共查询到20条相似文献,搜索用时 62 毫秒
1.
We describe the capabilities of coherent high resolution radar to observe remotely the effects of an upwelling subsurface
flow on the water surface. This observation is possible because the radar radiation backscatters very strongly from surface
features with dimensions similar to its wavelength, in this case X-band at 0.03 m. This technique provides imaging capability
with relatively high spatial resolution (∼0.3 m) and fast time sampling (∼0.006 s) over a large surface area. The processed
data reveal both the line-of-sight velocity spectrum of moving water surface features, and their water surface radar backscatter
cross-section. We believe that the surface features are generated by subsurface vortices oriented normal to the surface. The
vortices are advected with the bulk flow of the jet. Our radar observations of the down-stream flow from a submerged waterjet
that is directed parallel to the surface are consistent with those previously measured by laser velocimetry.
Received: 25 February 1994/Accepted: 16 May 1996 相似文献
2.
The character of transitional capillary flow is investigated using pressure-drop measurements and instantaneous velocity fields
acquired by microscopic PIV in the streamwise–wall-normal plane of a 536 μm capillary over the Reynolds-number range 1,800
≤ Re ≤ 3,400 in increments of 100. The pressure-drop measurements reveal a deviation from laminar behavior at Re = 1,900 with the differences between the measured and the predicted laminar-flow pressure drop increasing with increasing
Re. These observations are consistent with the characteristics of the mean velocity profiles which begin to deviate from the
parabolic laminar profile at Re = 1,900, interpreted as the onset of transition, by becoming increasingly flatter and fuller with increasing Re. A fully-turbulent state is attained at Re ≅ 3,400 where the mean velocity profile collapses onto the mean profile of fully-developed turbulent pipe flow from an existing
direct numerical simulation at Re = 5,300. Examination of the instantaneous velocity fields acquired by micro-PIV in the range 1,900 ≤ Re < 3,400 reveal that transitional flows at the microscale are composed of a subset of velocity fields illustrating a purely
laminar behavior and a subset of fields that capture significant departure from laminar behavior. The fraction of velocity
fields displaying non-laminar behavior increases with increasing Re, consistent with past observations of a growing number of intermittent turbulent spots bounded by nominally laminar flow
in macroscale pipe flow with increasing Re. Instantaneous velocity fields that are non-laminar in character consistently contain multiple spanwise vortices that appear
to streamwise-align to form larger-scale interfaces that incline slightly away from the wall. The characteristics of these
“trains” of vortices are reminiscent of the spatial features of hairpin-like vortices and hairpin vortex packets often observed
in fully-turbulent wall-bounded flow at both the macro- and micro-scales. Finally, single-point statistics computed from the
non-laminar subsets at each transitional Re, including root-mean-square velocities and the Reynolds shear stress, reveal a gradual and smooth maturation of the patches
of disordered motion toward a fully-turbulent state with increasing Re. 相似文献
3.
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
4.
《Applied Scientific Research》1996,57(3-4):279-290
This paper presents a new technique to produce controlled stretched vortices. Intense elliptical vortices are created by stretching
of an initial vorticity sheet. The initial vorticity comes from a laminar boundary layer flow and the stretching is parallel
to the vorticity vectors. This low velocity flow enables direct observation of the formation and destabilization of vortices.
Visualizations are combined with quasi-instantaneous measurements of a full velocity profile. The velocity profile is obtained
with an ultrasonic pulsed Doppler velocimeter. The evolution of the central diameter of the vortices is related to the stretching.
It is observed that destabilization occurs by pairing of two vortices, by hairpin deformation, and by breakdown of vortices
into a “coil shape”. 相似文献
5.
Late-Stage Transitional Boundary-Layer Structures. Direct Numerical Simulation and Experiment 总被引:1,自引:0,他引:1
V.I. Borodulin V.R. Gaponenko Y.S. Kachanov D.G.W. Meyer U. Rist Q.X. Lian C.B. Lee 《Theoretical and Computational Fluid Dynamics》2002,15(5):317-337
This paper is devoted to direct comparisons of related, detailed experimental and numerical studies of the non-linear, late
stages of laminar-turbulenttransition in a boundary layer including flow breakdown and the beginning offlow randomization.
