Pitch-up Motion Research Articles

Development and interaction of vortices over a very low aspect-ratio wing under pitch-up motion

The vortical structures over a thin rectangular wing with a very low aspect ratio of 0.277 were investigated in a wind tunnel at an effective Reynolds number of $3 \times 10^6$ . When applying pitch-up motion pivoted at mid-chord, the maximum lift angle was increased with an increase in the pitch rate, but the maximum lift coefficient was reduced. The pitching motion also caused delay in the vortical development over the wing, which was increased with an increase in the pitch rate. The delay in the leading-edge vortex development due to the pitching motion was nearly identical to that in the tip vortex development, indicating that the dynamics of the leading-edge vortex was strongly influenced by the tip vortex. This was confirmed by particle image velocimetry measurements, which demonstrated that the tip vortex over a very low aspect-ratio wing induced strong downwash to influence the development of the leading-edge vortex during the pitching motion, which led to a delay in flow separation.

Pitch-Up Motion Mechanism With Heat Welding by Soft Inflatable Growing Robot

Soft growing robots have recently attracted considerable interest. They are expected to operate in complicated environments leveraging their flexibility and adaptability. However, some of them are unable to maintain their bending shape without contact with an object, and some of their mechanisms are complex. In this article, we propose a novel heat welding mechanism for pitch-up motion without any support. The proposed robot uses a gusset folded tube and welds its gusset parts. The welded tube is bent, and the robot performs a pitch-up motion. In addition, we experimentally investigate the characteristics of welded bending tubes and develop a stiffness model and bending angle model. The results demonstrate that they depend on the tube diameter, weld shape, and inner pressure. We conducted experiments to evaluate the performance of the robot and confirmed that the maximum bending moment based on which the robot can maintain its bending shape increases under a higher inner pressure, and a large bending angle is obtained by continuous bending. Further, we confirmed that proposed models can adequately simulate the pitch-up motion of the robot. The proposed mechanism enables the robot to perform novel pitch-up motion and expands the application of growing robots.

The reverse flight of a monarch butterfly (Danaus plexippus) is characterized by a weight-supporting upstroke and postural changes.

Butterflies are agile fliers which use inactive and active upstrokes (US). The active US plays a secondary role to the downstroke (DS), generating both thrust and negative vertical force. However, whether their active halfstroke function is fixed or facultative has not been clarified. We showed that during multiple backward flights of an individual, postural adjustments via body angles greater than 90°, with pitch-down and pitch-up motions in the DS and US, respectively, reoriented the stroke plane and caused the reversal of the aerodynamic functions of the halfstrokes compared with forward flight. The US and DS primarily provided weight support and horizontal force, respectively, and a leading edge vortex (LEV) was formed in both halfstrokes. The US's LEV was a Class II LEV extending from wingtip to wingtip, previously reported albeit during the DS in forward flight. The US's net force contribution increased from 32% in forward to 60% in backward flight. Likewise, US weight support increased from 8 to 85%. Despite different trajectories, body postures and force orientations among flight sequences in the global frame, the halfstroke-average forces pointed in a uniform direction relative to the body in both forward and backward flight.

Influence of pitch rate on freely translating perching airfoils

We numerically investigated the unsteady dynamics of a two-dimensional airfoil undergoing a continuous, prescribed pitch-up motion and freely translating as a response to aerodynamic forces and the gravity field. The pitch-up motion was applied about an axis located $1/6$ chord away from the leading edge and was parameterized using the shape change number, with a Reynolds number set to 2000. It was shown that the minimum kinetic energy reached by the airfoil depends stochastically and asymptotically on shape change numbers for values below and above 1, respectively. Very low kinetic energy levels (close to zero) can be reached in both stochastic and asymptotic regions but high shape change numbers are accompanied by significant gain in altitude which may be undesirable from a practical perspective. Rather, shape change numbers in the range [0.1–0.3] allow us to reach relatively low levels of kinetic energy for close perching locations. We showed that highly nonlinear fluid–structure interactions induced by massive flow separations and strong vortices are conducive to low kinetic energy, but responsible for the stochastic dependence of kinetic energy to shape change number, which can make perching manoeuvres hardly controllable for flying vehicles.

