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1.
A fibre Bragg grating (FBG) sensor was proposed as an alternative to strain gauges to measure the strain ɛ of a vibrating
cylinder in a uniform cross flow. In order to validate the measurements of the FBG sensor, the transverse fluctuating bending
displacement Y of the cylinder was also measured using a laser vibrometer. The two measurements were found to be consistent in terms of
the natural frequency of the fluid–structure system and the vortex shedding frequency. The spectral coherence between ɛ and
Y at the same point of the cylinder attains 1 at these frequencies, thus indicating a near perfect correlation between the
two quantities. When the transverse bending displacement is small, the measured ɛ and Y are linearly related. Therefore, the results indicate that the FBG sensor can be used with confidence to measure the fluctuating
strain arising from the vortex-induced forces on a structure in a uniform cross flow. As such, it can be used in conjunction
with a laser Doppler anemometer to study fluid–structure interactions in flow-induced vibration problems. Furthermore, it
is expected that the FBG sensor, because of its physical uniqueness, will have an important role to play in the study of fluid–structure
interaction problems with multiple structures arranged in an array.
Received: 17 August 1998 / Accepted 27 January 1999 相似文献
2.
Flow development in the wake of a dual step cylinder has been investigated experimentally using Laser Doppler Velocimetry
and flow visualization. The dual step cylinder model is comprised of a large diameter cylinder (D) mounted at the mid-span of a small diameter cylinder (d). The experiments have been performed for a Reynolds number (Re
D
) of 1,050, a diameter ratio (D/d) of 2, and a range of large cylinder aspect ratios (L/D). The results show that the flow development is highly dependent on L/D. The following four distinct flow regimes can be identified based on vortex dynamics in the wake of the large cylinder: (1)
for L/D ≥ 15, three vortex shedding cells form in the wake of the large cylinder, one central cell bounded by two cells of lower
frequency, (2) for 8 < L/D ≤ 14, a single vortex shedding cell forms in the wake of the large cylinder, (3) for 2 < L/D ≤ 6, vortex shedding from the large cylinder is highly three-dimensional. When spanwise vortices are shed, they deform substantially
and attain a hairpin shape in the near wake, (4) for 0.2 ≤ L/D ≤ 1, the large cylinder induces vortex dislocations between small cylinder vortices. The results show that for Regimes I
to III, on the average, the frequency of vortex shedding in the large cylinder wake decreases with L/D, which is accompanied by a decrease in coherence of the shed vortices. In Regime IV, small cylinder vortices connect across
the large cylinder wake, but these connections are interrupted by vortex dislocations. With decreasing L/D, the frequency of dislocations decreases and the dominant frequency in the large cylinder wake increases toward the small
cylinder shedding frequency. 相似文献
3.
This paper is concerned with a liquid metal flow driven by a rotating magnetic field inside a stationary cylinder. We consider
especially the secondary meridional flow during the time when the fluid spins up from rest. The developing flow is investigated
experimentally and by direct numerical simulations. The vertical profiles of the axial velocity are measured by means of the
ultrasound Doppler velocimetry. Evolving instabilities in the form of Taylor–G?rtler vortices have been observed just above
the instability threshold (Ta ≥ 1.5· Ta
cr). The rotational symmetry may survive over a distinct time even if a first Taylor–G?rtler vortex pair has been formed as
closed rings along the cylinder perimeter. The transition to a three-dimensional flow in the side layers results from the
advection or a precession and splitting of the Taylor–G?rtler vortex rings. The predictable behaviour of the Taylor–G?rtler
vortices disappears with increasing magnetic field strength. The numerical simulations agree very well with the flow measurements. 相似文献
4.
Vortex pairs are studied using a dye tracing visualisation technique and a particle tracking velocimetry system. The vortex
pairs are produced by gravity induced inlets of water issued through a uniform gap. The inlet Reynolds number is Re=Ud/ν≈875 in all tests (d being the gap width and U the cross sectional mean velocity), i.e. the flow is in the laminar regime. Initially, the dipolar vortex structure is two-dimensional,
but after travelling a distance of a few times its own width, the flow structure becomes unstable, breaks up and changes into
a three-dimensional flow structure. The breakup appears to be caused by an axial flow in the core centres of each vortex of
the dipolar structure. These axial flows are induced by boundary effects related to the von Karman viscous pump. After the
breakup, it is believed that a vortex ring is formed through reconnection of rudiments from the dipolar structure mediated
by the wall induced vorticity.
