John B. Blottman

An active energy harvesting scheme with an electroactive polymer

We investigate the energy harvesting with an electrostrictive polymer, possessing high electromechanical response and elastic energy density, which make it possible to generate high electric energy density and attractive for the active energy harvesting scheme. It is shown that combining the active energy harvesting scheme and high electromechanical response of the polymer yields a harvested electric energy density of ∼40mJ∕cm3 with a 10% efficiency.

A computationally efficient finite element model with perfectly matched layers applied to scattering from axially symmetric objects

A frequency-domain finite-element (FE) technique for computing the radiation and scattering from axially symmetric fluid-loaded structures subject to a nonsymmetric forcing field is presented. The Berenger perfectly matched layer (PML), applied directly at the fluid-structure interface, makes it possible to emulate the Sommerfeld radiation condition using FE meshes of minimal size. For those cases where the acoustic field is computed over a band of frequencies, the meshing process is simplified by the use of a wavelength-dependent rescaling of the PML coordinates. Quantitative geometry discretization guidelines are obtained from a priori estimates of small-scale structural wavelengths, which dominate the acoustic field at low to mid frequencies. One particularly useful feature of the PML is that it can be applied across the interface between different fluids. This makes it possible to use the present tool to solve problems where the radiating or scattering objects are located inside a layered fluid medium. The proposed technique is verified by comparison with analytical solutions and with validated numerical models. The solutions presented show close agreement for a set of test problems ranging from scattering to underwater propagation.

A bio-inspired shape memory alloy composite (BISMAC) actuator

A beam-shape composite actuator using shape memory alloy (SMA) wires as the active component, termed a Bio-Inspired Shape Memory Alloy Composite (BISMAC), was designed to provide a large deformation profile. The BISMAC design was inspired by contraction of a jellyfish bell, utilizing the rowing mechanism for locomotion. Characterization of maximum deformation in underwater conditions was performed for different actuator configurations to analyze the effect of different design parameters, including silicone thickness, flexible steel thickness and distance between the SMA and flexible steel. A constant cross-section (CC)-BISMAC of length 16 cm was found to achieve deformation with a radius of curvature of 3.5 cm. Under equilibrium conditions, the CC-BISMAC was found to achieve 80% of maximum deformation, consuming 7.9 J/cycle driven at 16.2 V/0.98 A and a frequency of 0.25 Hz. A detailed analytical model was developed using the transfer matrix method and a 1D finite beam element (FE) model to simulate the behavior of the BISMAC incorporating gravity, buoyancy and SMA parameters. The FE and transfer matrix models had a maximum deformation error norm of 1.505 and 1.917 cm in comparison with experimentally observed beam deformation in the CC-BISMAC. The mean curvatures predicted by the FE and transfer matrix methods were 0.292 cm−1 and 0.295 cm−1 compared to a mean experimental curvature of 0.294 cm−1, a percentage error of −5.4% and 2.77%, respectively. Using the developed analytical model, an actuator design was fabricated mimicking the maximum deformation profile of jellyfish of the species Aurelia aurita (AA). The designed AA-BISMAC achieved a maximum curvature of 0.428 cm−1 as compared to 0.438 cm−1 for A. aurita with an average square root error of 0.043 cm−1, 10.2% of maximum A. aurita curvature.

Biomimetic and Live Medusae Reveal the Mechanistic Advantages of a Flexible Bell Margin

Flexible bell margins are characteristic components of rowing medusan morphologies and are expected to contribute towards their high propulsive efficiency. However, the mechanistic basis of thrust augmentation by flexible propulsors remained unresolved, so the impact of bell margin flexibility on medusan swimming has also remained unresolved. We used biomimetic robotic jellyfish vehicles to elucidate that propulsive thrust enhancement by flexible medusan bell margins relies upon fluid dynamic interactions between entrained flows at the inflexion point of the exumbrella and flows expelled from under the bell. Coalescence of flows from these two regions resulted in enhanced fluid circulation and, therefore, thrust augmentation for flexible margins of both medusan vehicles and living medusae. Using particle image velocimetry (PIV) data we estimated pressure fields to demonstrate a mechanistic basis of enhanced flows associated with the flexible bell margin. Performance of vehicles with flexible margins was further enhanced by vortex interactions that occur during bell expansion. Hydrodynamic and performance similarities between robotic vehicles and live animals demonstrated that the propulsive advantages of flexible margins found in nature can be emulated by human-engineered propulsors. Although medusae are simple animal models for description of this process, these results may contribute towards understanding the performance of flexible margins among other animal lineages.