Preceding non-linear stages of the transition process arealso well documented and compared with previous studies. The experiments
wereconducted with the help of a hot-wire anemometer. The numerical study wascarried out by direct numerical simulation (DNS)
of the flow employing theso-called spatial approach. Both the experiments and the DNS were performed atcontrolled disturbance
conditions with an excitation of instability waves inthe flat-plate boundary layer. In the two cases, the primary disturbanceconsists
of a time-harmonic, two-dimensional Tollmien--Schlichting wave thathas a very weak initial spanwise modulation. Despite somewhat
differentinitial disturbance conditions used in the experiment and simulation, thesubsequent flow evolution at late non-linear
stages is found to be practicallythe same. Detailed qualitative and quantitative comparisons of theinstantaneous velocity
and vorticity fields are performed for twocharacteristic stages of the non-linear flow breakdown: (i) “one-spike stage” and
(ii) “three-spike stage.” The twoapproaches clearly show in detail the process of development of the Γ-structure, a periodical
formation of ring-like vortices, the evolution of the surrounding flow field, and the beginning of flowrandomization. In particular,
it is found experimentally and numerically thatthe ring-like vortices (associated with the well-known spikes) induce somerather
intensive positive velocity fluctuations (positive spikes) in thenear-wall region which have the same scales as the ring-like
vortices and propagate downstream with the same high (almost free-stream) speed. The positive spikes form a new high-shear
layer in the near-wall region. In the experiment the induced near-wall perturbationshave a significant irregular low-frequency
component. These non-periodicalmotions play an important role in the process of flow randomization and finaltransition to
turbulence that starts under the ring-like vortices in thevicinity of the peak position.
Received 13 December 2000 and accepted 30 October 2001 相似文献
6.
A new technique to produce controlled stretched vortices is presented. The initial vorticity comes from a laminar boundary
layer flow and the stretching is parallel to the initial vorticity. This low velocity flow enables direct observations of
the formation and destabilization of vortices. Visualizations are combined with quasi-instantaneous measurements of a full
velocity profile obtained with an ultra-sonic pulsed Doppler velocimeter. Several modes of destabilization are observed and
include pairing of two vortices, hairpin deformation, and vortex breakdown into a coil shape.
Received: 3 April 1996/ Accepted: 4 October 1996 相似文献
7.
C. B. Lee Z. X. Hong Y. S. Kachanov V. I. Borodulin V. V. Gaponenko 《Experiments in fluids》2000,28(3):243-251
This study is concerned with transition in flat plate boundary layer flow. Sets of results are obtained as follows: (1) Very
clear pictures of the formation and the development of the butterfly-like structures rather than ∧-structures in the K-regime of boundary layer transition are obtained. (2) A chain of ring like vortices, which generate the high-frequency spikes
on the time traces of velocity and still present periodical behaviour, at the tip of each ∧-vortex, which is the part of the
butterfly-like structure, are visualized for the first time. (3) A wave-like structure is observed to occupy the whole boundary
layer, extending from the near-wall region to the outer edge of the boundary layer.
Received: 24 September 1998/Accepted: 24 April 1999 相似文献
8.
The near-wall flow structures of a turbulent boundary layer over a riblet surface with semi-circular grooves were investigated
experimentally for the cases of drag decreasing (s
+=25.2) and drag increasing (s
+=40.6). One thousand instantaneous velocity fields over riblets were measured using the velocity field measurement technique
and compared with those above a smooth flat plate. The field of view was 6.75 × 6.75 mm2 in physical dimension, containing two grooves. Those instantaneous velocity fields were ensemble averaged to get turbulent
statistics including turbulent intensities and turbulent kinetic energy. To see the global flow structure qualitatively, flow
visualization was also carried out using the synchronized smoke-wire technique under the same experimental conditions. For
the case of drag decreasing (s
+=25.2), most of the streamwise vortices stay above the riblets, interacting with the riblet tips frequently. The riblet tips
impede the spanwise movement of the streamwise vortices and induce secondary vortices. The normalized rms velocity fluctuations
and turbulent kinetic energy are small near the riblet surface, compared with those over a smooth flat plate. Inside the riblet
valleys, these are sufficiently small that the increased wetted surface area of the riblets can be compensated. In addition,
in the outer region (y
+ > 30), these values are almost equal to or slightly smaller than those for the smooth plate. For the case of drag increasing
(s
+=40.6), however, most of the streamwise vortices stay inside the riblet valleys and contact directly with the riblet surface.
The high-speed down-wash flow penetrating into the riblet valley interacts actively with the wetted riblet surface and increases
the skin friction. The rms velocity fluctuations and turbulent kinetic energy have larger values compared with those over
a smooth flat plate.
Received: 24 March 1999/Accepted: 10 March 2000 相似文献
9.
