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Smooth camber morphing aircraft offer increased control authority and improved aerodynamic efficiency. Smart material actuators have become a popular driving force for shape changes, capable of adhering to weight and size constraints and allowing for simplicity in mechanical design. As a step towards creating uncrewed aerial vehicles (UAVs) capable of autonomously responding to flow conditions, this work examines a multifunctional morphing airfoil’s ability to follow commands in various flows. We integrated an airfoil with a morphing trailing edge consisting of an antagonistic pair of macro fiber composites (MFCs), serving as both skin and actuator, and internal piezoelectric flex sensors to form a closed loop composite system. Closed loop feedback control is necessary to accurately follow deflection commands due to the hysteretic behavior of MFCs. Here we used a deep reinforcement learning algorithm, Proximal Policy Optimization, to control the morphing airfoil. Two neural controllers were trained in a simulation developed through time series modeling on long short-term memory recurrent neural networks. The learned controllers were then tested on the composite wing using two state inference methods in still air and in a wind tunnel at various flow speeds. We compared the performance of our neural controllers to one using traditional position-derivative feedback control methods. Our experimental results validate that the autonomous neural controllers were faster and more accurate than traditional methods. This research shows that deep learning methods can overcome common obstacles for achieving sufficient modeling and control when implementing smart composite actuators in an autonomous aerospace environment.

Many oscillatory systems of engineering and scientific interest (e.g., mechanical metastructures) exhibit nonproportional damping, wherein the mass-normalized damping and stiffness matrices do not commute. A new modal analysis technique for nonproportionally damped systems, referred to as the “dual-oscillator approach to complex-stiffness damping,” was recently proposed as an alternative to the current standard method originally developed by Foss and Traill-Nash. This article presents a critical comparison of the two approaches, with particular emphasis on the time required to compute the resonant frequencies of nonproportionally damped linear systems. It is shown that, for degrees-of-freedom greater than or equal to nine, the dual-oscillator approach is significantly faster (on average) than the conventional approach, and that the relative computation speed actually improves with the system’s degree-of-freedom. With 145 degrees-of-freedom, for example, the dual-oscillator approach is about 25% faster than the traditional approach. The difference between the two approaches is statistically significant, with attained significance levels less than machine precision. This suggests that the dual-oscillator approach is the faster of the two algorithms for computing resonant frequencies of nonproportionally damped discrete linear systems with large degrees-of-freedom, at least within the limits of the present study. The approach is illustrated by application to a model system representative of a mechanical metastructure.

In the present work, we investigate dynamic interaction and response of flexible bio-inspired morphing wing structure to a low Reynolds aerodynamic load. The aspects of inspiration are as follows. First, the segmentation of the wing into rigid and flexible segments. Considering a leading edge constitution of bone and muscle. In addition to a flexible trailing edge composed of feathers. Second, the material properties provided by experimental biology in literature are adopted such as the bending stiffness and Young's modulus. The development of numerical models allowing non uniform distribution of properties are developed and implemented into an OpenFoam finite volume solver that couples fluid dynamics to a structural solid dynamics solver through the FSI interface. In the course of this work, the validation is performed for a NACA6409 airfoil considering a rigid segment of 40\% and flexible segment 60\% chord length in order to test the aero-structure behavior for an aerodynamic load of air flow at low Reynolds number of $\boldsymbol{5\times 10^5}$ for the fluid and feather inspired material properties. The results suggest that bio-inspired techniques can be reproduced in engineering configurations., Comment: This is the author's version of an article that has been published at 2023 AIAA SciTech Forum - 23-27 January 2023. The final version of record is available at: \url{https://arc.aiaa.org/doi/abs/10.2514/6.2023-0826}

