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1.
Insect wings are subjected to fluid, inertia and gravitational forces during flapping flight. Owing to their limited rigidity,
they bent under the influence of these forces. Numerical study by Hamamoto et al. (Adv Robot 21(1–2):1–21, 2007) showed that
a flexible wing is able to generate almost as much lift as a rigid wing during flapping. In this paper, we take a closer look
at the relationship between wing flexibility (or stiffness) and aerodynamic force generation in flapping hovering flight.
The experimental study was conducted in two stages. The first stage consisted of detailed force measurement and flow visualization
of a rigid hawkmoth-like wing undergoing hovering hawkmoth flapping motion and simple harmonic flapping motion, with the aim
of establishing a benchmark database for the second stage, which involved hawkmoth-like wing of different flexibility performing
the same flapping motions. Hawkmoth motion was conducted at Re = 7,254 and reduced frequency of 0.26, while simple harmonic flapping motion at Re = 7,800 and 11,700, and reduced frequency of 0.25. Results show that aerodynamic force generation on the rigid wing is governed
primarily by the combined effect of wing acceleration and leading edge vortex generated on the upper surface of the wing,
while the remnants of the wake vortices generated from the previous stroke play only a minor role. Our results from the flexible
wing study, while generally supportive of the finding by Hamamoto et al. (Adv Robot 21(1–2):1–21, 2007), also reveal the existence
of a critical stiffness constant, below which lift coefficient deteriorates significantly. This finding suggests that although
using flexible wing in micro air vehicle application may be beneficial in term of lightweight, too much flexibility can lead
to deterioration in flapping performance in terms of aerodynamic force generation. The results further show that wings with
stiffness constant above the critical value can deliver mean lift coefficient almost the same as a rigid wing when executing
hawkmoth motion, but lower than the rigid wing when performing a simple harmonic motion. In all cases studied (7,800 ≤ Re ≤ 11,700), the Reynolds number does not alter the force generation significantly. 相似文献
2.
The ultra-low Reynolds number airfoil wake 总被引:1,自引:0,他引:1
Lift force and the near wake of an NACA 0012 airfoil were measured over the angle (α) of attack of 0°–90° and the chord Reynolds
number (Re
c
), 5.3 × 103–5.1 × 104, with a view to understand thoroughly the near wake of the airfoil at low- to ultra-low Re
c
. While the lift force is measured using a load cell, the detailed flow structure is captured using laser-Doppler anemometry,
particle image velocimetry, and laser-induced fluorescence flow visualization. It has been found that the stall of an airfoil,
characterized by a drop in the lift force, occurs at Re
c
≥ 1.05 × 104 but is absent at Re
c
= 5.3 × 103. The observation is connected to the presence of the separation bubble at high Re
c
but absence of the bubble at ultra-low Re
c
, as evidenced in our wake measurements. The near-wake characteristics are examined and discussed in detail, including the
vortex formation length, wake width, spanwise vorticity, wake bubble size, wavelength of K–H vortices, Strouhal numbers, and
their dependence on α and Re
c
. 相似文献
3.
