共查询到20条相似文献,搜索用时 31 毫秒
1.
The present study deals with the experimental investigations of static pressure and mean velocity fields obtained as a result
of the interaction of two plane turbulent jets at impingement angles of α equal to 30° and 45°, with an additional central
jet in a confined space. The investigation is carried out for the velocity ratios of U
c/U
o=1.0, 2.0 and 3.0, where U
c and U
o are the velocities in the central plane at the exit of the central jet and the outer jets, respectively. The introduction
of the central jet alters the various recirculation zones present in the flow field for all the cases considered above. Also,
the change in the velocity ratio U
c/U
o has a significant effect on the pressure and mean velocity flow fields. Flow visualisation results are presented which give
a better physical insight into the flow field considered.
Received: 26 July 1999/Accepted: 14 February 2000 相似文献
2.
Mixing by secondary flow is studied by particle image velocimetry (PIV) in a developing laminar pulsating flow through a circular curved pipe. The pipe curvature ratio is η = r
0/r
c
= 0.09, and the curvature angle is 90°. Different secondary flow patterns are formed during an oscillation period due to
competition among the centrifugal, inertial, and viscous forces. These different secondary-flow structures lead to different
transverse-mixing schemes in the flow. Here, transverse mixing enhancement is investigated by imposing different pulsating
conditions (Dean number, velocity ratio, and frequency parameter); favorable pulsating conditions for mixing are introduced.
To obviate light-refraction effects during PIV measurements, a T-shaped structure is installed downstream of the curved pipe.
Experiments are carried out for the Reynolds numbers range 420 ≤ Rest ≤ 1,000 (Dean numbers 126.6 ≤ Dn ≤ 301.5) corresponding to non-oscillating flow, velocity component ratios 1 ≤ (β = U
max,osc/U
m,st) ≤ 4 (the ratio of velocity amplitude of oscillations to the mean velocity without oscillations), and frequency parameters
8.37 < (α = r
0(ω/ν)0.5) < 24.5, where α2 is the ratio of viscous diffusion time over the pipe radius to the characteristic oscillation time. The variations in cross-sectional
average values of absolute axial vorticity (|ζ|) and transverse strain rate (|ε|) are analyzed in order to quantify mixing.
The effects of each parameter (Rest, β, and α) on transverse mixing are discussed by comparing the dimensionless vorticities (|ζ
P
|/|ζ
S
|) and dimensionless transverse strain rates (|ε
P
|/|ε
S
|) during a complete oscillation period. 相似文献
3.
A specially constructed hot-wire probe was used to obtain very near-wall velocity measurements in both a fully developed
turbulent channel flow and flat plate boundary layer flow. The near-wall hot-wire probe, having been calibrated in a specially
constructed laminar flow calibration rig, was used to measure the mean streamwise velocity profile, distributions of streamwise
and spanwise intensities of turbulence and turbulence kinetic energy k in the viscous sublayer and beyond; these distributions compare very favorably with available DNS results obtained for channel
flow. While low Reynolds number effects were clearly evident for the channel flow, these effects are much less distinct for
the boundary layer flow. By assuming the dissipating range of eddy sizes to be statistically isotropic and the validity of
Taylor's hypothesis, the dissipation rate ɛ
iso in the very near-wall viscous sublayer region and beyond was determined for both the channel and boundary layer flows. It
was found that if the convective velocity U
c in Taylor's hypothesis was assumed to be equal to the mean velocity Uˉ at the point of measurement, the value of (ɛ+
iso)1 thus obtained agrees well with that of (ɛ
+)DNS for y
+ ≥ 80 for channel flow; this suggests the validity of assuming U
c=Uˉ and local isotropy for large values of y
+. However, if U
c was assumed to be 10.6u
τ
, the value of (ɛ+
iso)2 thus obtained was found to compare reasonably well with the distribution of (ɛ+
iso)DNS for y
+≤ 15.
