Sudib K. Mishra

An improved tuned mass damper (SMA-TMD) assisted by a shape memory alloy spring

The tuned mass damper (TMD) is a well acclaimed passive control device for vibration control of structures. However, the requirement of a higher mass ratio restricts its applicability for seismic vibration control of civil engineering structures. Improving the performance of TMDs has been attempted by supplementing them with nonlinear restoring devices. In this regard, the ability of shape memory alloy (SMA) in dissipating energy through a hysteretic phase transformation of its microstructure triggered by cyclic loading is notable. An improved version of TMD assisted by a nonlinear shape memory alloy (SMA) spring, referred as SMA-TMD, is studied here for seismic vibration mitigation. Extensive numerical simulations are conducted based on nonlinear random vibration analysis via stochastic linearization of the nonlinear force–deformation hysteresis of the SMA. A design optimization based on minimizing the root mean square displacement of the main structure is also carried out to postulate the optimal design parameters for the proposed system. The viability of the optimal design is verified with respect to its performance under recorded earthquake motions. Significant improvements of the control efficiency and a reduction of the TMD displacement at a much reduced mass ratio are shown to be achieved in the proposed SMA-TMD over those in the linear TMD.

Modeling the coupling of reaction kinetics and hydrodynamics in a collapsing cavity

We introduce a model of cavitation based on the multiphase Lattice Boltzmann method (LBM) that allows for coupling between the hydrodynamics of a collapsing cavity and supported solute chemical species. We demonstrate that this model can also be coupled to deterministic or stochastic chemical reactions. In a two-species model of chemical reactions (with a major and a minor species), the major difference observed between the deterministic and stochastic reactions takes the form of random fluctuations in concentration of the minor species. We demonstrate that advection associated with the hydrodynamics of a collapsing cavity leads to highly inhomogeneous concentration of solutes. In turn these inhomogeneities in concentration may lead to significant increase in concentration-dependent reaction rates and can result in a local enhancement in the production of minor species.

Compliant liquid column damper modified by shape memory alloy device for seismic vibration control

Liquid column dampers (LCDs) have long been used for the seismic vibration control of flexible structures. In contrast, tuning LCDs to short-period structures poses difficulty. Various modifications have been proposed on the original LCD configuration for improving its performance in relatively stiff structures. One such system, referred to as a compliant-LCD has been proposed recently by connecting the LCD to the structure with a spring. In this study, an improvement is attempted in compliant LCDs by replacing the linear spring with a spring made of shape memory alloy (SMA). Considering the dissipative, super-elastic, force-deformation hysteresis of SMA triggered by stress-induced micro-structural phase transition, the performance is expected to improve further. The optimum parameters for the SMA-compliant LCD are obtained through design optimization, which is based on a nonlinear random vibration response analysis via stochastic linearization of the force-deformation hysteresis of SMA and dissipation by liquid motion through an orifice. Substantially enhanced performance of the SMA–LCD over a conventional compliant LCD is demonstrated, the consistency of which is further verified under recorded ground motions. The robustness of the improved performance is also validated by parametric study concerning the anticipated variations in system parameters as well as variability in seismic loading.

Spatiotemporal Compound Wavelet Matrix Framework for Multiscale/Multiphysics Reactor Simulation: Case Study of a Heterogeneous Reaction/Diffusion System

We present a mathematical method for efficiently compounding information from different models of species diffusion from a chemically reactive boundary. The proposed method is intended to serve as a key component of a multiscale/multiphysics framework for heterogeneous chemically reacting processes. An essential feature of the method is the merging of wavelet representations of the different models and their corresponding time and length scales. Up-and-down-scaling of the information between the scales is accomplished by application of a compounding wavelet operator, which is assembled by establishing limited overlap in scales between the models. We show that the computational efficiency gain and potential error associated with the method depend on the extent of scale overlap and wavelet filtering used. We demonstrate the method for an example problem involving a two-dimensional chemically reactive boundary and first order reactions involving two species.

Optimal performance of base isolated building considering limitation on excessive isolator displacement

The past studies on optimisation of isolation bearings emphasise on the allowable limit on the isolator displacement without incorporating it in the optimisation process. Large bearing displacement of the isolator during strong motion is often linked to isolator damage and pounding to adjacent units. The present study addresses the limitation on the excessive isolator displacement in the optimisation process of the isolator. It is intended not only to assure optimal performance in terms of vibration mitigation, but also to reveal the importance of constraining excessive isolator displacements. The stochastic response required for the optimisation study is evaluated through nonlinear random vibration analysis. The consistency of the optimal response behaviour obtained through stochastic optimisation is validated for recorded earthquake ground motions. A Lead Rubber Bearing (LRB), isolating a multi-story shear building is used to elucidate the effect of this constraint on the optimum parameters and system performance. The results clearly indicate the existence of significant disparities among the conventional unconstrained and the proposed constrained optimisation approach.

