Alper Ertürk

We leverage space- and time-periodic resonators to produce tunable acoustic radiation in fluid-loaded metamaterials, where the propagation angle becomes a controllable degree of freedom for spatial focusing. Under such a modulation, the waveguide generates leaky waves that radiate energy across a wide angular span—from shallow emission to broadside— through selective activation of Floquet harmonics that concentrate energy into desired acoustic beams. To characterize this behavior, we develop numerical methods that resolve the complex-valued dispersion relation, which, in turn, captures the radiation properties in spatially and temporally modulated thin elastic waveguides loaded with water. We demonstrate that the tunable radiation is inherently governed by the modulation parameters and the resulting wavenumber and frequency transformations, with predictions corroborated by time-domain simulations. By exploring the physics of fluid-loaded resonant metamaterials with spatially- and temporally-varying properties, this paper points toward opportunities in areas involving structures coupled with acoustically heavy fluid (e.g. water), such as underwater communication to transcranial ultrasound for imaging, diagnosis, and therapy.

In biology, neighboring soft and stiff domains can grow at different times, so the growth of one domain influences the subsequent growth of the next. To isolate key factors controlling this complex spatiotemporal behavior, we model gels undergoing biomimetic stepwise growth. The gel's top surface is patterned with a stiff cross, while the underlying non-patterned domains are less crosslinked and softer. At ambient pressure, if growth of the stiff cross occurs before growth of soft layers, the structure displays a concave shape; reversing the growth order yields a gel exhibiting a convex structure. The findings reveal how the shapes and properties of these heterogeneous materials co-evolve as they reach equilibrium morphologies. By increasing the hydrostatic pressure, we also isolate morphologies that remain pressure resistant. Our findings reveal an approach to control a material's geometric patterning and mechanical properties within these patterns and can provide insight into physicochemical factors affecting biological morphogenesis.

Electronic systems contained within sealed metallic enclosures, such as nuclear waste containers where wire penetration is not an option, require a through-wall data transfer technology to communicate. One such technology utilizes piezoelectric ultrasonic transducers to communicate through the barrier using elastic waves. Currently demonstrated ultrasonic through-wall data communication techniques have relied on having a direct interface with the ultrasonic transducers on either side. This work studies the coupling of an antenna to such an ultrasonic communication system. A computationally efficient model of a piezo-barrier ultrasonic communication system and antenna using a simplified antenna model and a transfer matrix piezo-barrier model is utilized to perform a representative parameter space search. The system efficiency and bandwidth trends are shown for the system parameters as well as operating frequency. The trade-off between efficiency and bandwidth is also presented. A system configuration within the parameter space is fabricated and demonstrated to achieve a data rate of up to 100 kbps.

1.2 to 1.8 million people worldwide experience a new intracerebral hemorrhage (ICH) each year. ICH is a subtype of stroke associated with high morbidity and mortality in which nearly 40% of patients experience significant hematoma growth within the first 24 hours. Hematoma expansion is associated with worse clinical outcomes, thus tracking of hematoma evolution may inform critical management decisions. Conventional transcranial ultrasound imaging is limited in its ability to image the periphery of the brain close to the skull and thus cannot visualize peripheral hematomas. To address these limitations, we designed and fabricated a 60-element hybrid phased array transducer capable of dual-mode operation: (1) conventional B-mode imaging at ~1 MHz and (2) guided Lamb wave generation and controlled acoustic leakage into targeted regions close to the skull. The fabricated array had a center frequency of 1.3 MHz and a -6 dB bandwidth of 51%. At depth of 5 cm, the measured axial resolution was 0.96 mm, the lateral resolution was 1.94 mm, and the SNR was 24.7 dB. In addition, in simulations and experiments, the array successfully generated and controlled Lamb wave propagation in a skull-mimicking aluminum plate. By electrically tuning individual elements with shunt inductors, the system could toggle between "leaky" and "non-leaky" guided wave modes, effectively acting as an active acoustic metasurface on the skull. This could improve ultrasound delivery to locations inside the skull that are typically inaccessible.

