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1.
Kyung Chun Kim Sang Youl Yoon Sang Moon Kim H. H. Chun Inwon Lee 《Experiments in fluids》2006,40(6):876-883
An experimental investigation was carried out regarding a three-dimensional topology of a zero-pressure gradient turbulent boundary layer. In this study, the polarization separation technique has been applied to the PIV measurements. Two mutually perpendicular measurement planes have been employed in x–y and x–z planes, respectively. Synchronization between a stereoscopic PIV with another plane PIV system was made toward the detection of such salient features of the coherent structure as the legs and the head of the hairpin vortices. Polarization rotation via a half-waveplate and subsequent particle image separation using polarizer minimized the spurious particle images. The PIV results clearly demonstrate the presence of hairpin-like coherent vortical structures and coincidence between the near-wall quasi-streamwise vortex pair and the legs of the hairpin vortex. 相似文献
2.
Wind turbines operate in the surface layer of the atmospheric boundary layer, where they are subjected to strong wind shear
and relatively high turbulence levels. These incoming boundary layer flow characteristics are expected to affect the structure
of wind turbine wakes. The near-wake region is characterized by a complex coupled vortex system (including helicoidal tip
vortices), unsteadiness and strong turbulence heterogeneity. Limited information about the spatial distribution of turbulence
in the near wake, the vortex behavior and their influence on the downwind development of the far wake hinders our capability
to predict wind turbine power production and fatigue loads in wind farms. This calls for a better understanding of the spatial
distribution of the 3D flow and coherent turbulence structures in the near wake. Systematic wind-tunnel experiments were designed
and carried out to characterize the structure of the near-wake flow downwind of a model wind turbine placed in a neutral boundary
layer flow. A horizontal-axis, three-blade wind turbine model, with a rotor diameter of 13 cm and the hub height at 10.5 cm,
occupied the lowest one-third of the boundary layer. High-resolution particle image velocimetry (PIV) was used to measure
velocities in multiple vertical stream-wise planes (x–z) and vertical span-wise planes (y–z). In particular, we identified localized regions of strong vorticity and swirling strength, which are the signature of helicoidal
tip vortices. These vortices are most pronounced at the top-tip level and persist up to a distance of two to three rotor diameters
downwind. The measurements also reveal strong flow rotation and a highly non-axisymmetric distribution of the mean flow and
turbulence structure in the near wake. The results provide new insight into the physical mechanisms that govern the development
of the near wake of a wind turbine immersed in a neutral boundary layer. They also serve as important data for the development
and validation of numerical models. 相似文献
3.
Three-dimensional vortical structures have been measured in a circular-cylinder wake using particle imaging velocimetry (PIV) for the Reynolds number range of 2×103 to 1×104. The PIV was modified, compared with the conventional one, in terms of its light sheet arrangement to capture reliably streamwise vortices. While in agreement with previous reports, the presently measured spanwise structures complement the data in the literature in the streamwise evolution of the near-wake spanwise vortex in size, strength, streamwise and lateral convection velocities, shedding new light upon vigorous interactions between oppositely signed spanwise structures. The longitudinal vortices display mushroom patterns in the (x, z)-plane in the immediate proximity to the cylinder. Their most likely inclination in the (x, y)-plane is inferred from the measurements in different (x, z)-planes. The longitudinal vortices in the (y, z)-plane show alternate change in sign, though not discernible at x/d > 15. They decay in the maximum vorticity and circulation rapidly from x/d = 5 to 10 and slowly for x/d > 10, and are further compared with the spanwise vortices in size, strength and rate of decay. 相似文献
4.
