Over the past decades, the research on structural vibration control has mainly focused on 'energy dissipation' strategy using various dampers for hazard mitigation. This paper proposes a novel application of linear motion electromagnetic (EM) devices, termed linear EM dampers in this paper, for both vibration damping and energy harvesting. The kinetic energy caused by earthquakes, wind or traffic loads is not only dissipated by EM dampers, but also stored by energy-harvesting electric circuits connected to EM dampers. The green and regenerative energy output may provide an alternative power supply to portable and wireless devices at remote sites. This paper presents a theoretical and experimental study of linear EM dampers connected with four representative circuits. The dynamic characteristics of linear EM dampers, including parasitic damping, EM damping, energy conversion efficiency and effective output power, are modeled and discussed systematically in each case. The modeling is further verified by a series of dynamic testing of a small-scale linear EM damper, which is cyclically tested on a MTS machine at different frequencies and amplitudes. A good match between the modeling and testing results clearly demonstrates that the described model can predict the performance of the linear EM damper and energy harvesting circuit very well. The promises and challenges of using EM dampers in future civil infrastructure for both vibration damping and energy harvesting are discussed based on the outcome of this study.

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A renewed interest in vertical axis wind turbines (VAWTs) has been seen recently. Computational fluid dynamics (CFD) is regarded as a promising technique for aerodynamic studies of VAWTs. In particular, 2D unsteady Reynolds-averaged Navier-Stokes (URANS) is commonly adopted, although past studies on VAWTs revealed the limited accuracy of 2D URANS. This paper investigated the feasibility and accuracy of three different CFD approaches, namely 2D URANS, 2.5D URANS and 2.5D Large Eddy Simulations (LES), in the aerodynamic characterization of straight-bladed VAWT (SBVAWT), with a focus on the capability of the 2.5D LES approach in CFD simulation of high angle of attack (AOA) flow. The 2.5D model differs from a full 3D model in that only a certain length of blades is modeled with periodic boundaries at the two extremities of the domain. The applications of these three approaches were systematically examined in the aerodynamic simulations of a single static airfoil and a 3-blade SBVAWT at different rotating speeds. Their capabilities to predict the aerodynamic forces were evaluated through a comparison with the wind tunnel results obtained by other researchers, with particular attention to high AOA flow beyond stall. Among the three methods, 2.5D LES yielded the best agreement with the experimental results in both cases. Detailed examinations of simulated flow field revealed that 2.5D LES produces more realistic 3D vortex diffusion after flow separation, resulting in more accurate predictions of aerodynamic coefficients in static or dynamic stall situations. It is noteworthy that 2.5D LES cannot capture the effect of tip vortex and vertical flow divergence in VAWTs, which used to be regarded by some researchers as the major cause of overprediction of VAWT power in 2D URANS. In this study, the considerably improved results achieved by 2.5D LES imply that the poor accuracy of URANS method is mainly due to its inherent limitation in vortex modeling. In general, 2.5D LES showed good agreement with experimental results at a relatively low tip speed ratio (TSR), but only fair agreement at a high TSR. Compared with the other two approaches, 2.5D LES is regarded as a more promising and effective CFD tool for investigating the aerodynamic characteristics of VAWTs, particularly their self-starting features corresponding to very low rotation speeds.

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This paper presents a special shape memory alloy-based hysteretic damper with distinctive features such as tunable hysteretic behavior and ability to withstand several design level earthquakes. Superelastic nitinol stranded wires are used for energy dissipation in this damping device, termed a reusable hysteretic damper (RHD). By adjusting its design parameters, the hysteretic behavior of the RHD can be modified to best fit the needs for passive structural control applications. Adjustable design parameters of the RHD include the inclination angle of the nitinol wires, pretension level, and friction effect. A simulation-based parametric study was carried out to examine the effects of these design parameters of the RHD on its energy dissipating performance. The effectiveness of the RHD in passive seismic response control of civil engineering structures is examined through a nonlinear dynamic analysis of a three-story steel frame building with and without an RHD. The simulation results suggest that it can effectively reduce the structural response of building structures subjected to strong earthquakes. With proper design, an RHD can be reused for several strong earthquakes without the need for repair, due to the high fatigue life of nitinol wires.

Summary Given their excellent self‐centering and energy‐dissipating capabilities, superelastic shape memory alloys (SMAs) become an emerging structural material in the field of earthquake engineering. This paper presents experimental and numerical studies on a scaled self‐centering steel frame with novel SMA braces (SMAB), which utilize superelastic Ni–Ti wires. The braces were fabricated and cyclically characterized before their installation in a two‐story one‐bay steel frame. The equivalent viscous damping ratio and ‘post‐yield’ stiffness ratio of the tested braces are around 5% and 0.15, respectively. In particular, the frame was seismically designed with nearly all pin connections, including the pinned column bases. To assess the seismic performance of the SMA braced frame (SMABF), a series of shake table tests were conducted, in which the SMABF was subjected to ground motions with incremental seismic intensity levels. No repair or replacement of structural members was performed during the entire series of tests. Experimental results showed that the SMAB could withstand several strong earthquakes with very limited capacity degradation. Thanks to the self‐centering capacity and pin‐connection design, the steel frame was subjected to limited damage and zero residual deformation even if the peak interstory drift ratio exceeded 2%. Good agreement was found between the experimental results and numerical simulations. The current study validates the prospect of using SMAB as a standalone seismic‐resisting component in critical building structures when high seismic performance or earthquake resilience is desirable under moderate and strong earthquakes. Copyright © 2016 John Wiley & Sons, Ltd.

The recent increased use of shape memory alloys (SMAs) for civil engineering applications manifests the need for a high-fidelity constitutive model which considers the material’s strong dependence on the loading rate. This paper presents an improved thermomechanical constitutive model with strain-rate dependence for predicting the uniaxial superelastic behavior of shape memory alloys. The proposed constitutive model, which is formulated within a thermomechanical framework, is comprised of three principal parts: a mechanical law, an energy balance equation, and a transformation kinetics rule. The analytical derivation of the model and experimental test results for superelastic NiTi wires are described in this paper. The prediction made by this phenomenological model shows good agreement with experimental data for superelastic NiTi wires at various loading rates. Through a comparison with experimental results, the proposed constitutive model was evaluated for several key characteristics of superelastic behavior such as reduction of hysteresis area, increase of transformation plateau, and temperature change with strain rate. The proposed constitutive model offers a useful tool for the design and simulation of superelastic SMA-based devices in civil engineering applications.

This communication presents the design principle and experimental validation of two novel configurations of magnetic negative stiffness dampers (MNSDs), both of which are composed of several permanent magnets arranged in a conductive pipe. The MNSD, as a passive device, efficiently integrates negative stiffness and eddy-current damping in a simple and compact design, in which the negative stiffness behavior depends on the different arrangements of the permanent magnets. When applied to structural vibration control, passive MNSD may achieve a performance comparable with semi-active or active control in some applications. Laboratory experiments of small-scale prototypes successfully verified the proposed MNSD design concept.