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1.
The dye visualization experiments show that a dual leading-edge vortex (LEV) structure exists on the suction side of a simplified
butterfly model of Papilio ulysses at α = 8°−12°. Furthermore, the results of particle image velocimetry (PIV) measurement indicate that the axial velocity of the
primary (outer) vortex core reaches the lower extreme value while a transition from a “wake-like” to a “jet-like” axial velocity
profile occurs. The work reveals for the first time the existence of dual LEV structure on the butterfly-like forward-sweep
wing configuration. 相似文献
2.
Results are presented from an experimental investigation into the interaction of a planar shock wave with a vortex ring. A
free-falling spherical soap bubble is traversed by the incident shock wave and develops into a vortex ring as a result of
baroclinically deposited vorticity (?r×?p 1 0{\nabla\rho\times\nabla p \neq 0}). The vortex ring translates with a velocity relative to the particle velocity behind the shock wave due to circulation.
After the shock wave reflects from the tube end wall, it traverses the vortex ring (this process is called “reshock”) and
deposits additional vorticity. Planar Mie scattering is used to visualize the atomized soap film at high frame rates (up to
10,000 fps). Particle image velocimetry (PIV) was performed for an argon bubble in nitrogen accelerated by a M = 1.35 shock wave. Circulation was determined from the PIV velocity field and found to agree well with Kelvin’s vortex ring
model. 相似文献
3.
By direct numerical simulation of the Navier-Stokes equations we investigate the reconnection of two antiparallel vortex tubes.
A new type of perturbation of the initial vorticity field is given which is different from that presented in Refs. [8] and
[9]. The formation and the evolution of the “curved vortex belts”, their mutual action with the “bridges” are found. These
are important phenomena not studied by others.
The project supported by the LNM of Institute of Mechanics, Academia Sinica and The National Natural Science Foundation of
China 相似文献
4.
Singular value decomposition (SVD) is often used as a tool to analyze particle image velocimetry (PIV) data. However, experimental error tends to corrupt higher SVD modes, in which the root mean square velocity value is
smaller than the experimental error. Therefore, we suggest that the threshold criterion, $s_k >\sqrt{DT}\epsilon,$s_k >\sqrt{DT}\epsilon, can be used as a rough limit of the validity of SVD modes extracted from experimental data (where s
k
is the singular value of mode k, D and T are the number of data sites and time steps, respectively, and e\epsilon is the root mean square PIV error). By synthesizing the relationship between the general SVD procedure and its two special
cases—biorthogonal decomposition (BOD) and proper orthogonal decomposition (POD)—we show that our criterion can be used to assess modes extracted by either BOD or POD. We apply our threshold criterion
to PIV data of the wake behind a live swimming Giant Danio (Danio aequipinnatus). The biorthogonal decomposition of the fish wake, which is a reverse-Kármán street, reveals that the first four modes are
similar to the modes of a regular Kármán street created in the wake of a stationary cylinder and that higher modes are corrupted
by experimental error. 相似文献
5.
Oscillating flow near the end of a stack of parallel plates placed in a standing wave resonator is investigated using particle
image velocimetry (PIV). The Reynolds number, Re
d
, based on the plate thickness and the velocity amplitude at the entrance to the stack, is controlled by varying the acoustic
excitation (so-called drive ratio) and by using two configurations of the stacks. As the Reynolds number changes, a range
of distinct flow patterns is reported for the fluid being ejected from the stack. Symmetrical and asymmetrical vortex shedding
phenomena are shown and two distinct modes of generating “vortex streets” are identified. 相似文献
6.
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
7.
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
8.
