Into turbulent air: size-dependent effects of von Kármán vortex streets on hummingbird flight kinematics and energetics

Abstract

Animal fliers frequently move through a variety of perturbed flows during their daily aerial routines. However, the extent to which these perturbations influence flight control and energetic expenditure is essentially unknown. Here, we evaluate the kinematic and metabolic consequences of flight within variably sized vortex shedding flows using five Anna's hummingbirds feeding from an artificial flower in steady control flow and within vortex wakes produced behind vertical cylinders. Tests were conducted at three horizontal airspeeds (3, 6 and 9 m s 21) and using three different wake-generating cylinders (with diameters equal to 38, 77 and 173% of birds' wing length). Only minimal effects on wing and body kinematics were demonstrated for flight behind the smallest cylinder, whereas flight behind the medium-sized cylinder resulted in significant increases in the variances of wingbeat frequency, and variances of body orientation, especially at higher airspeeds. Metabolic rate was, however, unchanged relative to that of unperturbed flight. Hummingbirds flying within the vortex street behind the largest cylinder exhibited highest increases in variances of wingbeat frequency, and of body roll, pitch and yaw amplitudes at all measured airspeeds. Impressively, metabolic rate under this last condition increased by up to 25% compared with control flights. Cylinder wakes sufficiently large to interact with both wings can thus strongly affect stability in flight, eliciting compensatory kinematic changes with a consequent increase in flight metabolic costs. Our findings suggest that vortical flows frequently encountered by aerial taxa in diverse environments may impose substantial energetic costs.

Hummingbird flight stability and control in freestream turbulent winds

“…The large pitch and yaw rotation rates of the tail in turbulent airflow almost certainly reflect a combination of passive interactions with the imposed airflow and active compensatory rotations produced by the hummingbirds to correct for perturbations. Consistent with this interpretation, tail pitch angle has previously been shown to be more variable during flight in vortex streets as well (Ortega-Jimenez et al, 2014).…”

“…to create freestream turbulence. The flow conditions generated here are fundamentally different from those used in previous experiments on flight in unsteady flows (Ortega-Jimenez et al, 2013, 2014Ravi et al, 2013), in which bumblebees, hawkmoths and hummingbirds were flown in the unsteady, structured flow present in the wake of a cylinder, where discrete alternating vortices are shed at a constant frequency. These structured wakes rapidly break down into the type of freestream turbulence generated in the present study, which consists of random variations in wind speed and direction that impose unpredictable perturbations at all frequencies and in all directions.…”

“…Previous studies have shown that hummingbirds display statistically significant but modest increases in flapping frequency to increase force production during hovering (∼4-10% increase in reduced air density or up to 19% with added loads; Chai and Dudley, 1995;Altshuler and Dudley, 2003) and while flying in the unsteady wake behind a cylinder (∼10% increase, OrtegaJimenez et al, 2014), but display no significant change in frequency with increased flight speeds in smooth flow (Tobalske et al, 2007). Hummingbirds flying in unsteady vortex streets also display increased variability in flapping frequency (Ortega-Jimenez et al, 2014), as in our study. When flying in turbulent flow, the hummingbirds also displayed a significant, but fairly modest (∼7%) increase in mean stroke amplitude, as well as greater stroke-to-stroke variability and bilateral asymmetry.…”

“…∼25% increase from 8 to 12 m s −1 ; Tobalske et al, 2007). When flying in the unsteady wake behind a cylinder, hummingbirds do not increase mean stroke amplitude, but variability and bilateral asymmetry in amplitude increase significantly (Ortega-Jimenez et al, 2014). Thus, our data show that hummingbirds flying in fully mixed, freestream turbulence display some of the same kinematic adjustments in stroke amplitude as those seen during flight in unsteady vortex streets (increased variability and bilateral asymmetry), as well as those seen when hummingbirds increase force production during hovering or forward flight (increased mean amplitude).…”

Moving in a moving medium: new perspectives on flight

“…The convention of treating the three components separately serves the biological community well, as u and w are linked to the mechanical power requirements of flight, with vertical motion affecting induced power costs, and wind causing power use to vary in relation to distance. Sustained turbulence or gusts can also influence flight costs when the turbulent components are of similar scales to the animals themselves, as kinematic or aerodynamic adjustments may be required to maintain flight stability [13,14]. Yet despite the ubiquity of turbulent features, our understanding of how turbulence affects flight performance remains limited.…”