Flap-bounding is a common flight style in small birds in which flapping phases alternate with flexed-wing bounds. Body lift
is predicted to be essential to making this flight style an aerodynamically attractive flight strategy. To elucidate the contributions
of the body and tail to lift and drag during the flexed-wing bound phase, we used particle image velocimetry (PIV) and measured
properties of the wake of zebra finch (Taeniopygia guttata, N = 5), flying at 6–10 m s−1 in a variable speed wind tunnel as well as flow around taxidermically prepared specimens (N = 4) mounted on a sting instrumented with force transducers. For the specimens, we varied air velocity from 2 to 12 m s−1 and body angle from −15° to 50°. The wake of bounding birds and mounted specimens consisted of a pair of counter-rotating
vortices shed into the wake from the tail, with induced downwash in the sagittal plane and upwash in parasagittal planes lateral
to the bird. This wake structure was present even when the tail was entirely removed. We observed good agreement between force
measures derived from PIV and force transducers over the range of body angles typically used by zebra finch during forward
flight. Body lift:drag (L:D) ratios averaged 1.4 in live birds and varied between 1 and 1.5 in specimens at body angles from 10° to 30°. Peak (L:D) ratio was the same in live birds and specimens (1.5) and was exhibited in specimens at body angles of 15° or 20°, consistent
with the lower end of body angles utilized during bounds. Increasing flight velocity in live birds caused a decrease in C
L
and C
D
from maximum values of 1.19 and 0.95 during flight at 6 m s−1 to minimum values of 0.70 and 0.54 during flight at 10 m s−1. Consistent with delta-wing theory as applied to birds with a graduated-tail shape, trimming the tail to 0 and 50% of normal
length reduced L:D ratios and extending tail length to 150% of normal increased L:D ratio. As downward induced velocity is present in the sagittal plane during upstroke of flapping flight, we hypothesize that
body lift is produced during flapping phases. Future efforts to model the mechanics of intermittent flight should take into
account that flap-bounding birds may support up to 20% of their weight even with their wings fully flexed. 相似文献
4.
This paper reports results of DPIV measurements on a two-dimensional elliptic airfoil rotating about its own axis of symmetry
in a fluid at rest and in a parallel freestream. In the former case, we examined three rotating speeds (Re
c,Ω = 400, 1,000 and 2,000), and in the later case, four rotating speeds (Ro
c,Ω = 2.4, 1.2, 0.6 and 0.4), together with two freestream velocities (Re
c,u
= 200 and 1,000) and two starting configurations of the airfoil (i.e., chord parallel to (α
0 = 0°) or normal (α
0 = 90°) to the freestream). Results show that a rotating airfoil in a stationary fluid produces two distinct types of vortex
structures depending on the Reynolds number. The first type occurs at the lowest Reynolds number (Re
c,Ω = 400), where vortices shed from the two edges or tips of the airfoil dissipated quickly, resulting in the airfoil rotating
in a layer of diffused vorticity. The second type occurs at higher Reynolds numbers (i.e., Re
c,Ω = 1,000 and 2,000), where the corresponding vortices rotated together with the airfoil. Due to the vortex suction effect,
the torque characteristics are likely to be heavily damped for the first type because of the rapidly subsiding vortex shedding,
and more oscillatory for the second type due to persistent presence of tip vortices. In a parallel freestream, increasing
the tip-speed ratio (V/U) of the airfoil (i.e., decreasing the Rossby number, Ro
c,Ω) transformed the flow topology from periodic vortex shedding at Ro
c,Ω = 2.4 to the generation of a “hovering vortex” at Ro
c,Ω = 0.6 and 0.4. The presence of the hovering vortex, which has not been reported in literature before, is likely to enhance
the lift characteristics of the airfoil. Freestream Reynolds number is found to have minimal effect on the vortex formation
and shedding process, although it enhances shear layer instability and produces more small-scale flow structures that affect
the dynamics of the hovering vortex. Likewise, initial starting configuration of the airfoil, while affecting the flow transient
during the initial phase of rotation, has insignificant effect on the overall flow topology. Unfortunately, technical constraint
of our apparatus prevented us from carrying out complimentary force measurements; nevertheless, the results presented herein,
which are more extensive than those computed by Lugt and Ohring (1977), will provide useful benchmark data, from which more advanced numerical calculations can be carried out to ascertain the
corresponding force characteristics, particularly for those conditions with the presence of hovering vortex. 相似文献
5.
