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1.
The impact of 12 spoiler–tab configurations, of different heights and widths, on the tip vortex generated by an oscillating
NACA 0015 wing was investigated experimentally. For an oscillating wing equipped with a spoiler, the peak tangential velocity
and core and total circulation were greatly reduced compared to a tab, regardless of its width, while the core radius remained
largely unaffected with its center displaced vertically above the baseline wing. The most noticeable impact of a spoiler with
a reduced height was its potential in alleviating the blade–vortex interaction (BVI) strength. Meanwhile, the largest favorable
impact on the critical vortex flow parameters was achieved via a 25%-span spoiler–tab combination with a height of 5 and 2.5%
chord, respectively. A contrary effect on the BVI suppression, especially during pitch-up, was, however, observed. The impact
on the BVI can be improved by reducing the height of the spoiler at the expense of unfavorable change in the vortex strength
and displacement. 相似文献
2.
Douglas L. Altshuler Marko Princevac Hansheng Pan Jesse Lozano 《Experiments in fluids》2009,46(5):835-846
The flow fields of slowly flying bats and faster-flying birds differ in that bats produce two vortex loops during each stroke,
one per wing, and birds produce a single vortex loop per stroke. In addition, the circulation at stroke transition approaches
zero in bats but remains strong in birds. It is unknown if these difference derive from fundamental differences in wing morphology
or are a consequence of flight speed. Here, we present an analysis of the horizontal flow field underneath hovering Anna’s
hummingbirds (Calypte anna) to describe the wake of a bird flying at zero forward velocity. We also consider how the hummingbird tail interacts with
the wake generated by the wings. High-speed image recording and analysis from three orthogonal perspectives revealed that
the wing tips reach peak velocities in the middle of each stroke and approach zero velocity at stroke transition. Hummingbirds
use complex tail kinematic patterns ranging from in phase to antiphase cycling with respect to the wings, covering several
phase shifted patterns. We employed particle image velocimetry to attain detailed horizontal flow measurements at three levels
with respect to the tail: in the tail, at the tail tip, and just below the tail. The velocity patterns underneath the wings
indicate that flow oscillates along the ventral–dorsal axis in response to the down- and up-strokes and that the sideways
flows with respect to the bird are consistently from the lateral to medial. The region around the tail is dominated by axial
flows in dorsal to ventral direction. We propose that these flows are generated by interaction between the wakes of the two
wings at the end of the upstroke, and that the tail actively defects flows to generate moments that contribute to pitch stability.
The flow fields images also revealed distinct vortex loops underneath each wing, which were generated during each stroke.
From these data, we propose a model for the primary flow structures of hummingbirds that more strongly resembles the bat model.
Thus, pairs of unconnected vortex loops may be shared features of different animals during hovering and slow forward flight. 相似文献
3.
Gonzalo Arranz Alejandro Gonzalo Markus Uhlmann Oscar Flores Manuel García-Villalba 《Flow, Turbulence and Combustion》2018,101(2):477-497
In this study the flow around a winged-seed in auto-rotation is characterized using direct numerical simulations (DNS) at Reynolds number in the range 80–240, based on the descent speed and a characteristic chord length. In this range, the flow is approximately steady when observed from a reference frame fixed to the seed. For all cases, the flow structure consists of a wing tip vortex which describes a helical path, a vortex shed behind the nut of the seed and a stable leading edge vortex above the wing surface which merges with the tip vortex. With increasing Reynolds number, the leading edge vortex becomes more intense and gets closer to the wing surface. The simulation results also show the formation of a spanwise flow on the upper surface of the wing, moving fluid towards the wing tip in a region downstream and beneath the leading edge vortex. This spanwise flow is rather weak inside the core of the leading edge vortex, and the analysis of the streamlines show a very weak transport of vorticity along the vortex for the cases under consideration. The analysis of the flow suggests that the stabilization of the leading edge vortex is mainly due to non-inertial accelerations, although viscous effects may contribute, specially at lower Re. Furthermore, the leading edge vortex has been characterized by analysing the flow variables averaged along cross-sections of the vortex. While some quantities, like the spanwise velocity or the pressure inside the vortex, are rather insensitive to the threshold used to define the leading edge vortex, the same is not true for the circulation of the vortex or its averaged spanwise vorticity, due to the viscous nature of the vortex. Finally, it is observed that the spanwise vorticity scales with the angular rotation of the seed for the different Re. 相似文献
4.