M. Sugawara E. Tamura Y. Satoh Y. Komatsu M. Tago H. Beer 《Heat and Mass Transfer》2007,43(10):1009-1018
Visual observations reveal a complicated flow in the liquid melt and a melting front configuration resulting from horizontal
ice plate melting from above into a 20 wt% calcium chloride aqueous solution. The initial temperature of the ice plate and
the mixture are both −5°C. Small scale “mountain and valley” structures (∼1 mm) appear on the flat melting front just after
melting begins, which have been called “sharkskin”. Innumerable upward and downward flows appear near the sharkskin and are controlled by its “mountain and valley” structure. These typical flows will considerably promote the melting of the
ice plate to be 30% larger as compared to the numerically predicted results assuming a flat melting front (i.e., without the
sharkskin), and also by three times larger compared with the results for melting from below. 相似文献
9.
V. B. Kurzin 《Journal of Applied Mechanics and Technical Physics》2006,47(3):346-358
A model for separated incompressible flow past thin airfoils in the neighborhood of the “shockless entrance” condition is
constructed based on the averaging of the vortex shedding flow past the airfoil edges. By approximation of the vortex shedding
by two vortex curves, determination of the average hydrodynamic parameters is reduced to a twofold solution of an integral
singular equation equivalent to the equation describing steady-state nonseparated airfoil flow. In this case, the calculation
time is two orders of magnitude smaller than the time required for the solution of the corresponding evolution problem. The
results of a test calculation using the proposed method are in fair agreement with available results of calculations and experiments.
__________
Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 47, No. 3, pp. 49–63, May–June, 2006. 相似文献
10.
The evolution of wake structures and variation of the forces on a flat plate in harmonic oscillatory and in-line combined
flows are obtained numerically by improved discrete vortex method. For the oscillatory oncoming flow cases, wyenKc number varies from 2 to 40, the vortex pattern changes from a “harmonic wave” shaped (in a range of smallKc number) to a slight inclined “harmonic wave” shaped (in a range of moderateKc numbers), then to inclined vortex clusters with an angle of 50° to the oncoming flow direction (atKc=20), at last, asKc number becomes large, the vortex pattern is like a normal Karman vortex street. The well predicted drag and inertia force
coefficients are obtained, which are more close to the results of Keulegan & Carpenter's experiment as compared with previous
vortex simulation by other authors. The existence of minimum point of inertia force coefficientC
m
nearKc=20 is also well predicted and this phenomenon can be interpreted according to the vortex structure. For steady-oscillatory
in-line combined flow cases, the vortex modes behave like a vortex street, exhibit a “longitudinal wave” structure, and a
vortex cluster shape corresponding to the ratios ofU
m
toU
0 which are ofO (10−1)O(1) andO(10), respectively. The effect on the prediction of forces on the flat plate from the disturbance component in a combined
flow has been demonstrated qualitatively. In addition to this, the lock in phenomenon of vortex shedding has been checked.
The project supported by National Natural Science Foundation of China & LNM, Institute of Mechanics, CAS 相似文献
11.
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
12.
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
13.
V. L. Berdichevsky 《Continuum Mechanics and Thermodynamics》2008,20(4):219-229
Ideal incompressible fluid is a Hamiltonian system which possesses an infinite number of integrals, the circulations of velocity
over closed fluid contours. This allows one to split all the degrees of freedom into the driving ones and the “slave” ones,
the latter to be determined by the integrals of motions. The “slave” degrees of freedom correspond to “potential part” of
motion, which is driven by vorticity. Elimination of the “slave” degrees of freedom from equations of ideal incompressible
fluid yields a closed system of equations for dynamics of vortex lines. This system is also Hamiltonian. The variational principle
for this system was found recently (Berdichevsky in Thermodynamics of chaos and order, Addison-Wesly-Longman, Reading, 1997;
Kuznetsov and Ruban in JETP Lett 67, 1076–1081, 1998). It looks striking, however. In particular, the fluid motion is set
to be compressible, while in the least action principle of fluid mechanics the incompressibility of motion is a built-in property.
This striking feature is explained in the paper, and a link between the variational principle of vortex line dynamics and
the least action principle is established. Other points made in this paper are concerned with steady motions. Two new variational
principles are proposed for steady vortex flows. Their relation to Arnold’s variational principle of steady vortex motion
is discussed.