Received: 20 November 1995/Accepted: 14 November 1996 相似文献
5.
We investigate the impact of different boundary conditions on the flow field developing around a tilted rectangular cylinder
with two different values of the aspect ratio (l/s=3 and 4). We are mainly interested in analyzing the changes in force coefficients and in the vortex shedding Strouhal number
when the cylinder is placed at various distances from a bottom wall and different values of attack angle. The angle of attack
ranges between −30° and +30° and the cylinder elevation above the bottom wall is varied between almost zero and 5 times the
thickness of the cylinder. A large body of experimental results is related to the small elevation conditions at different
attack angles, where the presence of the wall has a non-negligible effect on the behavior of the force coefficients and Strouhal
number of the vortex shedding. 相似文献
6.
Experiments were conducted in water and wind tunnels on spheres in the Reynolds number range 6 × 103 to 6.5 × 105 to study the effect of natural ventilation on the boundary layer separation and near-wake vortex shedding characteristics.
In the subcritical range of Re (<2 × 105), ventilation caused a marginal downstream shift in the location of laminar boundary layer separation; there was only a small
change in the vortex shedding frequency. In the supercritical range (Re > 4 × 105), ventilation caused a downstream shift in the mean locations of boundary layer separation and reattachment; these lines
showed significant axisymmetry in the presence of venting. No distinct vortex shedding frequency was found. Instead, a dramatic
reduction occurred in the wake unsteadiness at all frequencies. The reduction of wake unsteadiness is consistent with the
reduction in total drag already reported. Based on the present results and those reported earlier, the effects of natural
ventilation on the flow past a sphere can be categorized in two broad regimes, viz., weak and strong interaction regimes.
In the weak interaction regime (subcritical Re), the broad features of the basic sphere are largely unaltered despite the large addition of mass in the near wake. Strong
interaction is promoted by the closer proximity of the inner and outer shear layers at supercritical Re. This results in a modified and steady near-wake flow, characterized by reduced unsteadiness and small drag.
Received: 8 September 1998 / Accepted: 1 January 2000 相似文献
7.
Drag Reduction of a Circular Cylinder Using an Upstream Rod 总被引:3,自引:0,他引:3
Experimental studies on the drag reduction of the circular cylinder were conducted by pressure measurement at a Reynolds number
of 82 000 (based on the cylinder diameter). A rod was placed upstream of and parallel to the cylinder to control the flow
around the cylinder. The upstream rod can reduce the resultant force of the cylinder at various spacing between the rod and
the cylinder for α < 5∘(α defined as the staggered angle of the rod and the cylinder). For α > 10∘, the resultant force coefficient has a large value, so the upstream rod cannot reduce the force on the cylinder any more.
For α = 0∘ and d/D = 0.5 (where d and D are the diameter of the rod and the cylinder, respectively), the maximum drag of the cylinder reduces to 2.34% that of the
single cylinder. The mechanism of the drag reduction of the cylinder with an upstream rod in tandem was presented by estimating
the local contributions to the drag reduction of the pressure variation. In the staggered arrangement, the flow structures
have five flow patterns (they are the cavity mode, the wake splitting mode, the wake merge mode, the weak boundary layer interaction
mode and the negligible interaction mode) according to the pressure distribution and the hydrogen bubble flow visualization.
The half plane upwind of the cylinder can be divided to four regions, from which one can easily estimates the force acting
on the circular cylinder with an upstream rod in staggered arrangement. 相似文献
8.
