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1.
The effects of exit-wedge angle on evolution, formation, pinch-off, propagation and diffusive mass entrainment of vortex rings
in air were studied using digital particle image velocimetry. Vortex rings were generated by passing a solenoid-valve-controlled
air jet through a cylindrical nozzle. Experiments were performed over a wide range of exit-wedge angles (10° ≤ α ≤ 90°) of the cylindrical nozzle, initial Reynolds numbers (450 ≤ Re ≤ 4,580) and length-to-diameter ratios (0.9 ≤ L/D ≤ 11) of the air jet. For sharp edges (α ≤ 10°), a secondary ring may emerge at high Reynolds numbers, which tended to distort the vortex ring if ingested into it.
For blunt edges (α ≥ 45°), by contrast, stable vortex rings were produced. The formation phase of a vortex ring was found to be closely related
to its evolution pattern. An exit-wedge angle of 45° was found to be optimal for rapid pinch-off and faster propagation and
better stability of a vortex ring. Diffusive mass entrainment was found to be between 35% and 40% in the early stages of a
vortex ring propagation and it gradually increased throughout the course of vortex ring propagation. Entrainment fraction
was found to be sensitive to the L/D ratio of the initial jet and decreases when the L/D ratio is increased. 相似文献
2.
《Journal of Fluids and Structures》2007,23(1):85-100
Multi-resolution analysis (MRA) was applied to the large-scale coherent structure in a turbulent separation bubble affected by an unsteady wake. The unsteady wake was generated using a spoked-wheel type wake generator, which was installed in front of the separation bubble. The wake generator was rotated either clockwise (CW) or counter-clockwise (CCW) with a normalized passing frequency of StH=0.2. The Reynolds number based on the half-thickness of the blunt body was ReH=5600. To show the unsteady dynamic flow structures between the ‘cutting’ and ‘wrapping’ regimes, a MRA using the maximal overlap discrete wavelet transform (MODWT) was performed. This method enabled delineation of the coherent structure of the turbulent separation bubble through a scale-resolved analysis. Reconstruction of the flow field in combination with conditional averaging was attempted. Flapping motions as well as sawtooth movements of the unsteady separation bubble were analyzed using the MODWT. The unsteady wakes decayed faster in the system with CCW rotation than in that with CW rotation. 相似文献
3.
A. Klaczak 《Heat and Mass Transfer》2001,37(4-5):443-448
This article presents the results of laboratory research on heat exchange while heating water in horizontal and vertical
tubes with twisted-tape inserts.
The scope of the research:
70 ≤ Re ≤ 4000
3.6 ≤ Pr ≤ 5.9
8.6 ≤ Gz ≤ 540
The research was held for three cases:
– horizontal experimental tube
– vertical experimental tube, the direction of flow according to the free convection vector
– vertical experimental tube, the direction of flow not in accordance with the free convection vector
For such cases the correlation equation was defined NuT=f(Gz; y), Nu = f(Gz) and the proportion NuT/Nu was analysed.
Received on 30 March 2000 相似文献
4.
Rheological and mechanical properties of silica colloids: from Newtonian liquid to brittle behaviour
Rheological and mechanical properties of aqueous mono-disperse silica suspensions (Ludox? HS40) are investigated as a function
of particle volume fraction (ϕ
p ranging from 0.22 to 0.51) and water content, using shear rate tests, oscillatory methods, indentation and an ultrasonic
technique. As the samples are progressively dried, four regimes are identified; they are related to the increasing particle
content and the existence and behaviour of the electrical double layer (EDL) around each particle. For 0.22 ≤ ϕ
p ≤ 0.30), the suspensions are stable due to the strong electrostatic repulsion between particles and show Newtonian behaviour
(I). As water is removed, the solution pH decreases and the ionic strength increases. The EDL thickness therefore slowly decreases,
and screening of the electrostatic repulsion increases. For 0.31 ≤ ϕ
p ≤ 0.35, the suspensions become turbid and exhibit viscoelastic (VE) shear thinning behaviour (II), as they progressively
flocculate. For 0.35 ≤ ϕ
p ≤ 0.47, the suspensions turn transparent again and paste-like, with VE shear thinning behaviour and high elastic modulus
(III). At higher particle concentration, the suspensions undergo a glass transition and behave as an elastic brittle solid
(IV, ϕ
p = 0.51). 相似文献
5.
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
6.
