Bumblebee Flight in Heavy Turbulence

Abstract

High-resolution numerical simulations of a tethered model bumblebee in forward flight are performed superimposing homogeneous isotropic turbulent fluctuations to the uniform inflow. Despite tremendous variation in turbulence intensity, between 17% and 99% with respect to the mean flow, we do not find significant changes in cycle-averaged aerodynamic forces, moments or flight power when averaged over realizations, compared to laminar inflow conditions. The variance of aerodynamic measures, however, significantly increases with increasing turbulence intensity, which may explain flight instabilities observed in freely flying bees.Insect flight currently receives considerable attention from both biologists and engineers. This growing interest is fostered by the recent trend in miniaturization of unmanned air vehicles that naturally incites reconsidering flapping flight as a bio-inspired alternative to fixed-wing and rotary flight. For all small flyers it is challenging to fly outdoors in an unsteady environment, and it is essential to know how insects face that challenge.Field studies show variations of insect behavior with changing weather conditions, including the atmospheric turbulence [1]. Earlier laboratory research on aerodynamics of insect flight assumed quiescent air, and only some more recent experiments focused on the effect of different kinds of unsteady flows. The behavior of orchid bees flying freely in a turbulent air jet has been studied in [2]. The authors found that turbulent flow conditions have a destabilizing effect on the body, most severe about the animal's roll axis. In response to this flow, bees try to compensate the induced moments by an extension of their hindlegs, increasing the roll moment of inertia. Interaction of bumblebees with wake turbulence has also been considered in [3]. These experiments were performed in a von Kármán-type wake behind cylinders. The bees displayed large rolling motions, pronounced lateral accelerations, and a reduction in their upstream flight speed. In [4] a comparative study on the sensitivity of honeybees and stalk-eye flies to localized wind gusts was performed. The study found that bees and stalkeye flies respond differently to aerial perturbations, either causing roll instabilities in bees or significant yaw rotations in stalk-eye flies. In [5] feeding flights of hawkmoths in vortex streets past vertical cylinders were analyzed. Depending on distance of the animal from the cylinder and cylinder size, destabilizing effects on yaw and roll and a reduction in the animal's maximum flight speed have been observed. Kinematic responses to large helical coherent structures were also found in hawkmoths flying in a vortex chamber [6]. A study on the energetic significance of kinematic changes in hummingbird feeding flights further demonstrated a substantial increase in metabolic rate during flight in turbulent flows, compared to flight in undisturbed laminar inflow [7,8]. All studies reported significant changes in the behavior of insects when they fly in turbulent...

Biomechanics and biomimetics in insect-inspired flight systems

“…( d ) Flexible wings aerodynamics in hovering hawkmoth: fluid–structure interaction model and near-field vortex dynamics [19]. ( e ) Vortex dynamics around a bumblebee flying in turbulence [34]. …”

“…Ravi et al [63] reported that bees are most sensitive to the lateral perturbations induced by a cylinder positioned vertically, displaying large rolling motions, pronounced lateral accelerations and a reduction in the upstream flight speed. Engels et al [34] conducted computational investigations (figure 3 e ) of a tethered-bumblebee model in isotropic turbulence and revealed no significant changes in the statistical averages of the aerodynamic forces, moments or aerodynamic power, compared with steady inflow conditions. The instantaneous aerodynamic forces, however, showed larger fluctuations, consistent with the flight instabilities observed in freely flying bees.…”

Foraging in an unsteady world: bumblebee flight performance in field-realistic turbulence

“…A single previous indoor wind tunnel study has shown that hummingbirds display flight instabilities in freestream turbulence at a relatively high flow speed (5 m s 21 ), and that birds alter several aspects of their wing kinematics in response [18]. In addition, recent computational work [21] has suggested that turbulence may have only minimal effects on the mean aerodynamic properties of flying insects, despite leading to increased fluctuations of instantaneous aerodynamic forces. It thus remains unclear whether turbulence has a significant impact on insect flight performance at environmentally relevant levels.…”

“…Previous work has shown that turbulence limits top flight speed and increases drag (and presumably associated energetic costs) in orchid bees [12]. A single previous indoor wind tunnel study has shown that hummingbirds display flight instabilities in freestream turbulence at a relatively high flow speed (5 m s 21 ), and that birds alter several aspects of their wing kinematics in response [18]. In addition, recent computational work [21] has suggested that turbulence may have only minimal effects on the mean aerodynamic properties of flying insects, despite leading to increased fluctuations of instantaneous aerodynamic forces.…”

Insect and insect-inspired aerodynamics: unsteadiness, structural mechanics and flight control

“…In the case of insects, although the aerodynamics of merely a few archetypal species have been thoroughly scrutinized (recent examples for: mosquitoes [10]; bees [11]; dragonflies [12]; hawkmoths [13–15]; fruit flies [16]; hover flies [17,18]; blow flies [19]; desert locusts [20,21]), it is reasonable to say that the key aerodynamic mechanisms have been now identified, including, in addition to the aforementioned LEV and clap-and-fling dynamics, other subtle mechanisms related to added-mass, rotational circulation or wake capture [2,6,10]. To understand the role of each of these mechanisms in any specific case, one must recall that flow separation and vortex dynamics are ruled by the relative importance of inertial versus viscous forces: a balance determined, in the language of fluid mechanics, by the Reynolds number ( Re = ρUL/μ , where ρ and μ are the air density and viscosity, respectively, and U and L represent characteristic velocity and length scales).…”