The adverse pressure gradient induced by a surface-mounted obstacle in a turbulent boundary layer causes the approaching flow
to separate and form a dynamically rich horseshoe vortex system (HSV) in the junction of the obstacle with the wall. The Reynolds
number of the flow (Re) is one of the important parameters that control the rich coherent dynamics of the vortex, which are known to give rise to
low-frequency, bimodal fluctuations of the velocity field (Devenport and Simpson, J Fluid Mech 210:23–55, 1990; Paik et al., Phys Fluids 19:045107, 2007). We carry out detached eddy simulations (DES) of the flow past a circular cylinder mounted on a rectangular channel for
Re = 2.0 × 104 and 3.9 × 104 (Dargahi, Exp Fluids 8:1–12, 1989) in order to systematically investigate the effect of the Reynolds number on the HSV dynamics. The computed results are compared
with each other and with previous experimental and computational results for a related junction flow at a much higher Reynolds
number (Re = 1.15 × 105) (Devenport and Simpson, J Fluid Mech 210:23–55, 1990; Paik et al., Phys Fluids 19:045107, 2007). The computed results reveal significant variations with Re in terms of the mean-flow quantities, turbulence statistics, and the coherent dynamics of the turbulent HSV. For Re = 2.0 × 104 the HSV system consists of a large number of necklace-type vortices that are shed periodically at higher frequencies than
those observed in the Re = 3.9 × 104 case. For this latter case the number of large-scale vortical structures that comprise the instantaneous HSV system is reduced
significantly and the flow dynamics becomes quasi-periodic. For both cases, we show that the instantaneous flowfields are
dominated by eruptions of wall-generated vorticity associated with the growth of hairpin vortices that wrap around and disorganize
the primary HSV system. The intensity and frequency of these eruptions, however, appears to diminish rapidly with decreasing
Re. In the high Re case the HSV system consists of a single, highly energetic, large-scale necklace vortex that is aperiodically disorganized
by the growth of the hairpin mode. Regardless of the Re, we find pockets in the junction region within which the histograms of velocity fluctuations are bimodal as has also been
observed in several previous experimental studies. 相似文献
9.
Effect of local forcing on a turbulent boundary layer 总被引:6,自引:0,他引:6
An experimental study is performed to analyze flow structures behind local suction and blowing in a flat-plate turbulent
boundary layer. The local forcing is given to the boundary layer flow by means of a sinusoidally oscillating jet issuing from
a thin spanwise slot at the wall. The Reynolds number based on the momentum thickness is about Re
θ
=1700. The effects of local forcing are scrutinized by altering the forcing frequency (0.011 ≤ f +≤ 0.044). The forcing amplitude is fixed at A
0=0.4. It is found that a small local forcing reduces the skin friction and the skin friction reduction increases with the
forcing frequency. A phase-averaging technique is employed to capture the large-scale vortex evolution. An organized spanwise
vortical structure is generated by the local forcing. The cross-sectional area of vortex and the time fraction of vortex are
examined by changing the forcing frequency. An investigation of the random fluctuation components reveals that turbulent energy
is concentrated near the center of vortical structures.
Received: 17 March 2000/Accepted: 3 April 2001 相似文献
10.
Accuracy of out-of-plane vorticity measurements derived from in-plane velocity field data 总被引:2,自引:0,他引:2
A study of the errors in out-of-plane vorticity (ω
z
) calculated using a local χ2 fitting of the measured velocity field and analytic differentiation has been carried out. The primary factors of spatial
velocity sampling separation and random velocity measurement error have been investigated. In principle the ω
z
error can be decomposed into a bias error contribution and a random error contribution. Theoretical expressions for the transmission
of the random velocity error into the random vorticity error have been derived. The velocity and vorticity field of the Oseen
vortex has been used as a typical vortex structure in this study. Data of different quality, ranging from exact velocity vectors
of analytically defined flow fields (Oseen vortex flow) sampled at discrete locations to computer generated digital image
frames analysed using cross-correlation DPIV, have been investigated in this study. This data has been used to provide support
for the theoretical random error results, to isolate the different sources of error and to determine their effect on ω
z
measurements. A method for estimating in-situ the velocity random error is presented. This estimate coupled with the theoretically derived random error transmission results
for the χ2 vorticity calculation method can be used a priori to estimate the magnitude of the random error in ω
z
. This random error is independent of a particular flow field. The velocity sampling separation is found to have a profound
effect on the precise determination of ω
z
by introducing a bias error. This bias error results in an underestimation of the peak vorticity. Simple equations, which
are based on a local model of the Oseen vortex around the peak vorticity region, allowing the prediction of the ω
z
bias error for the χ2 vorticity calculation method, are presented. An important conclusion of this study is that the random error transmission
factor and the bias error cannot be minimised simultaneously. Both depend on the velocity sampling separation, but with opposing
effects. The application of the random and bias vorticity error predictions are illustrated by application to experimental
velocity data determined using cross-correlation DPIV (CCDPIV) analysis of digital images of a laminar vortex ring.
