Kaushik Sampath

This study characterizes the effects of passive flexibility on the wake of a tandem bio-inspired flapping fin. Soft fins made of polydimethylsiloxane with 10:1 and 20:1 mixing ratios, corresponding to elastic moduli ( E ) of 820 kPa and 380 kPa, respectively, were evaluated in comparison to a rigid fin of identical geometry made of nylon ( E = 50 MPa). To investigate how material stiffness and tandem fin phasing influence wake flow structure, we measured thrust and conducted two-dimensional particle image velocimetry. Experiments were performed at three phase offsets ( − 112 ∘ , 0 ∘ , 67 ∘ ) and at three wake planes behind the front fin ( x / c = 2.1, 5 and 8, normalized by the chord length). The results showed a substantial effect of the fin stiffness on the wake strength and expansion rate downstream. An intermediate level of passive flexibility produced higher thrust and greater wake coherence, whereas both the rigid and highly flexible fins performed poorly in comparison. These findings indicate the existence of a ‘goldilocks’ range of fin flexibility and further clarify how fin stiffness and tandem phasing shape the downstream wake signature.

Advanced applications of polymer additive manufacturing (AM) require knowledge of the material acoustic and complex elastic properties. Recently, ultrasound nondestructive evaluation (NDE) methods have been applied to the understanding of AM polymers manufactured with a variety of methods. Nonetheless, the available information is still limited to a few materials and frequency ranges, and knowledge of shear acoustic properties and complex elastic properties of AM polymers is lacking. In this study, ultrasound measurements of compressional and shear phase velocity and attenuation are used to experimentally determine the complex elastic properties of 14 AM photopolymers manufactured using PolyJet systems. The results provided here are expected to aid in design of advanced polymer AM structures, and highlight the value of ultrasound NDE for the characterization of AM polymers.

This study introduces a framework for the design and implementation of acoustic black holes (ABHs) in airfoils. A generalized multiparameter damped-ABH generation function is mapped onto airfoils based on the National Advisory Committee for Aeronautics series. Representative geometries and a uniformly distributed baseline, all with the same mass of structure and damping are fabricated using multimaterial PolyJet three-dimensional printing. Laser Doppler vibrometer measurements along the airfoil chord in response to a broadband 0.1–12 kHz excitation show a decrease in trailing-edge vibrations by as much as 10 dB, a broadband 5-dB reduction across the entire chord, substantial spatial and temporal modulation of flexural waves by ABH foils. Finite-element analysis (FEA) models are developed and validated based on the measured data. Furthermore, a parametric FEA study is performed on a set of comparable designs to elucidate the scope of modulation achievable. These findings are applicable to trailing-edge noise reduction, flow control, structural enhancement, and energy harvesting for airfoils.

Due to the unprecedented increase in telework requirements, the motivation to further automate and remotely control experiments has become apparent. This work documents the technical development of creating a two-dimensional (2D) Laser Doppler Vibrometry (LDV)measurement using a single-point LDV system through an automated and remotely controllable process. This report aims to assist in rapid development of setups for similar use cases. The setup described is also modular, and has been used to analyze the modal response of samples actuated through air-based acoustic signals as well as those mechanically induced.

We show how phasing between tandem bioinspired fins flapping at high-stroke amplitudes modulates rear fin thrust production and wake characteristics. Load cell thrust measurements show that the rear fin generates 25% more thrust than the front fin when it lags the latter by a quarter cycle, and performs 8% worse when it leads the front fin by the same amount. The flow interactions between the fins responsible for these observations are analyzed using two-dimensional particle image velocimetry measurements and three-dimensional computational fluid dynamics simulations. Distributions of velocity elucidate variations in the effective flow induced on the rear fin for different phase offsets. Vortex structure interactions and particle rakes reveal the contributions of the leading- and trailing-edge vortices shed by the front fin in modulating the suction at the rear fin leading edge. Furthermore, the wake structure far downstream of the fins changes in its coherence, axial and radial extents for the different phase offsets. These findings are relevant for the design and performance optimization of various unmanned underwater vehicles that utilize such tandem systems.