Laser-Doppler velocimeter measurements of a wing/body junction flow field made within a plane to the side of the wing/wall junction and perpendicular both to a 3:2 elliptical nose—NACA 0020 tail wing, and a flat wall are presented. Reynolds number of the approach boundary layer was, Reθ = 5940, and free-stream air velocity was, Uref = 27.5 m/s. A large vortical structure residing in the outer region redirects the low-turbulence free-stream flow to the vicinity of the wing/wall junction, resulting in thin boundary layers with velocity magnitudes higher than free-stream flow. Lateral pressure gradients result in a three-dimensional separation on the uplifting side of the vortex. Additionally, a high vorticity vortical structure with opposite sense to the outer-layer vortex forms beneath the outer-layer vortex. Normal and shear stresses increase to attain values an order of magnitude larger compared to values measured in a three-dimensional boundary layer just outside the junction vortex. Bimodal histograms of the w fluctuating velocity occur under the outer-layer vortex near the wall due to the time-dependent nature of the horseshoe vortex. In such a flow the shear-stress angle (SSA) highly lags the flow-gradient angle (FGA), and the turbulence diffusion is highly altered due to presence of vortical structures. 相似文献
5.
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
6.
A laminar wall jet undergoing transition is investigated using the particle image velocimetry (PIV) technique. The plane
wall jet is issued from a rectangular channel, with the jet-exit velocity profile being parabolic. The Reynolds number, based
on the exit mean velocity and the channel width, is 1450. To aid the understanding of the global flow features, laser-sheet/smoke
flow visualizations are performed along streamwise, spanwise, and cross-stream directions. Surface pressure measurements are
made to correlate the instantaneous vorticity distribution with the surface pressure fluctuations. The instantaneous velocity
and vorticity field measurements provide the basis for understanding the formation of the inner-region vortex and the subsequent
interactions between the outer-region (free-shear-layer region) and inner-region (boundary-layer region) vortical structures.
Results show that under the influence of the free-shear-layer vortex, the local boundary layer becomes detached from the surface
and inviscidly unstable, and a vortex is formed in the inner region. Once this vortex has formed, the free-shear-layer vortex
and the inner-region vortex form a vortex couple and convect downstream. The mutual interactions between these inner- and
outer-region vortical structures dominate the transition process. Farther downstream, the emergence of the three-dimensional
structure in the free shear layer initiates complete breakdown of the flow.
Received: 8 November 1995/Accepted: 6 November 1996 相似文献
7.
Three-dimensional flow structures and associated turbulence in the tip region of a waterjet pump rotor blade 总被引:3,自引:0,他引:3
Stereo particle image velocimetry measurements focus on the flow structure and turbulence within the tip leakage vortex (TLV)
of an axial waterjet pump rotor. Unobstructed optical access to the sample area is achieved by matching the optical refractive
index of the transparent pump with that of the fluid. Data obtained in closely spaced planes enable us to reconstruct the
3D TLV structure, including all components of the mean vorticity and strain-rate tensor along with the Reynolds stresses and
associated turbulence production rates. The flow in the tip region is highly three-dimensional, and the characteristics of
the TLV and leakage flow vary significantly along the blade tip chordwise direction. The TLV starts to roll up along the suction
side tip corner of the blade, and it propagates within the passage toward the pressure side of the neighboring blade. A shear
layer with increasing length connects the TLV to the blade tip and initially feeds vorticity into it. During initial rollup,
the TLV involves entrainment of a few vortex filaments with predominantly circumferential vorticity from the blade tip. Being
shed from the blade, these filaments also have high circumferential velocity and appear as swirling jets. The circumferential
velocity in the TLV core is also substantially higher than that in the surrounding passage flow, but the velocity peak does
not coincide with the point of maximum vorticity. When entrainment of filaments stops in the aft part of the passage, newly
forming filaments wrap around the core in helical trajectories. In ensemble-averaged data, these filaments generate a vortical
region that surrounds the TLV with vorticity that is perpendicular to that in the vortex core. Turbulence within the TLV is
highly anisotropic and spatially non-uniform. Trends can be traced to high turbulent kinetic energy and turbulent shear stresses,
e.g., in the shear layer containing the vortex filaments and the contraction region situated along the line where the leakage
backflow meets the throughflow, causing separation of the boundary layer at the pump casing. Upon exposure to adverse pressure
gradients in the aft part of the passage, at 0.65–0.7 chord fraction in the present conditions, the TLV bursts into a broad
turbulent array of widely distributed vortex filaments. 相似文献
8.
