Effects of wall compliance on the laminar–turbulent transition of torsional Couette flow

Torsional Couette flow between a rotating disk and a stationary wall is studied experimentally. The surface of the disk is either rigid or covered with a compliant coating. The influence of wall compliance on characteristic flow instabilities and on the laminar–turbulent flow transition is investigated. Data obtained from analysing flow visualizations are discussed. It is found that wall compliance favours two of the three characteristic wave patterns associated with the transition process and broadens the parameter regime in which these patterns are observed. The results for the effects of wall compliance on the third pattern are inconclusive. However, the experiments indicate that the third pattern is not a primary constituent of the laminar–turbulent transition process of torsional Couette flow

Experimental Study of the Stability of a Travelling Roll System in a Rotating Disk Flow

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Thrust generation by a heaving flexible foil: Resonance, nonlinearities, and optimality

International audienceFlexibility of marine animal fins has been thought to enhance swimming performance. However, despite numerous experimental and numerical studies on flapping flexible foils, there is still no clear understanding of the effect of flexibility and flapping amplitude on thrust generation and swimming efficiency. Here, to address this question, we combine experiments on a model system and a weakly nonlinear analysis. Experiments consist in immersing a flexible rectangular plate in a uniform flow and forcing this plate into a heaving motion at its leading edge. A complementary theoretical model is developed assuming a two-dimensional inviscid problem. In this model, nonlinear effects are taken into account by considering a transverse resistive drag. Under these hypotheses, a modal decomposition of the system motion allows us to predict the plate response amplitude and the generated thrust, as a function of the forcing amplitude and frequency. We show that this model can correctly predict the experimental data on plate kinematic response and thrust generation, as well as other data found in the literature. We also discuss the question of efficiency in the context of bio-inspired propulsion. Using the proposed model, we show that the optimal propeller for a given thrust and a given swimming speed is achieved when the actuating frequency is tuned to a resonance of the system, and when the optimal forcing amplitude scales as the square root of the required thrust

Towards a Self-contained Soft Robotic Fish: On-Board Pressure Generation and Embedded Electro-permanent Magnet Valves

Abstract. This paper details the design, fabrication and experimental verification of a complete, tetherless, pressure-operated soft robotic platform. Miniature CO2 cartridges in conjunction with a custom pressure regulating system are used as an onboard pressure source and embeddable electro-permanent magnet (EPM) [9] valves [13] are used to address supporting hardware requirements. It is shown that this system can repeatedly generate and regulate supply pressure while driving a fluidic elastomer actuator (FEA) [7, 14, 13]. To demonstrate our approach in creating tetherless soft mobile robots, this paper focuses on an example case-study: a soft robotic fish. An underactuated propulsion system emulating natural caudal fin and peduncle movement is designed, fabricated, and subsequently experimentally characterized.