Reflected wave manipulation by aeroacoustic metasurfaces in sheared grazing flows

Acoustic metasurfaces are mostly designed in a static medium, ignoring the influence of flow characteristics. However, in actual aeroacoustic noise reduction, e.g., aircraft engine liner design, the background flow can have effects on the sound performance of acoustic metasurfaces, especially for a viscous shear flow. The effect of a viscous shear flow is often neglected in previous studies on the design and sound field prediction of acoustic metasurfaces. For considering the viscous and thermal dissipation effects, an analytical model is developed to predict the sound field of a periodic metasurface in a viscous shear boundary layer. In this model, the effective impedance based on the high-frequency limits is utilized to consider both the actual impedance of the acoustic metasurface and the effect of a finite-thickness viscous shear boundary layer. An acoustic metasurface designed in the static medium or even redesigned with only the effect of an inviscid shear flow is not suitable for wave manipulation when the Reynolds number (Re) changes significantly, since the viscosity is an important and non-negligible factor affecting the sound performance. For the cases in this work, the sound performance gradually deteriorates with the decrease in Re when Re≥5×106. When Re≤1×106, especially at Re=1×105, the existence of viscous shear flows could result in the destruction of expected anomalous reflection and significant intensity change of the reflected waves. This research provides a method for the design of acoustic metasurfaces under viscous shear flow conditions, which is significant for future aeroacoustic applications.

Metasurfaces are a family of novel wavefront-shaping devices with planar profile and subwavelength thickness. Acoustic metasurfaces with ultralow profile yet extraordinary wave manipulating properties would be highly desirable for improving the performance of many acoustic wave-based applications. However, designing acoustic metasurfaces with similar functionality to their electromagnetic counterparts remains challenging with traditional metamaterial design approaches. Here we present a design and realization of an acoustic metasurface based on tapered labyrinthine metamaterials. The demonstrated metasurface can not only steer an acoustic beam as expected from the generalized Snell's law, but also exhibits various unique properties such as conversion from propagating wave to surface mode, extraordinary beam-steering and apparent negative refraction through higher-order diffraction. Such designer acoustic metasurfaces provide a new design methodology for acoustic signal modulation devices and may be useful for applications such as acoustic imaging, beam steering, ultrasound lens design and acoustic surface wave-based applications.

Acoustic metasurfaces are attractive due to the feasibility of unprecedented modulation for reflected or refracted waves. However, most proposed acoustic metasurfaces are planar, which limits the application of metasurfaces. An acoustic metasurface with a curved surface shape is rarely reported, especially for the three-dimensional case. In this paper, the three-dimensional generalized Snell’s law for reflection at an arbitrarily curved surface is derived. Then tunable curved metasurfaces composed of the corrugated holes unit-cells are designed. And anomalous reflection, focusing and ground illusion are numerically demonstrated. The design methodology proposed in this paper expands the application range of acoustic metasurfaces and paves the way to some special application realms, such as wearable acoustic devices and acoustic devices with curved surfaces.

An ultra-thin acoustic metasurface with multiply resonant units

The advent of acoustic metasurfaces (AMs), which are the two-dimensional equivalents of metamaterials, has opened up new possibilities in wave manipulation using acoustically thin structures. Through the interaction between the acoustic waves and the subwavelength scattering, AMs exhibit versatile capabilities to control acoustic wave propagation such as by steering, focusing, and absorption. In recent years, this vibrant field has expanded to include tunable, reconfigurable, and programmable control to further expand the capacity of AMs. This paper reviews recent developments in AMs and summarizes the fundamental approaches for achieving tunable control, namely, by mechanical tuning, active control, and the use of field-responsive materials. An overview of basic concepts in each category is first presented, followed by a discussion of their applications and details about their performance. The review concludes with the outlook for future directions in this exciting field.

Complex structures, large size and limited manipulation of acoustic waves are the problems that restrict the development of acoustic metasurfaces. Here, we report a transmission-type acoustic metasurface based on local resonance mechanism, which is composed of meta-atomic units called dumbbell-shaped double-split hollow spheres (DSDSHS). This metasurface with subwavelength scale has the advantage of simple structure and easy preparation, and can realize the full manipulation of sound waves. Negative refraction with different transmission angles and high intensity plate focusing lens are realized in the air environment of audible frequency. The proposed metasurface has potential applications in the miniaturization and integration of sound transmission and sound energy collection, opening a new opportunity for manipulation of acoustic wavefront.