Zhang Hongquan 《Acta Mechanica Sinica》1998,14(2):104-112
The physical mechanism for generation of streamwise vortices (or rib vortices) in the cylinder wake is numerically investigated
with a finite-difference scheme. Rayleigh's theory of centrifugal instability for inviscid axisymmetric flow is extended to
analyze the 2-D primary flows. Accordingly, an analytical dimensionless groupRay=−(r/v
θ)∂v
θ/∂r−1 is derived, wherev
θ represents the velocity of a fluid element relative to the oncoming flow,r is the local curvature radius of the element pathline. Centrifugal instability occurs whenRay>0. Stability analyses are carried out with this discriminant for primary flows at different time levels in a half shedding
period of the von Kármán (or vK) vortices. Unstable areas are identified and the locations of rib vortices are coincident
well with the unstable areas within the first wavelength of vK vortices behind the cylinder. The numerical results also show
that rib vortices experience amplification in this region. It is apparent that centrifugal instability plays an important
role in the generation of rib vortices in the cylinder wake.
The project spported by the National Natural Science Foundation of China 相似文献
10.
Shiyao Bian James F. Driscoll Brian R. Elbing Steven L. Ceccio 《Experiments in fluids》2011,51(1):51-63
High Reynolds number, low Mach number, turbulent shear flow past a rectangular, shallow cavity has been experimentally investigated
with the use of dual-camera cinematographic particle image velocimetry (CPIV). The CPIV had a 3 kHz sampling rate, which was
sufficient to monitor the time evolution of large-scale vortices as they formed, evolved downstream and impinged on the downstream
cavity wall. The time-averaged flow properties (velocity and vorticity fields, streamwise velocity profiles and momentum and
vorticity thickness) were in agreement with previous cavity flow studies under similar operating conditions. The time-resolved
results show that the separated shear layer quickly rolled-up and formed eddies immediately downstream of the separation point.
The vortices convect downstream at approximately half the free-stream speed. Vorticity strength intermittency as the structures
approach the downstream edge suggests an increase in the three-dimensionality of the flow. Time-resolved correlations reveal
that the in-plane coherence of the vortices decays within 2–3 structure diameters, and quasi-periodic flow features are present
with a vortex passage frequency of ~1 kHz. The power spectra of the vertical velocity fluctuations within the shear layer
revealed a peak at a non-dimensional frequency corresponding to that predicted using linear, inviscid instability theory. 相似文献
11.
The baseline and forced flow around a bluff body with semi-elliptical D-shape was investigated by solving the 2D Navier–Stokes
equations at low Reynolds numbers. A D-shape rather than the canonic circular-cylinder was selected due to the fixed separation
points in the latter, enabling to study a pure wake rather than boundary-layer control. The correlation between Strouhal and
Reynolds numbers, the mean drag, the lift and drag oscillations vs. the Reynolds number and wake structure were investigated
and compared to experimental and numerical data. Effects of open-loop forcing, resulting from the influence of zero-mass-flux
actuators located at the fixed separation points, were studied at a Reynolds number of 150. Fluidic rather than body motion
or volume forcing was selected due to applicability considerations. The motivation for the study was to quantify the changes
in the flow field features, as captured by Proper Orthogonal Decomposition (POD) analysis, due to open-loop forcing, inside
and outside the “lock-in” regime. This is done in order to evaluate the suitability of low-order-models based on POD modes
of this changing flow field, for future feed-back flow control studies. The evolution of the natural and the excited vortices
in the Kármán wake were also investigated. The formation and convection regions of the vortex evolution were documented. It
was found that the forcing causes an earlier detachment of the vortices from the boundary-layers, but does not affect their
circulation or convection speeds. The results of the POD analysis of the near-wake flow show that the influence of the bluff
body shape (“D”-shaped versus circular cylinder) on the baseline POD wake modes is small. It was found that the eigenfunctions
(mode-shapes) of the POD velocity modes are less sensitive to slot excitation than the vorticity modes. As a result of the
open-loop excitation, two types of mode-shape-change were observed: a mode can be exchanged with a lower-energy mode or shifted
to a low energy level. In the latter case, the most energetic mode becomes the “actuator” mode. The evolution of one-slot
excitation on still fluid (“Synthetic jet”) was studied and compared to published data and to “actuator” modes with external
flow present. Based on the current findings, it is hypothesized that the cross-flow velocity POD modes are suitable for feedback
control of wake flow using periodic excitation, due to their low sensitivity to the excitation as compared to the streamwise
velocity or vorticity modes. 相似文献
12.