Thermo-mechanical strain rate–dependent behavior of shape memory alloys as vibration dampers and comparison to conventional dampers

A study on shape memory alloy materials as vibration dampers is reported. An important component is the strain rate–dependent and temperature-dependent constitutive behavior of shape memory alloy, which can significantly change its energy dissipation capacity under cyclic loading. The constitutive model used accounts for the thermo-mechanical strain rate–dependent behavior and phase transformation. With increasing structural flexibility, the hysteretic loop size of shape memory alloy dampers increases due to increasing strain rates, thus further decreasing the response of the structure to cyclic excitation. The structure examined is a beam, and its behavior with shape memory alloy dampers is compared to the same beam with conventional dampers. Parametric studies reveal the superior performance of the shape memory alloy over the conventional dampers even at the resonance frequency of the beam-damper system. An important behavior of the shape memory alloy dampers is discovered, in that they absorb energy from the fundamental and higher vibration modes. In contrast, the conventional dampers transfer energy to higher modes. For the same beam control, the stiffness requirement for the shape memory alloy dampers is significantly less than that of the conventional dampers. Response quantities of interest show improved performance of the shape memory alloy over the conventional dampers under varying excitation intensity, frequency, temperature, and strain rate.

Response of Bridges Isolated by Shape Memory–Alloy Rubber Bearing

AbstractThe present paper deals with optimum performance of the shape memory–alloy-based rubber bearing (SMARB) compared with the conventionally adopted lead rubber bearing (LRB) for isolating the bridge deck against a random earthquake. Specifically, a systematic study on the optimum performance of SMARB is conducted and compared with the LRB to minimize the acceleration of the isolated deck subjected to a stochastic earthquake by addressing the limitation on the excessive isolator displacements. The optimal characteristic strength of the bearing and respective responses are obtained by solving a bi-objective optimization problem. The responses required for this are obtained by nonlinear random vibration analysis via stochastic linearization of the cyclic nonlinear force-deformation behavior of the shape memory?alloy restrainers. The robustness of the improved performances of the SMARB was studied through extensive parametric studies with respect to the variation of the system parameters and the scenario...

Optimal Performance of Buildings Isolated By Shape-Memory-Alloy-Rubber-Bearing (SMARB) Under Random Earthquakes

Shape Memory Alloy (SMA)-based bearing has been proposed recently for improved base isolation by optimal choice of its transformation strength. Presently, superior performances of the Shape-Memory-Alloy-Rubber-Bearing (SMARB) over the elastomeric bearing are established in mitigating seismic vibration under constraint on maximum isolator displacement. The optimal transformation strengths are proposed through constrained optimization based on stochastic responses. Numerical simulation reveals that Lead Rubber Bearings (LRB) either fails to provide feasible parameters or leads to large floor acceleration, compromising the isolation efficiency. Contrarily, optimal SMARB can efficiently enforce such constraint without greatly affecting the isolation efficiency. Evidence of robustness of SMARB over LRB is also established.

Damage Detection in Railway Bridges Under Moving Train Load

In vibration based structural health monitoring, measured response data is used to detect structural damage. This study considers monitoring of railway bridges using response data under moving train loads. The effect of train-bridge interaction, especially with heavy trains, makes the train-bridge system time-varying, and modal identification challenging. The problem becomes even more complex when only the bridge response data is available, and characteristics of the train load (mass, speed etc.) are unknown. To avoid this complexity we engage into a strictly data based technique. Signal energies of the measured responses from healthy and damaged systems are compared statistically to detect the damage in the system. This comparison accounts for: (a) operational variability from different train masses and speeds, and (b) uncertainty from limited instrumentation and unknown input. The technique is validated though numerical simulations and the results promise faster detection of damage.

Spectral characterization of the stochastically simulated vehicle queue on bridges

Monte Carlo simulation in conjunction with Fourier transform based spectral windowing is used to model the live load on bridges. Vehicles are classified into a few groups and the probability distributions of axle weight and length associated with each group are estimated. The vehicle arriving at an instant is determined through Monte Carlo simulation, which uses a vehicle group density function derived from measurement data on the relative contribution of each group in total vehicles. The weight and length of the arriving vehicle is also simulated by Monte Carlo using the distribution function for the corresponding group. Vehicle arrivals are modeled by the Poisson distribution. The vehicle velocities are realized through spectral simulation based on decaying power spectra of the velocity time series. The simulations are performed for a sufficient time interval in several lanes, thus the ensemble sampling of load is obtained. Fourier transform based windowing is used to characterize the power spectra of mechanical load on the bridge. The study shows the white noise nature of the load spectral density, which is in agreement with the assumptions of previous investigators. Parametric sensitivity of the spectra is also performed and recommendations are made to include site-specific parameters in the model. Finally, applications are illustrated for frequency domain random vibration analysis of a simple model of bridge structures.