Acoustic metasurface has become one of the most promising platforms for manipulating acoustic waves with the advantage of ultra-thin geometry. The conventional design method of acoustic metasurface relies on numerical, trial-and-error methods to retrieve effective properties of the locally resonant unit cells. It is often inefficient and requires significant efforts to investigate the enormous number of possible structures with different physical and geometric parameters, which demands huge computational resources. This is especially when modeling narrow cavities where thermoviscous loss has to be considered. In this paper, a deep learning-based acoustic metasurface absorber modeling approach is introduced to significantly reduce the characterization time while maintaining accuracy. Based on a convolutional neural network (CNN), the proposed network can model wide absorption spectrum response in the timescale of milliseconds. The performance of the implemented network is compared with other classical machine learning methods. Using CNN, we have demonstrated an ultrathin metasurface absorber having perfect absorption at an extremely low frequency of 38.6 Hz with an ultrathin thickness down to λ/684 (1.3 cm). The total path length for the propagating waves inside the channel is about λ/5.7 which breaks the quarter-wavelength resonator theory. The network prediction is validated using the experiments to demonstrate the effectiveness of this physical mechanism. Furthermore, we propose a broadband low-frequency metasurface absorber by coupling unit cells exhibiting different properties based on the supercell concept. This approach is attractive for applications necessitating fast on-demand design and optimization of a metasurface acoustic absorber.

Acoustic metasurfaces display unprecedented potential for their unique and flexible capabilities of wave front manipulations. Although rapid progress and significant developments have been achieved in this field, it still remains a significant challenge to obtain an acoustic metasurface that can perform different functions in three-dimensional (3D) space, as desired. Here, a tunable acoustic metasurface composed of Helmholtz-resonator-like digital-coding meta-atoms is presented to overcome the limitation and realize 3D dynamic wave manipulations. The digital meta-atom is constructed from two cylindrical cavities controlled by a motor. By changing the depth of the bottom cavity with the motor automatically, the reflection phase of the meta-atom can be adjusted continuously over the range of nearly 360\ifmmode^\circ\else\textdegree\fi{}, realizing the required digital states. To build the relationship between the digital-coding profiles and scattering patterns, convolution and addition operations are implemented. Based on such operations, various fascinating functionalities, such as arbitrary scattering-pattern shift and superposition of different scattering beams, are achieved. This study paves the way for studying acoustic metasurfaces from the digital perspective and provides efficient methods for realizing spatial acoustic wave control.

Acoustic metasurfaces (AMs) are artificial structures that can realize exotic properties, such as negative refraction and perfect absorption, etc., in sub-wavelength thickness scales. Most of the previous studies on AMs were conducted in laboratory conditions with a static medium, limiting their applications in practical conditions, e.g., the aircraft engines where an aerodynamic background flow is present. To fill this gap, the effect of a grazing shear flow on the reflection performance of AMs with periodic construction is studied. An AMs design method is proposed to realize effective wavefront manipulation with the consideration of the shear flow. The strategy is to form a linear reflected phase-shifting covering 0 to 2π at the interference plane between the uniform and the shear flow regions. To validate the effectiveness of the proposed design method, two acoustic porous metasurfaces (APMs), APM1 for the uniform flow and APM2 for the shear flow, are designed and their reflection characteristics are numerically investigated. Results show that the presence of the shear layer deteriorates the effectiveness of APM1 under oblique incidences due to the considerable refraction effect. In contrast, APM2 realizes reflection wavefront manipulation, such as anomalous reflection and surface wave conversion, under the same conditions. This study provides a guideline for AMs design in flow conditions, which offers potential for noise control in aeroacoustic problems.

Acoustic metasurfaces provide a way to manipulate wavefronts at anomalous reflection or refraction angles through subwavelength structures. Here, based on the generalized Snell's refraction law for acoustic metasurfaces and the classical acoustic phased array (PA) theory, a broadband acoustic PA with a subwavelength active tube array has been proposed to form a special acoustic beam and to determine the directivity characteristics of the acoustic source. Theoretical analysis shows that the dispersionless wavefront manipulation can be realized by the gradient model of the active tube array, and a wide working frequency band can be obtained in practical applications from the simulated and experimental results. The numerical results of forming a special acoustic beam and establishing an acoustic focus model with an arbitrary focal position are consistent with the theoretical predictions. The experimental results agree well with the simulated results in the model of forming the acoustic beam of 45°. By combining acoustic metamaterials and conventional acoustic PA, the model of the active tube array paves a way to design a composite acoustic PA with high radiation efficiency and system robustness without the need for any complex circuit control system. This design concept is expected to be used in the design of ultrasonic therapy devices and high-efficiency transducers.