On the dynamics of perching manoeuvres with low-aspect-ratio planforms

The dynamics of a simple perching manoeuvre are investigated using circular and aspect-ratio-two elliptical flat plates, as abstractions of low-aspect-ratio planforms observed in highly-manoeuvrable birds. The perching kinematic investigated in this study involves a pitch-up motion from an angle of attack of to , while simultaneously decelerating. This motion is defined by the shape change number, , which acts as a measure of the relative contributions of added-mass and circulatory effects. This motion has been observed in natural flyers during controlled landings, and has recently been explored through the use of a nominally two-dimensional airfoil. The parameter space of low-aspect-ratio plates therefore serves to elucidate how realistic free-end conditions affect the timescales of vortex evolution, and therefore the relative contributions between added mass and circulation. The results presented herein suggest that for the low-aspect-ratio plates, the shedding of vortices occurs more rapidly than for equivalent two-dimensional cases, and therefore faster pitching motions are required to compensate for the lower levels of lift and drag. Furthermore, the vortex topology and instantaneous forces that arise during the rapid-area changes show no sensitivity to aspect ratio, and strong collapse is observed between both flat plates. Similar aerodynamic advantages may therefore be exploited during perching manoeuvres by birds of various scale regardless of wing aspect ratio.

Three-Dimensionality Effects due to Change in the Aspect Ratio for the Flow around an Impulsively Pitching Flat Plate

AbstractThree-dimensionality effects have been investigated for a flat plate undergoing an impulsive pitch-up motion in a large-scale, free-surface water channel with the presence and absence of an...

Dynamic- and post-stall characteristics of pitching airfoils at extreme conditions

Post-stall flow structure and surface pressures are evaluated to determine the effects of large angles of attack, perching like manoeuvres on the flow about a NACA 0021 airfoil exposed to dynamic stall. Phase-averaged particle image velocimetry was performed to assess the load development during the constant angular velocity pitch-up motion and in post-stall conditions. Evaluation of the resultant aerodynamic loads indicates that initial airfoil rotation generates significant delays in force response. Furthermore, the reduced frequency is shown to influence the angle of attack at which deep stall is initiated, to the extent that fully separated flows are delayed to an angle of attack of 60°. Vortex structures are linked to lower surface pressures with increased angle of attack and also for post-stall flow conditions. Likewise, the presence of the vortex structures shifts the centre of pressure significantly along the airfoil chordline immediately after cessation of the airfoil rotation. At the maximum angle of attack, the centre of pressure is shown to move aft for fully separated flow conditions. The variation in location of the centre of pressure, not only changes the moment generation and aero-elastic characteristics of the airfoil, but also increases structural torsional loading and fluctuations that result in increased fatigue of helicopter rotor shafts and horizontal-axis wind turbines.

Unsteady flow structure and loading of a pitching low-aspect-ratio wing

High-fidelity simulations examine dynamic stall over a pitching aspect-ratio-two wing. Dynamic stall leads to an arch-type vortex which can sustain high lift. The arch vortex persists after pitch-up motion ceases. This results from the self-induced velocity of the combined vortex/image system.

Dynamic stall control on flapping wing airfoils

Flapping wing efficiency is limited by flow separation effects. The time dependent development of a leading edge vortex (LEV) during rapid pitch-up motion of a retreating helicopter rotor blade is known as dynamic stall vortex. Movement of this vortex along the airfoil upper surface first increases lift but later the vortex lifts off the airfoil surface causing strong drag rise, severe nose-down pitching moments, and possibly negative aerodynamic damping. Very similar effects can be observed on flapping airfoils and wings experiencing combined plunging (heaving) motion and pitching motion. With increasing plunge amplitude the flow on the flapping wing starts to separate and concentrated dynamic stall vortices may develop on both upper and lower wing surfaces. Under these conditions it is shown that wing propulsion efficiency is considerably reduced. Recent investigations of dynamic stall control have shown that a strong vortex may be avoided by appropriate airfoil deformation. It will be shown in the present paper that with dynamic airfoil deformation the propulsion efficiency can be improved considerably. The validity of the numerical calculations is first tested against existing data from literature.

