Gil Iosilevskii

Experimental and Numerical Study of Cyclogiro Aerodynamics

A combined experimental and numerical study of a cyclogiro rotor operating at Reynolds numbers about 40,000 has been conducted. The study reveals complex flow field with complex (unsteady) interactions between the blades and the wakes of other blades. In spite of this complexity, time-averaged integral forces acting on the rotor can be predicted by a simple momentum theory, properly corrected for thrust-producing area of the rotor and for large Magnus effect. The effective thrust producing area was estimated to be about half of its projected area, suggesting that the effectiveness of a cyclogiro rotor may be comparable with that of a heavy-loaded helicopter rotor.

AERODYNAMICS OF THE CYCLOGIRO

A combined numerical and experimental study of cyclogiro aerodynamics has been conducted. Experiments have been carried out on a small, 4.5 inch span cyclogiro rotor in 2, 4 and 6 blades conflguration. Numerical study has been carried out on a comparable rotor in 4-blades conflguration. In general, the numerical results are in a good agreement with experimental ones. Both suggest increasing propulsion losses with increasing number of blades. It is argued that these losses stem from unsteady aerodynamic interactions between blades and vortical wakes left by other blades.

Relations between morphology, buoyancy and energetics of requiem sharks

Sharks have a distinctive shape that remained practically unchanged through hundreds of millions of years of evolution. Nonetheless, there are variations of this shape that vary between and within species. We attempt to explain these variations by examining the partial derivatives of the cost of transport of a generic shark with respect to buoyancy, span and chord of its pectoral fins, length, girth and body temperature. Our analysis predicts an intricate relation between these parameters, suggesting that ectothermic species residing in cooler temperatures must either have longer pectoral fins and/or be more buoyant in order to maintain swimming performance. It also suggests that, in general, the buoyancy must increase with size, and therefore, there must be ontogenetic changes within a species, with individuals getting more buoyant as they grow. Pelagic species seem to have near optimally sized fins (which minimize the cost of transport), but the majority of reef sharks could have reduced the cost of transport by increasing the size of their fins. The fact that they do not implies negative selection, probably owing to decreased manoeuvrability in confined spaces (e.g. foraging on a reef).

Flow over NACA-0009 and Eppler-61 Airfoils at Reynolds Numbers 5000 and 60,000

The flow about NACA-0009 and Eppler-61 airfoils at (chord-based) Reynolds numbers between 5000 and 60,000 is studied using flow visualizations and incompressible, time-accurate, two-dimensional, laminar Navier-Stokes simulations. Good agreement is found between experimental and numerically simulated flow patterns, as long as the flow remained nonturbulent. The study outlines three phases of angle-of-attack-dependent transition from fully laminar to partly turbulent flow around the airfoils, which are universally applicable at these Reynolds numbers. The first phase is characterized by a laminar wake that appears stable for at least a few chords behind the airfoil; in the second phase, the wake reforms into a well-ordered vortex street; in the third phase, this street becomes unstable and a separation bubble is formed on the suction side of the airfoil. The pattern of the flow changes phases as the angle of attack increases; the exact angles at which the change occurs are airfoil- and Reynolds-number-dependent.

Forward flight of birds revisited. Part 1: aerodynamics and performance

This paper is the first part of the two-part exposition, addressing performance and dynamic stability of birds. The aerodynamic model underlying the entire study is presented in this part. It exploits the simplicity of the lifting line approximation to furnish the forces and moments acting on a single wing in closed analytical forms. The accuracy of the model is corroborated by comparison with numerical simulations based on the vortex lattice method. Performance is studied both in tethered (as on a sting in a wind tunnel) and in free flights. Wing twist is identified as the main parameter affecting the flight performance—at high speeds, it improves efficiency, the rate of climb and the maximal level speed; at low speeds, it allows flying slower. It is demonstrated that, under most circumstances, the difference in performance between tethered and free flights is small.

On the Onset of Transition at Low Reynolds Number Flow Over Airfoils

The flow about NACA-0009 and Eppler-61 airfoils at Reynolds numbers between 5,000 and 60,000 is studied using a combination of water-tunnel visualizations and incompressible, time-accurate laminar Navier-Stokes simulations. Water-tunnel tests seem to justify the use of laminar Navier-Stokes simulations for the cases considered and elucidate the laminar-toturbulent transition. Both the water-tunnel tests and the Navier-Stokes simulations outline three-phase transition from fully laminar to partly turbulent flow around the airfoils at Reynolds numbers between 20,000 and 60,000. At low angles of attack, the vorticity layers created at the suction and pressure surfaces of an airfoil are stable in the vicinity of the airfoil (phase 1). As the angle-of-attack is increased, these layers re-form into a stable vortex street (phase 2). At yet higher angles this street becomes unstable, and the separation bubble at the suction side of the airfoil reattaches (phase 3).

