Research
Energy Harvesting: Due to the recent advances in the fields of fabrication, computing, and electronics, small size and low power-consumption sensors and actuators have become a reality. New devices that operate with minimal power requirements invaded the consumer market and are now an essential part of our daily life. To cope with the rapid growth in low-power consumption technologies, researchers have established a new and flourishing area in vibrations known as vibration-based energy harvesting. This science deals with scavenging of otherwise waste mechanical energy and transforming it into electrical energy to self-power and maintain a device operation. Energy harvesting may prove very useful in powering remote sensor networks, bio-implants, and security devices.

Our research interest lies in the modeling analysis and optimization of energy harvesting devices. This includes formulating a deep understanding of the effect of design parameters on the optimal power, utilizing different active materials and comparing their effectiveness, and addressing the effect of the nonlinearities on the output power.

Performance of Nonlinear Energy Harvesters in Stochastic Environments:

Funded through an NSF CAREER grant, one of our current major thrusts of research focuses on understanding the influence of nonlinearities on the performance of energy harvesters operating in realistic environments (e.g. random and non-stationary). As of today, there is a clear lack of understanding of how to design efficient vibratory energy harvesters for realistic excitations and how to optimize their performance in such environments. Additionally, it is still not clear whether the commonly adopted steady-state harmonic fixed-frequency analysis constitutes an accurate performance indicator.  In fact, some of our initial studies have shown that such simplified understanding can yieldincorrect conclusions about the actual performance. To resolve this issue, we aim to formulate systematic methodologies: analytical (approximate solutions of the Fokker-Plank-Kolmagorov equation), numerical (Monte Carlo simulations) and experimental to build the fundamental understanding necessary for efficient energy harvesting under random and time-varying frequency excitations. Research results are expected to provide the missing link which describes how electromechanical transduction is affected by the nature of the excitation and nonlinearities in the design.

Micro-Power Generation Using Flow-induced Self Excited Oscillations:

Funded through an NSF sensor and sensing systems grant, and motivated by the obvious need for compact, scalable, and low-maintenance micro-power generators, we introduced a new concept for power generation which uses wind and other dynamic flow fields to power and maintain low-power consumption electronics. The generator is inspired by the operation principle of music-playing harmonicas, where a cantilever (piezoelectric) beam is embedded within a cavity to mimic the vibration of reeds in a harmonica when subjected to air blow. Variations in the air pressure within the cavity due to variations in the aperture area between the vibrating beam and the supporting structure causes the beam to undergo limit-cycle oscillations that can be used to harvest energy from the surrounding flow. The objectives of this work are two folds: i) Using theories from continuous-systems vibrations, piezoelectricity, and fluid dynamics, we have developed an analytical model that describes the aero-electromechanical behavior of the system by invoking several assumptions on the fluid-structural interactions. The model is currently being validated against a full computational analysis to identify the operation regions where the analytical model is valid. ii) Using a combination of perturbation methods, a spectral balance algorithm, and Floquet theory, we are currently investigating the influence of the design parameters on the cut-on wind speed (Hopf bifurcation point and its nature) and the output power of the device (amplitude and stability of limit cycle oscillations) with the goal of minimizing the cut-on speed and maximizing the power. 

Understanding the Stick-Slip Dynamics in Ultrasonic Consolidation:

Ultrasonic Consolidation is a promising additive manufacturing process that is currently being used to build complex structures by joining thin metal films. In principle, layers of metal foil are joined by being compressed under moderate pressure using a rollingultrasonic horn which vibrates at a very high frequency in a direction transverse to the rolling direction. The stick-slip motion at the foil-foilinterface combined with moderate heating results in pure metal contact which causes atomic bonding. Additional layers are built until the required shape is realized.  Ultrasonic consolidation has recently demonstrated a critical shortcoming in its operation. Specifically, for a certain range of build height, additional layers cannot be built. We hypothesize that the complex stick-slip dynamic interactions between the high frequencyexcitations of the ultrasonic horn and resonances of the feature at the critical build heights are responsible for process degradation. We are currently testing this hypothesis through an NSF construction and machine equipment grant. The specific objectives of our current work are to: i) develop models that describe the dynamics and resonances of the build feature under ultrasonic excitations, ii)develop models for the frictional stresses at the interface and study the stick-slip response of the feature as function of the process parameters using nonlinear methods (e.g. Poincare maps), and iii) conceptualize and test passive and active strategies to eliminate bond degradation based on the understanding attained through completion of the previous objectives.                                                                                                                                                                                                 

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