相似文献
14.
K. Prestridge P. M. Rightley P. Vorobieff R. F. Benjamin N. A. Kurnit 《Experiments in fluids》2000,29(4):339-346
We describe a highly-detailed experimental characterization of the Richtmyer-Meshkov instability (the impulsively driven
Rayleigh-Taylor instability) (Meshkov 1969; Richtmyer 1960). In our experiment, a vertical curtain of heavy gas (SF6) flows into the test section of an air-filled, horizontal shock tube. The instability evolves after a Mach 1.2 shock passes
through the curtain. For visualization, we pre-mix the SF6 with a small (∼10−5) volume fraction of sub-micron-sized glycol/water droplets. A horizontal section of the flow is illuminated by a light sheet
produced by a combination of a customized, burst-mode Nd:YAG laser and a commercial pulsed laser. Three CCD cameras are employed
in visualization. The “dynamic imaging camera” images the entire test section, but does not detect the individual droplets.
It produces a sequence of instantaneous images of local droplet concentration, which in the post-shock flow is proportional
to density. The gas curtain is convected out of the test section about 1 ms after the shock passes through the curtain. A
second camera images the initial conditions with high resolution, since the initial conditions vary from test to test. The
third camera, “PIV camera,” has a spatial resolution sufficient to detect the individual droplets in the light sheet. Images
from this camera are interrogated using Particle Image Velocimetry (PIV) to recover instantaneous snapshots of the velocity
field in a small (19 × 14 mm) field of view. The fidelity of the flow-seeding technique for density-field acquisition and
the reliability of the PIV technique are both quantified in this paper. In combination with wide-field density data, PIV measurements
give us additional physical insight into the evolution of the Richtmyer-Meshkov instability in a problem which serves as an
excellent test case for general transition-to-turbulence studies.
Received: 26 June 1999/Accepted: 29 October 1999 相似文献
15.
Numerical investigation on the flowfield of "swallowtail" cavity for supersonic mixing enhancement 总被引:1,自引:0,他引:1
A "swallowtail" cavity for the supersonic combustor was proposed to serve as an efficient flame holder for scramjets by enhancing the mass exchange between the cavity and the main flow. A numerical study on the "swallow- tail" cavity was conducted by solving the three-dimensional Reynolds-averaged Navier-Stokes equations implemented with a k-e turbulence model in a multi-block mesh. Turbu- lence model and numerical algorithms were validated first, and then test cases were calculated to investigate into the mechanism of cavity flows. Numerical results demonstrated that the certain mass in the supersonic main flow was sucked into the cavity and moved spirally toward the combustor walls. After that, the flow went out of the cavity at its lateral end, and finally was efficiently mixed with the main flow. The comparison between the "swallowtail" cavity and the conventional one showed that the mass exchanged between the cavity and the main flow was enhanced by the lateral flow that was induced due to the pressure gradient inside the cavity and was driven by the three-dimensional vortex ring generated from the "swallowtail" cavity structure. 相似文献
16.
A new model is proposed for the entrainment rate by vortices across stratified interfaces. In the model, different entrainment
regimes are distinguished by the conventional parameters Richardson, Reynolds, and Schmidt number as well as a new parameter,
the “vortex persistence”. Vortex persistence is defined as the number of rotations a vortex makes during the time it moves
its own diameter with respect to the interface. It is further proposed that the concept of vortex persistence is important
whenever a vortex is near any kind of surface, either stratified or solid. The model is in accord with most field and laboratory
observations in a variety of stratified and bounded flows, including measurements of wall heat transfer and vortex formation
in starting jets. 相似文献
17.