A laminar separation bubble occurs on the suction side of the SD7003 airfoil at an angle of attack α = 4–8° and a low Reynolds
number less than 100,000, which brings about a significant adverse aerodynamic effect. The spatial and temporal structure
of the laminar separation bubble was studied using the scanning PIV method at α = 4° and Re = 60,000 and 20,000. Of particular interest are the dynamic vortex behavior in transition process and the subsequent vortex
evolution in the turbulent boundary layer. The flow was continuously sampled in a stack of parallel illuminated planes from
two orthogonal views with a frequency of hundreds Hz, and PIV cross-correlation was performed to obtain the 2D velocity field
in each plane. Results of both the single-sliced and the volumetric presentations of the laminar separation bubble reveal
vortex shedding in transition near the reattachment region at Re = 60,000. In a relatively long distance vortices characterized by paired wall-normal vorticity packets retain their identities
in the reattached turbulent boundary layer, though vortices interact through tearing, stretching and tilting. Compared with
the restricted LSB at Re = 60,000, the flow at Re = 20,000 presents an earlier separation and a significantly increased reversed flow region followed by “huge” vortical structures. 相似文献
5.
The near wake of square cylinders with different corner radii was experimentally studied based on particle imaging velocimetry
(PIV), laser doppler anemometry (LDA) and hotwire measurements. Four bluff bodies, i.e., r/d=0 (square cylinder), 0.157, 0.236, 0.5 (circular cylinder), where r is corner radius and d is the characteristic dimension of the bluff bodies, were examined. A conditional sampling technique was developed to obtain
the phase-averaged PIV data in order to characterize quantitatively the effect of corner radii on the near-wake flow structure.
The results show that, as r/d increases from 0 to 0.5, the maximum strength of shed vortices attenuates, the circulation associated with the vortices decreases
progressively by 50%, the Strouhal number, St, increases by about 60%, the convection velocity of the vortices increases along
with the widening of the wake width by about 25%, the vortex formation length and the wake closure length almost double in
size. Meanwhile, both the vortex wavelength, λ
x
, and the lateral spacing, λ
y
, decrease as r/d increases, but the ratio of λ
y
to λ
x
is approximately 0.29, irrespective of r/d, which is close to the theoretical value of 0.281 for a stable Karman vortex street. The decrease in wavelength is probably
responsible for the change in the flow structure from the approximately circular-shaped vortex at r/d=0 to the laterally stretched vortex at r/d=0.5. The leading edge corner radius is more important than the trailing one in influencing the near wake structure since
it determines to a great extent the behavior of the streamlines, the separation angle and the base pressure. It is further
found that the ratio of the mean drag coefficient to the total shed circulation, C
d/Γ0, approaches a constant, about 0.25 for different bluff bodies in the subcritical flow regime. The streamwise evolution of
vortices and the streamwise fluctuating velocity along the centerline for rounded cylinders are also discussed. 相似文献
6.
High-resolution two-dimensional (2D) measurements on a large plane mixing layer provide new quantitative information of its
spatial and temporal evolution to turbulence. Periodic acoustic excitation with three frequencies was used to stabilize the
fundamental instability of the mixing layer (roll-up) and its first and second subharmonics (vortex pairings). Phase-locked
velocity measurements of the time evolution in 2D space (x, y, t) reveal accurate spatially resolved primary (2D) instabilities of the mixing layer and turbulence transition. The measurements
unveil new quantitative details of the initial Kelvin–Helmholtz waves and their spatial and temporal evolution into vortex
shedding and the effect of the second subharmonic on the first vortex pairing. The second-subharmonic effect hastens alternate
first pairings of the rollers, with the result that pairing is completed at two downstream locations. The pairings that occur
closer to the knife-edge are more organized (laminar) than those occurring farther downstream (transitional). This effect
is corroborated using Taylor’s hypothesis to compute the vorticity distributions from the measured velocity field and a pseudo-spectral
simulation of the temporal evolution of the mixing layer.
Received: 26 March 1998/Accepted: 2 March 1999 相似文献
7.
In the present study, we employed stereoscopic particle image velocimetry (PIV) to investigate the characteristics of turbulence
structures in a drag-reduced turbulent channel flow with addition of surfactant. The tested drag-reducing fluid was a CTAC/NaSal/Water
(CTAC: cetyltrimethyl ammonium chloride; NaSal: sodium salicylate) system at 25°C. The weight concentration of CTAC was 30 ppm.
