Study of Mosquito Aerodynamics for Imitation as a Small Robot and Flight in a Low-Density Environment

In terms of their flight and unusual aerodynamic characteristics, mosquitoes have become a new insect of interest. Despite transmitting the most significant infectious diseases globally, mosquitoes are still among the great flyers. Depending on their size, they typically beat at a high flapping frequency in the range of 600 to 800 Hz. Flapping also lets them conceal their presence, flirt, and help them remain aloft. Their long, slender wings navigate between the most anterior and posterior wing positions through a stroke amplitude about 40 to 45°, way different from their natural counterparts (>120°). Most insects use leading-edge vortex for lift, but mosquitoes have additional aerodynamic characteristics: rotational drag, wake capture reinforcement of the trailing-edge vortex, and added mass effect. A comprehensive look at the use of these three mechanisms needs to be undertaken—the pros and cons of high-frequency, low-stroke angles, operating far beyond the normal kinematic boundary compared to other insects, and the impact on the design improvements of miniature drones and for flight in low-density atmospheres such as Mars. This paper systematically reviews these unique unsteady aerodynamic characteristics of mosquito flight, responding to the potential questions from some of these discoveries as per the existing literature. This paper also reviews state-of-the-art insect-inspired robots that are close in design to mosquitoes. The findings suggest that mosquito-based small robots can be an excellent choice for flight in a low-density environment such as Mars.

Hybrid metachronal rowing augments swimming speed and acceleration via increased stroke amplitude

Numerous aquatic invertebrates use drag-based metachronal rowing for swimming, in which closely spaced appendages are oscillated starting from the posterior, with each appendage phase-shifted in time relative to its neighbor. Continuously swimming species such as Antarctic krill generally use “pure metachronal rowing” consisting of a metachronal power stroke and a metachronal recovery stroke, while burst swimming species such as many copepods and mantis shrimp typically use “hybrid metachronal rowing” consisting of a metachronal power stroke followed by a synchronous or nearly synchronous recovery stroke. Burst swimming organisms need to rapidly accelerate in order to capture prey and/or escape predation, and it is unknown whether hybrid metachronal rowing can augment acceleration and swimming speed compared to pure metachronal rowing. Simulations of rigid paddles undergoing simple harmonic motion showed that collisions between adjacent paddles restrict the maximum stroke amplitude for pure metachronal rowing. Hybrid metachronal rowing similar to that observed in mantis shrimp (Neogonodactylus bredini) permits oscillation at larger stroke amplitude while avoiding these collisions. We comparatively examined swimming speed, acceleration, and wake structure of pure and hybrid metachronal rowing strategies by using a self-propelling robot. Both swimming speed and peak acceleration of the robot increased with increasing stroke amplitude. Hybrid metachronal rowing permitted operation at larger stroke amplitude without collision of adjacent paddles on the robot, augmenting swimming speed and peak acceleration. Hybrid metachronal rowing generated a dispersed wake unlike narrower, downward-angled jets generated by pure metachronal rowing. Our findings suggest that burst swimming animals with small appendage spacing, such as copepods and mantis shrimp, can use hybrid metachronal rowing to generate large accelerations via increasing stroke amplitude without concern of appendage collision.

Acute Comparative Effect of Foam Rolling and Static Stretching on Range of Motion in Rowers

Rowers’ anthropometric characteristics and flexibility are fundamental to increase stroke amplitude and optimize power transfer. The aim of the present study was to analyze the effect of foam rolling and static stretching on the range of motion over time. Eight university rowers (24.8 ± 3.4 yrs., height 182.3 ± 6.5 cm, body mass 79.3 ± 4.6 kg) participated in an alternating treatment design study with two-way repeated measures ANOVA. The sit and reach test was used to measure the range of motion. Both in the foam rolling and in the static stretching method, a pre-test (T0), a post-test (T1), and a post-15-min test (T2) were performed. A significant effect was observed on the range of motion over time (p < 0.001), but not for time x method interaction (p = 0.680). Significant differences were found between T0 and T1 with foam rolling and static stretching (p < 0.001, d = 0.4); p < 0.001, d = 0.6). The differences between T0 and T2 were also significant with both methods (p = 0.001, d = 0.4; p < 0.001, d = 0.4). However, no significant difference was observed between T1 and T2 (p = 1.000, d = 0.1; p = 0.089, d = 0.2). Foam roller and static stretching seem to be effective methods to improve the range of motion but there seems to be no differences between them.

