共查询到20条相似文献,搜索用时 0 毫秒
1.
Active and passive flow control methods have been studied for decades, but there have been only a few studies of flow control methods using ion wind, which is the bulk motion of neutral molecules driven by locally ionized air from a corona discharge. This paper describes an experimental study of ion wind wake control behind a circular cylinder. The experimental conditions consisted of a range of electrohydrodynamic numbers—the ratio of an electrical body force to a fluid inertial force—from 0 to 2 and a range of Reynolds numbers from 4×103 to 8×103. Pressure distributions over the cylinder surface were measured and flow visualizations were carried out using a smoke-wire method. The flow visualizations confirmed that ion wind significantly affects the wake structure behind a circular cylinder, and that the pressure drag can be dramatically reduced by superimposing ion wind.List of symbols
BR
blockage ratio
-
C
d
coefficient of the pressure drag
-
C
p
coefficient of the surface pressure, 2(p–p
0)/(U
0
2)
-
C
pb
coefficient of the base surface pressure, 2(p
b–p
0)/(U
0
2)
-
D
diameter of the cylinder
-
D
P
pressure drag
-
d
p
diameter of particle
-
E
the electric field
-
F
e
Coulombian force (qE)
-
F
v
viscous force
-
H
wire-to-cylinder spacing
-
I
total electric current (A)
-
L
the axial length of cylinder (m)
-
N
EHD
electrohydrodynamic number
-
p
b
base pressure of cylinder at =180°
-
p
0
reference static pressure at 10D upstream
-
q
the charge on the particle
-
R
radius of the cylinder
-
V
applied voltage (kV)
-
U
0
mean flow velocity (m/s)
-
ion mobility in air (m2/(s V))
-
0
permittivity of free space
-
viscosity of fluid (kg/ms)
-
density of fluid (kg/m3)
-
installation angle of a wire electrode (°) 相似文献
2.
Adjoint formulation is employed for the optimal control of flow around a rotating cylinder, governed by the unsteady Navier–Stokes equations. The main objective consists of suppressing Karman vortex shedding in the wake of the cylinder by controlling the angular velocity of the rotating body, which can be constant in time or time‐dependent. Since the numerical control problem is ill‐posed, regularization is employed. An empirical logarithmic law relating the regularization coefficient to the Reynolds number was derived for 60?Re?140. Optimal values of the angular velocity of the cylinder are obtained for Reynolds numbers ranging from Re=60 to Re=1000. The results obtained by the computational optimal control method agree with previously obtained experimental and numerical observations. A significant reduction of the amplitude of the variation of the drag coefficient is obtained for the optimized values of the rotation rate. Copyright © 2002 John Wiley & Sons, Ltd. 相似文献
3.
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
4.
Feedback control of vortex shedding from a circular cylinder 总被引:3,自引:0,他引:3
X. Y. Huang 《Experiments in fluids》1996,20(3):218-224
This paper presents an experimental study on the suppressing of vortex shedding from a circular cylinder by feedback sound. Experiments were performed in a wind tunnel, and the feedback sound was generated inside the cylinder and locally introduced into the flow through a thin slit on the cylinder surface. In this way, the shear flow nearest to the slit was directly manipulated during the control. The results show that the suppression of vortex shedding can be achieved at Reynolds numbers ranging from 4×103 to 1.3× 104, according to signals from a hot-wire checking throughout the wake and signals from a remote microphone. This local and one-sided feedback, being different from other control techniques, allows a better understanding of the control mechanism, which in this case probably causes a destructive interaction between two shear flows separated from both sides of the cylinder. The technique has been useful to deepen our understanding to the wake instabilities behind the cylinder.This work was done when the author was a visiting Scientist at the Institute of Technical Acoustics, Technical University Berlin. The author wishes to express his gratitude to Prof. M. Heckl and Prof. M. Möser for the arrangement and encouragement in this research. The help from Mr. M. Hansen in the experiment is acknowledged. Thanks also go to the German Science Foundation for the financial support. 相似文献
5.
Ling Guocan 《Acta Mechanica Sinica》1988,4(3):211-222
In the present paper the mechanism involved in vortex shedding flows is investigated in detail. In the early stage of the
unsteady separated flow the interaction between secondary vortices and primary vortices is quite strong. In the later stage
of the flow, corresponding to the vortex shedding the recirculating flow region on each side of the aft body goes through
such a cycle: growth-contraction-growth, the secondary separation occurs periodically rather than continuously. The reduction
of circulation is taken into account in three cases with different decay factors to study its influence on the prediction
of main flow characteristics. Results show that to simulate vortex shedding flow it is necessary to include the reduction
of circulation to bring the calculated results into good agreement with experiments. An improved discrete vortex model is
suggested in which both the secondary separation and the reduction are incorporated. The processes of vortex shedding, the
forces prediction and other flow characteristics are given and some discussions are made.