Stereoscopic PIV measurement was performed for a water flow (Re=1.1×104) and a CTAC solution flow (Re=1.5×104 with 54% drag reduction) in both the streamwise–spanwise and wall-normal-spanwise planes, respectively. The three-dimensionality
of hairpin vortex structures in the near-wall region for wall-bounded turbulent flow was reproduced by conditionally averaging
the stereoscopic two-dimensional-three-component velocity fields. A series of wall-normal vortex cores were found to align
with the near-wall low-speed streaks with opposite vorticity signals at both sides of the streaks and with the vorticity decreased
on average by about one order of magnitude in CTAC solution flow compared with water flow; the spanwise spacing between the
near-wall low-speed streaks in the solution flow is increased by about 46%. The streamwise vorticity of the vortex cores appearing
in the wall-normal-spanwise plane was also decreased by the use of drag-reducing surfactant additives. 相似文献
8.
The flow characteristics of the propeller wake behind a container ship model with a rotating propeller were investigated using a two-frame PIV (Particle Image Velocimetry) technique. Ensemble-averaged mean velocity fields were measured at four different blade phases and ensemble-averaged to investigate the flow structure in the near-wake region. The mean velocity fields in longitudinal planes show that a velocity deficit is formed in the regions near the blade tips and hub. As the flow develops in the downstream direction, the trailing vortices formed behind the propeller hub move upward slightly due to the presence of the hull wake and free surface. Interaction between the bilge vortices and the incoming flow around the hull causes the flow structure to be asymmetric. Contour plots of the vorticity give information on the radial distribution of the loading on the blades. The radial velocity profiles fluctuate to a greater extent under the heavy (J=0.59) and light loading (J=0.88) conditions than under the design loading condition (J=0.72). The turbulence intensity has large values around the tip and trailing vortices. As the wake develops in the downstream direction, the strength of the vorticity diminishes and the turbulence intensity increases due to turbulent diffusion and active mixing between the tip vortices and the adjacent wake flow. 相似文献
9.
《European Journal of Mechanics - B/Fluids》2005,24(1):33-48
This paper reports an experimental investigation of the vortex shedding wake behind a long flat plate inclined at a small angle of attack to a main flow stream. Detailed velocity fields are obtained with particle-image velocimetry (PIV) at successive phases in a vortex shedding cycle at three angles of attack, α=20°, 25° and 30°, at a Reynolds number Re≈5,300. Coherent patterns and dynamics of the vortices in the wake are revealed by the phase-averaged PIV vectors and derived turbulent properties. A vortex street pattern comprising a train of leading edge vortices alternating with a train of trailing edge vortices is found in the wake. The trailing edge vortex is shed directly from the sharp trailing edge while there are evidences that the formation and shedding of the leading edge vortex involve a more complicated mechanism. The leading edge vortex seems to be shed into the wake from an axial location near the trailing edge. After shedding, the vortices are convected downstream in the wake with a convection speed roughly equal to 0.8 the free-stream velocity. On reaching the same axial location, the trailing edge vortex, as compared to the leading edge vortex, is found to possess a higher peak vorticity level at its centre and induce more intense fluid circulation and Reynolds stresses production around it. It is found that the results at the three angles of attack can be collapsed into similar trends by using the projected plate width as the characteristic length of the flow. 相似文献
10.
K.M. Lam 《Journal of Fluids and Structures》2009,25(2):245-262
This paper investigates flow past a rotating circular cylinder at 3600?Re?5000 and α?2.5. The flow parameter α is the circumferential speed at the cylinder surface normalized by the free-stream velocity of the uniform cross-flow. With particle image velocimetry (PIV), vortex shedding from the cylinder is clearly observed at α<1.9. The vortex pattern is very similar to the vortex street behind a stationary circular cylinder; but with increasing cylinder rotation speed, the wake is observed to become increasing narrower and deflected sideways. Properties of large-scale vortices developed from the shear layers and shed into the wake are investigated with the vorticity field derived from the PIV data. The vortex formation length is found to decrease with increasing α. This leads to a slow increase in vortex shedding frequency with α. At α=0.65, vortex shedding is found to synchronize with cylinder rotation, with one vortex being shed every rotation cycle of the cylinder. Vortex dynamics are studied at this value of α with the phase-locked eduction technique. It is found that although the shear layers at two different sides of the cylinder possess unequal vorticity levels, alternating vortices subsequently shed from the cylinder to join the two trains of vortices in the vortex street pattern exhibit very little difference in vortex strength. 相似文献
11.