Several species of birds have been known to invert in flight to lose altitude — a behavior known as whiffling. When the bird flies inverted, the flight feathers twist open to create gaps in the trailing edge of the wing, decreasing the lift produced by the wing. Gaps along the trailing edge of an aircraft wing were inspired by the feather rotation mechanism during whiffling, and asymmetrically applying these gaps on only one side of the wing produces a rolling moment due to the lift differential across the full wing. Previous experimental data and analytical estimates showed that whiffling-inspired gapped wings can produce a larger rolling moment coefficient than a conventional aileron, for a fraction of the actuation work. In the current work, we perform a computational study using Siemens STAR-CCM+ to estimate the work required to actuate nine gaps along the trailing edge of a whiffling-inspired wing and compare it to that of a representative aileron configuration. We show that the results of the simulation agree well with the prior experimental results. The results indicated that the work on the entire gap area may be higher than the work to deflect an aileron, however, the analytically estimated work on a smaller, more realistic, area corresponding to a gap cover was substantially lower than the work to deflect an aileron. These results provide evidence that sliding gaps that open in the plane of a wing require less work than deflecting an aileron into the flow for rolling moment coefficients above 0.0139. This computational validation is the first step in determining if smart materials can be used for this type of wing morphing. In all, the whiffling-inspired gapped wings could provide a far more energy-efficient method of roll control for energy-constrained fixed-wing uncrewed aerial vehicles than conventional ailerons, particularly at higher rolling moment coefficients.

Macro fiber composites (MFC) have proven useful as multifunctional material actuators for implementation in intelligent aerospace structures; however, due to nonlinear behaviors such as hysteresis and creep, incorporating sensor feedback is necessary to achieve sufficient control. In our work we use an antagonistic MFC unimorph system to produce a smooth trailing edge deflection in a morphing airfoil. In the past, piezoelectric flex sensors have been used to provide position feedback for the airfoil trailing edge, but these sensors also suffer from hysteresis. To achieve accurate position measurements from the flex sensors, we use time-series machine learning to model the relationship between flex voltage output and the true deflection. After testing offline, a long short-term memory (LSTM) neural network is implemented for inference on the hardware system and compared to a traditional linear model. True deflection information is obtained through external laser measurements, providing a context for comparing the accuracy of the mentioned state inference methods. Additionally, the sensor models are used in conjunction with a PD feedback controller to determine performance when controlling the aileron. Another difficulty in MFC actuator application is the change in performance under loading. Therefore, we perform final performance comparisons in a wind tunnel to simulate a realistic environment for the airfoil. We find the presented methods improve state inference performance over the traditional linear method, allowing for more accurate tip control under the aerodynamic loading.

Birds perform astounding aerial maneuvers by actuating their shoulder, elbow, and wrist joints to morph their wing shape. This maneuverability is desirable for similar-sized uncrewed aerial vehicles (UAVs) and can be analyzed through the lens of dynamic flight stability. Quantifying avian dynamic stability is challenging as it is dictated by aerodynamics and inertia, which must both account for birds’ complex and variable morphology. To date, avian dynamic stability across flight conditions remains largely unknown. Here, we fill this gap by quantifying how a gull can use wing morphing to adjust its longitudinal dynamic response. We found that it was necessary to adjust the shoulder angle to achieve trimmed flight and that most trimmed configurations were longitudinally stable except for configurations with high wrist angles. Our results showed that as flight speed increases, the gull could fold or sweep its wings backward to trim. Further, a trimmed gull can use its wing joints to control the frequencies and damping ratios of the longitudinal oscillatory modes. We found a more damped phugoid mode than similar-sized UAVs, possibly reducing speed sensitivity to perturbations, such as gusts. Although most configurations had controllable short-period flying qualities, the heavily damped phugoid mode indicates a sluggish response to control inputs, which may be overcome while maneuvering by morphing into an unstable flight configuration. Our study shows that gulls use their shoulder, wrist, and elbow joints to negotiate trade-offs in stability and control and points the way forward for designing UAVs with avian-like maneuverability.