We show how phasing between tandem bioinspired fins flapping at high-stroke amplitudes modulates rear fin thrust production and wake characteristics. Load cell thrust measurements show that the rear fin generates 25% more thrust than the front fin when it lags the latter by a quarter cycle, and performs 8% worse when it leads the front fin by the same amount. The flow interactions between the fins responsible for these observations are analyzed using two-dimensional particle image velocimetry measurements and three-dimensional computational fluid dynamics simulations. Distributions of velocity elucidate variations in the effective flow induced on the rear fin for different phase offsets. Vortex structure interactions and particle rakes reveal the contributions of the leading- and trailing-edge vortices shed by the front fin in modulating the suction at the rear fin leading edge. Furthermore, the wake structure far downstream of the fins changes in its coherence, axial and radial extents for the different phase offsets. These findings are relevant for the design and performance optimization of various unmanned underwater vehicles that utilize such tandem systems.

Submersible robotics have improved in efficiency and versatility by incorporating features found in aquatic life, ranging from thunniform kinematics to shark skin textures. To fully realize these benefits, sensor systems must be incorporated to aid in object detection and navigation through complex flows. Again, inspiration can be taken from biology, drawing on the lateral line sensor systems and neuromast structures found on fish. To maintain a truly soft-bodied robot, a man-made flow sensor must be developed that is entirely complaint, introducing no rigidity to the artificial “skin.” We present a capacitive cupula inspired by superficial neuromasts. Fabricated via lost wax methods and vacuum injection, our 5 mm tall device exhibits a sensitivity of 0.5 pF/mm (capacitance versus tip deflection) and consists of room temperature liquid metal plates embedded in a soft silicone body. In contrast to existing capacitive examples, our sensor incorporates the transducers into the cupula itself rather than at its base. We present a kinematic theory and energy-based approach to approximate capacitance versus flow, resulting in equations that are verified with a combination of experiments and COMSOL simulations.

Bubble bursting is a primary source of marine aerosols, yet little is known about particle emissions due to the bubble bursting in slicks containing oil‐dispersant mixtures. In this study, bubbles with mode sizes of 86 μm (denoted as small), 178 μm (medium), and 595 μm (large) are injected into a seawater column covered by slicks of crude oil, pure dispersant, and dispersant premixed with oil at a ratio of 1:25. The aerosol size distributions are monitored in the 0.5‐ to 20‐μm and 10‐ to 380‐nm ranges both in clean and ambient air environments. In ambient air, a tenfold increase in submicron particle concentration occurs when large bubbles burst on slicks of 500‐μm dispersant premixed with oil at a ratio of 1:25 oil or 50‐μm pure dispersant. Yet, in multiple tests performed at different ambient particle concentrations, the elevated size distributions persistently maintain the same shape as that of the ambient air. In contrast, smaller bubbles and tests not involving dispersants do not cause such an increase. Nanodroplets are also generated by large bubbles in particle‐free air, but their concentrations are much lower. All plumes generate micron‐sized aerosols, but trends vary. For the same contaminant, the microdroplet concentration decreases with increasing slick thickness. Particularly striking is a reduction of 2 orders of magnitude in the microdroplet concentration when medium and small bubbles burst on 500‐μm crude oil slicks. Chemical analysis of air and particulates collected from filters sampling the particles confirms the presence of airborne oil above the slicks.

We present the design, fabrication, and testing of stretchable pressure sensing membranes. Two sensing techniques are demonstrated: resistive and capacitive. Both designs are incorporated in 400μm-thick films and are fabricated with thin film application of silicone and stencil/mask deposition of conductive materials. The resistive sensor utilizes room temperature liquid metal while the capacitive sensor utilizes multi-walled carbon nanotubes. Tests are performed with 18mm-diameter samples of each. Point load tests and acoustic response in an impedance tube provide feedback on sensor performance. The resistive sensor demonstrates a sensitivity of 0.045Ω/mm, and the sensor’s response has been characterized for in the 30Hz to 10kHz range with varying degrees of sensitivity. The capacitive sensor has a small point-load-deflection sensitivity ranging from 0.018pF/mm to 0.044pF/mm depending on capacitor diameter. Acoustic response are shown for 5Hz to 40 Hz, limited by external electronics. These devices are progress towards developing sensor networks capable of tracking aqueous turbulence.