We present detailed experimental results examining “negative wakes” behind spheres settling along the centerline of a tube
containing a viscoelastic aqueous polyacrylamide solution. Negative wakes are found for all Deborah numbers (2.43≤De(˙γ)≤8.75) and sphere-to-tube aspect ratios (0.060≤a/R≤0.396) examined. The wake structures are investigated using laser-Doppler velocimetry (LDV) to examine the centerline fluid
velocity around the sphere and digital particle image velocimetry (DPIV) for full-field velocity profiles. For a fixed aspect
ratio, the magnitude of the most negative velocity, U
min
, in the wake is seen to increase with increasing De. Additionally, as the Deborah number becomes larger, the location of this minimum velocity shifts farther downstream. When
normalized with the sphere radius and the steady state velocity of the sphere, the axial velocity profiles become self-similar
to the point of the minimum velocity. Beyond this point, the wake structure varies weakly with aspect ratio and De, and it extends more than 20 radii downstream. Inertial effects at high Reynolds numbers are observed to shift the entire
negative wake farther downstream. Using DPIV to investigate the transient kinematic response of the fluid to the initial acceleration
of the sphere from rest, it is seen that the wake develops from the nonlinear fluid response at large strains. Measurements
of the transient uniaxial extensional viscosity of this weakly strain-hardening fluid using a filament stretching rheometer
show that the existence of a negative wake is consistent with theoretical arguments based on the opposing roles of extensional
stresses and shearing stresses in the wake of the sphere.
Received: 10 November 1997 Accepted: 1 May 1998 相似文献
7.
Experimental investigation of mixed convection heat transfer in a horizontal and inclined rectangular channel 总被引:2,自引:0,他引:2
Mixed convection heat transfer in rectangular channels has been investigated experimentally under various operating conditions.
The lower surface of the channel is subjected to a uniform heat flux, sidewalls are insulated and adiabatic, and the upper
surface is exposed to the surrounding fluid. Experiments were conducted for Pr=0.7, aspect ratios AR=5 and 10, inclination
angles 0° ≤ θ ≤ 30°, Reynolds numbers 50 ≤ Re ≤ 1000, and modified Grashof numbers Gr*=7.0 × 105 to 4.0 × 107. From the parametric study, local Nusselt number distributions were obtained and effects of channel inclination, surface
heat flux and Reynolds number on the onset of instability were investigated. Results related to the buoyancy affected secondary
flow and the onset of instability have been discussed. Some of the results obtained from the experimental measurements are
also compared with the literature, and a good agreement was observed. The onset of instability was found to move upstream
for increasing Grashof number and increasing aspect ratio. On the other hand, onset of instability was delayed for increasing
Reynolds number and increasing inclination angle.
Received on 19 March 2001 / Published online: 29 November 2001 相似文献
8.
Transient laminar natural convection over a sphere which is subjected to a constant heat flux has been studied numerically
for high Grashof numbers (105 ≤ Gr ≤ 109) and a wide range of Prandtl numbers (Pr = 0.02, 0.7, 7, and 100). A plume with a mushroom-shaped cap forms above the sphere and drifts upward continuously with time.
The size and the level of temperature of the transient cap and plume stem decrease with increasing Gr and Pr. Flow separation and an associated vortex may appear in the wake of the sphere depending on the magnitude of Gr and Pr. A recirculation vortex which appears and grows until “steady state” is attained was found only for the very high Grashof
numbers (105 ≤ Gr ≤ 109) and the lowest Prandtl number considered (Pr = 0.02). The appearance and subsequent disappearance of a vortex was observed for Gr = 109 and Pr = 0.7. Over the lower hemisphere, the thickness of both the hydrodynamic (δH) and the thermal (δT) boundary layers remain nearly constant and the sphere surface is nearly isothermal. The surface temperature presents a local
maximum in the wake of the sphere whenever a vortex is established in the wake of the sphere. The surface pressure recovery
in the wake of the sphere increases with decreasing Pr and with increasing Gr. For very small Pr, unlike forced convection, the ratio δT/δH remains close to unity. The results are in good agreement with experimental data and in excellent agreement with numerical
results available in the literature. A correlation has also been presented for the overall Nusselt number as a function of
Gr and Pr. 相似文献
9.
A. Klaczak 《Heat and Mass Transfer》2000,36(3):195-199
Heat transfer for laminar flow of water in an air-cooled vertical copper pipe with four twisted-tape inserts was determined
experimentally. The tests were executed for laminar flow within 110 ≤ Re ≤ 1500, 8.1 ≤ Gz ≤ 82.0 and 1.62 ≤ y ≤ 5.29. The correlation equation for heat transfer was defined for the tested range. The obtained results were compared to
the results of other authors.
Received on 28 April 1998 相似文献
10.
