Marine animals are known to have developed adaptations to minimize drag and energy expenditure. Among these are passive material properties, such as the streamwise-aligned riblets found on shark skin, as well as the active modulation of the viscoelastic skin layer, which is thought to give dolphins their hydrodynamic edge. These adaptations serve to delay the transition from laminar to turbulent flow in the boundary layer around the body, minimize boundary layer turbulence, and reduce frictional drag. Transition to turbulence in the boundary layer happens via the development of two-dimensional instabilities, so-called Tollmien-Schlichting (TS) waves, which break down into fully developed turbulence. One mechanism to delay transition, is to counteract TS waves, reduce their amplitude, and delay their breakdown. This can be achieved by actively modulating a deformable membrane as part of the boundary near which the instabilities develop. We investigate boundary layer flow and transition to turbulence, and the effect of actuated boundaries, in a laminar to turbulent flow tank. To test the impact of a deformable boundary on the flow, a hydrofoil is outfitted with fluid chambers overlaid by an optical quality PDMS membrane, which can be actuated in response to the flow. Flow over the hydrofoil is visualized with dye experiments and quantified with Particle Image Velocimetry. The impact of boundary actuation, as well as different boundary materials, on the flow is characterized. Achieving a reduction of boundary layer turbulence on operational scales would have profound implications for platform energy efficiency, as well as signature and acoustic noise reduction.
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.
We present a liquid flow sensor inspired by cupula structures found on a variety of fish. Our 5mm x 5mm x 1.75mm artificial cupula uniquely comprises a pair of differential liquid metal capacitors encased in silicone. Deflection of the structure – manually or by fluid flow – increases capacitance on one side and decreases on the other. To fabricate the complex internal structure, a commercial 3D printer is used to create a mold out of a sacrificial wax-like material. After casting uncured rubber, internal mold structures are melted and dissolved away, leaving channels and voids for liquid metal vacuum injection. The measured sensitivity of ~0.05pF/mm is compared to theoretical capacitance versus deflection values based on kinematics. To test behavior under water flow, a custom flow channel consisting of a 7.5mm x 7.5mm cross-section is employed with rates up to 1L/min. The parabolic capacitive response as a function of flowrate is compared to analytic theory based on kinematics and drag as well as to fluid-structure interaction (FSI) simulations using COMSOL. This device has future applications in the control of bio-inspired soft robotics. [Work sponsored by the Office of Naval Research.]