A purely alternating jet without mean mass flux and a mixed pulsed jet containing an additional blowing component were investigated
by particle image velocimetry (PIV). The jets issued from a two-dimensional slit connected to a converging nozzle, opening
normally from a flat wall. The pulsation was driven by a loudspeaker. The mean velocity fields were characterized by the combination
of downstream directional blowing and omni-directional suction. The velocity fluctuations were dominated by contra-rotating
eddy pairs synchronized with the pulsation and formed at the jet edges during blowing. Phase-synchronized measurements permit
the investigation of the averaged patterns and the cycle-to-cycle fluctuations of these vortices. The mean trajectories of
vortex centers during a whole injection cycle show how large lateral jet expansions are achieved. For a purely alternating jet, the expansion takes place close to the slit. For a mixed pulsed jet, the vortices develop farther from the orifice. In addition, proper orthogonal mode decomposition demonstrates that only
a few modes are required to represent the main events of the flow dynamics.
Received: 10 August 1999 / Accepted: 10 January 2001 相似文献
13.
To better understand mixing by hairpin vortices, time-series particle image velocimetry (PIV) was applied to the wake of
a trapezoidal-shaped passive mixing tab mounted at the bottom of a square turbulent channel (Re
h
=2,080 based on the tab height). Instantaneous velocity/vorticity fields were obtained in sequences of 10 Hz in the tab wake
in the center plane (x–y) and in a plane (x–z) parallel to the wall. Periodically-shed hairpin vortices were clearly identified and seen to rise as they advected downstream.
Experimental evidence shows that the vortex-induced ejection of the near-wall viscous fluid to the immediate upstream is important
to the dynamics of hairpin vortices. It can increase the strength of the hairpin vortices in the near tab region and cause
generation of secondary hairpin vortices further downstream when the hairpin heads are farther away from the wall. Measurements
also reveal the existence of a type of new secondary vortice with the opposite-sign spanwise vorticity. The distribution of
vortex loci in the x–y plane shows that the hairpin vortices and the reverse vortices are spatially segregated in distinct layers. Turbulence statistics,
including mean velocity profiles, Reynolds stresses, and turbulent kinetic energy dissipation rate distributions, were obtained
from the PIV data. These statistical quantities clearly reveal imprints of the identified vortex structures and provide insight
into mixing effectiveness.
Received: 24 February 2000/Accepted: 24 October 2000 相似文献
14.
The detailed flow structure behind an impulsively started circular cylinder has been investigated experimentally. The Reynolds
number based on the steady state velocity and the diameter of the cylinder was 500 to 3,000. This work is unique in that unsteady
spatial velocities were measured simultaneously by a quantitative visualization technique — Laser Induced Photochemical Anemometry
(LIPA). The surface vorticity at g/q = π/2 and vorticity distribution behind the cylinder in the Lagrangian coordinates (i.e. coordinates fixed on the cylinder)
were calculated from the measured velocities. The surface vorticity shows in the early stage of flow development a close agreement
with the previous results obtained by analytical and numerical approaches. The large-field velocity and vorticity information
provides an insight into the formation process of the vortices downstream of the cylinder. In addition to the quantitative
information, the results of visualized flow pattern obtained by LIPA technique are also presented.
A preliminary version of this paper was presented at the Twelfth Symposium on Turbulence, University of Missouri-Rolla, Sept.
24–26, 1990 相似文献
15.
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
16.
The near-ground flow structure of tornadoes is of utmost interest because it determines how and to what extent civil structures
could get damaged in tornado events. We simulated tornado-like vortex flow at the swirl ratios of S = 0.03–0.3 (vane angle θv = 15°–60°), using a laboratory tornado simulator and investigated the near-ground-vortex structure by particle imaging velocimetry.
Complicated near-ground flow was measured in two orthogonal views: horizontal planes at various elevations (z = 11, 26 and 53 mm above the ground) and the meridian plane. We observed two distinct vortex structures: a single-celled
vortex at the lowest swirl ratio (S = 0.03, θv = 15°) and multiple suction vortices rotating around the primary vortex (two-celled vortex) at higher swirl ratios (S = 0.1–0.3, θv = 30°–60°). We quantified the effects of vortex wandering on the mean flow and found that vortex wandering was important
and should be taken into account in the low swirl ratio case. The tangential velocity, as the dominant velocity component,
has the peak value about three times that of the maximum radial velocity regardless of the swirl ratio. The maximum velocity
variance is about twice at the high swirl ratio (θv = 45°) that at the low swirl ratio (θv = 15°), which is contributed significantly by the multiple small-scale secondary vortices. Here, the results show that not
only the intensified mean flow but greatly enhanced turbulence occurs near the surface in the tornado-like vortex flow. The
intensified mean flow and enhanced turbulence at the ground level, correlated with the ground-vortex interaction, may cause
dramatic damage of the civil structures in tornadoes. This work provides detailed characterization of the tornado-like vortex
structure, which has not been fully revealed in previous field studies and laboratory simulations. It would be helpful in
improving the understanding of the interaction between the tornado-like vortex structure and the ground surface, ultimately
leading to better predictions of tornado-induced wind loads on civil structures. 相似文献
17.