Received: 31 May 1999/Accepted: 20 December 1999 相似文献
4.
A two-dimensional flow generated by the interaction of two opposing, symmetric curved wall jets is investigated experimentally. The overall flow field can be divided into the curved wall jet region, the interaction region, and the merged jet region; thus, the results of the measurement are discussed to characterize these three distinct regions. For the curved wall jet region, the Reynolds stress distribution, the correlation coefficient,
, and the ratio of normal stresses,
, are presented and the effects of curvature and adverse pressure gradient on these distributions are discussed. The Reynolds stress distributions in the interaction region are analyzed in detail to illuminate the negative production of the turbulent kinetic energy. The developing jet in this region is found to accelerate owing to the very high pressure arising from the collision of the two wall jets. A counter-gradient shear flow situation is also observed in this interacting region. Measured data in the merged jet region are often compared to those of plane jets and the development of the merged jet is discussed in that respect. The spreading rate of the present merged jet is found to be much larger than that of the plane jets. To account for the larger spreading rate, the intermittency distribution is also investigated.List of symbols
b
position of y where U = U
c/2
-
f
turbulent/non-turbulent interface crossing rate
-
f
max
maximum interface crossing rate
-
h
slot height of the wall jet, 10 mm
-
L
u
integral length scale
-
P, P
a
static and atmospheric pressure, respectively
-
P
u
2
production rate of longitudinal normal stress
-
P
v
2
production rate of lateral normal stress
-
r
radial distance from the cylinder surface
-
R
radius of curvature of the cylinder, 100 mm
-
r
1/2
position of r where U=U
m/2
-
U
streamwise velocity
-
U
c
centerline velocity of the merged jet
-
U
m
maximum velocity of the curved wall jet
-
U
0
exit velocity
-
\]
Reynolds stresses
-
V
lateral velocity in the merged jet
-
x
distance along the centerline of the merged jet
-
y
lateral distance from the centerline of the merged jet
-
intermittency factor 相似文献
5.
Coherent structures in countercurrent axisymmetric shear flows 总被引:1,自引:0,他引:1
The dynamical behaviors of coherent structures in countercurrent axisymmetric shear flows are experimentally studied.The forward velocity U1 and the velocity ratio R=(U1-U2)/(U1+U2),where U2 denotes the suction velocity,are consldered as the control parameters.Two kinds of vortex structures,i.e.,axisymmetric and helical structures,were discovered with respect to different reginmes in the R versus U1 diagram .In the case of U1 rangjing from 3 to 20m/s and R from 1 to 3,the axisymmetric structures plan an important role.Based on the dynamical behaviors of axisymmetric structures,a critical forward velocity U1cr=6.8m/s was defined,subsequently,the subcritical velocity regime:U1>U1cr and the supercritical velocity regime:U1<U1er,In the subcritical velocity regine,the flow system contains shear layer self-excited oscillations in a certain range of the velocity ratio with respect to any forward velocity.In the supercritical velocity regime,the effect of the velocity ratio could be explained by the relative movement and the spatial evolution of the axisymmetric structure undergoes the following stages:(1) Kelvin-Helmholtz instability leading to vortex rolling up,(2) first time vortex agglomeration.(3) jet colunn self-excited oscillation,(4) shear layer self-excited oscillation,(5)“ordered tearing“,(6) turbulence in the case of U1<4m/s (the “ordered tearing“ does not exist when U1>4m/s),correspondingly,the spatial evolution of the temporal asymptotic behavior of a dynamical system can be described as follows:(1) Hopt bifurcation,(5) chaos(“weak turbulence“)in the case of U1<4m/s(superharmonic bifurcation does not exist when U1>4m/s).The proposed new terms,superharmonic and reversed superbarmonic bifurcations,are characterized of the frequency doubling rather than the period doubling.A kind of unfamiliar vortices referred to as the helical structure was discovered experimentally when the forward velocity around 2m/s and the velocity range from 1.1 to 2.3,There are two base frequencies contained in the flow system and they could coexist as indicated by the Wigner-Ville-Distribution and the temporal asymptotic behavior of the dynamical system corresponding to the helical vortex could be described as 2-torus as indicted by the 3D reconstructed phase trajectory and correlation dimension.The scenario of the spatial evolution of helical structures could be described as follows:the jet column is separated into two parts at a certain spatial location and they entangle each other to form the helical vortex until the occurrence of those separated jet columns to reconnect further downstream with the result that the flow system evolves into turbulence in a catastrophic form.Correspondingly,the dynamical system evolves directly into 2-tiorus through the supercritical Hopf bifurcation followed by a transition from a quasi-periodic attractor to a strange attractor.In the case of U1=2m/s,the parametric evolution of the temporal asymptotic behavior of the dynamical system as the velocity ratio increases from 1 to 3 could be described as follows:(1)2-torus(Hopf bifurcation),(2) limit cycle(reversed Hopf bifurcation),(3) strange attractor (subbarmonic bifurcation). 相似文献
6.