High-impedance nanoparticle inclusions provide a potential means to control wave propagation in plastic composites while maintaining local thermodynamic equilibrium across phases.We use forward-ballistic plate impact experiments to compare wave dissipation characteristics for composites of various thickness, comprised of high-density polyethylene (HDPE) and ceria nanoparticles at concentrations of 0, 10 and 20 vol%.The rise time of the shock front is found to increase with composite thickness and to be significantly greater for the two nanocomposites than for pure HDPE.The results are corroborated by acoustic wave transmission measurements at near-ambient pressure that also indicate increased wave attenuation with nanoparticle addition.The findings are discussed in terms of independent structural characterizations of the HDPE and ceria phases.

This work presents our design, fabrication, and characterization studies of a bioinspired hybrid robotic fish as an underwater swimmer. Macro-fiber composite (MFC) piezoelectric structures strike a balance between the deformation and actuation force capabilities to generate hydrodynamic propulsion, enabling a low-form-factor actuator for the tail region. Here, by coupling the MFC actuation with a servomotor in the head region, extra inphase or out-of-phase motion (relative to the tail) is provided for improved maneuverability. This work builds upon the previous generation of piezoelectric fish, which utilized a pair of MFC laminates as a caudal fin for the robotic fish, where it acted like an artificial muscle that could be driven out of phase to expand and contract on each side of the polymer substrate to simulate bending. The coupling of the servomotor with the MFCs allows for enhancement of thrust and ease of control for turning with combined DC (head) and AC (tail) actuation. Following design, fabrication, hardware/software aspects, resonant dynamics in quiescent fluid, as well as free (unconstrained) locomotion characterizations are explored for a range of piezo and servo actuation parameters, which are ongoing.

In structures where direct pressure measurement is impossible, dynamic pressure can still be estimated from the vibrational response of the system. Nevertheless, because this is an inverse load identification problem, several critical challenges must be overcome to ensure good accuracy and consistency in the estimated pressure field. Namely, to avoid poor numerical conditioning, many sensors must be used to overdetermine the system, especially to reconstruct a spatially varying distributed pressure input. To address this challenge, we perform numerical and experimental investigations into distributed pressure estimation using a large piezoelectric sensor array. To estimate the pressure, we construct a distributed pressure basis using the mode shapes of the structure, creating a model that relates each basis function to the piezoelectric sensor output. During operation, we regularize and invert this model to obtain the weighting of each basis function and reconstruct the distributed pressure on the structure. We show how the number and placement of sensors can be optimized to improve accuracy, and we determine the required number of sensors for a given frequency and wavelength of p

A piezoelectric metamaterial concept for the control of surface acoustic waves is demonstrated experimentally. An array of inductive-shunted piezoelectric elements bonded to an aluminum block mode-converts or prevents the propagation of incident Rayleigh waves depending on the inductive-shunt profile and frequency content of the waves. Varying the inductance values in space is shown to enable bandgap formation or cause mode conversion of the incident surface acoustic waves to bulk shear waves.

The ability of a wave to pass through a material boundary can be improved by adding a tuned middle layer, known as an impedance-matching layer. However, in many situations, it is unfeasible to modify the physical system itself. This paper demonstrates that virtual impedance matching without added tuned layers is possible by allowing the frequencies of incident waves to take on complex values. The resulting tailored waveforms directly excite the zeros of the reflection coefficient and lead to complex generalizations of Fabry-Pérot resonances and quarter-wavelength matching. This is demonstrated experimentally whereby the reflection coefficient for an ultrasound beam incident on a bi-layer plate immersed in water is reduced by more than an order of magnitude. While the technique is naturally limited in temporal duration due to the quasi-steady state nature of the signals, it provides an alternative approach to traditional impedance matching by eliminating the need for extra tuned layers and may prove useful in applications where reduction of reflections is desired without modifying the system itself.

As industrial activities increasingly impact the environment, the need for resource conservation and long-term energy security becomes paramount, positioning sustainable electricity production as a critical focus area. In this regard, the application of Machine Learning, a branch of Artificial Intelligence, has revealed considerable promise in optimizing various dimensions of energy generation, especially within renewable sectors such as wind energy. This research presents a case study that showcases the implementation of Machine Learning techniques in forecasting the failures and establishing predictive maintenance plans for wind energy systems. In this study, we employed Logistic Regression, Decision Tree, Bagging, Random Forest, Gradient Boosting, Adaboost, and XGBoost to analyze more than 40,000 rows of data from 40 different system parts. At the end of the analysis, Tuned XGBoost model has turned out to be the best model having the highest Recall value that correctly predicts the Failures. Through the application of diverse methodologies, we demonstrate the effectiveness of these techniques in anticipating potential failures and prioritizing maintenance or replacement of parts, which in turn facilitates substantial resource savings in terms of labor, costs, and time, thereby advancing sustainability and strategic planning.