The emergence of the use of mechanical metamaterials for vibration suppression and the creation of frequency gaps in structures require an understanding of the fundament underlying dynamics partial differential equations coupled to ordinary differential equations. Essentially periodic structures consist of a distributed parameter structure connected (embedded) to a series of spring-mass-dampers. Such systems in the past have been studied as combined dynamical systems. This work deals with the modal analysis of non-conservative combined dynamic systems formed by assembling distributed parameter structures and linear, viscously damped, lumped parameter oscillators. The mathematical model of the forced response of such dynamic systems is presented via differential operators. The related non-linear eigenproblem is formulated next and a proper solution is provided. Furthermore, the orthogonality of the eigenfunctions is studied and the completeness of the generated solution space is verified. Coupled modal coordinate differential equations result through modal analysis, thus revealing the non-proportional damping configuration, while the proportional damping conditions are also derived and discussed. The theory is applied to non-conservative Euler–Bernoulli beams subject to different types of boundary conditions and coupled to linear, viscously damped oscillators. A numerical example yields interesting conclusions about the non-proportionality and the applicability of the associated methods to solving the coupled differential equations.

Some bird species fly inverted, or whiffle, to lose altitude. Inverted flight twists the primary flight feathers, creating gaps along the wing’s trailing edge and decreasing lift. It is speculated that feather rotation-inspired gaps could be used as control surfaces on uncrewed aerial vehicles (UAVs). When implemented on one semi-span of a UAV wing, the gaps produce roll due to the asymmetric lift distribution. However, the understanding of the fluid mechanics and actuation requirements of this novel gapped wing were rudimentary. Here, we use a commercial computational fluid dynamics solver to model a gapped wing, compare its analytically estimated work requirements to an aileron, and identify the impacts of key aerodynamic mechanisms. An experimental validation shows that the results agree well with previous findings. We also find that the gaps re-energize the boundary layer over the suction side of the trailing edge, delaying stall of the gapped wing. Further, the gaps produce vortices distributed along the wingspan. This vortex behavior creates a beneficial lift distribution that produces comparable roll and less yaw than the aileron. The gap vortices also inform the change in the control surface’s roll effectiveness across angle of attack. Finally, the flow within a gap recirculates and creates negative pressure coefficients on the majority of the gap face. The result is a suction force on the gap face that increases with angle of attack and requires work to hold the gaps open. Overall, the gapped wing requires higher actuation work than the aileron at low rolling moment coefficients. However, above rolling moment coefficients of 0.0182, the gapped wing requires less work and ultimately produces a higher maximum rolling moment coefficient. Despite the variable control effectiveness, the data suggest that the gapped wing could be a useful roll control surface for energy-constrained UAVs at high lift coefficients.

The impressive maneuverability demonstrated by birds has so far eluded comparably sized uncrewed aerial vehicles (UAVs). Modern studies have shown that birds’ ability to change the shape of their wings and tail in flight, known as morphing, allows birds to actively control their longitudinal and lateral flight characteristics. These advances in our understanding of avian flight paired with advances in UAV manufacturing capabilities and applications has, in part, led to a growing field of researchers studying and developing avian-inspired morphing aircraft. Because avian-inspired morphing bridges at least two distinct fields (biology and engineering), it becomes challenging to compare and contrast the current state of knowledge. Here, we have compiled and reviewed the literature on flight control and stability of avian-inspired morphing UAVs and birds to incorporate both an engineering and a biological perspective. We focused our survey on the longitudinal and lateral control provided by wing morphing (sweep, dihedral, twist, and camber) and tail morphing (incidence, spread, and rotation). In this work, we discussed each degree of freedom individually while highlighting some potential implications of coupled morphing designs. Our survey revealed that wing morphing can be used to tailor lift distributions through morphing mechanisms such as sweep, twist, and camber, and produce lateral control through asymmetric morphing mechanisms. Tail morphing contributes to pitching moment generation through tail spread and incidence, with tail rotation allowing for lateral moment control. The coupled effects of wing–tail morphing represent an emerging area of study that shows promise in maximizing the control of its morphing components. By contrasting the existing studies, we identified multiple novel avian flight control methodologies that engineering studies could validate and incorporate to enhance maneuverability. In addition, we discussed specific situations where avian-inspired UAVs can provide new insights to researchers studying bird flight. Collectively, our results serve a dual purpose: to provide testable hypotheses of flight control mechanisms that birds may use in flight as well as to support the design of highly maneuverable and multi-functional UAV designs.