A NACA 0015 airfoil with and without a Gurney flap was studied in a wind tunnel with Re
c = 2.0 × 105 in order to examine the evolving flow structure of the wake through time-resolved PIV and to correlate this structure with time-averaged measurements of the lift coefficient. The Gurney flap, a tab of small length (1–4% of the airfoil chord) that protrudes perpendicular to the chord at the trailing edge, yields a significant and relatively constant lift increment through the linear range of the C
L
versus α curve. Two distinct vortex shedding modes were found to exist and interact in the wake downstream of flapped airfoils. The dominant mode resembles a Kàrmàn vortex street shedding behind an asymmetric bluff body. The second mode, which was caused by the intermittent shedding of fluid recirculating in the cavity upstream of the flap, becomes more coherent with increasing angle of attack. For a 4% Gurney flap at α = 8°, the first and second modes corresponded with Strouhal numbers based on flap height of 0.18 and 0.13. Comparison of flow around ‘filled’ and ‘open’ flap configurations suggested that the second shedding mode was responsible for a significant portion of the overall lift increment. 相似文献
6.
Lift and power requirements of hovering insect flight 总被引:6,自引:0,他引:6
Lift and power requirements for hovering flight of eight species of insects are studied by solving the Navier-Stokes equation numerically. The solution provides velocity and pressure fields, from which unsteady aerodynamic forces and moments are obtained. The inertial torque of wing mass are computed analytically. The wing length of the insects ranges from 2 mm (fruit fly) to 52 mm (hawkmoth); Reynolds numbers Re (based on mean flapping speed and mean chord length) ranges from 75 to 3850. The primary findings are shown in the following: (1) Either small (R = 2mm, Re = 75), medium (R ≈ 10 mm, Re ≈ 500) or large (R ≈ 50 mm, Re ≈ 4 000) insects mainly employ the same high-lift mechanism, delayed stall, to produce lift in hovering flight. The midstroke angle of attack needed to produce a mean lift equal to the insect weight is approximately in the range of 25° to 45°, which is approximately in agreement with observation. (2) For the small insect (fruit fly) and for the medium and large insects with relatively small wingbeat frequency (cranefly, ladybird and hawkmoth), the specific power ranges from 18 to 39W·kg^-1 , the major part of the power is due to aerodynamic force, and the elastic storage of negative work does not change the specific power greatly. However for medium and large insects with relatively large wingbeat frequency (hover fly, dronefly, honey bee and bumble bee), the specific power ranges from 39 to 61 W·kg^-1 , the major part of the power is due to wing inertia, and the elastic storage of negative work can decrease the specific power by approximately 33%. (3) For the case of power being mainly contributed by aerodynamic force (fruit fly, cranefly, ladybird and hawkmoth), the specific power is proportional to the product of the wingbeat frequency, the stroke amplitude, the wing length and the drag-to-lift ratio. For the case of power being mainly contributed by wing inertia (hoverfly, dronefly, honey bee and bumble bee), the specific power (without elastic storage) is proportional to the product of the cubic of wingbeat frequency, the square of the stroke amplitude, the square of the wing length and the ratio of wing mass to insect mass. 相似文献
7.
Davide Lasagna Raffaele Donelli Fabrizio De Gregorio Gaetano Iuso 《Experiments in fluids》2011,51(5):1369-1384
The effects of a trapped vortex cell (TVC) on the aerodynamic performance of a NACA0024 wing model were investigated experimentally
at Re = 106 and 6.67×1056.67\times 10^{5}. The static pressure distributions around the model and the wake velocity profiles were measured to obtain lift and drag
coefficients, for both the clean airfoil and the controlled configurations. Suction was applied in the cavity region to stabilize
the trapped vortex. For comparison, a classical boundary layer suction configuration was also tested. The drag coefficient
curve of the TVC-controlled airfoil showed sharp discontinuities and bifurcative behavior, generating two drag modes. A strong
influence of the angle of attack, the suction rate and the Reynolds number on the drag coefficient was observed. With respect
to the clean airfoil, the control led to a drag reduction only if the suction was high enough. Compared to the classical boundary
layer suction configuration, the drag reduction was higher for the same amount of suction only in a specific range of incidence,
i.e., α = −2° to α = 6° and only for the higher Reynolds number. For all the other conditions, the classical boundary layer
suction configuration gave better drag performances. Moderate increments of lift were observed for the TVC-controlled airfoil
at low incidence, while a 20% lift enhancement was observed in the stall region with respect to the baseline. However, the
same lift increments were also observed for the classical boundary layer suction configuration. Pressure fluctuation measurements
in the cavity region suggested a very complex interaction of several flow features. The two drag modes were characterized
by typical unsteady phenomena observed in rectangular cavity flows, namely the shear layer mode and the wake mode. 相似文献
8.
