VISUALISATION OF ACOUSTIC STREAMING USING PIV IN NEWTONIAN AND NON-NEWTONIAN LIQUIDS

Abstract

The effect of fluid rheology on acoustic streaming was studied experimentally using a low frequency (600Hz–15kHz) underwater acoustic transducer. The fluid rheology was compared with deionized water and non-Newtonian fluid polyanionic cellulose (PAC). Streaming effect generated by the trans- ducer in a static liquid medium was visualized by particle image velocimetry (PIV) method. The motion of fluid was optically visualized using light scattering ‘seeding’ particles. Velocity profiles induced by the acoustic streaming have different shapes and range of magnitudes. First, the acoustic stream- ing in deionized water was visualized for different frequencies and pressure amplitudes (voltages). A maximum of 1 g/L PAC was then introduced in smaller steps for some selected frequency and voltage settings. The streaming disappeared completely when the total concentration of the fluid medium reached 0.19 g/L PAC. The measured streaming velocities are found to be in the range of 2.1 to 9.9 cm/s for water and it is proportional to the applied voltage and the operating frequency of the transducer. When introducing PAC, the streaming velocity within water gradually decreased until zero due to the attenuation of acoustic waves by viscous effects. This confirms that the streaming velocity is approximately inversely proportional to the bulk viscosity of the medium. The velocity vectors and the streaming velocity maps illustrate the induced non-linearities of the fluid medium due to the acoustic propagation. The results are part of a comprehensive study aimed at investigating the influence of acoustic vibration on particle settling in non-Newtonian