Experimental study of acoustic streaming generated by ultrasonic excitation

Abstract

The paper is devoted to an experimental study of the acoustic streaming generated by an ultrasonic (US) irradiation in a confined fluid volume. The container is a parallelepiped, partially filled with liquid (water), into which a piezoelectric US emitter is submerged from the top. To transmit the oscillations into the fluid, a waveguide is used whose thickness is considerably smaller than the US wavelength. The structure and velocity of the acoustic streaming, which forms in the bulk, are studied by methods of particle image velocimetry and photography. The observations are carried out in the vertical cross section, which contains the beam axis, with the use of a light sheet created by means of a continuous green laser. The illumination produces the heating in the plane of the light sheet and leads to the onset of free convection. The latter is studied: the flow structure and velocity and the time of their settling are considered at different power of illumination. The information on the natural convection properties is used during the analysis of the structure of acoustic streaming. In general, in the considered region of interest, the thermal convection does not have a substantial influence on the outcomes of the present study, but it determines the minimal velocity, which is limiting the possibility of investigation of slow flows. The observed acoustic streaming may be conventionally divided into two kinds. The first is localized near the waveguide surface and is steady-state. It possesses a regular structure and its characteristic velocity is not high. The streaming of the second kind occupies the entire fluid bulk and transforms continuously. Its velocity magnitude is higher by an order than that of first-kind streaming. An interesting peculiarity of the non-stationary acoustic streaming is that its sources may be located on the container boundaries and not only on the waveguide. This finding points to an important contribution of the reflected US waves in the fluid motion at the sufficient intensity of the US irradiation. The obtained results are going to be used during the investigation of the emulsions behavior in US fields.