Leading Edge Vortex Circulation Development on Finite Aspect Ratio Pitch-Up Wings

We study the leading edge vortex (LEV) development and LEV circulation of pitch-up wings at a low Reynolds number of 500. Wings of different aspect ratios are linearly pitched up from 0 to 90 deg at two reduced pitch rates of 0.1 and 0.2. The flowfield is described by solving the unsteady three-dimensional incompressible Navier–Stokes equations on composite overlapping grids. The -criterion is used to isolate the LEV structure from shear layer. The simulation shows that forces increase with the aspect ratio and the pitch rate. The stall angle increases with the pitch rate but decreases with the aspect ratio. also It is shown that the LEV circulation depends on the wing aspect ratio: increasing wing aspect ratio increases the rate of LEV circulation generation during the pitch-up motion. It also shows that the reduced pitch rate could delay the LEV circulation development. Furthermore, the study shows that the LEV circulation distribution better matches the spanwise force distribution for higher aspect ratio wings.

MEMS flexible thermal flow sensors for measurement of unsteady flow above a pitching wind turbine blade

MEMS (Micro-Electro-Mechanical System) thermal flow sensors have been applied widely in boundary-layer studies and aerodynamic flow sensing due to high spatial resolutions and fast response times as well as minimal interference with fluid flow. In this study, self-made MEMS thermal flow sensors were designed and fabricated on a flexible skin. The steady laminar separation was investigated on two-dimensional LS (1) 0417 airfoil by using thermal flow sensors at various angles of attack with validation by hot wires and flow visualization. The unsteady flow on the pitching airfoil was experimentally investigated to simulate the dynamic stall condition of VAWT (Vertical Axis Wind Turbine). Based on variations of the mean value and standard deviation of the thermal flow sensor signals, nine stages of unsteady flow developing events are identified with further evidence from flow visualization. As the reduced frequency (k) increases, the incipient transition is delayed to higher angles of attack during the pitch-up motion; the re-laminarization is postponed to lower angles of attack during the pitch-down motion. The hysteresis is more pronounced at higher k where the oscillating time scale plays a more significant role in determining the unsteady flow pattern than the convective time scale. The phase difference between transition and re-laminarization was enlarged from Δα≈4.9° for k=0.009 to Δα≈13.5° for k=0.027 at Re=6.3×104.

An optimal trajectory design for the lunar vertical landing

A research for designing the optimal lunar vertical landing trajectory to reduce the total energy or mass of propellant is addressed in this paper. Most of these problems can be divided into two phases: breaking and approach phase. The optimal landing trajectory in general does not consider the pitch-up motion so that the landing problem has been only solved in the breaking phase. For this reason, there are some attempts to find the optimal trajectory including the final vertical landing phase by including the equations of angular motion of the vehicle. However, the optimal solution using this approach depends on the scale factor of a cost function because the cost function consists of two different mechanical parameters such as the final mass and total control torque. The final control constraints are augmented for vertical lunar landing instead of the equations of angular motion. The obtained optimal trajectory has an additional positive effect of the image acquisition as well as the final vertical landing.