Guang-Kun Tan Gong-Xin Shen Shuo-Qiao Huang Wen-Han Su Yu Ke 《Experiments in fluids》2007,43(5):811-821
When swimming in water by flapping its tail, a fish can overcome the drag from uniform flow and propel its body. The involved
flow mechanism concerns 3-D and unsteady effects. This paper presents the investigation of the flow mechanism on the basis
of a 3-D robotic fish model which has the typical geometry of body and tail with periodic flapping 2-freedom kinematical motion
testing in the case of St = 0.78, Re = 6,600 and phase delay mode (φ = −75°), in which may have a greater or maximum propulsion (without consideration of the optimal efficiency). Using a special
technique of dye visualization which can clearly show vortex sheet and vortices in detail and using the inner 3-component
force balance and cable supporting system with the phase-lock technique, the 3-D flow structure visualized in the wake of
fish and the hydrodynamic force measurement were synchronized and obtained. Under the mentioned flapping parameters, we found
the key flow structure and its evolution, a pair of complex 3-D chain-shape vortex (S–H vortex-rings, S1–H1 and S2–H2, and their legs L1 and L2) flow structures, which attach the leading edge and the trailing edge, then shed, move downstream and outwards and distribute
two anti-symmetric staggering arrays along with the wake of the fish model in different phase stages during the flapping period.
It is different with in the case of St = 0.25–0.35. Its typical flow structure and evolution are described and the results prove that they are different from the
viewpoints based on the investigation of 2-D cases. For precision of the dynamic force measurement, in this paper it was provided
with the method and techniques by subtracting the inertial forces and the forces induced by buoyancy and gravity effect in
water, etc. from original data measured. The evolution of the synchronized measuring forces directly matching with the flow
structure was also described in this paper. 相似文献
9.
A study on the mechanism of high-lift generation by an airfoil in unsteady motion at low reynolds number 总被引:3,自引:0,他引:3
The aerodynamic force and flow structure of NACA 0012 airfoil performing an unsteady motion at low Reynolds number (Re=100) are calculated by solving Navier-Stokes equations. The motion consists of three parts: the first translation, rotation
and the second translation in the direction opposite to the first. The rotation and the second translation in this motion
are expected to represent the rotation and translation of the wing-section of a hovering insect. The flow structure is used
in combination with the theory of vorticity dynamics to explain the generation of unsteady aerodynamic force in the motion.
During the rotation, due to the creation of strong vortices in short time, large aerodynamic force is produced and the force
is almost normal to the airfoil chord. During the second translation, large lift coefficient can be maintained for certain
time period and
, the lift coefficient averaged over four chord lengths of travel, is larger than 2 (the corresponding steady-state lift coefficient
is only 0.9). The large lift coefficient is due to two effects. The first is the delayed shedding of the stall vortex. The
second is that the vortices created during the airfoil rotation and in the near wake left by previous translation form a short
“vortex street” in front of the airfoil and the “vortex street” induces a “wind”; against this “wind” the airfoil translates,
increasing its relative speed. The above results provide insights to the understanding of the mechanism of high-lift generation
by a hovering insect.
The project supported by the National Natural Science Foundation of China (19725210) 相似文献
10.