This study investigates the enhancement of the laminar forced convection characteristics of backward-facing step flow in a two-dimensional channel through the installation of solid and slotted baffles onto the channel wall. The effects of the height of baffle H
b, inclination of baffle installation ϕb, height of slot in baffle H
t, inclination of slot in baffle ϕt, and distance between the backward-facing step and baffle D on the flow structure, temperature distribution and Nusselt number variation for the system at various Re are numerically explored. Results show that a slotted baffle can enhance the average Nusselt number for the heating section of channel plate by the maximum 190% when Pr=0.7, H
s=0.5, L=5, H
b ≤ 0.3, W
b ≤ 0.2, 0.1 ≤ D ≤ 0.5, 0° ≤ ϕb ≤ 45°, H
t ≤ 0.1, 0° ≤ ϕt ≤ 45° and 50 ≤ Re ≤ 400. As for the solid baffle, the enhancement may be up by 230%. The solid baffle might cause the re-separation of main stream, and consequently result in poor local heat transfer coefficient in the end region of heating section. This disadvantage can be obviously improved as the baffle is slotted. Besides the penalty of increase in pressure drop due to the baffle installation is much higher for the situation with solid baffle. 相似文献
7.
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
8.
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
9.
Simultaneous velocity and concentration fields in a confined liquid-phase rectangular jet with a Reynolds number based on
the hydraulic diameter of 50,000 (or 10,000 based on the velocity difference between streams and the jet exit dimension) and
a Schmidt number of 1,250 were obtained by means of a combined particle image velocimetry (PIV) and planar laser-induced fluorescence
(PLIF) system. Data were collected at the jet exit and six further downstream locations. The velocity and concentration field
data were analyzed for flow statistics such as turbulent fluxes, turbulent viscosity and diffusivity, and turbulent Schmidt
number (Sc
T
). The streamwise turbulent flux was found to be larger than the transverse turbulent flux, and the mean concentration gradient
was not aligned with the turbulent flux vector. The average Sc
T
was found to vary both in streamwise and in cross stream directions and had a mean value around 0.8, a value consistent with
the literature. Spatial correlation fields of turbulent fluxes and concentration were then determined. The R
u′ϕ′ correlation was elliptical in shape with a major axis tilted downward with respect to the streamwise axis, whereas the R
v′ϕ′ correlation was an ellipse with a major axis aligned with the cross-stream direction. Negative regions of R
u′ϕ′ were observed in the outer streams, and these negatively correlated regions decayed with downstream distance and finally
disappeared altogether. The R
ϕ′ϕ′ correlation field was found to be an ellipse with the major axis inclined at about 45° with respect to the streamwise direction.
Linear stochastic estimation was used to interpret spatial correlation data and to determine conditional flow structures.