Oksana Stalnov Vitali Palei Ilan Fono Kelly Cohen Avi Seifert 《Experiments in fluids》2007,42(4):531-542
The effectiveness of a small array of body-mounted sensors, for estimation and eventually feedback flow control of a D-shaped
cylinder wake is investigated experimentally. The research is aimed at suppressing unsteady loads resulting from the von-Kármán
vortex shedding in the wake of bluff-bodies at a Reynolds number range of 100–1,000. A low-dimensional proper orthogonal decomposition
(POD) procedure was applied to the stream-wise and cross-stream velocities in the near wake flow field, with steady-state
vortex shedding, obtained using particle image velocimetry (PIV). Data were collected in the unforced condition, which served
as a baseline, as well as during influence of forcing within the “lock-in” region. The design of sensor number and placement
was based on data from a laminar direct numerical simulation of the Navier-Stokes equations. A linear stochastic estimator
(LSE) was employed to map the surface-mounted hot-film sensor signals to the temporal coefficients of the reduced order model
of the wake flow field in order to provide accurate yet compact estimates of the low-dimensional states. For a three-sensor
configuration, results show that the estimation error of the first two cross-stream modes is within 20–40% of the PIV-generated
POD time coefficients. Based on previous investigations, this level of error is acceptable for a moderately robust controller
required for feedback flow control. 相似文献
18.
19.
A computational study of spatially evolving two-dimensional free shear flows has been performed using direct numerical simulation
of the Navier–Stokes equations in order to investigate the ability of these two-dimensional simulations to predict the overall
flow-field quantities of the corresponding three-dimensional “real” turbulent flows. The effects of inflow forcing on these
two-dimensional flows has also been studied. Simulations were performed of shear layers, as well as weak (large co-flow and
relatively weak shear) and strong (small co-flow and relatively strong shear) jets. Several combinations of discrete forcing
with and without a broadband background spectrum were used. Although spatially evolving direct simulations of shear layers
have been performed in the past, no such simulations of the plane jet have been performed to the best of our knowledge.
It was found that, in the two-dimensional shear layers, external forcing led to a strong increase in the initial growth of
the shear-layer thickness, followed by a region of decreased growth as in physical experiments. The final downstream growth
rate was essentially unaffected by forcing. The mean velocity profile and the naturally evolving growth rate of the shear
layer in the case of broadband forcing compare well with experimental data. However, the total and transverse fluctuation
intensities are larger in the two-dimensional simulations with respect to experimental data.
In the weak-jet simulations it was found that symmetric forcing completely overwhelms the natural tendency to transition to
the asymmetric jet column mode downstream. It was observed that two-dimensional simulations of “strong” jets with a low speed
co-flow led to a fundamentally different flow with large differences even in mean velocity profiles with respect to experimental
data for planar jets. This was a result of the dominance of the two-dimensional mechanism of vortex dipole ejection in the
flow due to the lack of spanwise instabilities. Experimental studies of planar jets do not show vortex dipole formation and
ejection. A three-dimensional “strong”-jet simulation showed the rapid evolution of three-dimensionality effectively preventing
this two-dimensional mechanism, as expected from experimental results.
Received: 25 November 1996 and accepted 17 April 1997 相似文献
20.
The axisymmetric vortex sheet model developed by Nitsche & Krasny (1994) has been extended to study the formation of vortex
rings (pairs) at the edge of circular (2D) tube and opening. Computations based on this model are in good agreement with the
experiments (Didden (1979) for circular tube and Auerbach (1987) for 2D tube and opening). Using this new model, evidences
are provided to show that the main failure of the similarity theory (the false prediction of axial trajectory of vortex ring)
is due to its ignorance of the self-induced ring velocity (mutual induction for vortex pair). We further reason why the similarity
theory succeeds in its prediction of radial movement of vortex ring. The effects of various parameters such as turning angle
α and piston speedU
p
(t) on the formation of vortex ring are investigated. Numerical result shows that turning angle α has no effect on circulation
shed τ. We also discuss Glezer (1988)'s summary on the influence ofU
p
upon the shedding circulation, and finally give the variation of core distribution of vortex ring with α andU
p
(t).
The project is supported by National Natural Science Foundation of China and Doctoral Program of Institution of Higher Education 相似文献