Closer appendage spacing augments metachronal swimming speed by promoting tip vortex interactions

Numerous species of aquatic invertebrates, including crustaceans, swim by oscillating multiple closely spaced appendages. The coordinated, out-of-phase motion of these appendages, known as "metachronal paddling", has been well-established to improve swimming performance relative to synchronous paddling. Invertebrates employing this propulsion strategy cover a wide range of body sizes and shapes, but the ratio of appendage spacing (G) to the appendage length (L) has been reported to lie in a comparatively narrow range of 0.2 < G/L ≤ 0.65. The functional role of G/L on metachronal swimming performance is unknown. We hypothesized that for a given Reynolds number and stroke amplitude, hydrodynamic interactions promoted by metachronal stroke kinematics with small G/L can increase forward swimming speed. We used a dynamically scaled self-propelling robot to comparatively examine swimming performance and wake development of metachronal and synchronous paddling under varying G/L, phase lag, and stroke amplitude. G/L was varied from 0.4 to 1.5, with the expectation that when G/L is large, there should be no performance difference between metachronal and synchronous paddling due to a lack of interaction between vortices that form on the appendages. Metachronal stroking at non-zero phase lag with G/L in the biological range produced faster swimming speeds than synchronous stroking. As G/L increased and as stroke amplitude decreased, the influence of phase lag on the swimming speed of the robot was reduced. For smaller G/L, vortex interactions between adjacent appendages generated a horizontally-oriented wake and increased momentum fluxes relative to larger G/L, which contributed to increasing swimming speed. We find that while metachronal motion augments swimming performance for closely spaced appendages (G/L < 1), moderately spaced appendages (1.0 ≤ G/L ≤ 1.5) can benefit from metachronal motion only when the stroke amplitude is large.

Parametric study of a corrugated airfoil in a forward flight at ultra-low Reynolds number

In the current study, the mid cross section of the dragonfly forewing was simulated at ultra-low Reynolds number. The study aims to understand better the contribution of corrugations found along the wing on the aerodynamic performance during a forward flight. Different flapping parameters were employed. FLUENT solver was used to solve unsteady, two-dimensional, laminar, incompressible Navier–Stokes equations. The results revealed that any stroke amplitude less than 1cm generated no thrust force. The stroke amplitude had to be increased to form the reversed Kármán vortices responsible for generating thrust force. The highest propulsive efficiency was found in the Strouhal number range 0.2 < St < 0.4 with a peak efficiency of 57% at St = 0.39. Changing the phase difference between pitching and plunging motions from advanced to synchronized caused lift force to drop 91% and thrust force to increase by 15%. On the other hand, changing the phase difference from synchronized to delayed caused lift and thrust forces to increase by 89% and 36%, respectively, and propulsive efficiency to deteriorate significantly. In all performed simulations, the airfoil was assumed to start motion from rest with no initial angle of attack. The increase in initial angle of attack generates a very high lift force with a fair loss for both thrust force and propulsive efficiency. The decomposition of flapping motion into its elementary motions revealed that the aerodynamic forces generated are a non-linear superposition from both pure pitching and pure plunging aerodynamic forces. This can be attributed to the non-linear interaction between unsteady vortices generated from these decomposed motions.

Flight muscle power increases with strain amplitude and decreases with cycle frequency in zebra finches (Taeniopygia guttata)

ABSTRACTBirds that use high flapping frequencies can modulate aerodynamic force by varying wing velocity, which is primarily a function of stroke amplitude and wingbeat frequency. Previous measurements from zebra finches (Taeniopygia guttata) flying across a range of speeds in a wind tunnel demonstrate that although the birds modulated both wingbeat kinematic parameters, they exhibited greater changes in stroke amplitude. These two kinematic parameters contribute equally to aerodynamic force, so the preference for modulating amplitude over frequency may instead derive from limitations of muscle physiology at high frequency. We tested this hypothesis by developing a novel in situ work loop approach to measure muscle force and power output from the whole pectoralis major of zebra finches. This method allowed for multiple measurements over several hours without significant degradation in muscle power. We explored the parameter space of stimulus, strain amplitude and cycle frequencies measured previously from zebra finches, which revealed overall high net power output of the muscle, despite substantial levels of counter-productive power during muscle lengthening. We directly compared how changes to muscle shortening velocity via strain amplitude and cycle frequency affected muscle power. Increases in strain amplitude led to increased power output during shortening with little to no change in power output during lengthening. In contrast, increases in cycle frequency did not lead to increased power during shortening but instead increased counter-productive power during lengthening. These results demonstrate why at high wingbeat frequency, increasing wing stroke amplitude could be a more effective mechanism to cope with increased aerodynamic demands.