It is believed that a vortex street formed near the splitter plate is responsible for the negatively correlated region observed
in the R
u′ϕ′ spatial correlations of turbulent fluxes. A positive concentration fluctuation event was observed to correspond to a finger
of nearly uniform concentration fluid reaching out into the outer stream, whereas a negative event corresponds to a pocket
of nearly uniform fluid being entrained from the outer stream into the center jet region. Large-scale vortical structures
were observed in the conditional velocity fields with an elliptical shape and a streamwise major axis. The growth of the structure
size increased linearly initially but then grew more slowly as the flow transitioned toward channel flow.
Support of this work was provided by the National Science Foundation through grants CTS-9985678 and CTS-0336435 and by the
Dow Chemical Company. The author greatly acknowledge Charles Lipp at Dow Chemical and Ken Junk at Emerson Fisher for their
valuable assistance in the design and construction of the flow system. 相似文献
10.
Velocity profile measurements in zero pressure gradient, turbulent boundary layer flow were made on a smooth wall and on two
types of rough walls with a wide range of roughness heights. The ratio of the boundary layer thickness (δ) to the roughness
height (k) was 16≤δ/k≤110 in the present study, while the ratio of δ to the equivalent sand roughness height (k
s) ranged from 6≤δ/k
s≤91. The results show that the mean velocity profiles for all the test surfaces agree within experimental uncertainty in velocity-defect
form in the overlap and outer layer when normalized by the friction velocity obtained using two different methods. The velocity-defect
profiles also agree when normalized with the velocity scale proposed by Zagarola and Smits (J Fluid Mech 373:33–70, 1998). The results provide evidence that roughness effects on the mean flow are confined to the inner layer, and outer layer similarity
of the mean velocity profile applies even for relatively large roughness. 相似文献
11.
A linear stability analysis is used to study the conditions marking the onset of secondary flow in the form of longitudinal
vortices for plane Poiseuille flow of water in the thermal entrance region of a horizontal parallel-plate channel by a numerical
method. The water temperature range under consideration is 0∼30°C and the maximum density effect at 4°C is of primary interest.
The basic flow solution for temperature includes axial heat conduction effect and the entrance temperature is taken to be
uniform at far upstream location jackie=−∞ to allow for the upstream heat penetration through thermal entrance jackie=0. Numerical results for critical Rayleigh number are obtained for Peclet numbers 1, 10, 50 and thermal condition parameters
(λ
1, λ
2) in the range of −2.0≤λ
1≤−0.5 and −1.0≤λ
2≤1.4. The analysis is motivated by a desire to determine the free convection effect on freezing or thawing in channel flow
of water. 相似文献
12.
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
13.
Carlos B. da Silva Pedro Neto José C. F. Pereira 《Theoretical and Computational Fluid Dynamics》2009,23(4):287-296
Direct and large-eddy simulations (DNS/LES) of accelerating round jets are used to analyze the effects of acceleration on
the kinematics of vortex rings in the near field of the jet (x/D < 12). The acceleration is obtained by increasing the nozzle jet velocity with time, in a previously established (steady)
jet, and ends once the inlet jet velocity is equal to twice its initial value. Several acceleration rates (α = 0.02–0.6) and Reynolds numbers (Re
D
= 500–20000) were simulated. Acceleration maps were used to make a detailed study of the kinematics of vortex rings in accelerating
jets. One of the effects of the acceleration is to cause a number of new primary and secondary vortex merging events that
are absent from steady jets. As the acceleration rate α increases, both the number of primary merging events between rings
and the axial position where these take place decreases. The statistics for the speed of the starting ring that forms at the start of the acceleration phase for each simulation, agree well with the statistics for the “front” speed
observed by Zhang and Johari (Phys Fluids 8:2185–2195, 1996). Acceleration maps and flow visualizations show that during the
acceleration phase the near field coherent vortices become smaller and are formed at an higher frequency than in the steady
jet, and their (mean) shedding frequency increases linearly with the acceleration rate. Finally, it was observed that the
acceleration decreases the spreading rate of the jet, in agreement with previous experimental works.