Porject is supported by National Natural Science Foundation of China. 相似文献
6.
It has been observed by researchers in the past that vortex shedding behind circular cylinders can be altered, and in some cases suppressed, over a limited range of Reynolds numbers by proper placement of a second, much smaller, ‘control’ cylinder in the near wake of the main cylinder. Results are presented for numerical computations of some such situations. A stabilized finite element method is employed to solve the incompressible Navier–Stokes equations in the primitive variables formulation. At low Reynolds numbers, for certain relative positions of the main and control cylinder, the vortex shedding from the main cylinder is completely suppressed. Excellent agreement is observed between the present computations and experimental findings of other researchers. In an effort to explain the mechanism of control of vortex shedding, the streamwise variation of the pressure coefficient close to the shear layer of the main cylinder is compared for various cases, with and without the control cylinder. In the cases where the vortex shedding is suppressed, it is observed that the control cylinder provides a local favorable pressure gradient in the wake region, thereby stabilizing the shear layer locally. Copyright © 2001 John Wiley & Sons, Ltd. 相似文献
7.
Feedback control of vortex shedding from a circular cylinder in a uniform flow at moderate Reynolds numbers is studied experimentally with the cylinder subjected to feedback cylinder oscillations in cross-flow direction. The cylinder oscillation is digitally phase shifted with respect to the shedding vortex and is controlled by velocity feedback from the shear layer of the cylinder wake. Possible attenuation of vortex shedding is demonstrated by hot-wire measurements of the flow field and its mechanisms are studied by simultaneous data sampling and flow visualization with the smoke wire method and a laser-sheet illumination technique. Measurement results reveal substantial reduction in the fluctuating reference velocity at the optimum phase control. Flow visualization study indicates that the shear layer roll-up and the eventual vortex formation are dynamically attenuated under the control which results in a modification of the near wake.List of symbols
A
amplitude of cylinder oscillation
-
D
cylinder diameter
-
E
u
power spectrum function for fluctuating velocity u
-
frequency
-
R
radius of circular cylinder
-
t
time
-
u
streamwise mean velocity
-
u
streamwise fluctuating velocity
-
U
streamwise mean velocity of main flow
-
u
r
mean reference velocity
-
u
r
fluctuating reference velocity
-
u
rf
fluctuating reference velocity after filtering
-
y
c
cylinder displacement
-
x, y, z
coordinates from the cylinder center (Fig. 1)
-
feedback coefficient
-
phase lag
The authors would like to express thanks to Professor K. Nagaya for his advice for designing electromagnetic actuators in the present experiments. 相似文献
8.
The effect of the streamwise vortex generators on the near-wake flow structure of a circular cylinder was experimentally investigated.
Digital particle image velocimetry (DPIV) measurements were performed in a large circulating water tunnel facility at a Reynolds
number of 41,300 where the flow around a bare cylinder was expected to be at the sub-critical flow state. In order to capture
various flow properties and to provide a detailed wake flow topology, the DPIV images were analysed with three different but
complementary flow field decomposition techniques which are Reynolds averaging, phase averaging and proper orthogonal decomposition
(POD). The effect of the vortex generators was clearly demonstrated both in qualitative and in quantitative manner. Various
topological features such as vorticity and stress distribution of the flow fields as well as many other key flow characteristics
including the length scales and the Strouhal number were discussed in the study. To the best of the authors’ knowledge, the
study presents the first DPIV visualization of the near-wake flow of a circular cylinder fitted with the vortex generators
in the open literature. 相似文献
9.
A vortex structure consisting of two concentric vortex rings located in a circular cylinder is studied. The rings touch each other and have a uniform vorticity. The geometric and dynamic parameters of the rings satisfy the condition of zero total intensity. A linear stability analysis of this structure is performed, including the limiting cases when the inner ring transforms into a circle or the outer ring is adjacent to the boundary of the cylinder. The results of numerical simulation of the evolution of unsteady flows are presented for a wide range of variation of governing parameters of the problem. It is established that the evolution of unstable vortex flows results in the formation of several types of quasistationary structures, the topology of which depends mainly on the dominant disturbance mode. 相似文献
10.