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
12.
Alexandros Romeos Georgios Lemonis Thrassos Panidis Demos D. Papailiou 《Flow, Turbulence and Combustion》2009,83(2):153-183
Experimental evidence is reported, regarding the formation of a pair of co-rotating tip vortices by a split wing configuration,
consisting of two half wings at equal and opposite angles of attack. Simultaneous measurements of the three-dimensional vector
fields of velocity and vorticity were conducted on a cross plane at a downstream distance corresponding to 0.3 cord lengths
(near wake), using an in-house constructed 12-sensor hot wire anemometry vorticity probe. The probe consists of three closely
separated orthogonal 4-wire velocity sensor arrays, measuring simultaneously the three-dimensional velocity vector at three
closely spaced locations on a cross plane of the flow filed. This configuration makes possible the estimation of spatial velocity
derivatives by means of a forward difference scheme of first order accuracy. Velocity measurements obtained with an X-wire
are also presented for comparison. In this near wake location, the flow field is dictated by the pressure distribution established
by the flow around the wings, mobilizing large masses of air and leading to the roll up of fluid sheets. Fluid streams penetrating
between the wings collide, creating on the cross plane flow a stagnation point and an “impermeable” line joining the two vortex
centres. Along this line fluid is directed towards the two vortices, expanding their cores and increasing their separation
distance. This feeding process generates a dipole of opposite sign streamwise mean vorticity within each vortex. The rotational
flow within the vortices obligates an adverse streamwise pressure gradient leading to a significant streamwise velocity deficit
characterizing the vortices. The turbulent flow field is the result of temporal changes in the intensity of the vortex formation
and changes in the position of the cores (wandering). 相似文献
13.
Particle image velocimetry (PIV) measurements were carried out on a backward-facing step flow at a Reynolds number of Reh=UXh/9=4,660 (based on step height and freestream velocity). In-plane velocity, out-of-plane vorticity, Reynolds stress and turbulent kinetic energy production measurements in the x-y and x-z planes of the flow are presented. Proper orthogonal decomposition was performed on both the fluctuating velocity and vorticity fields of the x-y plane PIV data using the method of snapshots. Low-order representations of the instantaneous velocity fields were reconstructed using the velocity modes. These reconstructions provided insight into the contribution that the various length scales make to the spatial distribution of mean and turbulent flow quantities such as Reynolds stress and turbulent kinetic energy production. Large scales are found to contribute to the Reynolds stresses and turbulent kinetic energy production downstream of reattachment, while small scales contribute to the intense Reynolds stresses in the vicinity of reattachment. 相似文献
14.
The vortical structure of near-wake behind a sphere is investigated using a PIV technique in a circulating water channel at
Re = 11,000. The measured velocity fields show a detailed vortical structure in the recirculation region such as recirculation
vortices, reversed velocity zone, and out-of-plane vorticity distribution. The vorticity distribution of the sphere wake shows
waviness in cross-sectional planes. The time-averaged turbulent structures are consistent with the visualized flow showing
the onset of shear layer instability. The spatial distributions of turbulent intensities provide turbulent statistics for
validating numerical predictions. 相似文献
15.