Three-dimensional vorticity in the wake of an inclined stationary circular cylinder was measured simultaneously using a multi-hot
wire vorticity probe over a streamwise range of x/d = 10–40. The study aimed to examine the dependence of the wake characteristics on cylinder inclination angle α (=0°–45°).
The validity of the independence principle (IP) for vortex shedding was also examined. It was found that the spanwise mean
velocity which represents the three-dimensionality of the wake flow, increases monotonically with α. The root-mean-square (rms) values
of the streamwise (u) and spanwise (w) velocities and the three vorticity components decrease significantly with the increase of α, whereas the transverse velocity
(v) does not follow the same trend. The vortex shedding frequency decreases with the increase of α. The Strouhal number (St
N), obtained by using the velocity component normal to the cylinder axis, remains approximately a constant within the experimental
uncertainty (±8%) when α is smaller than about 40°. The autocorrelation coefficients ρ
u
and ρ
v
of the u and v velocity signals show apparent periodicity for all inclination angles. With increasing α, ρ
u
and ρ
v
decrease and approach zero quickly. In contrast, the autocorrelation coefficient ρ
w
of w increases with α in the near wake, implying an enhanced three-dimensionality of the wake. 相似文献
11.
The near-field flow structure of a tip vortex behind a sweptback and tapered NACA 0015 wing was investigated and compared
with a rectangular wing at the same lift force and Re=1.81×105. The tangential velocity decreased with the downstream distance while increased with the airfoil incidence. The core radius
was about 3% of the root chord c
r, regardless of the downstream distance and α for α<8°. The core axial velocity was always wake-like. The core Γc and total Γo circulation of the tip vortex remained nearly constant up to x/c
r=3.5 and had a Γc/Γo ratio of 0.63. The total circulation of the tip vortex accounted for only about 40% of the bound root circulation Γb. For a rectangular wing, the axial flow exhibited islands of wake- and jet-like velocity distributions with Γc/Γo=0.75 and Γo/Γb=0.70. For the sweptback and tapered wing tested, the inner region of the tip vortex flow exhibited a self-similar behavior
for x/c
r≥1.0. The lift force computed from the spanwise circulation distributions agreed well with the force-balance data. A large
difference in the lift-induced drag was, however, observed between the wake integral method and the inviscid lifting-line
theory. 相似文献
12.
The streamwise evolution of an inclined circular cylinder wake was investigated by measuring all three velocity and vorticity
components using an eight-hotwire vorticity probe in a wind tunnel at a Reynolds number Red of 7,200 based on free stream velocity (U
∞) and cylinder diameter (d). The measurements were conducted at four different inclination angles (α), namely 0°, 15°, 30°, and 45° and at three downstream
locations, i.e., x/d = 10, 20, and 40 from the cylinder. At x/d = 10, the effects of α on the three coherent vorticity components are negligibly small for α ≤ 15°. When α increases further
to 45°, the maximum of coherent spanwise vorticity reduces by about 50%, while that of the streamwise vorticity increases
by about 70%. Similar results are found at x/d = 20, indicating the impaired spanwise vortices and the enhancement of the three-dimensionality of the wake with increasing
α. The streamwise decay rate of the coherent spanwise vorticity is smaller for a larger α. This is because the streamwise
spacing between the spanwise vortices is bigger for a larger α, resulting in a weak interaction between the vortices and hence
slower decaying rate in the streamwise direction. For all tested α, the coherent contribution to [`(v2)] \overline{{v^{2}}} is remarkable at x/d = 10 and 20 and significantly larger than that to [`(u2)] \overline{{u^{2}}} and [`(w2)]. \overline{{w^{2}}}. This contribution to all three Reynolds normal stresses becomes negligibly small at x/d = 40. The coherent contribution to [`(u2)] \overline{{u^{2}}} and [`(v2)] \overline{{v^{2}}} decays slower as moving downstream for a larger α, consistent with the slow decay of the coherent spanwise vorticity for
a larger α. 相似文献
13.