Triply periodic minimal surface (TPMS)-based continuous structures have recently attracted increased attention due to their remarkable mechanical properties, such as high strength-to-weight ratio, impact resistance, and energy absorption capabilities. In this study, we investigate topological interface modes in I-WP (Wrapped Package) TPMS geometry. Inspired by a one-dimensional (1D) Su–Schrieffer–Heeger (SSH) model, we design 1D elastic Phononic Crystals (PCs) made of sheet-based I-WP minimal surface geometry. By manipulating the geometry of the I-WP minimal surface, we open the degeneracies formed at the edges of the Brillouin zone to create band-folding-induced bandgaps. We then design a 1D dimerized chain of two topologically distinct unit cells of I-WP minimal surface to create an interface and introduce topological interface modes. Numerical simulations are performed to study the band structure and topological transition properties of the proposed 1D PC. In addition, we show that hybridizing alternative I-WP unit cells of different relative densities can also break the inversion symmetry of the periodic structure in contrast to manipulating the geometry. The 1D PC made of hybridized I-WP geometry is then used to realize topological interface modes. The proposed 1D PCs are additively manufactured to experimentally validate the existence of topological interface modes. Our work provides an efficient method for TPMS structures to produce multifunctional devices that can support superior load-bearing capabilities as well as robust topological phase properties. • A novel phononic crystal is proposed using triply periodic minimal surface (TPMS) geometry. • Scaling and hybridization techniques are used to break the space inversion symmetry. • 3D-printed I-WP phononic crystals show the existence of interface modes. • This novel TPMS phononic crystal exhibits superior mechanical and wave guiding properties.

Antenna miniaturization is applicable to many mobile communications applications where size, and weight, and power consumption are important design parameters. Recently there has been interest in electrically small antennae that leverage mechanical resonance and the piezoelectric effect to achieve several orders of magnitude performance improvements over metallic counterparts. This paper proposes a novel piezoelectric antenna structure utilizing stacked alternating polarization layers capable of scaling to larger electrical sizes while maintaining a usable impedance. The proposed structure was evaluated using a full wave piezoelectric finite element model at a size of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$ka=7\times 10^{-3}$</tex> achieving a real impedance of 52.5 ohms and a theoretical radiation efficiency of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$3.08\times 10^{-5}$</tex>.

Developing persistent and smart underwater markers is critical for improving navigation accuracy and communication capabilities of autonomous underwater vehicles (AUVs). A wireless acoustic identification tag, which uses a piezoelectric transducer tuned in the broadband ultrasonic range (200-500 kHz), was experimentally demonstrated to achieve highly efficient power transfer (source-to-tag electrical power efficiency of >2% at 6 m) and concurrent high data rate and backscatter level communication (>83.3 kbit s-1, >170 dB sound pressure level at 6 m) with potential operating range ≈ 10 m based on analytical extrapolations. Parameter selection considerations dictated by the desired range and data-rate requirements in communication are presented. The transducer piezoelectric element selection, impedance matching approach, and simulation-based circuit optimization for frequency multiplexed operation are also detailed. Experimental tests benchmarking performance sensitivity to source and tag misalignment are introduced and implications for AUV operations are discussed.