The main objective of this research study was to investigate the aerodynamic forces of an avian flapping wing model system.
The model size and the flow conditions were chosen to approximate the flight of a goose. Direct force measurements, using
a three-component balance, and PIV flow field measurements parallel and perpendicular to the oncoming flow, were performed
in a wind tunnel at Reynolds numbers between 28,000 and 141,000 (3–15 m/s), throughout a range of reduced frequencies between
0.04 and 0.20. The appropriateness of quasi-steady assumptions used to compare 2D, time-averaged particle image velocimetry
(PIV) measurements in the wake with direct force measurements was evaluated. The vertical force coefficient for flapping wings
was typically significantly higher than the maximum coefficient of the fixed wing, implying the influence of unsteady effects,
such as delayed stall, even at low reduced frequencies. This puts the validity of the quasi-steady assumption into question.
The (local) change in circulation over the wing beat cycle and the circulation distribution along the wingspan were obtained
from the measurements in the tip and transverse vortex planes. Flow separation could be observed in the distribution of the
circulation, and while the circulation derived from the wake measurements failed to agree exactly with the absolute value
of the circulation, the change in circulation over the wing beat cycle was in excellent agreement for low and moderate reduced
frequencies. The comparison between the PIV measurements in the two perpendicular planes and the direct force balance measurements,
show that within certain limitations the wake visualization is a powerful tool to gain insight into force generation and the
flow behavior on flapping wings over the wing beat cycle. 相似文献
9.
The impact of Gurney flaps, of different heights and perforations, on the growth and development of a tip vortex, both along
the tip and in the near field of a finite NACA 0012 wing, at Re = 1.05 × 105 was investigated by using particle image velocimetry (PIV). Wind-tunnel force balance measurements were also made to supplement
the PIV results. This study is a continuation of the work of Lee and Ko (Exp Fluids 46(6):1005–1019, 2009) on the near-wake measurements behind perforated Gurney flaps. The present results show that along the tip, the overall behavior
of the secondary vortices and their interaction with the primary, or tip, vortex remained basically unchanged, regardless
of flap height and perforation. The peak vorticity of the tip vortex, however, increased with flap height and always exhibited
a local maximum at x/c = 0.8 (from the leading edge). In the near field, the strength and structure of the near-field tip vortex were found to vary
greatly with the flap height and perforation. The small flaps produced a more concentrated tip vortex with an increased circulation,
while the large Gurney flaps caused a disruption of the tip vortex. The disrupted vortex can, however, be re-established by
the addition of flap perforation. The larger the flap perforation the more organized the tip vortex. The Gurney flaps have
the potential to serve as an alternative off-design wake vortex control device. 相似文献
10.
Ultrasound time-of-flight methods employing counter-propagating ultrasonic pulses are utilized for the direct measurement
of circulation in vortical flows. Two schemes are described here which involve either a single straight path or a closed path.
Both techniques are shown to result in time differences, between the counter-propagating pulses around the path, linearly
proportional to the circulation enclosed by the ultrasound path. The ultrasound methods of circulation measurement do not
require calibration constants and can be non-invasive. The reliability of the closed path ultrasound method was assessed by
comparing the measured circulation values with those calculated from the lift measurements of a NACA 0012 airfoil. Two examples
are also presented where the closed path ultrasound method has been applied to the flow over a delta wing and a free-surface
vortex in a cylindrical tank.
Received: 8 October 1997/Accepted: 23 April 1998 相似文献
11.
A vortex ring impinging on a three-dimensional bump is studied using large eddy simulation for a Reynolds number Re = 4 × 104 based on the initial translation speed and diameter of the vortex ring. The effects of bump height on the vortical flow phenomena and the underlying physical mechanisms are investigated. Based on the analysis of the evolution of vortical structures, two typical kinds of vortical structures, i.e., the wrapping vortices and the hair-pin vortices, are identified and play an important role in the flow state evolution. The circulation of the primary vortex ring reasonably elucidates some typical phases of flow evolution. Furthermore, the mechanism of flow transition from laminar to turbulent state has been revealed based on analysis of turbulent kinetic energy. 相似文献
12.