Received: 31 October 1997/Accepted: 6 February 1998 相似文献
11.
Hot-wire measurements have been carried out in the turbulent flow around a rotating circular cylinder in still air for Reynolds
numbers Re=∣U
w
∣D/ν=1.5×104 to 105. The experimental results confirm the analysis derived by asymptotic theory for high Reynolds numbers. Two different ways
of deriving the friction law from the experiments (via shear stress and via velocity distribution) resulted practically in
the same law. It is shown, that in spite of the curvature of the streamlines the universal logarithmic velocity distribution
is still valid near the wall.
Received: 8 August 1996/Accepted: 24 April 1998 相似文献
12.
We study flow and heat transfer to a cylinder in cross flow at Re = 3,900–80,000 by means of three-dimensional transient RANS (T-RANS) simulations, employing an RNG k − ε turbulence model. Both the case of a bare solid cylinder and that of a solid cylinder surrounded at some fixed distance by
a thin porous layer have been studied. The latter configuration is a standard test geometry for measuring the insulating and
protective performance of garments. In this geometry, the flow in the space between the solid cylinder and the porous layer
is laminar but periodic, whereas the outer flow is transitional and characterized by vortex shedding in the wake of the cylinder.
The results from the T-RANS simulations are validated against data from Direct Numerical Simulations and experiments. It is
found that T-RANS is very well suited for simulating this type of flow. The transient nature of the flow underneath the porous
layer is well reproduced, as well as the influence of vortex shedding on the heat transfer in the downstream stagnation zone.
T-RANS results are found to be in much better agreement with DNS and experimental data than results from steady-state RANS. 相似文献
13.
The surface vorticity method (SVM), which is a fast and practical grid-free two-dimensional (2-D) method, and a fluid–structure interaction model incorporating the effects of cylinder motions and displacements is used to simulate the vortex-induced vibration of cylinder arrays at sub-critical Reynolds number Re=2.67×104. The SVM is found to be most suitable for simulating a 2-D cylinder row with large-amplitude vibrations where the vorticity field and the fluid forces of the cylinder row change drastically, and the effect of the stream on the transverse direction vibration is very significant. The fluidelastic instability of a flexible cylinder row at small pitch ratio is also investigated, and the critical reduced velocity of the cylinder row at a reduced damping parameter SG=1.29 is calculated, which is in good agreement with experimental and analytical results of the unsteady model. Vortex-induced vibration of a staggered cylinder array is simulated using different structural parameters. When the cylinders are relatively more flexible, the flow pattern changes dramatically and the fluid–structure interaction has a dominant impact on the flow field. Compared with grid-based methods, the grid-free SVM is a fast and practical method for the simulation of the FIV of cylinder arrays due to vortex shedding at sub-critical Reynolds numbers. 相似文献
14.
Relation between the surface flow pattern and the local mass transfer characteristic on the free end surface of a finite
circular cylinder mounted on a flat plate was investigated using the oil surface flow visualization and the naphthalene sublimation
technique. The aspect ratio [the ratio of the cylinder length to the cylinder diameter (H/D)] was 1.25 and the Reynolds number based on the diameter was 1.48 × 105. Several kinds of critical points on the free-end surface were identified by the visualization and the mass transfer measurement
around those critical points through the naphthalene sublimation technique was then performed to investigate the relation
between the flow and mass transfer. The free-end surface flow was topologically characterized by the presence of a saddle
point and two focal points in the fore-half portion of the surface and two nodes and a saddle point in the rear-half portion.
It was found that the rate of mass transfer was largest in the vicinity of the nodes in the rear-half of the free-end surface.
Received on 26 April 2000 相似文献
15.