Waste energy harvesting is a method of generating small amounts of energy, usually through vibrations or thermal energy. Macro-fiber Composites (MFCs) have been employed in energy harvesting applications utilizing the piezoelectric effect, however, energy harvesting using the pyroelectric effect in MFCs has not been thoroughly explored. This paper takes strides in investigating the energy generated using the pyroelectric effect with P1 and P2 MFCs using two different oscillating temperature rates at 5°C/min and 10°C/min. Numerical temperature data along with analytical temperature functions were used to model the pyroelectric effect and the result was compared to experimental tests. Depending on the size of the MFC, type, resistor used, and the temperature rate, the energy generated varies from 0.4 to 24 µJ for the P1 MFC, and 8 to 459 µJ for the P2 MFC. The maximum specific power was also estimated analytically, numerically, and experimentally. A resistor sweep was performed using the numerical model to calculate the optimal resistance that would provide the most energy. The resistor was in the Gigaohm (GΩ) to Teraohm (TΩ) range for the specified temperature profiles. The required resistance to generate the maximum energy decreased as the temperature rate and the size of the MFC increased. This was validated with experiments conducted at varying resistances. Because this is energy generated from the ambient environment, pyroelectric harvesting can be used to power devices without cost to the source. This form of harvesting can be used in any place where there is a natural thermal cycle (i.e. due to weather or machine giving off thermal energy).

Forpractical considerations reinforcement learning has proven to be a difficult task outside of simulation when applied to a physical experiment. Here we derive an optional approach to model free reinforcement learning, achieved entirely online, through careful experimental design and algorithmic decision making. We design a reinforcement learning scheme to implement traditionally episodic algorithms for an unstable 1-dimensional mechanical environment. The training scheme is completely autonomous, requiring no human to be present throughout the learning process. We show that the pseudo-episodic technique allows for additional learning updates with off-policy actor-critic and experience replay methods. We show that including these additional updates between periods of traditional training episodes can improve speed and consistency of learning. Furthermore, we validate the procedure in experimental hardware. In the physical environment, several algorithm variants learned rapidly, each surpassing baseline maximum reward. The algorithms in this research are model free and use only information obtained by an onboard sensor during training.

Structural Health Monitoring (SHM) techniques have an important role in the performance of mechanical structures. In particular, piezoelectric transducers (PZT) have been employed for establishing damage detection processes in SHM systems. However, despite their wide use, there is limited information in the literature regarding important characteristics for SHM applications, such as the shape, dimensions and frequencies to use, which can affect the performance of the system used to detect the damage. This article investigates longitudinal and flexural waves for applications using circular piezoelectric transducers bonded to thin plates. The methodology allows one to obtain optimal frequencies to create and capture both types of waves and understand the influence of the geometric characteristics of these transducers on the damage detection. Numerical simulations are carried out to show that a change in the parameters of the piezoelectric transducers can maximize the waves amplitudes incoming at the sensor. Results from experimental tests are presented to demonstrate the proposed methodology. New equations are introduced and they compute output voltage and determine optimal frequencies to monitor the structure. The findings contribute to establishing a more efficient design of a damage detection process involving plate-like structures in SHM systems based on wave propagation.

Some bird species exhibit a flight behavior known as whiffling, in which the bird flies upside-down during landing, predator evasion, or courtship displays. Flying inverted causes the flight feathers to twist, creating gaps in the wing’s trailing edge. It has been suggested that these gaps decrease lift at a potentially lower energy cost, enabling the bird to maneuver and rapidly descend. Thus, avian whiffling has parallels to an uncrewed aerial vehicle (UAV) using spoilers for rapid descent and ailerons for roll control. However, while whiffling has been previously described in the biological literature, it has yet to directly inspire aerodynamic design. In the current research, we investigated if gaps in a wing’s trailing edge, similar to those caused by feather rotation during whiffling, could provide an effective mechanism for UAV control, particularly rapid descent and banking. To address this question, we performed a wind tunnel test of 3D printed wings with a varying amount of trailing edge gaps and compared the lift and rolling moment coefficients generated by the gapped wings to a traditional spoiler and aileron. Next, we used an analytical analysis to estimate the force and work required to actuate gaps, spoiler, and aileron. Our results showed that gapped wings did not reduce lift as much as a spoiler and required more work. However, we found that at high angles of attack, the gapped wings produced rolling moment coefficients equivalent to upwards aileron deflections of up to 32.7° while requiring substantially less actuation force and work. Thus, while the gapped wings did not provide a noticeable benefit over spoilers for rapid descent, a whiffling-inspired control surface could provide an effective alternative to ailerons for roll control. These findings suggest a novel control mechanism that may be advantageous for small fixed-wing UAVs, particularly energy-constrained aircraft.