Flight agility, resistance to gusts, capability to hover coupled with a low noise generation might have been some of the reasons
why insects are among the oldest species observed in nature. Biologists and aerodynamicists focused on analyzing such flight
performances for diverse purposes: understanding the essence of flapping wings aerodynamics and applying this wing concept
to the development of micro-air vehicles (MAVs). In order to put into evidence the fundamentally non-linear unsteady mechanisms
responsible for the amount of lift generated by a flapping wing (Dickinson et al. in Science 284:1954–1960, 1999), experimental and numerical studies were carried out on typical insect model wings and kinematics. On the other hand, in
the recent context of MAVs development, it is of particular interest to study simplified non-biological flapping configurations
which could lead to lift and/or efficiency enhancement. In this paper, we propose a parametrical study of a NACA0012 profile
undergoing asymmetric hovering flapping motions at Reynolds 1000. On the contrary to normal hovering, which has been widely
studied as being the most common configuration observed in the world of insects, asymmetric hovering is characterized by an
inclined stroke plane. Besides the fact that the vertical force is hence a combination of both lift and drag (Wang in J Exp
Biol 207:1137–1150, 2004), the specificity of such motions resides in the vortex dynamics which present distinct behaviours, whether the upstroke
angle of attack leads to a partially attached or a strong separated flow, giving more or less importance to the wake capture
phenomenon. A direct consequence of the previous remarks relies on the enhancement of aerodynamic efficiency with asymmetry.
If several studies reported results based on the asymmetric flapping motion of dragonfly, only few works concentrated on parametrizing
asymmetric motions (e.g. Wang in Phys Rev Lett 85:2216–2219, 2000). The present study relies on TR-PIV measurements which allow determination of the vorticity fields and provide a basis to
evaluate the resulting unsteady forces through the momemtum equation approach. 相似文献
14.
The mean wake of a surface-mounted finite-height square prism was studied experimentally in a low-speed wind tunnel to explore the combined effects of incidence angle (α) and aspect ratio (AR). Measurements of the mean wake velocity field were made with a seven-hole pressure probe for finite square prisms of AR = 9, 7, 5 and 3, at a Reynolds number of Re = 3.7 × 104, for incidence angles from α = 0° to 45°. The relative thickness of the boundary layer on the ground plane, compared to the prism width, was δ/D = 1.5. As the incidence angle increases from α = 0° to 15°, the mean recirculation zone shortens and the mean wake shifts in the direction opposite to that of the mean lift force. The downwash is also deflected to this side of the wake and the mean streamwise vortex structures in the upper part of the wake become strongly asymmetric. The shortest mean recirculation zone, and the greatest asymmetry in the mean wake, is found at the critical incidence angle of αcritical ≈ 15°. As the incidence angle increases from α = 15° to 45°, the mean recirculation zone lengthens and the mean streamwise vortex structures regain their symmetry. These vortices also elongate in the wall-normal direction and become contiguous with the horseshoe vortex trailing arms. The mean wake of the prism of AR = 3 has some differences, such as an absence of induced streamwise vorticity near the ground plane, which support its classification as lying below the critical aspect ratio for the present flow conditions. 相似文献
15.
In the present study, we perform a wind-tunnel experiment to investigate the aerodynamic performance of a gliding swallowtail-butterfly
wing model having a low aspect ratio. The drag, lift and pitching moment are directly measured using a 6-axis force/torque
sensor. The lift coefficient increases rapidly at attack angles less than 10° and then slowly at larger attack angles. The
lift coefficient does not fall off rapidly even at quite high angles of attack, showing the characteristics of low-aspect-ratio
wings. On the other hand, the drag coefficient increases more rapidly at higher angles of attack due to the increase in the
effective area responsible for the drag. The maximum lift-to-drag ratio of the present modeled swallowtail butterfly wing
is larger than those of wings of fruitfly and bumblebee, and even comparable to those of wings of birds such as the petrel
and starling. From the measurement of pitching moment, we show that the modeled swallowtail butterfly wing has a longitudinal
static stability. Flow visualization shows that the flow separated from the leading edge reattaches on the wing surface at
α < 15°, forming a small separation bubble, and full separation occurs at α ≥ 15°. On the other hand, strong wing-tip vortices are observed in the wake at α ≥ 5° and they are an important source of the lift as well as the main reason for broad stall. Finally, in the absence of
long hind-wing tails, the lift and longitudinal static stability are reduced, indicating that the hind-wing tails play an
important role in enhancing the aerodynamic performance. 相似文献
16.