• A supercapacitor charged by a vibration energy harvester (VEH) was investigated. • A wireless sensor node (WSN) powered by a piezoelectric VEH was modelled. • Nonlinear piezoelectricity, three-branch model, and diode model were considered. • Analysis technique was established based on differential algebraic equation (DAE). • Simulation predicted charging/discharging characteristics for the WSN accurately. A supercapacitor or an electric double layer capacitor (EDLC) is an essential component of a high-performance wireless sensor node (WSN) that can transmit the three-axis acceleration waveform data of structural vibrations and is powered by a piezoelectric vibration energy harvester (VEH). However, the intrinsically slow charging rate of the piezoelectric VEH triggers charging of extra capacitors formed inside the complex-shaped electrodes of the supercapacitor, which complicates the charging/discharging characteristics and thus estimation of the power generation and energy storage in the supercapacitor. Therefore, in this work, we establish an analytical methodology that allows accurate prediction of the charging/discharging characteristics of the supercapacitor built into the WSN powered by the piezoelectric VEH, considering (1) the nonlinear piezoelectricity, (2) a three-branch circuit model of the supercapacitor, and (3) the current–voltage relation equation for the diode. We develop a parameter identification technique for both the nonlinear piezoelectric VEH and the supercapacitor, and then establish a coupled analysis technique based on the differential algebraic equation (DAE). Using the DAE, we investigate the charging/discharging characteristics of the supercapacitor by considering the actual working steps of the WSN. We revealed that the second branch in the three-branch model contributes not only to an increase in the effective capacitance of the supercapacitor, but also to faster recovery of the voltage across the supercapacitor during WSN operation. We also revealed that a longer charging time led to more energy being stored in the capacitor in the third branch, from which it is difficult to extract energy within a short time period such as the WSN’s transmission process due to its long time constant. The contribution of the second and third branches must be considered to enable accurate prediction of the charging/discharging characteristics of the supercapacitor when built into the WSN powered by the piezoelectric VEH.

The dynamics of topological boundary modes in both periodic and quasi-periodic electromechanical metastructures is investigated, with a focus on their applications to energy harvesting and vibration reduction. The metastructure analyzed in this study is based on a shunted array of piezoelectric patches, with electrical parameters modulated according to the 1D Aubry–André–Harper model. As a result of this modulation, a fractal spectrum is generated near the central frequency of the resonators, a hallmark of nontrivial topology that enables the emergence of digitally controllable edge states and ensuing localization phenomena at subwavelength frequencies. In this framework, a detailed analysis of the metastructure spectral characteristics is conducted to investigate the influence of the modulation parameters on mode localization, both at the boundaries and within the interior of the beam. Such localization effects are then studied in relation to the energy harvesting, attenuation, and wave transport capabilities of the system. These functionalities point toward the realization of self-powered structures with low frequency and digitally controllable vibration attenuation capabilities, and are considered of significant technological interest in applications involving elastic waves and vibrations, where the ability to precisely control and harness these phenomena could lead to innovative solutions in energy-efficient and adaptive systems. • A fractal spectrum is found in piezoelectric-based electromechanical metastructures. • Low-frequency topological edge modes are tunable using shunt circuits. • The formation of a fractal Hofstadter spectrum allows widening the attenuation band. • Edge-to-edge and edge-to-interior transitions are found by parameter variation. • The localization properties are functional for energy harvesting and wave transport.

International audience

Programmable piezoelectric metamaterials are attractive due to their ability to precisely tune band structures and thereby control waves in unprecedented ways. By connecting piezoelectric unit cells to shunt circuits, the effective stiffness of the piezoelectric metamaterial can be tuned locally, offering a higher degree of design freedom over mechanical metamaterials. On the other hand, with advances in topological insulators, and the introduction of topological properties such as quantum Hall, quantum spin Hall, quantum Valley-Hall, and Weyl physics in acoustic and elastic systems have opened new avenues to manipulate waves. Topological interface modes are of interest due to their resistance to backscattering, defects, and energy leakage. In this study, a 1D Su–Schrieffer–Heeger (SSH) type programmable piezoelectric metamaterial beam model is proposed to explore topological interface modes. Specifically, we use the band folding mechanism, by doubling the primitive unit cells we close the bands in the high symmetric points and develop two band folding points near the locally resonant bandgap. Furthermore, the effective stiffness of the two-unit cells is varied using inductive shunt circuits to break the space inversion symmetry and lift the degeneracies. We show that by tuning the inductive shunt resonant frequency of the two-unit cells near the locally resonant bandgap frequency, the two band foldinginduced bandgaps can be merged along with the locally resonant bandgap, and two topological interface modes are realized numerically. Our future work involves experimental demonstration of merging topological bandgaps to realize multiple interface states.

