Developing Broadband Wave Energy Converters

Harnessing wave energy has, for the longest times, been an area of continuous research around the globe. While point wave absorbers occupy 40 percent of the market share, they can only transduce energy efficiently within a very narrow wave frequency bandwidth leaving most of the wave energy unharnessed. To alleviate this problem, we are proposing a new design approach which can alleviate this shortcoming and increase the efficiency of point wave absorbers. This would push the performance limits of current point wave energy absorbers making the technology more viable especially for intermediate wave power levels similar to those observed in the Arabian gulf.  The key enabling feature of the proposed design is the use of nonlinear (bi-stable) restoring force elements to replace the linear springs currently used to create the restoring force in the design of the generator. The idea of using nonlinear restoring elements stems from the knowledge that stiffness nonlinearities can widen the response bandwidth of an oscillator by creating amplitude frequency dependence.  A similar concept has been applied successfully by our group to broaden the bandwidth of vibration-based energy harvesters which suffer from similar bandwidth issues. It is proposed here to adapt this same concept to point wave energy converters.

Structural Morphing Using Dynamic Instabilities


Structural morphing refers to the process by which the geometry of a given structure is altered in real time such that it is better suited to respond to an external stimulus while performing a desired task. Morphing has, for long, showed a great potential to improve the typical functionalities of aerospace vehicles. For example, the use of slats and flaps to morph the wing and tail structures of an aircraft has always been the method of choice for flight control. Today, morphing is also being exploited for accurate directional control of parachutes and parafoils used in joint precision airdrop systems. Despite the significant strides made over the last two decades to advance morphing technologies, a major challenge still exists. Morphing under large aerodynamics loads, similar to those experienced on parafoils, requires very large actuation forces that cannot be easily realized without compromising the simplicity of the design, the weight of the structure, and its cost. To face this challenge, novel approaches have recently emerged to design simple actuation mechanisms that result in large deformations without significant actuation effort. One promising approach lies in exploiting mechanical instabilities in multi-stable morphing structures. In such systems, the potential energy function of the structure possesses, at least, two stable equilibrium shapes separated by an energy barrier. The transition from one shape to the other requires an external perturbation to allow the dynamic trajectories to overcome the potential barrier.  Once the dynamic instability is activated, a structural snap-through motion follows, yielding a significant change in the shape of the structure. While all the previous research efforts focused on using static forces to achieve the desired shape change, we demonstrated that less input energy is required to overcome the potential barrier when the structure is subjected to dynamic loads instead. As such, we proposed, for the first time, using cross-flow induced instabilities to activate shape changes in multi-stable structures. This approach is expected to lower the energy input required to activate the system resulting in a much more sensitive and efficient morphing mechanism.

Dynamics of Origami-Based Structures


This constitutes a major thrust area of my group’s research at NYU Abu Dhabi. The idea is to investigate the potential of using three-dimensional origami-based structures to carry dynamic loads for the purpose of actuation and vibration mitigation. The beauty of origami is that a three-dimensional structure can be built from a sheet  of a certain foldable material without the need for cutting or adding new material; a process when automated can reduce the cost and complexity of manufacturing processes. Furthermore, three-dimensional origami based structures can be folded into a thin, mostly two-dimensional sheets, when not in use and expanded into their original shape when in demand. Our group is among the first in the world to investigate using foldable bellows as dynamic load carrying elements. To this end, the dynamic behavior of foldable springs and damping elements based on different types of origami cells is currently being characterized in my group at NYUAD.

Improving the Performance of Flow Energy Harvesters Via Trailing-Edge Passive Flow Manipulation


Motivated by the obvious need for compact, sensitive, and highly-efficient flow energy harvesters (FEHs), this research endeavor is based on passively manipulating the flow at the trailing edge of galloping-based FEHs to create flow patterns that are favorable for energy generation. This will lead to considerable improvements in the steady-state output power levels and to enhancements in the transient behavior of the harvester in response to unsteady flow conditions. To achieve this goal, the research implements an integrated research framework involving analytical, computational, and experimental tools to resolve the influence of the flow characteristics on the harvester's performance. The research marks the first attempt to introduce passive flow manipulation at the trailing edge to enhance the performance of galloping FEHs. In the process, it builds the fundamental understanding necessary to resolve the relationship between the flow characteristics and the output power. Furthermore, it introduces, for the first time, the transient characteristics, particularly the time required to achieve steady state, as a critical performance criterion. This is essential to assess performance under unsteady flow conditions typically encountered in locations where such harvesters are designed to operate. Research results will provide a validated mathematical model, which describes the basic physics of the driving mechanism.

Exploiting Liquid-State Transduction Materials for Energy Harvesting


Funded through an NSF sensor and sensing systems grant, and motivated by the obvious need for compact, scalable, and low-maintenance micro-power generators, this research aims to build the fundamentals necessary to evaluate and maximize the transduction efficiency of vibratory energy harvesters that utilize liquid-state materials, namely ferrofluids, as the transduction element. To achieve this goal, a systematic three-level framework, which combines theories in fluid dynamics and electromagnetics with computational tools, is used to model and analyze the electro-magneto-hydrodynamic behavior of liquid-state energy harvesters. At the first level, non-equilibrium molecular dynamics simulations are implemented to determine the relationship between the microscopic behavior of the ferrofluid nanoparticles and the associated bulk material properties of the fluid under non-equilibrium/dynamic conditions. At the second level, a reduced-order analytical model of the harvesting system is developed. The model, which invokes several justifiable assumptions on the dynamics, is used to provide a qualitative insight into the influence of the design
parameters on the output power for simple device geometries. At the third level, a comprehensive computational model is developed and used to quantify the output power of the harvester for complex device geometries and at different scales.

Exploiting Nonlinearities and Non-Traditional Design Concepts to Improve the Performance of Flow Energy Harvesters


Cross-flow instabilities such as vortex shedding and wake galloping have been recently utilized to develop scalable flow energy harvesters. Unlike traditional rotary-type generators which are known to suffer serious scalability issues because their efficiency drops significantly as their size decreases; flow energy harvesters based on cross-flow instabilities operate using a very simple motion mechanism, and, hence can be scaled down to meet the desired application. Nevertheless, flow energy harvesters have their own shortcomings. Typically, they have a very narrow lock-in region where they can harness flow energy effectively. As a result, they not perform well under the broad range of shedding frequencies normally associated with a variable flow speed. To overcome this critical problem, part of the research we performed over the past seven years involved introducing nonlinearities and other novel design concepts to broaden the steady-state bandwidth of flow energy harvester and, thereby to decrease their sensitivity to variations in the flow speed. Our main contributions are summarized in the following publications.

Performance of Nonlinear Energy Harvesters in Stochastic Environments


Funded through an NSF CAREER grant, our current major thrust 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 yield incorrect 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.

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 rolling ultrasonic horn which vibrates at a very high frequency in a direction transverse to the rolling direction. The stick-slip motion at the foil-foil interface 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 frequency excitations 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.