A supersonic exhaust plume test rig and a Rayleigh scattering system were developed. Molecular number densities in the supersonic
high-temperature exhaust plume with and without an annular base flow were investigated. The physical meaning of the inferred
mean temperature from the number density measurement in turbulent flows is clarified. For both flows, the potential core extends
up to about six nozzle diameters, and self-similarity of the radial density distributions is observed at downstream sections
Z/d=10–50. The recovery of the flow density deficit (or the decay of temperature) with the annular flow is faster than that without
the annular flow at upstream sections Z/d ≤ 10.
Received: 16 August 2000 / Accepted: 20 November 2001 相似文献
11.
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
12.
Linear viscoelastic properties of SiO2/(AP/EP) suspension with various SiO2 volume fractions (ϕ) in a blend of acrylic polymer (AP) and epoxy (EP) were investigated at various temperatures (T). The AP/EP contained 70 vol.% of EP. The SiO2 particles were treated with epoxy silane coupling agent. The effects of the SiO2 particles are more pronounced in the terminal zone: a transition from viscoelastic liquid (ϕ ≤ 30 vol.%) to viscoelastic solid (ϕ ≥ 40 vol.%) was observed which can be interpreted as a critical gelation occurring at a critical particle content and critical
gel temperature. The SiO2/(AP/EP) systems exhibited a critical gel behavior at ϕ ≅ 35 vol.% and T ≅ 100°C characterized with a power–law relationship between the storage and loss moduli (G
′ and G
″) and frequency (ω); G
′ = G
″/tan(nπ/2) ∝ ω
n
. The critical gel exponent (n) was estimated to be about 0.45. The gelation occurred with increasing T. 相似文献
13.
A new type of flow visualization method utilizing a smoke-wire, a high-speed camera with high framing rates and a laser light
sheet was employed to delineate the unsteady processes of large-scale vortices in the separated shear layer about a blunt-faced
flat plate at Re
H
= 560. The sequential images showed that the unsteady behavior of large-scale vortices in the separated shear layer varies
as the shedding phases of large-scale vortices alter. Particularly, at a certain phase, a vortex-merging process between the
two neighboring large-scale vortices took place.
Received: 17 November 1998/Accepted: 1 November 1999 相似文献
14.
An experimental study of transient boiling heat transfer during a cooling of a hot cylindrical block with an impinging water
jet has been made at atmospheric pressure. The experimental data were taken for the following conditions: a degree of subcooling
of ΔT
sub = 20–80 K, a jet velocity of u
j
= 5–15 m/s, a nozzle diameter of d
j
= 2 mm and three materials of copper, brass and carbon steel. The block was initially and uniformly heated to about 250 °C
and the transient temperatures in the block were measured at eight locations in r-direction at two different depths from the surface during the cooling of hot block. The surface heat flux distribution with
time was evaluated using a numerical analysis of 2-D heat conduction. Behavior of the wetting front, which is extending the
nucleate boiling region outward, is observed with a high-speed video camera. A position of wetting region is measured and
it is correlated well with a power function of time. The changes in estimated heat flux and temperature were compared with
the position of wetting region to clarify the effects of subcooling, jet velocity and thermal properties of block on the transient
cooling.
Received on 17 March 2000 相似文献
15.
Ki Byung Sunwoo Seung Joon Park Seong Jae Lee Kyung Hyun Ahn Seung Jong Lee 《Rheologica Acta》2002,41(1-2):144-153
Three-dimensional numerical simulation of viscoelastic coextrusion process has been performed and numerical results were
compared with the experimental data of Karagiannis et al. (1990). By varying the magnitude of the second normal stress difference
and its ratio of Fluid I and Fluid II, we were able to control the interface profile and the degree of encapsulation along
the downstream direction. By increasing the parameter α (αFluid I=αFluid II) from 0.1 to 0.4 in the Giesekus model and increasing the α ratio (αFluid IαFluid II) between Fluid I and Fluid II from 2.0 to 4.0 in the permissible range of realistic polymeric systems, the interface profile
and the degree of encapsulation along the downstream direction were fitted with the experimental results. There was little
difference between the numerical results and the experimental data in the interface profile and the degree of encapsulation
along the downstream direction when the α ratio was set to 3.0 (0.3:0.1). Fluid I with larger magnitude of the second normal
stress difference protrudes into Fluid II with smaller magnitude of the second normal stress difference around the symmetric
plane, while Fluid II wraps around Fluid I near the side walls. As the ξ
1
ratio (ξ
1
,Fluid Iξ
1
,Fluid II) increases from 1.0 to 3.0 for the two-mode Phan-Thien and Tanner model, it was found that the curvature of the interface
profile increased, and the difference between the numerical results and the experimental data in the interface profile and
the degree of encapsulation along the downstream direction was almost negligible when the ξ
1
ratio was set to 3.0 (0.54:0.18). Although the parameters of viscoelastic models were fitted by using the shear viscosity
data only, quantitative agreements between the numerical results and the experimental coextrusion data were quite satisfactory.