Analogous piezoelectric networks have been shown to be effective for multimodal vibration attenuation in structures, including beams, plates, and rings. Previous studies for rings have only accounted for in-plane transverse vibration attenuation and disregarded the out-of-plane vibration modes. Furthermore, these previous numerical models and experiments have only been studied on thin rings, which ignore the effects of shear deformation and rotary inertia. As a result, these networks are not suitable for attenuating vibrations in thick rings. This study enhances on the previous electrical networks, considering both shear deformation and rotary inertia for both in-plane and out-of-plane vibrations. A new passive network topology is developed for the out-of-plane dynamics of thick rings, and the existing passive analogous network of in-plane vibration of thin rings is enhanced by considering the effects of shear deformation and rotary inertia. Combined, these new networks are capable of multimodal vibration damping of a thick ring in three-dimensions, encompassing primarily six types of vibration modes: the inextensional bending modes, the extensional modes, the thickness-shear modes, the coupled twist-bending modes, the torsional modes, and the transverse thickness-shear modes. By using piezoelectric elements to couple two separate analogous passive electrical networks derived from both the in-plane and out-of-plane governing equations of a ring and optimizing the internal resistance in each unit cell, it becomes possible to replicate the dynamics and effectively attenuate different types of vibration modes. This study serves as a theoretical foundation for implementations of passive vibration attenuation in ring structures.

This Letter presents an experimental demonstration of tunable exceptional points (EPs) in an electromechanical waveguide. EPs are non-Hermitian singularities typically found in parity-time (PT) symmetric systems with balanced gain and loss. Here, piezoelectric transducers on an aluminum beam (waveguide) are shunted to synthetic impedance circuits that emulate negative and positive resistors (responsible for gain and loss) and inductors (for resonant tunability), whose properties can be programmed digitally. Specifically, an electrical mode is introduced via inductive shunts to electromechanically interact with target structural mode(s) to create degeneracy. While the internal structural damping of the waveguide has the effect of breaking PT symmetry inherently, we show that EPs can still be created by using non-trivial gain and loss combinations. The results in this Letter pave the way for practical realization of EPs in elastic media toward their application in enhanced sensing and asymmetric wave control, among others.

Abstract We report the concept and demonstration of a double-negative, resonant metamaterial characterized by both dynamic negative mass and stiffness for negative refraction of flexural wave modes by means of a lens designed using this concept. The negative equivalent material properties are obtained in the subwavelength regime by concurrently exploiting both the effect of mechanical resonators (negative mass) and of piezoelectric patches with inductive resonant shunts (negative stiffness), leading to double-negative behavior. Following the theoretical foundations based on a modal framework, we analytically derive the frequency-dependent mass and stiffness properties as a function of the electromechanical parameters. The findings are corroborated by numerical computation of dispersion properties and simulations showing the focusing of a point source. As a case study, energy harvesting performance enhancement by exploiting the piezoelectric effect at the focal spot is also discussed.

The strengthening link between universities and industry continues to strongly impact business processes and improve regional policies. Good results require the participation of all parties. For this purpose, establishing collaboration is analyzed as a decision-making process for the parties. Factors and behavioral barriers influencing cooperation between higher education institutions and business organizations are being studied. Based on the analysis are proposed supporting mechanisms to motivate, support, develop, and promote this cooperation to maintain sustainability and adaptability so that all stakeholders can benefit. In addition to teaching and research, universities are also engaged in technology transfer and community engagement. Implementing an information disclosure system through appropriate effortless access will improve the process of building the necessary trust in partnership relations with industry, create the necessary motivation base for students, and the adaptability of the established relationship will maintain the university's awareness of industry needs.

Abstract Analogous electrical networks were previously derived from the Euler–Bernoulli and Kirchhoff–Love theories to represent beams and plates, respectively, for use in multimodal structural vibration damping. However, these networks do not account for shear deformations or rotary inertia, which can result in suboptimal vibration damping performance when used on moderately thick beams and plates. In this paper, we investigate the incorporation of shear deformation and rotary inertia using Timoshenko–Ehrenfest beam theory and Mindlin–Reissner plate theory to develop improved electrical networks that can more accurately represent thick beams and plates. Our findings suggest that the inclusion of shear deformation and rotary inertia can significantly improve the frequency coherence of the electrical networks and multimodal vibration damping for thicker structures. The electrical analogs presented here are of use for various applications, especially to conveniently design complex circuit topologies in fields spanning from vibration attenuation to energy harvesting.

