Abstract
Small unmanned aerial vehicles and biological fliers can experience wind gusts of similar magnitude to the flight speed, which is detrimental to flight stability. For one encounter type, the low Reynolds number transverse gust, little is known about the fundamental fluid mechanics due in part to the difficulties in replicating the scenario experimentally or computationally. The aim of this work is thus to present the development and characterisation of an apparatus capable of generating and measuring the transient response of large amplitude transverse wing–gust encounters. The system is designed to produce a sharp-edged gust profile for direct comparison with the linear Küssner model. Particle image velocimetry (PIV) measurements show that the system successfully generated a steady top-hat shaped gust. A technique using inertial sensors has been used to minimise the effects of model vibration in measuring the unsteady forces. A wing–gust interaction with cross flow velocity equal to the flight speed is also presented. For this interaction, a strong leading edge vortex forms and vorticity of opposite sense is shed at the trailing edge. The trailing edge vorticity remains relatively planar, which is similar to the planar wake assumption of the Küssner model. Large deformation of the gust shear layers is visible upon wing entry, which is a deviation from the ‘rigid’ shear layers assumed by linear theory. Despite differences in flow topology between theory and experiment, the lift force coefficients match surprisingly well during entry into the gust, but deviate upon exit.Graphical abstract
Highlights
The development of small low-cost sensors and flight platforms has facilitated extensive growth in the use of unmanned aerial vehicles (UAVs) for military, commercial and consumer purpose
While the small size of UAVs is beneficial from a cost and accessibility standpoint, lies a limitation from a flight stability and safety perspective
Small UAVs operate at altitudes well within the atmospheric boundary layer and fly across high shear and wake regions behind local obstructions (Watkins et al 2006)
Summary
The development of small low-cost sensors and flight platforms has facilitated extensive growth in the use of unmanned aerial vehicles (UAVs) for military, commercial and consumer purpose. Small UAVs operate at altitudes well within the atmospheric boundary layer and fly across high shear and wake regions behind local obstructions (Watkins et al 2006) In this domain, the wind turbulence intensity can exceed 50% (Mohamed et al 2015; Walshe 1972). Watkins et al (2006) showed that for a typical radio aircraft sized fixed wing UAV, there can be rapid changes in angle of attack in excess of 25◦ , as well as span-wise variations in incidence of up to 15◦ which result in roll instability These problems are compounded with reducing vehicle size, as the optimal flight speed (U) reduces with decreasing mass (Spedding and Lissaman 1998), the gust ratio (V/U) increases for a given wind speed (V). The safe operating speed is reduced (Mohamed et al 2015; Watkins et al 2006, 2010; Spedding and Lissaman 1998; White et al 2012) as well as arguably the reliability and usefulness
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