Anders Hedenström F. T. Muijres R. von Busse L. C. Johansson Y. Winter G. R. Spedding 《Experiments in fluids》2009,46(5):923-932
Previous studies on wake flow visualization of live animals using DPIV have typically used low repetition rate lasers and
2D imaging. Repetition rates of around 10 Hz allow ~1 image per wingbeat in small birds and bats, and even fewer in insects.
To accumulate data representing an entire wingbeat therefore requires the stitching-together of images captured from different
wingbeats, and at different locations along the wing span for 3D-construction of wake topologies. A 200 Hz stereo DPIV system
has recently been installed in the Lund University wind tunnel facility and the high-frame rate can be used to calculate all
three velocity components in a cube, whose third dimension is constructed using the Taylor hypothesis. We studied two bat
species differing in body size, Glossophaga soricina and Leptonycteris curasoa. Both species shed a tip vortex during the downstroke that was present well into the upstroke, and a vortex of opposite sign
to the tip vortex was shed from the wing root. At the transition between upstroke/downstroke, a vortex loop was shed from
each wing, inducing an upwash. Vorticity iso-surfaces confirmed the overall wake topology derived in a previous study. The
measured dimensionless circulation, Γ/Uc, which is proportional to a wing section lift coefficient, suggests that unsteady
phenomena play a role in the aerodynamics of both species. 相似文献
13.
Near wake vortex dynamics of a hovering hawkmoth 总被引:1,自引:0,他引:1
Numerical investigation of vortex dynamics in near wake of a hovering hawkmoth and hovering aerodynamics is conducted to support the development of a biology-inspired dynamic flight simulator for flapping wingbased micro air vehicles. Realistic wing-body morphologies and kinematics are adopted in the numerical simulations. The computed results show 3D mechanisms of vortical flow structures in hawkmoth-like hovering. A horseshoe-shaped primary vortex is observed to wrap around each wing during the early down- and upstroke; the horseshoe-shaped vortex subsequently grows into a doughnut-shaped vortex ring with an intense jet-flow present in its core, forming a downwash. The doughnut-shaped vortex rings of the wing pair eventu- ally break up into two circular vortex rings as they propagate downstream in the wake. The aerodynamic yawing and rolling torques are canceled out due to the symmetric wing kinematics even though the aerodynamic pitching torque shows significant variation with time. On the other hand, the time- varying the aerodynamics pitching torque could make the body a longitudinal oscillation over one flapping cycle. 相似文献
14.
Qualitative and quantitative flow visualizations were performed on a flapping rigid plate to establish a quantitative method
for flow observation and evaluation of the force in the near field of a flapping wing. Flow visualization was performed qualitatively
with dye visualization and quantitatively with velocity measurements using stereo particle image velocimetry (PIV) on three
planes near the tip of the plate along its chord and oriented normally. By ensemble averaging the velocity fields of the same
phase angles, they represent a portion of the volume near the tip. Measurements were conducted with two flapping frequencies
to compare the flow structure. The second invariant of the deformation tensor visualized the leading edge and mid-chord vortices
around the plate appearing due to flow separation behind the plate while other vortical structures were visualized by streamlines.
These structures appear to be related to the dynamics of the leading edge vortex. Force analysis by integrating the phase-averaged
velocity field within a chosen control volume showed increases in the maxima of the magnitudes of the non-dimensional unsteady
force terms on the edge of the plate at the angles after the end of each stroke. The non-dimensional phase-averaged momentum
flux was similar for both flapping frequencies. 相似文献
15.
Ruimin Sun 《力学快报》2011,1(3):032001
The tip vortices and aerodynamics of a NACA0012 wing in the vicinity of the ground were studied in a wind tunnel. The wing tip vortex structures and lift/drag forces were measured by a seven-hole probe and a force balance, respectively. The evolution of the flow structures and aerodynamics with a ground height were analyzed. The vorticity of tip vortices was found to reduce with the decreasing of the ground height, and the position of vortex-core moved gradually to the outboard of the wing tip. Therefore, the down-wash flow induced by the tip vortices was weakened. However, vortex breakdown occurred as the wing lowered to the ground. From the experimental results of aerodynamics, the maximum lift-to-drag ratio was observed when the angle of attack was 2.5° and the ground clearance was 0.2. 相似文献
16.
A. V. Kazakov 《Fluid Dynamics》1998,33(3):338-345
The results of calculating the stability of a three-dimensional swirl flow of a viscous heat-conducting gas are presented.