Autonomous underwater vehicle (AUV) operations are limited by currently achievable underwater localization and navigation solutions; hence, the development of low-cost and passive (i.e., operable without an active power supply) acoustic underwater markers (or tags) can provide accurate localization information to AUVs improving their situational awareness, especially when operating in small scales or confined missions. This work presents an acoustic identification (AID) tag that can be powered wirelessly with ultrasonic power transfer from a remote acoustic source (e.g., mounted on an interrogating AUV) and provide localization information using backscatter communication. The AID tag harvests energy from the acoustic signal generated from the AUV and communicates by modulating the reflected signals from an embedded piezoelectric transducer. A scaled broadband AID tag prototype that achieves concurrent acoustic energy harvesting (tuned around 1.3 MHz) and backscatter communication (in wider frequency band 600 and 800 kHz) using frequency-domain multiplexing is implemented using a custom broadband impedance matching-based transducer design approach. During concurrent power and data operation, this prototype AID tag achieves data rates up to 200 kb/s using amplitude- and frequency-based modulation communication. The use of broadband schemes to achieve robust communications in low SNR (tested here down to -6 dB) is also demonstrated using linear frequency-modulated data carriers. Finally, the extension to full-scale devices of this AID tag concept and potential applications for short-range AUV routing and navigation such as homing and docking are discussed.

Effect of carbon nanotubes, aluminum hydroxide, and zinc borate

Abstract Structural vibrations can be reduced by coupling to a piezoelectric electrical network that exhibits analogous modal properties of the structure. This paper considers the multimodal vibration damping of a thin circular ring using this method. The electrical network is derived by applying a finite difference model to the governing equations of motion for a segment of a thin curved beam. An electromechanical analogy is then applied to the physical constants. The resulting passive electrical network unit cell is a topology of capacitors, inductors, and transformers analogous to the dynamics of a segment of curved beam. The electrical network for a curved beam is simplified by considering an inextensional assumption and combining edge components in adjacent unit cells. The resulting simplified discrete network for a curved beam segment is assembled into a complete network for a circular ring. The electrical network for a circular ring displays modal properties similar to its mechanical analogue in both the spatial and frequency domains. As a result of the analogous modal properties across the frequency spectrum, it is shown that the network can be used to achieve multimodal vibration attenuation across a large frequency spectrum. Piezoelectric patches are used to couple the two domains. Numerical simulation of the coupled system demonstrates the effectiveness of the broadband damping effects from the analogous network. Notably, this research establishes a novelty in the field, as it not only introduces experimental validation of curved beam analogues, but also extends the investigation to encompass the coupling between a circular ring and its piezoelectric electrical network counterpart. Further experimental network optimization demonstrate the possibility of tuning the network to adapt to an imperfect mechanical ring.

Non-invasive vibration measurements from the knee offer a convenient and affordable alternative to benchtop or biomechanics lab joint health monitoring systems. Recently, joint acoustic emissions (JAEs) measured from the knee were shown to be an indicator of knee health. However, the origin of JAEs is still not fully understood, which limits its acceptance and use by clinical experts. In this proof-of-concept study, rather than relying on the movements of the knee and corresponding frictional rubbing of internal surfaces to produce vibrations, we propose using an active vibration sensing approach with a known vibration source interrogating the knee. We aim to elucidate the linkage between knee vibration characteristics and structural changes in the joint following injuries. We measured tibial vibration responses of two participants using a laser vibrometer system to quantify the frequency band where the most repeatable tibial vibration measurement can be taken. Subsequently, a custom-designed wearable system measured mid-activity tibial vibration characteristics from four participants (five healthy knees and three knees with prior acute injury) during unloaded knee flexion-extensions. An active sensing knee health score was defined as the ratio of the changes in low- to high-frequency response during flexion-extension. Since changes in the boundary of tibia would alter low-frequency response more than high frequency response, we found that increased knee laxity with acute injuries resulted in an increased active sensing knee health score. Our findings demonstrate the potential of active vibration sensing as an interpretable, computationally inexpensive alternative to JAEs for wearable knee health assessment.