Majid Nabavi

Simultaneous measurement of acoustic and streaming velocities using synchronized PIV technique

Synchronized particle image velocimetry (PIV) technique has been applied to measure the acoustic and streaming velocity fields simultaneously, inside a standing-wave rectangular channel. In this technique, the velocity fields were sampled at a certain phase of the excitation waveform. The acoustic velocity fields were obtained by cross-correlating the two consecutive PIV images, whereas the streaming velocity fields were obtained by cross-correlating the alternative PIV images at the same phase. The experimental values of the mean acoustic velocity and RMS streaming velocities obtained from PIV are in good agreement with the theoretical values, showing that this novel approach can measure both acoustic and streaming velocities, accurately and simultaneously, in the presence of large amplitude acoustic wave.

Experimental investigation of the formation of acoustic streaming in a rectangular enclosure using a synchronized PIV technique

The formation process of acoustic streaming generated in an air-filled rigid-walled square channel subjected to acoustic standing waves of different frequencies and intensities is investigated experimentally. The walls of the resonator are maintained at isothermal boundary condition. The synchronized particle image velocimetry (PIV) technique has been used to measure the streaming velocity fields. The results show that the formation of classical streaming patterns depends on the frequency and vibrational displacement of the acoustic driver. It is found that to generate the classical streaming flow patterns, the streaming Reynolds number (Res1 = u2max/νω) should be greater than 7.

Effects of transverse temperature gradient on acoustic and streaming velocity fields in a resonant cavity

Effects of transverse temperature gradient on acoustic and streaming velocity fields inside a gas-filled rectangular enclosure subject to acoustic standing wave are investigated experimentally. Synchronized particle image velocimetry technique has been used to measure the acoustic and streaming velocity fields. The results show that the temperature difference between the top and the bottom walls deforms the symmetric streaming vortices about the channel’s centerline to the asymmetric ones. As the temperature difference increases, the amplitude of streaming velocity increases.

Analysis of the flow structure inside the valveless standing wave pump

The flow structure inside the valveless standing wave pump is investigated experimentally. The two-dimensional velocity fields inside the chamber of this novel pump at different phases of the excitation signal are measured using the synchronized particle image velocimetry technique. The variations in the pump flow rate, pressure loss coefficients, and rectification capability of the diffuser-nozzle element are analyzed. According to the results obtained in this paper, the net flow rate of the pump increases with an increase in the pressure (or Reynolds number). The interactions of three different flow fields inside the pump chamber (pumping flow, acoustic, and streaming velocities) are studied. It is found that, while the pumping flow has an effect on the acoustic velocity patterns only around the inlet and outlet orifices, the streaming velocity structures are drastically affected by the pumping flow.

A FOURTH-ORDER ACCURATE SCHEME FOR SOLVING ONE-DIMENSIONAL HIGHLY NONLINEAR STANDING WAVE EQUATION IN DIFFERENT THERMOVISCOUS FLUIDS

Combination of a fourth-order Pade compact finite difference discretization in space and a fourth- order Runge-Kutta time stepping scheme is shown to yield an effective method for solving highly nonlinear standing waves in a thermoviscous medium. This accurate and fast-solver numerical scheme can predict the pressure, particle velocity, and density along the standing wave resonator filled with a thermoviscous fluid from linear to strongly nonlinear levels of the excitation amplitude. The stability analysis is performed to determine the stability region of the scheme. Beside the fourth- order accuracy in both time and space, another advantage of the given numerical scheme is that no additional attenuation is required to get numerical stability. As it is well known, the results show that the pressure and particle velocity waveforms for highly nonlinear waves are significantly different from that of the linear waves, in both time and space. For highly nonlinear waves, the results also indicate the presence of a wavefront that travels along the resonator with very high pressure and velocity gradients. Two gases, air and CO2, are considered. It is observed that the slopes of the traveling velocity and pressure gradients are higher for CO2 than those for air. For highly nonlinear waves, the results also indicate the higher asymmetry in pressure for CO2 than that for air.

Valveless acoustic standing wave micropump for biomedical applications: A numerical study

The operation principle of the valveless acoustic standing wave micropump is described. Time-variant flow structures through the planar diffuser-nozzle element of this micropump for different values of the divergence angle of the diffuser-nozzle element at excitation frequency of ƒ = 20 kHz are numerically investigated. The variations of micropump flow rate, pressure loss coefficients of the nozzle and diffuser, and diffuser efficiency are shown as functions of θ. The higher micropump flow rate is found to be achieved at larger values of θ. However, increasing θ from 45° to 60° shows no significant increase in the net flow rate. The results also show that the maximum diffuser efficiency is achieved at θ = 45.