The stability characteristics are determined using the linear time-dependent theory of plane-parallel flow stability. The
main undisturbed axisymmetric vortex flow was determined numerically using a quasi-cylindrical approximation for the complete
set of Navier-Stokes equations. The circulation of the peripheral velocity in the cocurrent flow surrounding the viscous vortex
core was assumed to be constant. In analyzing the stability, nonaxisymmetric perturbations in the shape of waves traveling
along the vortex axis with both positive and negative wavenumbers were considered; in these two cases the perturbation rotation
is either the same or opposite in sense to the rotation in the vortex core. Neutral stability curves are determined for various
values of the swirling parameter and the cocurrent flow Mach number.
Moscow. Translated from Izvestiya Rossiiskoi Akademii Nauk, Mekhanika Zhidkosti i Gaza, No. 3, pp. 50–59, May–June, 1998. 相似文献
17.
In this study parallel blade–vortex interaction for a Schmidt-propeller configuration has been examined using particle image
velocimetry (PIV). This tandem configuration consists of a leading airfoil (forefoil), used to generate a vortical wake of
leading-edge vortices (LEVs) and trailing-edge vortices (TEVs) through a pitching or plunging motion, and a trailing airfoil
(hindfoil), held fixed with a specified angle of attack and vertical spacing in its wake. The hindfoil incidence (loading)
and not the vertical spacing to the incoming vortical wake has been found to dictate the nature of the interaction (inviscid
vs. viscous). For cases where the vortex–blade offset is small and the hindfoil is loaded, vortex distortion and vortex-induced
separations are observed. By tracking the circulation of the LEV and TEV, it has been found that the vortices are strengthened
for the tandem arrangement and in certain cases dissipate quicker in the wake when interacting with the hindfoil. Time-averaged
forces obtained using a standard control-volume analysis are then obtained and used to evaluate these vortex-interaction cases.
A subsequent analysis of the varying pressure distribution over the suction side of the hindfoil is performed by integrating
the Navier–Stokes equations through the velocity field. This allows for a direct comparison of the vortex-induced loading
for the various configurations. 相似文献
18.
The aerodynamic forces and flow structure of a model insect wing is studied by solving the Navier-Stokes equations numerically.
After an initial start from rest, the wing is made to execute an azimuthal rotation (sweeping) at a large angle of attack
and constant angular velocity. The Reynolds number (Re) considered in the present note is 480 (Re is based on the mean chord length of the wing and the speed at 60% wing length from the wing root). During the constant-speed
sweeping motion, the stall is absent and large and approximately constant lift and drag coefficients can be maintained. The
mechanism for the absence of the stall or the maintenance of large aerodynamic force coefficients is as follows. Soon after
the initial start, a vortex ring, which consists of the leading-edge vortex (LEV), the starting vortex, and the two wing-tip
vortices, is formed in the wake of the wing. During the subsequent motion of the wing, a base-to-tip spanwise flow converts
the vorticity in the LEV to the wing tip and the LEV keeps an approximately constant strength. This prevents the LEV from
shedding. As a result, the size of the vortex ring increases approximately linearly with time, resulting in an approximately
constant time rate of the first moment of vorticity, or approximately constant lift and drag coefficients. The variation of
the relative velocity along the wing span causes a pressure gradient along the wingspan. The base-to-tip spanwise flow is
mainly maintained by the pressure-gradient force.
The project supported by the National Natural Science Foundation of China (10232010) 相似文献
19.
Unsteady plunging (heaving) of a wing in the form of a flat plate can give rise to pronounced axial flow in the small-scale
leading-edge vortex, during its initial stage of formation. Opposing axial flows along the vortex interact at the plane of
symmetry giving rise to large-scale patterns of streamwise-oriented vorticity, which can dominate the tip vortices over part
of the oscillation cycle. 相似文献
20.
The numerical investigation has been performed to explore the feasibility of vortex control by leading edge sucking excitation
on a delta wing. The results reveal that the flow on the upper surface of the delta wing changes significantly in a wide range
of the angle of attack. For the vortical flow at moderate angle of attack, the secondary and tertiary vortices are weakened
or suppressed, and the total lift is almost unchanged. For the stalled flow at high angle of attack, the leading edge concentrated
vortex is recovered, and the lift is enhanced with increasing suction rate. For the bluff-body flow at even high angles of
attack, the lift can still be improved. The concentrated vortex disappears on the upper surface, and the load increment is
nearly unchanged along the chordwise direction.
The project supported by the National Natural Science Foundation of China (19802018). 相似文献