Received: 24 April 2001 Accepted: 5 June 2001 相似文献
16.
Shear layers of a circular cylinder with rotary oscillation 总被引:1,自引:0,他引:1
The behavior of the separated shear layers and the near wake of a circular cylinder with small-amplitude rotary oscillations
(Ω1 = 0.05−0.15 for f
f/f
o ≤ 1.25) were investigated experimentally at Re = 3,700. Measurements of an unforced cylinder were also made for 2,000 ≤ Re ≤ 10,000 to better understand the effects of rotary oscillations. The results show that the shear-layer vortices formed closer
to the cylinder and the distance separating them was found to decrease with cylinder oscillations. The shear-layer frequency,
however, increased with increasing forcing frequency f
f. The formation-region length l
f decreased significantly with increasing f
f while decreased to a lesser extent with increasing normalized oscillation amplitude Ω1. The shear layer also diffused to a length L
d larger than that of an unforced cylinder, while the l
f-L
d-Strouhal frequency offsetting mechanism was generally maintained. The near wake was of lower momentum compared to an unforced
cylinder, and the transverse velocity fluctuations associated with the unforced vortex-shedding frequency f
o always presented a local peak at f
f/f
o = 0.5, regardless of Ω1 tested. 相似文献
17.
Experiments are presented on the surge effect induced by cylindrical bodies piercing a free surface at constant velocity.
The study covers the following ranges: 0.1 < Froude2 < 7.3, 0.7 < Weber < 89, 46 < Reynolds < 6000; with the Goucher number (Go) evolving between 1.8 and 4.5. Free-surface profiles are compared with those issued from a boundary element method (BEM)
simulation based on potential theory and axisymmetric flow configuration. The free-surface deformation is accurately predicted
by the BEM approach for Reynolds numbers (Re) higher than 500. However, for Re less than about 200, computed results underestimate the interface elevation, except on the axis of symmetry, where they remain
accurate. Finally, the magnitudes of the interface deformation predicted by the numerical simulation are provided over a wide
range of Goucher numbers (0.001 < Go < 50) both for hemispherical and conical tips. Implications of these results for phase detection probes are discussed.
Received: 18 December 2000/Accepted: 25 January 2001 相似文献
18.
Three-dimensional (3D) trajectories of spherical air bubbles passing through a converging part of a rectangular channel have
been measured. Bubble diameters, d
b, were less than 1 mm and the Reynolds numbers, Re
b, for stagnant tapwater and for mean liquid velocity vˉ
L=0.25 m/s were in about same range.
Received: 15 January 2001 / Accepted: 12 June 2001 相似文献
19.
Experiments have been performed to assess the impact of an extended surface on the heat transfer enhancement for axisymmetric,
turbulent liquid jet impingement on a heated round disk. The disk, with an array of integral radial fins mounted on its surface,
is placed at the bottom of an open vertical circular cavity. Hydrodynamic and heat transfer data were obtained for a dielectric
fluorocarbon liquid FC-77. For a fixed circular heater of diameter D=22.23 mm, several geometric parameters were tested: the nozzle diameter (4.42≤d≤9.27 mm), the confining wall diameter of the vertical cavity (22.23≤D
c≤30.16 mm), and the nozzle-to-heater spacing (0.5≤S/d≤5.0). The FC-77 flow rates varied from =0.2 to 11.0 l/min producing Reynolds numbers in the wide interval 700≤Re
d
≤44,000. For d=4.42 mm, the heat transfer response to the separation distance S/d was small but increased gradually with increasing nozzle diameter up to d=9.27 mm. The thermal resistance R
th increased with the confining wall diameter D
c and also with the nozzle diameter d. A minimum value of the thermal resistance of R
th,min=0.4 cm2 K/W was attained for a combination of d=4.42 mm, D
c=22.23 mm, S/d=1, and =7.5 l/min. Based on a simplified heat transfer model, reasonable agreement was obtained between measured values of
the thermal resistance and the R
th-predictions. The total fin effectiveness ɛf was shown to increase with increasing nozzle diameter, but was invariant with the flow rate (or the jet exit velocity). More
than a three-fold heat transfer enhancement was realized through the addition of the array of integral radial fins on the
heated round disk.
Received on 30 August 2000 / Published online: 29 November 2001 相似文献
20.
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