This paper is motivated by the works of Dickinson et al. (Science 284:1954–1960, 1999) and Sun and Tang (J Exp Biol 205:55–70, 2002) which provided two different perspectives on the influence of wing–wake interaction (or wake capture) on lift generation
during flapping motion. Dickinson et al. (Science 284:1954–1960, 1999) hypothesize that wake capture is responsible for the additional lift generated at the early phase of each stroke, while Sun
and Tang (J Exp Biol 205:55–70, 2002) believe otherwise. Here, we take a more fundamental approach to study the effect of wing–wake interaction on the aerodynamic
force generation by carrying out simultaneous force and flow field measurements on a two-dimensional wing subjected to two
different types of motion. In one of the motions, the wing at a fixed angle of attack was made to follow a motion profile
described by “acceleration-constant velocity-deceleration”. Here, the wing was first linearly accelerated from rest to a predetermined
maximum velocity and remains at that speed for set duration before linearly decelerating to a stop. The acceleration and deceleration
phase each accounted for only 10% of the stroke, and the stroke covered a total distance of three chord lengths. In another
motion, the wing was subjected to the same above-mentioned movement, but in a back and forth manner over twenty strokes. Results
show that there are two possible outcomes of wing–wake interaction. The first outcome occurs when the wing encounters a pair
of counter-rotating wake vortices on the reverse stroke, and the induced velocity of these vortices impinges directly on the
windward side of the wing, resulting in a higher oncoming flow to the wing, which translates into a higher lift. Another outcome
is when the wing encounters one vortex on the reverse stroke, and the close proximity of this vortex to the windward surface
of the wing, coupled with the vortex suction effect (caused by low pressure region at the center of the vortex), causes the
net force on the wing to decrease momentarily. These results suggest that wing–wake interaction does not always lead to lift
enhancement, and it can also cause lift reduction. As to which outcome prevails depend very much on the flapping motion and
the timing of the reverse stroke. 相似文献
17.
Hybrid unsteady-flow simulation combining particle tracking velocimetry (PTV) and direct numerical simulation (DNS) is introduced
in the series of two papers. Particle velocities on a laser-light sheet acquired with time-resolved PTV in a water tunnel
are supplied to two-dimensional DNS with time intervals corresponding to the frame rate of the PTV. Hybrid velocity fields
then approach those representing the PTV data in the course of time, and the reconstructed velocity fields satisfy the governing
equations with the resolution comparable to numerical simulation. In part 2, by extending the capabilities of the hybrid simulation
to higher Reynolds numbers, we simulate flows past the NACA0012 airfoil over ranges of Reynolds numbers (Re ≤ 104) and angles of attack (−5° ≤ α ≤ 20°) and validate the proposed technique by comparing with experimental results in terms
of the lift and drag coefficients. We also compare the results with unsteady Reynolds-averaged Navier–Stokes (URANS) simulation
in two-dimensions and show the advantages of the hybrid simulation against two-dimensional URANS. 相似文献
18.