Thermal management of thermoacoustic sound projectors using a free-standing carbon nanotube aerogel sheet as a heat source

Carbon nanotube (CNT) aerogel sheets produce smooth-spectra sound over a wide frequency range (1-10(5) Hz) by means of thermoacoustic (TA) sound generation. Protective encapsulation of CNT sheets in inert gases between rigid vibrating plates provides resonant features for the TA sound projector and attractive performance at needed low frequencies. Energy conversion efficiencies in air of 2% and 10% underwater, which can be enhanced by further increasing the modulation temperature. Using a developed method for accurate temperature measurements for the thin aerogel CNT sheets, heat dissipation processes, failure mechanisms, and associated power densities are investigated for encapsulated multilayered CNT TA heaters and related to the thermal diffusivity distance when sheet layers are separated. Resulting thermal management methods for high applied power are discussed and deployed to construct efficient and tunable underwater sound projector for operation at relatively low frequencies, 10 Hz-10 kHz. The optimal design of these TA projectors for high-power SONAR arrays is discussed.

Aurelia aurita Inspired Artificial Mesoglea

In this preliminary study, we report the mechanical and dielectric properties of polyvinyl alcohol (PVA)-ferritin hydrogel. This material was found to exhibit close resemblance to Aurelia aurita (jellyfish) mesoglea in terms of stiffness modulus and water content. Systematic experiments were conducted on natural jellyfish to identify its compression modulus a function of deformation. In compressive testing Aurelia aurita mesoglea was found to exhibit nonlinear modulus in the range of −10 kPa to 70 kPa depending upon the compressive strain (0–50% strain). The negative stiffness is an artifact of tensile force experienced by the specimen at the beginning of the test due to surface tension. PVA hydrogels with 60% water to dimethyl sulfoxide (DMSO) ratio without ferritin particle (H60) and PVA hydrogels with 80% water to DMSO ratio with ferritin particle (F80) provided a good alternative to natural jellyfish mesoglea exhibiting shear modulus of 33.06 Pa and 39.99 Pa respectively as compared to 4.75 Pa for Aurelia aurita mesoglea. This is a significantly better match compared to the 1041.67 Pa shear modulus of Ecoflex, a soft polymer material commonly used in biomimetic robotics. A Mooney Rivlin model suggests that H60 and F80 compositions are about 6.9 times and 8.4 times stiffer than natural Aurelia aurtia mesoglea whereas Ecoflex is 219 times as stiff. Nanocomposite hydrogel consisting of PVA matrix and ferritin nanoparticles were found to exhibit higher durability over regular PVA hydrogels and had more consistent properties due to increased cross-linking at ferritin nanoparticle sites. The ferritin nanoparticles were found to act as springs, increasing the modulus by increasing the surface area of the cross-linked polymer chains and disrupting long linear chain patterns of the polymer. Natural Aurelia aurita was found to have water content of 96.3% with a standard deviation of 0.57% as compared to 85% water content of PVA-ferritin hydrogels. Use of this material in the design of biomimetic unmanned underwater vehicles is expected to reduce the power consumption, increase swimming efficiency, and better replicate the rowing kinematics of naturally occurring Aurelia aurita.

