Flow-induced vibrations (FIVs) can be utilized to harvest energy for micro-aerial vehicles. The purpose of this paper is to investigate the fluid–structure interaction in piezoelectric energy harvesting. A piezoelectric energy harvester for a membrane wing at Reynolds number Re = 8000 is studied based on an aero-electro-mechanical model using the computational fluid dynamics/computational structure dynamic coupling method. The updated Lagrangian formulation is applied for the large deformation of the flexible structure. The effects of the location of piezoelectric harvesters and the angle of attack (α=4∘–24°) on FIV response and energy harvesting performance are investigated. Average power density is defined to evaluate the energy harvesting performance of the harvester. The location of the piezoelectric harvester has a negligible effect on the energy harvesting performance under the same FIV response. However, the change in local stiffness caused by the location of the piezoelectric harvester may induce a noticeable difference in FIV response which impacts the energy harvesting performance. The simulation results indicate the strong coupling relationship among flow field, membrane structure, and electric field. There are two states of fluid–structure interaction at the angles of attack investigated. At α=4°–12°, the vibration response of the membrane wing is mainly driven by the natural frequency of the structure. At α=16°–24°, the convection and shedding of leading- and trailing-edge vortices play a dominant role in FIV response. The work presents the mechanism of fluid–structure interaction in energy harvesting from FIVs and provides a significant basis for designing energy harvesters of membrane wings.

The purpose of this paper is to investigate the feasibility of using dielectric barrier discharge plasma actuation for the pitch stability control of a National Advisory Committee for Aeronautics 0012 pitch oscillating airfoil at low Reynolds number. The effects of the plasma actuator on the flow are incorporated into Navier–Stokes equations as a body force source term. The plasma body force is modeled by a phenomenological approach. Solutions are obtained by two-dimensional numerical simulations using the γ−Reθ transition model. The flow control effects of plasma actuator locations, strengths, force directions, and laminar separation flutter (LSF) conditions are investigated. The energy extracted by the airfoil from the flow over one oscillation cycle is defined to evaluate the control performance. The LSF analysis based on energy maps is also implemented. The control effect of co-flow plasma configuration on pitch instability is better than the counterflow configuration. The co-flow plasma actuator located at 0.6c reaches the best control performance and the energy extracted by the airfoil from the flow over one oscillation cycle reduces by 726% relative to the baseline case. The flow feature inducing the pitch instability and the flow control mechanism of co-flow and counterflow plasma actuator is analyzed in terms of the flow structure and pressure distribution, respectively. The results show that the plasma flow control is effective to mitigate the pitch instability across a wide range of Reynolds numbers at which laminar separation flutter occurs without control.

Federated learning (FL) is a promising distributed framework for collaborative artificial intelligence model training while protecting user privacy. A bootstrapping component that has attracted significant research attention is the design of incentive mechanism to stimulate user collaboration in FL. The majority of works adopt a broker-centric approach to help the central operator to attract participants and further obtain a well-trained model. Few works consider forging participant-centric collaboration among participants to pursue an FL model for their common interests, which induces dramatic differences in incentive mechanism design from the broker-centric FL. To coordinate the selfish and heterogeneous participants, we propose a novel analytic framework for incentivizing effective and efficient collaborations for participant-centric FL. Specifically, we respectively propose two novel game models for contribution-oblivious FL (COFL) and contribution-aware FL (CAFL), where the latter one implements a minimum contribution threshold mechanism. We further analyze the uniqueness and existence for Nash equilibrium of both COFL and CAFL games and design efficient algorithms to achieve equilibrium solutions. Extensive performance evaluations show that there exists free-riding phenomenon in COFL, which can be greatly alleviated through the adoption of CAFL model with the optimized minimum threshold.

This paper investigates suppressing the pitching oscillation of a NACA 0012 airfoil at a Reynolds number of [Formula: see text] by using phase-shifted trailing-edge morphing. The latter considering geometric nonlinearity is simulated numerically based on spatiotemporal polynomial surface fitting. Flowfield results and aerodynamic forces are obtained by solving the unsteady Reynolds-averaged Navier–Stokes equations accompanied by a low-Reynolds-number modified-shear-stress transport model. Dynamic meshing combining a sliding mesh and Laplacian diffusion is developed to deal with the deformation of the computational grid due to the large-amplitude coupled pitching and morphing motion of the airfoil. The airfoil is subjected to stall-flutter analysis from an energy perspective by calculating the energy extracted by the airfoil from the freestream in an oscillation cycle. The flow control mechanism of the phase-offset trailing-edge morphing is analyzed in terms of the pressure distribution and moment contribution. The results suggest that trailing-edge motion with a phase of [Formula: see text] reduces the energy extraction by more than 300% and could be effective for suppressing the stall-flutter limit-cycle oscillation amplitude in specific ranges of amplitude and frequency.

This paper aims to gain new insight into the physical mechanism of laminar separation flutter (LSF) from the perspective of energy transfer and dynamic mode decomposition (DMD) modes of flow. An online DMD method accounting for the airfoil's pitch motion is developed and the relationship between the topology of energy map and DMD modes is established. Simulation results indicate that there are two limit cycle branches in energy map but only one branch is stable. The LSF time response state can be predicted accurately by the stable limit cycle branch. The topology of energy map is dominated by the DMD mode corresponding to the airfoil's pitch frequency. The developed DMD method can extract the variation of flow structures effectively. The pressure distribution of DMD mode corresponding to the pitch frequency is dominated by the leading-edge suction and bubble's suction. The bubble's suction is induced by the trailing-edge laminar separation bubble or laminar separation bubble (LSB). When the pitch amplitude is larger than 4 degrees, the trailing-edge laminar separation bubble transforms to LSB. The inherent mechanism is that the increasing of trailing-edge separation bubble's intensity promotes the energy extraction while the occurrence of LSB mitigates it.

Field-programmable photonic array-based chips will be key components in realizing high-performance THz communication in the future. However, the optical diffraction limit prohibits their integration in chip-level sizes. Although plasmonic waveguides combined with graphene are able to possess mode transmission at THz wavelengths, their operational problems such as complex device structures or high transmission losses still cannot be addressed effectively. In this paper, by relying on a mechanism of THz surface plasmon mode of graphene and an energy distribution mechanism of the refractive index difference region, a novel hybrid graphene plasmonic waveguide is proposed, whose core design part is a simple gap-slot region. This waveguide's transmission distance is one order of magnitude longer than that of the conventional waveguide while retaining the benefit high energy confinement, under appropriate parameters. Hence the proposed waveguide can be an efficient candidate component of field-programmable photonic arrays for THz communication.

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Flow-induced vibrations can be utilized to harvest energy for micro air vehicles (MAVs). A flexible membrane wing with an embedded piezoelectric energy harvester at an angle of attack of 12 degrees and Reynolds number (Re) of 8000 is studied by numerical simulations. An aero-electro-mechanical model is established to investigate the effect of the leading-edge (LE) and trailing-edge (TE) geometries on the fluid-structure interaction (FSI) modes, aerodynamic performance, and energy harvesting performance. A new correction method of structural frequency is proposed that considers both the aerodynamic stiffness effect and added mass effect corresponding to a specific FSI mode of interest. The results suggest that the mode transition accompanied by the performance changes is essentially caused by the FSI state transition, which is distinguished by the corrected structural frequency and the vortex shedding one. With the Fourier mode decomposition (FMD) method, the modes of membrane vibration and pressure fluctuation become clear. The LE geometry is found to affect the FSI state by influencing the leading-edge vortices, which further triggers the mode transition.