On vortex shedding behind a circular disk 总被引:1,自引:0,他引:1
Abtract Experiments were performed for individual realizations of the vortex shedding process behind a circular disk at Reynolds
numbers of 103–105, at which periodic vortex shedding prevails in the wake. The phase differences regarding the individual vortex shedding structures
detected at multiple circumferential locations in the wake were obtained by analyzing the hot-wire signals with a conditional-sampling
scheme. The phase differences of vortex shedding detected at circumferential positions 90° apart show a wide scatter, but
the anti-phase character is largely preserved in the individual vortex shedding process as detected at circumferential locations
180° apart. The randomness of phase differences involved in the vortex shedding process is noted to be essential in order
to satisfy the axisymmetric property of the global flow.
Received: 4 April 19969/Accepted: 29 January 1997 相似文献
11.
Transformation of hanging discontinuities into vortex systems in a stratified flow behind a cylinder
The mechanisms of formation of hanging discontinuities, vortex dipoles, and vortex arrays in the wave wake behind a cylinder moving at a constant velocity in a stratified fluid are investigated using various schlieren methods. The existence of discontinuities is attributable to the distortion of the internal-wave phase pattern in the shear flow and to the varying stratification and subsequent interaction of the waves with the appearing nonuniformities. Hanging discontinuities and vortex systems are low-velocity analogs of shock waves. An analysis of the internal-wave pattern indicates that the values of the normal velocity component differ on the upper and lower edges of the discontinuities. A regime diagram for flows of this kind is given. 相似文献
12.
圆柱绕流涡脱落诱发较大的振动和声,如何有效地抑制值得关注.利用大涡模拟技术求解了Navier-Stokes方程,得到了涡脱落频率,升力脉动幅值及平均阻力系数.计算表明二维模拟不能体现流动基本特征,三维计算与实验吻合较好.为了抑制涡脱落,在直径为D的圆柱表面装入间距为1D,直径为0.0167D的O型环.通过升力、速度谱分析以及柱向横截面流场分析可知,在光滑圆柱外表面加入O型环能诱发流体边界层分离,有效地抑制涡脱落现象,升力脉动和观测点速度脉动幅值几乎完全消失,阻力系数也略微降低,适合在实际工程中采用. 相似文献
13.
In the present study, numerical investigations were performed with optimisation to determine efficient non-uniform suction profiles to control the flow around a circular cylinder in the range of Reynolds numbers 4 < Re < 188.5. Several objectives were explored, namely the minimisation of the separation angle, total drag, and pressure drag. This was in an effort to determine the relationships between the characteristics of the uncontrolled flow and the parameters of the optimised suction control. A variety of non-uniform suction configurations were implemented and compared to the benchmark performance of uniform suction. It was determined that the best non-uniform suction profiles consisted of a distribution with a single locus and compact support. The centre of suction on the cylinder surface for the optimised control, and the quantity of suction necessary to achieve each objective, varied substantially with Reynolds number and also with the separation angle of the uncontrolled flows. These followed predictable relationships. Surprisingly, the location of optimised suction to eliminate separation did not follow the separation point as it moved with Re, but rather it moved in opposition to it towards the trailing edge of the cylinder. Non-uniform suction profiles were much more efficient at eliminating boundary layer separation, requiring the removal of less than half the volume of fluid as uniform control to achieve the same objective. Regardless of the method of control, less net suction was needed to minimise total drag than to eliminate separation, except at low Re. The results suggest that controlling the dynamic aspects of the flow has the most impact for reducing drag. This reinforces the usefulness of other studies that focus on the elimination of vortex shedding. The results show that the balance of drag components must be an important consideration when designing flow control systems and that, when done appropriately, substantial improvement can be seen in the flow characteristics. 相似文献
14.
On the basis of the two-dimensional Navier-Stokes equations, the initiation and development of the separated flow behind a
thermally insulated circular cylinder in a supersonic perfect-gas stream is investigated in relation to the Reynolds number.
It is shown that the entire Re-range can be subdivided into a number of intervals with their own characteristic features.
In particular, the conditions for the generation and development of global separation are established.
Moscow. Translated from Izvestiya Rossiiskoi Akademii Nauk, Mekhanika Zhidkosti i Gaza, No. 6, pp. 27–36, November–December,
1998.
The study was carried out with the support of the Russian Foundation for Basic Research (project No. 95-01-01129a). 相似文献
15.
It is common knowledge that flow around bluff bodies exhibits oscillatory behaviour. The aim of the present study is to compute the steady two-dimensional flow around a square cylinder at different Reynolds numbers and to determine the onset of unsteadiness through a linear stability analysis of the steady flow. Stability of the steady flow to small two-dimensional perturbations is analysed by computing the evolution of these perturbations. An analysis of various time-stepping techniques is carried out to select the most appropriate technique for predicting the growth of the perturbations and hence the stability of the flow. The critical Reynolds number is determined from the growth rate of the perturbations. Computations are then made for periodic unsteady flow at a Reynolds number above the critical value. The predicted Strouhal number agrees well with experimental data. Heat transfer from the cylinder is also studied for the unsteady laminar flow. 相似文献
16.