Locomotion of neutrally buoyant fish with flexible caudal fin

Historically, burst-and-coast locomotion strategies have been given two very different explanations. The first one was based on the assumption that the drag of an actively swimming fish is greater than the drag of the same fish in motionless glide. Fish reduce the cost of locomotion by swimming actively during a part of the swimming interval, and gliding through the remaining part. The second one was based on the assumption that muscles perform efficiently only if their contraction rate exceeds a certain threshold. Fish reduce the cost of locomotion by using an efficient contraction rate during a part of the swimming interval, and gliding through the remaining part. In this paper, we suggest yet a third explanation. It is based on the assumption that propulsion efficiency of a swimmer can increase with thrust. Fish reduce the cost of locomotion by alternating high thrust, and hence more efficient, bursts with passive glides. The paper presents a formal analysis of the respective burst-and-coast strategy, shows that the locomotion efficiency can be practically as high as the propulsion efficiency during burst, and shows that the other two explanations can be considered particular cases of the present one.

Forward flight of birds revisited. Part 2: short-term dynamic stability and trim.

Thrust generation by flapping is accompanied by alternating pitching moment. On the down-stroke, it pitches the bird down when the wings are above its centre of gravity and up when they are below; on the up-stroke, the directions reverse. Because the thrust depends not only on the flapping characteristics but also on the angle of attack of the birds body, interaction between the flapping and body motions may incite a resonance that is similar to the one that causes the swinging of a swing. In fact, it is shown that the equation governing the motion of the birds body in flapping flight resembles the equation governing the motion of a pendulum with periodically changing length. Large flapping amplitude, low flapping frequency, and excessive tilt of the flapping plane may incite the resonance; coordinated fore–aft motion, that uses the lift to cancel out the moment generated by the thrust, suppresses it. It is probably incited by the tumbler pigeon in its remarkable display of aerobatics. The fore–aft motion that cancels the pitching moment makes the wing tip draw a figure of eight relative to the birds body when the wings are un-swept, and a ring when the wings are swept back and fold during the upstroke.

Latent power of basking sharks revealed by exceptional breaching events

The fast swimming and associated breaching behaviour of endothermic mackerel sharks is well suited to the capture of agile prey. In contrast, the observed but rarely documented breaching capability of basking sharks is incongruous to their famously languid lifestyle as filter-feeding planktivores. Indeed, by analysing video footage and an animal-instrumented data logger, we found that basking sharks exhibit the same vertical velocity (approx. 5 m s−1) during breach events as the famously powerful predatory great white shark. We estimate that an 8-m, 2700-kg basking shark, recorded breaching at 5 m s−1 and accelerating at 0.4 m s−2, expended mechanical energy at a rate of 5.5 W kg−1; a mass-specific energetic cost comparable to that of the great white shark. The energy cost of such a breach is equivalent to around 1/17th of the daily standard metabolic cost for a basking shark, while the ratio is about half this for a great white shark. While breaches by basking sharks must serve a different function to white shark breaches, their similar breaching speeds questions our perception of the physiology of large filter-feeding fish.

Supplementary material from "Relations between morphology, buoyancy and energetics of requiem sharks"

Sharks have a distinctive shape that remained practically unchanged through hundreds of millions of years of evolution. Nonetheless, there are variations of this shape that vary between and within species. We attempt to explain these variations by examining the partial derivatives of the cost of transport of a generic shark with respect to buoyancy, span and chord of its pectoral fins, length, girth and body temperature. Our analysis predicts an intricate relation between these parameters, suggesting that ectothermic species residing in cooler temperatures must either have longer pectoral fins and/or be more buoyant in order to maintain swimming performance. It also suggests that, in general, the buoyancy must increase with size, and therefore, there must be ontogenetic changes within a species, with individuals getting more buoyant as they grow. Pelagic species seem to have near optimally sized fins (which minimize the cost of transport), but the majority of reef sharks could have reduced the cost of transport by increasing the size of their fins. The fact that they do not implies negative selection, probably owing to decreased manoeuvrability in confined spaces (e.g. foraging on a reef).