The acoustic black hole (ABH) effect produces drastic wave compression and energy trapping in thin-walled structures. Recent research has shown that the reflected wave in laboratory ABH structures can be reduced by attaching a thin absorbing layer in specific wave concentration areas. In this paper, a novel design and implementation of one-dimensional (1D) functionally graded acoustic black hole (FG-ABH) is presented, as an alternative to achieve low reflection effects. Two configurations of the FG-ABHs are demonstrated, i.e., an axially graded ABH and a thickness graded ABH. The FG-ABH beams are manufactured by 3D printing technology using an Objet Connex 500 printer. Materials with varying levels of viscoelastic characteristics are produced using two base materials called TangoPlus and VeroWhitePlus. The FG-ABH beams have both diminishing thickness and elastic modulus in the tip of the ABH wedge. Numerical investigation of wave propagation, attenuation and reflection are conducted utilizing a viscoelastic code: University of Michigan’s Local Interaction Simulation Approach (UM/LISA). The damping effect of the materials is included based on Kelvin-Voigt viscoelastic theory. Finally, the process of wave propagation in FG-ABHs, traditional ABH and uniform beam are experimentally investigated using a Scanning Laser Doppler Vibrometer (SLDV) system. The reflection coefficients, i.e. the ratios between the amplitude of reflected wave and incident wave, are computed. Results indicate that the FG-ABHs enhance the ABH effect, and lower reflection is observed when compared to a traditional ABH structure.

An experimental study of the single-well and twin-well, also referred to as intra-well and inter-well, respectively, nonlinear dynamics of a bistable composite laminate is presented. An asymmetric four-ply [0/90/0/90] carbon fiber laminate with two cylindrical stable equilibria supported at its center and free at all boundaries is used for the experimental testing. The mechanical bistability makes the laminate able to snap from one stable equilibrium to the other when displacement reaches a critical value. This snapthrough motion is highly nonlinear and associated with large-amplitude vibrations. This property opens chances for bistable laminates in morphing and energy-harvesting applications. An electromechanical shaker is used to excite the laminate at its center by a controlled amplitude and frequency excitation. The dynamic response of the laminate is measured using a Polytec laser vibrometer. In addition, four macro-fiber composite actuators are attached to the laminate to measure the output voltage due to vibration and hence assess its energy-harvesting capability. The laminate’s natural frequencies and damping under small-amplitude excitations that would match the natural frequencies of an underlying linear system are experimentally identified. A primary resonance excitation of the first bending mode is carried out using amplitude sweep and frequency sweep. It is shown that in both cases, the response starts as a small-amplitude single-well vibration around the static equilibrium position. Varying the excitation amplitude or frequency, the response exhibits a period-doubling bifurcation associated with higher levels of vibration. As the excitation conditions are varied further, the laminate snaps from one equilibrium position to the other. This snapthrough motion is characterized to be of a frequency that is half the natural frequency of the excited mode. It is shown that the period-doubling cascade is responsible for the escape from the single-well to the twin-well response. A secondary Hopf bifurcation that creates a period-three motion and a chaotic snapthrough were also identified. Two low-frequency interesting modes that are attributed to the elasticity of the midpoint support, referred to as rocking modes, were identified using a hammer. These modes got repeatedly excited via subharmonic resonance of order two. In terms of the energy harvesting, the snapthrough motion was found to greatly enhance the energy extraction capability. The frequency band of significant power generation was found to be about 25% of the excitation frequency, which is a good ratio for a single excitation frequency.

