In recent years, there has been an increased interest in achieving nonreciprocal transmission of acoustic or elastic waves for their applications in various fields, such as energy concentration and harvesting, signal processing, thermal management, and improved ultrasound imaging. Reciprocity can be defined as indistinguishable detection of sound waves in the forward and backward direction of propagation. A breakdown in the reciprocity principle leads to a difference in transmission in one direction compared to the other. Active acoustic systems with resonant cavities, nonlinear acoustic medium, or circulating fluidic system has been used to break reciprocity and achieve non-reciprocal acoustic wave transmission.&nbsp; Our group has demonstrated passive linear non-reciprocal transmission in phononic crystals due to the parity and time-symmetry violation induced by asymmetric scatterers. However, the frequency of operation of these linear non-reciprocal system are dependent on the wave dispersion of the acoustic wave within the phononic crystal and cannot be tuned. This work realizes an active linear nonreciprocal phononic crystal device that allows for tunability of transmission in the desired frequency range.The tunable device used in this study is a hollow asymmetric channel which is 3D printed using a Form2 3D printer, stereolithography(SLA). The channel is a 120° asymmetric section with a radius of 10 mm and a length of 60 mm. The transmission through the channel was measured using an underwater bistatic arrangement of two transducers, each with the center frequency of 100 kHz. Fluid flow in the channel was introduced using a water pump connected through hoses. The flow of the fluid normal to the acoustic wave propagation has a component of the Doppler shifted component that is reciprocal for symmetric scatterers, but non-reciprocal for the asymmetric shaped scatterers of the phononic crystal. The transmission was recorded for both forward and backward directions of propagation. In a phononic crystal with symmetric scatterers or in the absence of fluid flow in a crystal with asymmetric scatterers, the transmission of acoustic wave was observed to reciprocal. Phononic crystals with asymmetric scatterers demonstrated nonreciprocal wave propagation at particular frequencies when a fluid flow was introduced in the system. The extent of nonreciprocity also varied with a varying velocity of the fluid.This work provides a dynamic method for achieving non-reciprocity in a linear tunable asymmetric channel, having broken PT-symmetry. The P-symmetry is broken due to the asymmetric shape of the channel, whereas the broken T-symmetry is achieved using a fluid flow inside the channel. The fluid-flow has different velocity component in the forward and backward direction of propagation, creating a varying degree of Doppler shift in the wave's frequency based on the propagation direction. Additionally, this device can be tuned using different fluids of varying properties with varying velocities to fit the desired frequency range. Non-reciprocity at the desired frequency range also depends on the channel's angle with respect to the transducers and fluid velocity. In conclusion, this active phononic crystal device shows promising results in achieving tunable nonreciprocal sound propagation.

Nonreciprocal Acoustic Wave Propagation in Doppler-Shifted Phononic Crystals With Asymmetric Scatterers

Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2021/volume/85543

ASME 2021 International Mechanical Engineering Congress and Exposition (IMECE2021)

Supporting the need among researchers worldwide for additional online and multimedia content, ASME now offers the ASME Conference Video Collection. This platform will collect and provide long-term access to conference presentation videos, encompassing all technology areas within ASME's technical conference and events program. About 30,000-40,000 engineers across all mechanical engineering and related disciplines participate in ASME conferences each year. This includes authors of approximately 6,000 to 8,000 technical papers published by ASME, the majority of whom also submit video presentations for our virtual conferences. The video collection will be available as an add-on to existing product packages or as a separate subscription.