a high velocity region in which the flow is only pulsatory, forming jets and vortex rings. These regions can be conveniently represented by “phase diagrams.” Photographs of the various flow patterns in each region of the “phase diagram” have been taken for a number of orifices. Under each observed condition, the acoustic impedance of the orifice is determined by a conventional standing-wave measurement in the tube. Interaction between the streams and the sound field can be seen and are always associated with additional sound absorption even at low levels. Momentum measurements were also made by means of a small disk placed in front of the orifice. These momentum measurements corroborate the photographic studies and show, in addition, that at the high velocities the forces exerted by the jet on the disk are as high as 900 dynes.