Experimental dual plane particle image velocimetry (PIV) data are assessed using direct numerical simulation (DNS) data of a similar flow with the aim of studying the effect of averaging within the interrogation window. The primary reason for the use of dual plane PIV is that the entire velocity gradient tensor and hence the full vorticity vector can be obtained. One limitation of PIV is the limit on dynamic range, while DNS is typically limited by the Reynolds number of the flow. In this study, the DNS data are resolved more finely than the PIV data, and an averaging scheme is implemented on the DNS data of similar Reynolds number to compare the effects of averaging inherent to the present PIV technique. The effects of averaging on the RMS values of the velocity and vorticity are analyzed in order to estimate the percentage of turbulence intensity and enstrophy captured for a given PIV resolution in turbulent boundary layers. The focus is also to identify vortex core angle distributions, for which the two-dimensional and three-dimensional swirl strengths are used. The studies are performed in the logarithmic region of a turbulent boundary layer at z
+ = 110 from the wall. The dual plane PIV data are measured in a zero pressure gradient flow over a flat plate at Re
τ = 1,160, while the DNS data are extracted from a channel flow at Re
τ = 934. Representative plots at various wall-normal locations for the RMS values of velocity and vorticity indicate the attenuation of the variance with increasing filter size. Further, the effect of averaging on the vortex core angle statistics is negligible when compared with the raw DNS data. These results indicate that the present PIV technique is an accurate and reliable method for the purposes of statistical analysis and identification of vortex structures. 相似文献
16.
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
17.
Scanning PIV is applied to a laminar separation bubble to investigate the spanwise structure and dynamics of the roll-up of vortices within the bubble. The laminar flow separation with turbulent reattachment is studied on the suction side of an airfoil SD7003 at Reynolds numbers of 20,000–60,000. The flow is recorded with a CMOS high-speed camera in successive light-sheet planes over a time span of 1–2 s to resolve the temporal evolution of the flow in the different planes. The results show the quasi-periodic development of large vortex-rolls at the downstream end of the separation bubble, which have a convex structure and an extension of 10–20% chord length in the spanwise direction. These vortices possess an irregular spanwise pattern. The evolution process of an exemplary vortex structure is shown in detail starting from small disturbances within the separation bubble transforming into a compact vortex at the downstream end of the separation bubble. As the vortex grows in size and strength it reaches a critical state that leads to an abrupt burst of the vortex with a large ejection of fluid into the mean flow. 相似文献
18.
Large-Eddy simulations (LES) of spatially evolving turbulent buoyant round jets have been carried out with two different density
ratios. The numerical method used is based on a low-Mach-number version of the Navier–Stokes equations for weakly compressible
flow using a second-order centre-difference scheme for spatial discretization in Cartesian coordinates and an Adams–Bashforth
scheme for temporal discretization. The simulations reproduce the typical temporal and spatial development of turbulent buoyant
jets. The near-field dynamic phenomenon of puffing associated with the formation of large vortex structures near the plume
base with a varicose mode of instability and the far-field random motions of small-scale eddies are well captured. The pulsation
frequencies of the buoyant plumes compare reasonably well with the experimental results of Cetegen (1997) under different
density ratios, and the underlying mechanism of the pulsation instability is analysed by examining the vorticity transport
equation where it is found that the baroclinic torque, buoyancy force and volumetric expansion are the dominant terms. The
roll-up of the vortices is broken down by a secondary instability mechanism which leads to strong turbulent mixing and a subsequent
jet spreading. The transition from laminar to turbulence occurs at around four diameters when random disturbances with a 5%
level of forcing are imposed to a top-hat velocity profile at the inflow plane and the transition from jet-like to plume-like
behaviour occurs further downstream. The energy-spectrum for the temperature fluctuations show both −5/3 and −3 power laws,
characteristic of buoyancy-dominated flows. Comparisons are conducted between LES results and experimental measurements, and
good agreement has been achieved for the mean and turbulence quantities. The decay of the centreline mean velocity is proportional
to x
−1/3 in the plume-like region consistent with the experimental observation, but is different from the x
−1 law for a non-buoyant jet, where x is the streamwise location. The distributions of the mean velocity, temperature and their fluctuations in the near-field
strongly depend upon the ratio of the ambient density to plume density ρa/ρ0. The increase of ρa/ρ0 under buoyancy forcing causes an increase in the self-similar turbulent intensities and turbulent fluxes and an increase
in the spatial growth rate. Budgets of the mean momentum, energy, temperature variance and turbulent kinetic energy are analysed
and it is found that the production of turbulence kinetic energy by buoyancy relative to the production by shear is increased
with the increase of ρa/ρ0.