Three-component velocity field measurements of propeller wake using a stereoscopic PIV technique 总被引:1,自引:0,他引:1
A stereoscopic PIV (Particle Image Velocimetry) technique was used to measure the three-dimensional flow structure of the turbulent wake behind a marine propeller with five blades. The out-of-plane velocity component was determined using two CCD cameras with an angular displacement configuration. Four hundred instantaneous velocity fields were measured for each of four different blade phases, and ensemble averaged in order to find the spatial evolution of the propeller wake in the region from the trailing edge up to one propeller diameter (D) downstream. The influence of propeller loading conditions on the wake structure was also investigated by measuring the velocity fields at three advance ratios (J=0.59, 0.72 and 0.88). The phase-averaged velocity fields revealed that a viscous wake formed by the boundary layers developed along the blade surfaces. Tip vortices were generated periodically and the slipstream contracted in the near-wake region. The out-of-plane velocity component and strain rate had large values at the locations of the tip and trailing vortices. As the flow moved downstream, the turbulence intensity, the strength of the tip vortices, and the magnitude of the out-of-plane velocity component at trailing vortices all decreased due to effects such as viscous dissipation, turbulence diffusion, and blade-to-blade interaction. 相似文献
18.
Kendra V. Sharp David Hill Daniel Troolin Geoffrey Walters Wing Lai 《Experiments in fluids》2010,48(1):167-183
Volumetric three-component velocimetry measurements have been taken of the flow field near a Rushton turbine in a stirred
tank reactor. This particular flow field is highly unsteady and three-dimensional, and is characterized by a strong radial
jet, large tank-scale ring vortices, and small-scale blade tip vortices. The experimental technique uses a single camera head
with three apertures to obtain approximately 15,000 three-dimensional vectors in a cubic volume. These velocity data offer
the most comprehensive view to date of this flow field, especially since they are acquired at three Reynolds numbers (15,000,
107,000, and 137,000). Mean velocity fields and turbulent kinetic energy quantities are calculated. The volumetric nature
of the data enables tip vortex identification, vortex trajectory analysis, and calculation of vortex strength. Three identification
methods for the vortices are compared based on: the calculation of circumferential vorticity; the calculation of local pressure
minima via an eigenvalue approach; and the calculation of swirling strength again via an eigenvalue approach. The use of two-dimensional
data and three-dimensional data is compared for vortex identification; a ‘swirl strength’ criterion is less sensitive to completeness
of the velocity gradient tensor and overall provides clearer identification of the tip vortices. The principal components
of the strain rate tensor are also calculated for one Reynolds number case as these measures of stretching and compression
have recently been associated with tip vortex characterization. Vortex trajectories and strength compare favorably with those
in the literature. No clear dependence of trajectory on Reynolds number is deduced. The visualization of tip vortices up to
140° past blade passage in the highest Reynolds number case is notable and has not previously been shown. 相似文献
19.
Sébastien Michelin Stefan G. Llewellyn Smith 《Theoretical and Computational Fluid Dynamics》2009,23(2):127-153
A method is proposed for the study of the two-dimensional coupled motion of a general sharp-edged solid body and a surrounding
inviscid flow. The formation of vorticity at the body’s edges is accounted for by the shedding at each corner of point vortices
whose intensity is adjusted at each time step to satisfy the regularity condition on the flow at the generating corner. The
irreversible nature of vortex shedding is included in the model by requiring the vortices’ intensity to vary monotonically
in time. A conservation of linear momentum argument is provided for the equation of motion of these point vortices (Brown–Michael
equation). The forces and torques applied on the solid body are computed as explicit functions of the solid body velocity
and the vortices’ position and intensity, thereby providing an explicit formulation of the vortex–solid coupled problem as
a set of non-linear ordinary differential equations. The example of a falling card in a fluid initially at rest is then studied
using this method. The stability of broadside-on fall is analysed and the shedding of vorticity from both plate edges is shown
to destabilize this position, consistent with experimental studies and numerical simulations of this problem. The reduced-order
representation of the fluid motion in terms of point vortices is used to understand the physical origin of this destabilization.
相似文献
20.
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