Kinematics Measurement and Power Requirements of Fruitflies at Various Flight Speeds

Energy expenditure is a critical characteristic in evaluating the flight performance of flying insects. To investigate how the energy cost of small-sized insects varies with flight speed, we measured the detailed wing and body kinematics in the full speed range of fruitflies and computed the aerodynamic forces and power requirements of the flies. As flight speed increases, the body angle decreases and the stroke plane angle increases; the wingbeat frequency only changes slightly; the geometrical angle of attack in the middle upstroke increases; the stroke amplitude first decreases and then increases. The mechanical power of the fruitflies at all flight speeds is dominated by aerodynamic power (inertial power is very small), and the magnitude of aerodynamic power in upstroke increases significantly at high flight speeds due to the increase of the drag and the flapping velocity of the wing. The specific power (power required for flight divided by insect weigh) changes little when the advance ratio is below about 0.45 and afterwards increases sharply. That is, the specific power varies with flight speed according to a J-shaped curve, unlike those of aircrafts, birds and large-sized insects which vary with flight speed according to a U-shaped curve.

Natural barriers: waterfall transit by small flying animals

Waterfalls are conspicuous geomorphological features with heterogeneous structure, complex dynamics and multiphase flows. Swifts, dippers and starlings are well-known to nest behind waterfalls, and have been reported to fly through them. For smaller fliers, by contrast, waterfalls seem to represent impenetrable barriers, but associated physical constraints and the kinematic responses of volant animals during transit are unknown. Here, we describe the flight behaviour of hummingbirds (the sister group to the swifts) and of various insect taxa as they fly through an artificial sheet waterfall. We additionally launched plastic balls at different speeds at the waterfall so as to assess the inertial dependence of sheet penetration. Hummingbirds were able to penetrate the waterfall with reductions in both their translational speed, and stroke amplitude. The body tilted more vertically and exhibited greater rotations in roll, pitch and yaw, along with increases in tail spread and pitch. The much smaller plastic balls and some flies moving at speeds greater than 2.3 m s −1 and 1.6 m s −1 , respectively, also overcame effects of surface tension and water momentum and passed through the waterfall; objects with lower momentum, by contrast, entered the sheet but then fell along with the moving water. Waterfalls can thus represent impenetrable physical barriers for small and slow animal fliers, and may also serve to exclude both predators and parasites from nests of some avian taxa.

Kinematic flexibility allows bumblebees to increase energetic efficiency when carrying heavy loads

Foraging bees fly with heavy loads of nectar and pollen, incurring energetic costs that are typically assumed to depend on load size. Insects can produce more force by increasing stroke amplitude and/or flapping frequency, but the kinematic response of a given species is thought to be consistent. We examined bumblebees (Bombus impatiens) carrying both light and heavy loads and found that stroke amplitude increased in proportion to load size, but did not predict metabolic rate. Rather, metabolic rate was strongly tied to frequency, which was determined not by load size but by the bee’s average loading state and loading history, with heavily loaded bees displaying smaller changes in frequency and smaller increases in metabolic rate to support additional loading. This implies that bees can increase force production through alternative mechanisms; yet, they often choose the energetically costly option of elevating frequency, suggesting associated performance benefits that merit further investigation.

Living in a trash can: turbulent convective flows impair Drosophila flight performance

Turbulent flows associated with thermal convection are common in areas where the ground is heated by solar radiation, fermentation or other processes. However, it is unknown how these flow instabilities affect the locomotion of small insects, like fruit flies, that inhabit deserts and urban landscapes where surface temperatures can reach extreme values. We quantified flight performance of fruit flies ( Drosophila melanogaster ) traversing a chamber through still air and turbulent Rayleigh–Bénard convection cells produced by a vertical temperature gradient. A total of 34% of individuals were unable to reach the end of the chamber in convection, although peak flow speeds were modest relative to typical outdoor airflow. Individuals that were successful in convection were faster fliers and had larger wing areas than those that failed. All flies displayed higher pitch angles and lower mean flight speeds in convection. Successful individuals took longer to cross the chamber in convection, due to lower flight speeds and greater path sinuosity. All individuals displayed higher flapping frequencies in convection, and successful individuals also reduced stroke amplitude. Our results suggest that thermal convection poses a significant challenge for small fliers, resulting in increased travel times and energetic costs, or in some cases precluding insects from traversing these environments entirely.