Validated Unsteady Computational Fluid Dynamic Analysis of an Oscillating Bio-Inspired Airfoil

An unsteady, two-dimensional numerical study was conducted to investigate the aerodynamic and flow characteristics of a bio-inspired corrugated airfoil oscillating at 2Hz with an amplitude of 10°. The upstream flow was set such that the chord Re = 14,000. The computational results were validated against experimental results from a 2D particle image velocimetry (PIV) experiment on the same airfoil geometry. Complex flow structures such as the formation and shedding of trailing edge vortices have been revealed to have significant impacts on the lift and drag characteristics of the airfoil in oscillating motion. The shed vortices provide a low pressure region on the top surface of the airfoil throughout the period of oscillation, thus increasing lift of the airfoil. In particular, vortices formed and shed from the rear-most corrugation appear to have the largest effect. The pitch-down motion produces a lower absolute peak lift as compared the pitch-up motion which may be explained by the disruption of the high pressure zone on the top surface of the airfoil by a vortex forming in the corrugations. This results in a relatively lower high pressure region on the advancing side as compared to the pitch-up motion. In addition to the lift calculations, drag calculations indicate that net thrust is being produced during the oscillations and more thrust is produced on the pitch-up than the pitch-down motion.

Inversion of a Capsule Impacting Water: Flip by Resurge Jet

A scale-model blunt-cone capsule intended for ocean splashdown was projected into a water pool to evaluate impact loads and postimpact behavior. In a small region of the speed/angle parameter space, the capsule would reproducibly capsize, flipping forward (pitch-down), despite a pitch-up motion induced at impact. Inspection of high-speed video shows that the resurging central jet of the impact cavity is responsible. Capsize occurs when this jet is energetic enough (for which we develop a simple criterion), and is timed to lift the trailing edge of the vehicle. The same phenomenon was observed on the Apollo capsules, and may be relevant for lifeboat deployment from ships and offshore platforms.

Vortex dynamics during blade-vortex interactions

Vortex dynamics during parallel blade-vortex interactions (BVIs) were investigated in a subsonic wind tunnel using particle image velocimetry (PIV). Vortices were generated by applying a rapid pitch-up motion to an airfoil through a pneumatic system, and the subsequent interactions with a downstream, unloaded target airfoil were studied. The blade-vortex interactions may be classified into three categories in terms of vortex behavior: close interaction, very close interaction, and collision. For each type of interaction, the vortex trajectory and strength variation were obtained from phase-averaged PIV data. The PIV results revealed the mechanisms of vortex decay and the effects of several key parameters on vortex dynamics, including separation distance (h/c), Reynolds number, and vortex sense. Generally, BVI has two main stages: interaction between vortex and leading edge (vortex-LE interaction) and interaction between vortex and boundary layer (vortex-BL interaction). Vortex-LE interaction, with its small separation distance, is dominated by inviscid decay of vortex strength due to pressure gradients near the leading edge. Therefore, the decay rate is determined by separation distance and vortex strength, but it is relatively insensitive to Reynolds number. Vortex-LE interaction will become a viscous-type interaction if there is enough separation distance. Vortex-BL interaction is inherently dominated by viscous effects, so the decay rate is dependent on Reynolds number. Vortex sense also has great impact on vortex-BL interaction because it changes the velocity field and shear stress near the surface.

Leading edge vortex development on a pitch-up airfoil

The development of the lift generated by a leading edge vortex (LEV) across a flat plate experiencing a pitch-up motion is investigated to understand the LEV's influence on lift generation. The flo...

Flow structure on a simultaneously pitching and rotating wing

AbstractA technique of particle image velocimetry is employed to characterize the three-dimensional flow structure on a wing subjected to simultaneous pitch-up and rotational motions. Distinctive vortical structures arise, relative to the well-known patterns on a wing undergoing either pure pitch-up or pure rotation. The features associated with these simultaneous motions include: stabilization of the large-scale vortex generated at the leading edge, which, for pure pitch-up motion, rapidly departs from the leading-edge region; preservation of the coherent vortex system involving both the tip vortex and the leading-edge vortex (LEV), which is severely degraded for pure rotational motion; and rapid relaxation of the flow structure upon termination of the pitch-up component, whereby the relaxed flow converges to a similar state irrespective of the pitch rate. Three-dimensional surfaces of iso-$\def \xmlpi #1{}\def \mathsfbi #1{\boldsymbol {\mathsf {#1}}}\let \le =\leqslant \let \leq =\leqslant \let \ge =\geqslant \let \geq =\geqslant \def \Pr {\mathit {Pr}}\def \Fr {\mathit {Fr}}\def \Rey {\mathit {Re}}{Q}$and helicity are employed in conjunction with sectional representations of spanwise vorticity, velocity and vorticity flux to interpret the flow physics.

