In this paper(DOI: 10.1039/C5CP02406D), the research team describe a novel cleaning system reliant on cavitation action created in a free flowing fluid stream where ultrasonic transmission to a surface, through the stream, is achieved using careful design and control of the device architecture, sound field and the materials employed.
The research team used rho-c polymer and matched to the acoustic impedance of water, which allows efficient transmission of the ultrasound from the water in the cone into the water in the stream (both the polymer and the stream being surrounded by air on the outside).
If such acoustic matching is disturbed, the pressure in the stream is greatly reduced. For example if the tip of the nozzle is replaced with an aluminium section (as shown in red in Fig. 1(a)), the zero-to-peak acoustic pressure in the stream reduces from B300 kPa to B80 kPa (Fig. 1(b)).
I can't understand the result of this experiment. The common knowledge is that metal can propagate of ultrasound wave, but in this experiment isn't. So should I match the acoustic impedance for better propagtion of the ultrasound in fluid? like in this experiment?
Then, what cause made that polymer make great propagation?
Thanks for replied researchers
ps. I refer this paper !
DOI: 10.1039/C5CP02406D
The matching of the specific impedance (rho c) between transducer and fluid (water) is not a matter of propagation in the fluid, but a matter of optimization of the transfer of acoustic energy from the transducer to the fluid. Indeed, for normal incidence at the transducer (rho_1 c_1)-liquid interface (rho2 c_2) the reflection coefficient for plane waves is R=P_reflected/P_incident=( rho_2 c_2- rho_1 c_1)/(rho_2 c_2+rho_1 c_1). The transmission coefficient for the energy is T_E=1-R^2. The speed of sound in aluminium is higher than in water as well as the density. The equation for the reflection coefficient is a consequence of the continuity of pressure and particle velocity at the interface:
P_incident+P_reflected=P_transmitted
and for the velocity normal to the interface
v_1=(P_incident-P_reflected)/(rho_1 c_1)=P_transmitted/ (rho_2 c_2)=v_2
We assumed here that there is no back travelling wave in the medium (2). This is a semi-infinite medium. In the experiment discussed the situation is more complex as some reflections can be expected. This depends on the geometry and confinement of the flow.
Prof.Hirschberg, Thank you for your detailed reply!
Your comment means that the important thing is transfering acoustic energy from the transducer to fluid. And I agree with your comment. But I still can't understant why impidence matching material can transfer more energy (they only change the tip of nozzle, others are same).
The context of your comment is that the dominant cause is geometry or confinement of flow. So I would appreciate it if you could comment the detail of how to geometry or confinement of flow play dominant role in this case
I'm really really thank you for kind reply!
Dear shin Jeong Pin,
to answer your question one would need a detailed simulation of the acoustic behaviour of the device and fluid. A first step in this direction is a lumped element model. You will Find iformation about lumped element models in many textbooks.
Another step would be a finite element solution of the wave equation taking into account the geometry and material properties. I such a simulation, I would ignore the fluid velocity. I do not have much experience with such models. Standard commercial codes (Ansys, Comsol…) are very expensive but userfriendly. I expect that there exist other free ware codes such as Openfoam. But I am not familiar with them. Of course numerical simulation should be kept as simple as possible and one should check the numerical convergence of the model upon mesh refibement. The first step is setting up a simplified model for the geometry of the set up. Never try a very detailed model without considering simplified models first. Good luck. Mico
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