Particle Image Velocimetry (PIV) is a well-established technique for determining the flow direction and velocity magnitude of complex flows. This paper presents a methodology for executing this non-intrusive measurement technique to study a scaled-up turbine vane geometry within an annular cascade at engine-relevant conditions. Custom optical tools such as laser delivery probes and imaging inserts were manufactured to mitigate the difficult optical access of the test section and perform planar PIV. With the use of a burst-mode Nd:YAG laser and Photron FASTCAM camera, the frame straddling technique is implemented to enable short time intervals for the collection of image pairs and velocity fields at 10 kHz. Furthermore, custom image processing tools were developed to optimize the contrast and intensity balance of each image pair to maximize particle number and uniformity, while removing scattering and background noise. The pre-processing strategies significantly improve the vector yield under challenging alignment, seeding, and illumination conditions. With the optical and software tools developed, planar PIV was conducted in the passage of a high-pressure stator row, at mid-span, in an annular cascade. Different Reynolds number operating conditions were achieved by modifying the temperature and mass flow. With careful spatial calibration, the resultant velocity vector fields are compared with Reynolds Averaged Navier Stokes (RANS) simulations of the vane passage with the same geometry and flow conditions. Uncertainty analysis of the experimental results is also presented and discussed, along with prospects for further improvements. Lastly, the adaptations of the diagnostic technique needed to assess rotor tip technology in a rotating facility are detailed.

Train movements generate oscillations that are transmitted as waves through the track support system into its surroundings. The waves reaching nearby structures may cause distractions for humans and/or result in equipment malfunctions. Dynamic characteristics of surrounding media affect the level of vibrations experienced in nearby buildings. In this paper, the authors are presenting a finite element analysis study performed to assess vibration levels due to train movements in nearby subway tunnels. The closer subway tunnel is located at an average horizontal distance of 50 ft (15.2 m) from an existing office building. While the authors focus on assessing vibration levels at the foundations of the office building, a plane strain finite element model is created to simulate the tunnel, surrounding rock and the soil stratum above the rock layer. The loading function is modeled as a sinusoidal point source and simulations are run for sensitivity analysis.

Recent work concerned with passive vibration suppression in structural systems has focused on the development of metastructures. Metastructures (also called architected materials) are a type of mechanical metamaterial: an object arranged or constructed in such a manner that it has traits unachievable by its constituent material. Metastructures often consist of embedded spring mass systems made from small beams with a tip mass or chiral lattice structures with mass inserts. The goal of a metastructures design is to reduce vibrations of the host structure. It does this using the absorber action of the embedded spring mass systems. The embedded tuned mass dampers (TMDs) then become prone to sustaining failure by fatigue. While many recent papers discuss various geometric and material designs, very few if any address the issues of fatigue of the embedded absorbers. This paper makes an initial investigation into the relationship between the TMDs’ geometric parameters and their fatigue performance with the goal of improving the design and implementation metastructures. A sensitivity analysis was performed to identify the geometric TMD parameter with the largest impact on natural frequency and stress concentration. Analytical models for natural frequency and maximum stress were developed for the TMDs and compared to finite element models. The analytical models were substantiated by the FEA models for some of the perturbed parameters, indicating that further work would be needed to confirm that, for a given parameter, a certain trend in natural frequency indicates an anticipatable trend in stress.

