Aeroelastic energy harvesters are a promising technology for powering wireless sensors and microelectromechanical systems. In this letter, we present a harvester inspired by the trembling of aspen leaves in barely noticeable winds. The galloping energy harvester, a curved blade oriented perpendicular to the flow, is capable of producing self-sustained oscillations at uncharacteristically low wind speeds. The dynamics of the harvesting system are studied experimentally and compared to a lumped parameter model. Numerical simulations quantitatively describe the experimentally observed dynamic behaviour. Flow visualisation is performed to investigate the patterns generated by the device. Dissimilar to many other galloping harvester designs, the flow is found to be attached at the rear surface of the blade when the blade is close to its zero displacement position, hence acting more closely to aerofoils rather than to conventionally used bluff bodies. Simulations of the device combined with a piezoelectric harvesting mechanism predict higher power output than that of a device with the square prism.

A galloping energy harvester with flow attachment

The control action of a throttling air jet produced by a gas generator and of the energy sources positioned at the walls of an axisymmetric channel, which models the combustion-chamber unit of a ramjet in which a gas flow is decelerated, on the shock-wave structure of a supersonic gas flow in the channel was investigated for the purpose of formation of a lengthy transonic region in it. A heat energy was supplied to the gas flow in the indicated channel in the pulse-periodic regime, and the pressure in the gas generator was also changed periodically. The range of energies corresponding to a stable transonic flow regime in the channel was determined on the basis of solution of the nonstationary Euler equations in dimensionless variables. For comparison, the combustion of a gaseous fuel inflowing to the channel through a slot in the channel wall upstream of the control air jet was considered. A nonstationary gas flow in the channel was calculated on the basis of the Reynolds-averaged Navier–Stokes equations with the use of the k–ω SST model of turbulence. It is shown that a controllable energy release in the channel, providing the obtaining of a transonic flow regime in it analogous to that obtained on the basis of solution of the Euler equations, can be realized with the use of a control transverse air jet.

Control of the Formation of a Transonic Region in an Axisymmetric Supersonic Flow with the Use of a Jet and a Near-Wall Energy Supply

It is shown that, in the case where, into the back wall of a semicircular airfoil with an angle of attack of 5°, a vortex cell of diameter 0.2 in fractions of the airfoil chord is built in and the mean-mass rate of slot suction of the air from this cell is larger than 0.15 of the incident-flow velocity, the pattern of the turbulent flow over the airfoil is transformed, and, at an optimum suction rate of 0.75, the lift coefficient of the airfoil reaches a maximum value of the order of 1.7 at an aerodynamic efficiency of 10.

Control of the Periodic Turbulent Flow over a Semicircular Airfoil with the Use of the Slot Suction of the Air from a Circular Vortex Cell at Small Angles of Attack

S. I. Isaev

Papers

Control of the Periodic Turbulent Flow over a Semicircular Airfoil with the Use of the Slot Suction of the Air from a Circular Vortex Cell at Small Angles of Attack