Received 16 June 2000 and accepted 26 June 2001 相似文献
19.
An experimental study was conducted to characterize the dynamic wind loads and evolution of the unsteady vortex and turbulent
flow structures in the near wake of a horizontal axis wind turbine model placed in an atmospheric boundary layer wind tunnel.
In addition to measuring dynamic wind loads (i.e., aerodynamic forces and bending moments) acting on the wind turbine model
by using a high-sensitive force-moment sensor unit, a high-resolution digital particle image velocimetry (PIV) system was
used to achieve flow field measurements to quantify the characteristics of the turbulent vortex flow in the near wake of the
wind turbine model. Besides conducting “free-run” PIV measurements to determine the ensemble-averaged statistics of the flow
quantities such as mean velocity, Reynolds stress, and turbulence kinetic energy (TKE) distributions in the wake flow, “phase-locked”
PIV measurements were also performed to elucidate further details about evolution of the unsteady vortex structures in the
wake flow in relation to the position of the rotating turbine blades. The effects of the tip-speed-ratio of the wind turbine
model on the dynamic wind loads and wake flow characteristics were quantified in the terms of the variations of the aerodynamic
thrust and bending moment coefficients of the wind turbine model, the evolution of the helical tip vortices and the unsteady
vortices shedding from the blade roots and turbine nacelle, the deceleration of the incoming airflows after passing the rotation
disk of the turbine blades, the TKE and Reynolds stress distributions in the near wake of the wind turbine model. The detailed
flow field measurements were correlated with the dynamic wind load measurements to elucidate underlying physics in order to
gain further insight into the characteristics of the dynamic wind loads and turbulent vortex flows in the wakes of wind turbines
for the optimal design of the wind turbines operating in atmospheric boundary layer winds. 相似文献
20.
Investigation of the vortex induced unsteadiness of a separation bubble via time-resolved and scanning PIV measurements 总被引:1,自引:0,他引:1
A transitional separation bubble on the suction side of an SD7003 airfoil is considered. The transition process that forces
the separated shear layer to reattach seems to be governed by Kelvin–Helmholtz instabilities. Large scale vortices are formed
due to this mechanism at the downstream end of the bubble. These vortices possess a three-dimensional structure and detach
from the recirculation region, while other vortices are formed within the bubble. This separation of the vortex is a highly
unsteady process, which leads to a bubble flapping. The structure of these vortices and the flapping of the separation bubble
due to these vortices are temporally and spatially analyzed at angles of attack from 4° to 8° and chord-length based Reynolds
numbers Re
c = 20,000–60,000 using time-resolved PIV measurements in a 2D and a 3D set-up, i.e., stereo-scanning PIV measurements are
done in the latter case. These measurements complete former studies at a Reynolds number of Re
c = 20,000. The results of the time-resolved PIV measurements in a single light-sheet show the influence of the angle of attack
and the Reynolds number. The characteristic parameters of the separation bubble are analyzed focusing on the unsteadiness
of the separation bubble, e.g., the varying size of the main recirculation region, which characterizes the bubble flapping,
and the corresponding Strouhal number are investigated. Furthermore, the impact of the freestream turbulence is investigated
by juxtaposing the current and former results. The stereo-scanning PIV measurements at Reynolds numbers up to 60,000 elucidate
the three-dimensional character of the vortical structures, which evolve at the downstream end of the separation bubble. It
is shown that the same typical structures are formed, e.g., the c-shape vortex and the screwdriver vortex at each Reynolds
number and angle of attack investigated and the occurrence of these patterns in relation to Λ-structures is discussed. To
evidence the impact of the freestream turbulence, these results are compared with findings of former measurements. 相似文献