Thermal behavior of high‐power active devices with the ATILA (analysis of transducers by integration of LAplace equations) finite‐element code

Many active devices using piezoelectric ceramics are driven with very high power densities and long pulse lengths. Due to mechanical and dielectric losses in the materials, this produces heat, causing a temperature rise in the devices, which may lead to their mechanical failure. The thermal issues have been shown to be the limiting device design criteria over electric field and mechanical stress limits, yet the effect of the temperature on performance is generally not considered in the numerical models used during the design stage. A coupled electro‐mechanical thermal analysis is implemented in the ATILA code. For a steady‐state or transient solution, a thermal behavior is weakly coupled to the electromechanical response. The method may take advantage of the order‐of‐magnitude‐greater time constant for thermal effects compared to mechanical behavior. A two‐step analysis is performed whereby the electromechanical behavior is first computed, and the resulting dissipated power is then applied as a heat generator to determine the resulting temperature of the device. A high‐drive, 31‐mode, free flooded ring transducer and a sonar projector serve as validation of the numerical model. The approach addresses both the transient thermal response and the steady temperature profile that results from the high‐power, high‐duty‐cycle drive.

The Jellyfish: smart electro-active polymers for an autonomous distributed sensing node

The US Navy has recently placed emphasis on deployable, distributed sensors for Force Protection, Anti-Terrorism and Homeland Defense missions. The Naval Undersea Warfare Center has embarked on the development of a self-contained deployable node that is composed of electro-active polymers (EAP) for use in a covert persistent distributed surveillance system. Electro-Active Polymers (EAP) have matured to a level that permits their application in energy harvesting, hydrophones, electro-elastic actuation and electroluminescence. The problem to resolve is combining each of these functions into an autonomous sensing platform. The concept presented here promises an operational life several orders of magnitude beyond what is expected of a Sonobuoy due to energy conservation and harvesting, and at a reasonable cost. The embodiment envisioned is that of a deployed device resembling a jellyfish, made in most part of polymers, with the body encapsulating the necessary electronic processing and communications package and the tentacles of the jellyfish housing the sonar sensors. It will be small, neutrally buoyant, and will survey the water column much in the manner of a Cartesian Diver. By using the Electro-Active Polymers as artificial muscles, the motion of the jellyfish can be finely controlled. An increased range of detection and true node autonomy is achieved through volumetric array beamforming to focus the direction of interrogation and to null-out extraneous ambient noise.

A Finite‐Element Tool for Scattering from Localized Inhomogeneities and Submerged Elastic Structures

A steady‐state 3‐D finite‐element tool called FESTA (Finite‐Element STructural Acoustics), is being developed at the NATO Undersea Research Centre. The code is geared towards a variety of applications in underwater acoustics, such as multistatic scattering from localized inhomogeneities, scattering across interfaces between fluids and/or solids, and multistatic scattering from single and multiple fluid‐loaded elastic targets. The hp‐adaptive finite‐element technology used to develop FESTA allows the user to optimize the convergence as well as the demand on computing resources by selectively changing the element size (h‐refinement) and/or by increasing the order of the polynomial finite‐element shape functions (p‐enrichment). An efficient hybrid tool for the computation of multistatic scattering from targets buried, partially buried or proud inside shallow water waveguides is being developed in conjunction with MIT. In this hybrid tool FESTA is used to perform target computations in the near field of the s...

Progress in hybrid finite‐element/propagation tool modeling of scattering from objects in underwater waveguides

A steady‐state 3D finite‐element software called FESTA (Finite‐Element STructural Acoustics), is being developed at the NATO Undersea Research Centre. The code is geared towards a variety of applications in underwater acoustics, such as multistatic scattering from localized inhomogeneities, scattering across interfaces between fluids and/or solids, and multistatic scattering from single and multiple fluid‐loaded elastic targets. One issue of importance to researchers in underwater acoustics is the trade‐off between full 3D models and thin‐shell theory. To address this issue, FESTA results are compared to results from thin‐shell scattering codes for cylindrical bodies with spherical endcaps. Another part of the work focuses on the integration of the finite‐element tool with underwater propagation tools for the computation of multistatic scattering from targets buried, partially buried or proud inside shallow‐water waveguides. To achieve this, FESTA is coupled to the MIT underwater propagation tool OASES. O...