The wakes of elliptical cylinders are numerically investigated at a Reynolds number ReD = 150. ANSYS-Fluent, based on the finite volume method, is used to simulate two-dimensional Newtonian fluid flow. The cylinder cross-sectional aspect ratio (AR) is varied from 0.25 to 1.0 (circular cylinder), and the angle of attack (α) of the cylinder is changed as α = 0° – 90°. With the changes in AR and α, three distinct wake patterns (patterns I, II, III) are observed, associated with different characteristics of fluid forces. Steady wake (pattern I) is characterised by two steady bubbles forming behind the cylinder, occurring at AR < 0.37 and α < 2.5°. Time-mean drag and fluctuating lift coefficients are small. Pattern II refers to Karman wake followed by steady wake (AR ≥ 0.37 – 0.67, depending on α) with the Karman street transitioning to two steady shear layers downstream. An inflection angle αi is identified where the time-mean drag of the elliptical cylinder is identical to that of a circular cylinder. Pattern III is the Karman wake followed by secondary wake (AR ≤ 0.67, α > 52°), where the Karman street forming behind the cylinder is modified to a secondary vortex street with a low frequency. The Time-mean drag coefficient is maximum for this pattern. 相似文献
19.
Experimental investigation of the effect of chordwise flexibility on the aerodynamics of flapping wings in hovering flight 总被引:1,自引:0,他引:1
Ornithopters or mechanical birds produce aerodynamic lift and thrust through the flapping motion of their wings. Here, we use an experimental apparatus to investigate the effects of a wing's twisting stiffness on the generated thrust force and the power required at different flapping frequencies. A flapping wing system and an experimental set-up were designed to measure the unsteady aerodynamic and inertial forces, power usage and angular speed of the flapping wing motion. A data acquisition system was set-up to record important data with the appropriate sampling frequency. The aerodynamic performance of the vehicle under hovering (i.e., no wind) conditions was investigated. The lift and thrust that were produced were measured for different flapping frequencies and for various wings with different chordwise flexibilities. The results show the manner in which the elastic deformation and inertial flapping forces affect the dynamical behavior of the wing. It is shown that the generalization of the actuator disk theory is, at most, only valid for rigid wings, and for flexible wings, the power P varies by a power of about 1.0 of the thrust T. This aerodynamic information can also be used as benchmark data for unsteady flow solvers. 相似文献
20.
The flow past two identical circular cylinders in side-by-side arrangements at right and oblique attack angles is numerically investigated by solving the three-dimensional Navier–Stokes equations using the Petrov–Galerkin finite element method. The study is focused on the effect of flow attack angle and gap ratio between the two cylinders on the vortex shedding flow and the hydrodynamic forces of the cylinders. For an oblique flow attack angle, the Reynolds number based on the velocity component perpendicular to the cylinder span is defined as the normal Reynolds number ReN and that based on the total velocity is defined as the total Reynolds number ReT. Simulations are conducted for two Reynolds numbers of ReN=500 and ReT=500, two flow attack angles of α=0° and 45° and four gap ratios of G/D=0.5, 1, 3 and 5. The biased gap flow for G/D=0.5 and 1 and the flip-flopping bistable gap flow for G/D=1 are observed for both α=0° and 45°. For a constant normal Reynolds number of ReN=500, the mean drag and lift coefficients at α=0° are very close to those at α=45°. The difference between the root mean square (RMS) lift coefficient at α=0° and that at α=45° is about 20% for large gap ratios of 3 and 5. From small gap ratios of 0.5 and 1, the RMS lift coefficients at α=0° and 45° are similar to each other. The present simulations show that the agreement in the force coefficients between the 0° and 45° flow attack angles for a constant normal Reynolds number is better than that for a constant total Reynolds number. This indicates that the normal Reynolds number should be used in the implementation of the independence principle (i.e., the independence of the force coefficients on the flow attack angle). The effect of Reynolds number on the bistable gap flow is investigated by simulating the flow for ReN=100–600, α=0° and 45° and G/D=1. Flow for G/D=1 is found to be two-dimensional at ReN=100 and weak three-dimensional at ReN=200. While well defined biased flow can be identified for ReN=300–600, the gap flow for ReN=100 and 200 changes its biased direction too frequently to allow stable biased flow to develop. 相似文献