传统CFD方法在振动钝体绕流计算中常借助动网格技术,网格再生任务繁重。针对于此,本文利用可在静止网格中计算动边界绕流问题的浸入边界算法(IBM),编写数值模拟程序,分别对竖向强迫正弦振动方柱(Re=UD/v=103、振幅恒定、振动频率变化)以及桥梁断面(Re=UB/v=7.5×103、振幅、振动频率均变化)展开气动特性和流场特征结构分析。初步研究结果表明,振幅恒定为方柱高度的14%时,其涡脱锁定区长度为0.06~0.2,锁定区后端(Stc0.2)振动方柱涡脱频率回归静止涡脱频率;不同振幅下的桥梁断面阻力系数均在静止涡脱频率处产生峰值,桥梁断面升力系数则在此处均出现归零效应,且振幅越大,归零效应愈明显。 相似文献
16.
Turbulence measurements are reported on the three-dimensional turbulent boundary layer along the centerline of the flat endwall
in a 30° bend. Profiles of mean velocities and Reynolds stresses were obtained down to y
+≈2 for the mean flow and y
+≈8 for the turbulent stresses. Mean velocity data collapsed well on a simple law-of-the-wall based on the magnitude of the
resultant velocity. The turbulence intensity and turbulent shear stress magnitude both increased with increased three-dimensionality.
The ratio of these two quantities, the a
1 structure parameter, decreased in the central regions of the boundary layer and showed profile similarity for y
+<50. The shear stress vector angle lagged behind the velocity gradient vector angle in the outer region of the boundary layer,
however there was an indication that the shear stress vector tends to lead the velocity gradient vector close to the wall.
Received: 16 July 1996/Accepted: 14 July 1997 相似文献
17.
The problem of heat convection from a vertically oscillating cylinder in a quiescent fluid is investigated. The governing
equations of motion and energy are solved numerically in a non-inertial frame of references to determine the flow field and
heat transfer characteristics under different conditions. The main dominating parameters are Keulegan–Carpenter number, KC,
frequency parameter, β, Grashof number, Gr and Prandtl number, Pr. The ranges considered for these parameters are KC ≤ 10,
β≤40 and Gr ≤ 105 while Prandtl number is kept constant. The study revealed that the effect of amplitude and frequency of oscillation on heat
transfer is strongly influenced by the Grashof number range. In the forced convection regime (Gr = 0), the increase of KC
creates extensive vortex motion at all cylinder positions that leads to a significant increase in heat transfer. A similar
trend, but with a lesser extent, is also observed for the increase of β. However, at high Grashof numbers, the effect of oscillation
on heat convection is only significant at large values of KC.
Received on 5 June 2000 / Published online: 29 November 2001 相似文献
18.
The influence of a dilute solution of the cationic surfactant C14Sal on the flow past a cylinder was investigated by means of LDV and Toepler Schlieren optics for visualization of both the
flow and structure of the fluid. At low Reynolds numbers the flow is similar to the Newtonian Kármán vortex street. The periodic
vortex shedding disappears simultaneously with the occurrence of a shear-induced structure. The alteration of the turbulence
characteristics is especially pronounced in the turbulent velocity fluctuations with the u
rms being many times over the values in water, whereas the v
rms are drastically reduced.
Received: 18 May 2000 / Accepted: 25 July 2000 相似文献
19.
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
20.
《Journal of Fluids and Structures》2008,24(5):605-627
Flow characteristics in the near wake of a circular cylinder located close to a fully developed turbulent boundary layer are investigated experimentally using particle image velocimetry (PIV). The Reynolds number based on the cylinder diameter (D) is 1.2×104 and the incident boundary layer thickness (δ) is 0.4D. Detailed velocity and vorticity fields in the wake region (0<x/D<6) are given for various gap heights (S) between the cylinder and the wall, with S/D ranging from 0.1 to 1.0. Both the ensemble-averaged (including the mean velocity vectors and Reynolds stress) and the instantaneous flow fields are strongly dependent on S/D. Results reveal that for S/D⩾0.3, the flow is characterized by the periodic, Kármán-like vortex shedding from the upper and lower sides of the cylinder. The shed vortices and their evolution are revealed by analyzing the instantaneous flow fields using various vortex identification methods, including Galilean decomposition of velocity vectors, calculation of vorticity and swirling strength. For small and intermediate gap ratios (S/D⩽0.6), the wake flow develops a distinct asymmetry about the cylinder centreline; however, some flow quantities, such as the Strouhal number and the convection velocity of the shed vortex, keep roughly constant and virtually independent of S/D. 相似文献