Here, we reviewed published aerodynamic efficiencies of gliding birds and similar sized unmanned aerial vehicles (UAVs) motivated by a fundamental question: are gliding birds more efficient than comparable UAVs? Despite a multitude of studies that have quantified the aerodynamic efficiency of gliding birds, there is no comprehensive summary of these results. This lack of consolidated information inhibits a true comparison between birds and UAVs. Such a comparison is complicated by variable uncertainty levels between the different techniques used to predict avian efficiency. To support our comparative approach, we began by surveying theoretical and experimental estimates of avian aerodynamic efficiency and investigating the uncertainty associated with each estimation method. We found that the methodology used by a study affects the estimated efficiency and can lead to incongruent conclusions on gliding bird aerodynamic efficiency. Our survey showed that studies on live birds gliding in wind tunnels provide a reliable minimum estimate of a birds’ aerodynamic efficiency while simultaneously quantifying the wing configurations used in flight. Next, we surveyed the aeronautical literature to collect the published aerodynamic efficiencies of similar-sized, non-copter UAVs. The compiled information allowed a direct comparison of UAVs and gliding birds. Contrary to our expectation, we found that there is no definitive evidence that any gliding bird species is either more or less efficient than a comparable UAV. This non-result highlights a critical need for new technology and analytical advances that can reduce the uncertainty associated with estimating a gliding bird’s aerodynamic efficiency. Nevertheless, our survey indicated that species flying within subcritical Reynolds number regimes may inspire UAV designs that can extend their operational range to efficiently operate in subcritical regimes. The survey results provided here point the way forward for research into avian gliding flight and enable informed UAV designs.

Train movements generate oscillations that are transmitted as waves through the track support system into its surroundings. The vibration waves propagate through the soil layers and reach to nearby buildings creating distractions for human activities and causing equipment malfunctioning. Not only the train components and the rails, but also the surrounding tunnel, soil and rock strata have dynamic characteristics that play significant roles in the vibration levels felt in a nearby structure. This paper presents a finite element study conducted to investigate the vibrations resulting from train movements in nearby subway tunnels. The subway line is located at an average horizontal distance of 50 ft (15.2 m) from the structure in assessment, which is a six-story office building. The main goal of the work is to assess the train-induced vibrations at the ground level of the building through a case study and sensitivity analysis. A plane strain finite element model is built to represent the railroad tunnel embedded in the rock and the soil stratum above it. The one train loading function is applied to the model as a point source at the track level and compared to the two-train scenario. Other simulations are undertaken for sensitivity analysis involving increased loading, decreased damping and decreased distance to tunnels. Even though there are several numerical studies on the propagation of train induced vibrations in the literature; a finite element model accompanied with a sensitivity analysis has not been discussed in detail in a technical publication before. The paper not only presents the finite element modeling but also compares the results with the criteria of Transit Noise and Vibration Impact Assessment Manual, which was published by the Federal Transit Administration (FTA) of the U.S. Department of Transportation.

Although there have been numerous efforts into harnessing the snap through dynamics of bistable structures with piezoelectric transducers to achieve large energy conversion, these same dynamics are undesirable under morphing applications where stationary control of the structure’s configuration is paramount. To suppress cross-well vibrations that primarily result from periodic excitation at low frequencies, a novel control strategy is proposed and implemented on the piezoelectrically generated bistable laminate, which consists of only macro fiber composites (MFCs) in a [0MFC/90MFC]T layup. While under cross-well regimes such as subharmonic, chaotic, or limit cycle oscillations, a single MFC is actuated to the laminate’s limit voltage to eliminate one of its potential wells and force it into the remaining stable state. Simultaneously, a positive position feedback (PPF) controller suppresses the resulting single-well oscillations through the other MFC. This dual control strategy is numerically and experimentally demonstrated through the suppression of various cross-well regimes and results in significant reduction of amplitude. The active control capability of the laminate prevents snap through instability when under large enough external vibrations.

This article presents a novel morphing unit cell concept which relies on the combination of shape memory alloy wires for actuation and permanent magnets to enable multistability. Two distinct applications are investigated experimentally, consisting in a morphing beam metastructure made with three unit cells and a variable camber airfoil metastructure having six unit cells in the chord-wise direction. Tests are performed to assess the influence of the permanent magnets on the morphing behavior of the two referred metastructures. It is verified that the permanent magnets are able to provide new stable equilibrium configurations to the metastructure and to reduce the time necessary for morphing. Another interesting feature, which enables the reduction in energy consumption, is that, due to the magnetic interactions, the thermal activation of the SMA wires can be ceased once an equilibrium configuration is achieved. The paper describes the design premises, evaluates its limitations and devises future improvements.