The paper presents the results of an experimental study of the vortex street behind a circular cylinder oscillating across a water stream at Reynolds numbers less than the critical value at which a vortex street is formed behind a stationary cylinder. The asymptotic dependence of the nondimensional vortex separation frequency (the Strouhal number) on the main relevant parameters is obtained. The value of a constant coefficient appearing in this relationship has been found experimentally.Moscow. Translated from Izvestiya Rossiiskoi Akademii Nauk, Mekhanika Zhidkosti i Gaza, No. 1, pp. 51–56, January–February, 1995. 相似文献
17.
The unsteady incompressible Navier-Stokes equations have been accurately solved for the laminar flow past a circular cylinder in the Reynolds number range 50–200. A direct elliptic solver called the SEVP is used to rapidly advance the streamfunction in time, facilitating the overall convergence to the fully periodic or quasi-steady state. A new integral-series method is developed for the far-field streamfunction condition on a finite two-dimensional computational domain. The use of fourth-order Hermitian relations for the convection terms in the conservation-form vorticity transport equation has also contributed to the good comparison of the present results with the earlier experimental data. The vortex-shedding patterns visualized by the experimentalist are numerically reproduced here in the given Reynolds number range. Discussions that may be helpful in interpreting the behaviour of the shedding frequency are presented in the main text. 相似文献
18.
The separation and shock wave formation on the aft-body of a hypersonic adiabatic circular cylinder were studied numerically using the open source software OpenFOAM. The simulations of laminar flow were performed over a range of Reynolds numbers (\(8\times 10^3 < Re < 8\times 10^4\)) at a free-stream Mach number of 5.9. Off-body viscous forces were isolated by controlling the wall boundary condition. It was observed that the off-body viscous forces play a dominant role compared to the boundary layer in displacement of the interaction onset in response to a change in Reynolds number. A modified free-interaction equation and correlation parameter has been presented which accounts for wall curvature effects on the interaction. The free-interaction equation was manipulated to isolate the contribution of the viscous–inviscid interaction to the overall pressure rise and shock formation. Using these equations coupled with high-quality simulation data, the underlying mechanisms resulting in Reynolds number dependence of the lip-shock formation were investigated. A constant value for the interaction parameter representing the part of the pressure rise due to viscous–inviscid interaction has been observed at separation over a wide range of Reynolds numbers. The effect of curvature has been shown to be the primary contributor to the Reynolds number dependence of the free-interaction mechanism at separation. The observations in this work have been discussed here to create a thorough analysis of the Reynolds number-dependent nature of the lip-shock. 相似文献
19.
S. M. Drozdov 《Fluid Dynamics》2010,45(5):691-702
A new mechanism of the formation of spatially periodic structures on the nose surfaces of cylindrically blunted bodies in
a hypersonic transverse flow is investigated. According to this mechanism, a curved shock wave produces a vortex flow, while
the vortex, which is conserved in the presence of weak dissipation, acts on the shock and maintains its curved shape. The
realizability of this vortex formation mechanism is verified by direct numerical simulation using the FLUENT software package.
It is confirmed that in the case of uniform hypersonic freestream both plane and three-dimensional modes of the steady flow
past the cylinder nose can exist. The three-dimensional mode is characterized by periodic-in-span vortex structures and considerable
heat flux peaks on the nose surface. The calculated results are compared with the experimental data. 相似文献
20.
《Fluid Dynamics Research》1988,4(1):39-46
In the previous paper the authors reported observing the formation of a spoon-shaped vortex chain in a wake behind a circular cylinder as a coherent structure in turbulence. In this report numerical simulation is carried out based on the assumption that the structure is formed by deformation of the Kánnán vortices. The basic equation is the localized induction equation for a single vortex filament with an influence of the background mean flow. The vortex filament is given an initial deformation within a plane at an angle θ to the x−z plane (x is the mean flow direct and z the spanwise direction) with the width Zw, and the further deformation process of the filament is numerically traced. The first calculation is made with fixed Zw and various values of θ. The result shows that the vortex filament finally reaches a structure lying on a plane with a constant angle of 30° ~ 45° to the x-z plane irrespective of the initial values of θ. The second calculation is made with fixed θ and various values of Zw. In this case the final spanwise scale of the deformed region of the filament has almost constant values of about 4d−6d (d is diameter of the cylinder). These results indicate that the final structure of the vortex filament is stable and definite irrespective of the initial disturbances. 相似文献