Abstract: 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 should be greater than 7.
Abstract: A combination of a fourth-order Pade compact finite difference discretization in space and a fourthorder RungeâKutta 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 fourthorder accuracy in both time and space, another advantage of the given numerical scheme is that no additional attenuation is required to get numerical stability.
Abstract: The velocity fields of an acoustic standing wave in a rectangular channel are investigated. A new approach is used to measure the velocity fields using PIV. In this approach, the velocity fields are sampled at different phases in a given experimental run without synchronizing the PIV system with the excitation signal. The results show that the RMS velocities measured from this approach are in excellent agreement with the theoretical values, indicating that this new and simple approach accurately measures the RMS velocities. At the velocity antinode, the difference between the RMS measured and theoretical velocities is less than 2.4%. The results also show that this approach can be used to measure velocity fields of a standing wave in different small segments at different times and to reconstruct the entire waveform. That is, the basic statistical properties of the entire standing wave can be obtained without synchronizing PIV with the acoustic signal.
Abstract: Influence of differentially heated horizontal walls on shape and amplitude of acoustic streaming velocity field inside a gas-filled rectangular enclosure subject to acoustic standing wave are experimentally investigated. The synchronized particle image velocimetry (PIV) technique has been used to measure the streaming velocity fields. The results indicate that the as the temperature difference increases, the amplitude of streaming velocity increases.
Abstract: 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.
Abstract: The motion of gas within an air-filled rigid-walled square channel subjected to acoustic standing waves is experimentally investigated. The synchronized particle image velocimetry (PIV) technique has been used to measure the acoustic velocity fields at different phases over the excitation signal period. The acoustic velocity measurements have been conducted for two different acoustic intensities in the quasi-nonlinear
range (in which the nonlinear effects can be neglected in comparison with the dissipation effects), and one acoustic intensity in the finite-amplitude nonlinear range (in which both the nonlinear term and the dissipative term play a role in the wave equation). The experimental velocity fields for the quasi-nonlinear cases are compared with the analytical results obtained from the time-harmonic solution of the wave equation. Good agreement between the experimental and analytical velocity fields proves the ability of the synchronized PIV technique to accurately measure both temporal and spatial variations of the acoustic velocity fields. The verified technique is then used to measure the acoustic velocity fields of the finite-amplitude nonlinear case at different phases.
Abstract: Effects of transverse temperature gradient on both 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 channelâs centerline to the asymmetric ones. The transitional behavior of streaming vortices due to the gradual increase in the transverse temperature gradient is presented and physically explained.
Abstract: A new 9-point sixth-order accurate compact finite difference method for solving the Helmholtz equation in one and two dimensions, is developed and analyzed. This scheme is based on sixth-order approximation to the derivative calculated from the Helmholtz equation. A sixth-order accurate symmetrical representation for the Neumann boundary condition was also developed. The efficiency and accuracy of the scheme is validated by its application to two test problems which have exact solutions. Numerical results show that this sixth-order scheme has the expected accuracy and behaves robustly with respect to the wave number.
Abstract: Synchronized PIV (particle image velocimetry) 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.
Abstract: In this paper, we have implemented and compared two different fourth-order accurate methods
for solving the Helmholtz equation. Comparison is done in terms of accuracy and computational cost.
Abstract: Nonlinear standing waves inside an air-filled rigid-walled square
channel are numerically and experimentally investigated. The
acoustic pressure is measured along the channel axis with a pressure
transducer. The nonlinear differential equation for particle
displacement is solved numerically using an effective finite
difference method without truncation. A good agreement between the
experimental and numerical results was observed in time and space
domains. The analysis showed that the shape of the particle
displacement and pressure waveforms of highly nonlinear waves are
deviated from the sinusoidal from in both space and time.