We designed and demonstrated a gradient acoustic metasurface to manipulate the transmissive wavefront. The gradient metasurface is composed of eight elements based on membrane-type hybrid structures, whose thickness and width are about 1/5 and 1/20 of the incident wavelength, respectively. Here, we employ acoustic theory to analyze the transmission spectrum and phase gradient of the metasurface, the properties of high transmission efficiency and discrete phase shifts over the full 2pi range can be achieved simultaneously. By appropriate selection of the phase profile along the transverse coordinate of the metasurface or the angle of incident wave, the transmissive wavefront manipulations based on metasurface can be obtained as expected from the generalized Snell’s law, such as anomalous refraction, acoustic cloak based on flat focusing, acoustic self-bending beam, conversion of propagating wave to surface wave and negative refraction. Our gradient metasurface may have potential application in low-loss acoustic devices.

A parallel plate flow chamber-based flow system is a flow system that allows for microscopic observation of the endothelial cell (EC) monolayer behavior in response to different flow conditions in a controlled manner. ECs’ flow response has been a critical factor that determines atherosclerotic legion formation at specific locations of arteries with disturbed shear. While many flow systems have been developed for such behaviors, however, they have lacked a modality to measure mechanical forces exerted by the ECs in response to the onset of shear flow, which is an important linkage in mechano-transduction cascade in the ECs. In this thesis, a flow system based on parallel plate flow chamber is developed with addition of a force sensor, referred to as traction force microscopy (TFM), on the chamber bottom surface. The flow chamber was designed, and manufactured 3D-printed holder and polydimethylsiloxane (PDMS)-based internal flow channel that ensures leakage-proof design. Computational fluid dynamics simulation was performed to ensure generation of laminar flow and uniform shear stress inside the flow channel. Using the assembled chamber and peristaltic pump-based flow system, a pilot experiment was performed to test a hypothesis that the shear stress generated on the luminal surface of the ECs is transferred to the cell-ECM adhesion site via cell cytoskeleton. The traction force was measured via TFM in real-time. Effect of shear flow on ECs showed an increase in force as an immediate response to the onset of the flow. Short time (30 minutes) flow results indicated that force increases overtime under a laminar shear stress of 1.0 Pa. Other key factors were identified such as high shear flow, cell confluency and phototoxicity, which can greatly influence the traction force transmission.

The effect of symmetric sheared plasma flow parallel to the equilibrium magnetic field on the evolution of plasmoid instability is investigated by using two-dimensional MHD simulations. It is shown that as the velocity amplitude of a shear flow increases from sub-Alfvénic to the level of super-Alfvénic, the current sheet is stabilized against the plasmoid instability. Suppression of plasmoid instability also takes place for a sub-Alfvénic shear flow, provided that the shear length is sufficiently small compared to the magnetic shear. Therefore, an initially elongated thin current sheet with a Lundquist number greater than the critical value does not necessarily guarantee the development of the plasmoid instability. As a result, the Kelvin-Helmholtz (KH) instability is triggered. Although the rate of reconnection is quite fast in the nonlinear regime of the plasmoid instability, the reconnection of twisted field lines during the KH instability is very slow.

Some of our recent work concerned the interactions of twisted wave fronts of acoustic vortex beams with boundaries and inhomogeneities of media. Experimental measurements and numerical simulations were conducted to investigate vortex beam reflection from a water-air interface and vortex beam refraction when travelling in an inhomogeneous medium or through an acoustic metasurface. The results reveal spatial evolution of the wave fields and the carried angular momenta. In particular, the investigation observed a reversal of orbital angular momentum in the reflected wave field [Zou et al., Phys. Rev. Lett. 125(7), 074301 (2020)], and a shift of beam center and an asymmetric vortex pattern in the refracted wave field in the direction transverse to the plane of incidence [Fan et al., Phys. Rev. Res. 1(3), 032014 (2019); 3(1), 013251 (2021)]. The shift relates to conservation of angular momenta and coupling of the twisted wave fronts with the refraction and is termed as acoustic orbital Hall effect.

The effects of sheared equilibrium flow and magnetic field on the Rayleigh–Taylor instability (RTI) are investigated and the linear growth rate is obtained analytically in the presence of a sharp interface. It is shown that the shear flow acts as a driving force and is the dominating drive when Atwood number AT, wave number k, flow shear δu, and gravitational acceleration g satisfy k(1−AT2)δu2∕AT⪢g. As AT increases growth rate increases first and then falls down if (2kδu2)<g is satisfied, and otherwise it rises monotonically. When magnetic stabilizing effect governs, RTI only occurs in the long wave region and not only the permitted band, 0<k<gAT∕[vra2−δu2(1−AT2)], is extended but the instability is enhanced as δu increases or the reduced Alfven speed vra decreases. For regime governed by the destabilizing effect of shear flow, growth rate increases as k and δu increase.