Lattice Boltzmann simulations of a pitch-up and pitch-down maneuver of a chord-wise flexible wing in a free stream flow

A rapid pitch-up and pitch-down maneuver of a chord-wise flexible wing in a steady free stream is studied by using a lattice Boltzmann flexible particle method in a three-dimensional space at a chord based Reynolds number of 100. The pitching rates, flexibility, and wing density are systematically varied, and their effects on aerodynamic forces are investigated. It is demonstrated that the flexibility can be utilized to significantly improve lift forces. The flexible wing has a larger angular momentum due to elasticity and inertia and generates a larger leading edge vortex as compared with a rigid wing. Such lift enhancement occurs mainly during the pitch-down motion while a large stall angle is produced during the pitch-up motion. At a low pitch rate, the flexibility cannot improve lift.

Numerical study of the influence of the Reynolds-number on the lift created by a leading edge vortex

We present a numerical study on the influence of the Reynolds-number on the lift enhancing effect of a leading edge vortex. Our approach is based on a combination of large-eddy simulations and the immersed boundary technique. We determine the influence of the leading edge vortex on the unsteady lift by simulating a fast pitch-up motion of the plate and studying the lift evolution after holding the flat plate fixed at an angle of attack. Our results suggest that an optimal Reynolds-number exists that maximizes the lift of the leading edge vortex, but that the lift-to-drag ratio is largely independent of the Reynolds-number above a Reynolds-number of Rec > 2000.

Flow structure on finite-span wings due to pitch-up motion

AbstractThe flow structure on low-aspect-ratio wings arising from pitch-up motion is addressed via a technique of particle image velocimetry. The objectives are to: determine the onset and evolution of the three-dimensional leading-edge vortex; provide complementary interpretations of the vortex structure in terms of streamlines, projections of spanwise and surface-normal vorticity, and surfaces of constant values of the second invariant of the velocity gradient tensor (iso-$Q$ surfaces); and to characterize the effect of wing planform (rectangular versus elliptical) on this vortex structure. The pitch-up motion of the wing (plate) is from 0 to $4{5}^{\ensuremath{\circ} } $ over a time span corresponding to four convective time scales, and the Reynolds number based on chord is 10 000. Volumes of constant magnitude of the second invariant of the velocity gradient tensor are interpreted in conjunction with three-dimensional streamline patterns and vorticity projections in orthogonal directions. The wing motion gives rise to ordered vortical structures along its wing surface. In contrast to development of the classical two-dimensional leading-edge vortex, the flow pattern evolves to a strongly three-dimensional form at high angle of attack. The state of the vortex system, after attainment of maximum angle of attack, has a similar form for extreme configurations of wing planform. Near the plane of symmetry, a large-scale region of predominantly spanwise vorticity dominates. Away from the plane of symmetry, the flow is dominated by two extensive regions of surface-normal vorticity, i.e. swirl patterns parallel to the wing surface. This similar state of the vortex structure is, however, preceded by different sequences of events that depend on the magnitude of the spanwise velocity within the developing vortex from the leading edge of the wing. Spanwise velocity of the order of one-half the free stream velocity, which is oriented towards the plane of symmetry of the wing, results in regions of surface-normal vorticity. In contrast, if negligible spanwise velocity occurs within the developing leading-edge vortex, onset of the regions of surface-normal vorticity occurs near the tips of the wing. These extremes of large and insignificant spanwise velocity within the leading-edge vortex are induced respectively on rectangular and elliptical planforms.