相似文献
14.
The ‘plug’ flow emerging from a long rotating tube into a large stationary reservoir was used in the experimental investigation
of swirling jets with Reynolds numbers, Re = 600, 1,000 and 2,000, and swirl numbers, S = ΩR/U, in the range 0–1.1, to cover flow regimes from the non-rotating jet to vortex breakdown. Here Ω is the nozzle rotation rate,
R is the radius of the nozzle exit, and U is the mean mass axial velocity. The jet was more turbulent and eddies shed faster at larger Re. However the flow criticality and shear layer morphology remained unchanged with Re. After the introduction of sufficient rotation, co-rotating and counter-winding helical waves replaced vortex rings to become
the dominant vortex structure. The winding direction of the vortex lines suggests that Kelvin–Helmholtz and generalized centrifugal
instability dominated the shear layer. A quantitative visualization study has been carried out for cases where the reservoir
was rotating independently with S
a
= Ω
a
R/U = ±0.35, ±0.51 and ±0.70 at Re = 1,000 and 2000, where Ω
a
is the rotation rate of the reservoir. The criterion for breakdown was found to be mainly dependent on the absolute swirl
number of the jet, S. This critical swirl number was slightly different in stationary and counter-swirl surroundings but obviously smaller when
the reservoir co-rotated, i.e. S
c
= 0.88, 0.85 and 0.70, respectively. These results suggest that the flow criticality depends mainly on the velocity distributions
of the vortex core, while instabilities resulting from the swirl difference between the jet and its ambient seem to have only
a secondary effect. 相似文献
15.
The qualitative and quantitative behaviour of double row vortical structures in the near field region of a plane wall jet are studied experimentally by flow visualization and hot-wire measurements. Ensemble averaging is employed to investigate the interaction of vortices with the wall. In the flow visualization study, a double row vortical structure, which includes a primary vortex formed in the outer layer region and a secondary vortex induced in the inner layer region, and the vortex lift-off phenomenon are clearly observed during the development of the wall jet. The phase averaged results of the velocity measurements show that the instability leading to induction of the secondary vortex is stimulated by the primary vortex. In the early stage of wall jet transition, the inflection point of the inner layer velocity profile moves transversely from the wall surface to the inner layer region due to passage of the well-organized primary vortex in the outer layer region. The inner layer instability is thus induced and the instability wave rolls up to form the secondary vortex. Furthermore, the secondary vortex will convect downstream faster than the primary vortex, and this difference in convective speed will lead to the subsequent phenomenon of vortex lift-off from the wall surface.List of symbols A1, A2, . . .
primary vortex
-
B1,B2, . . .
secondary vortex
-
fe
forcing frequency
-
f
fundamental frequency
-
H
nozzle exit height
-
Re
Reynolds number,U
j
H/
-
T
period of the referred signal (=13.5 ms)
-
t, t
time scale
-
U
streamwise mean velocity
-
U
c
convection speed
-
U
j
jet exit velocity
-
U
m
local maximum velocity
-
ut'
streamwise turbulence intensity
-
uv
turbulent shear stress
-
V
transverse mean velocity
-
v
transverse turbulence intensity
-
X
streamwise coordinate
-
Y
transverse coordinate
-
X
Ai
streamwise location of vortexAi
-
X
Bi
streamwise location of vortexBi
-
X
ave
averaged streamwise location of the vortex
-
Y
m
wall jet inner layer width, the distance from wall to whereU=U
m
-
Y
1/2
wall jet half-width, the distance from wall to whereU=1/2U
m
in outer layer region
- t
time interval (=0.267 s)
-
phase averaged value 相似文献
16.
This study focuses on the self-sustained oscillatory flow characteristics between two tandem circular cylinders of equal diameter
placed in a uniform inflow. The Reynolds number (Re
D
), based on the cylinder diameter, was around 1,000 and all experiments were performed in a recirculating water channel. The
streamwise distance between two tandem cylinders ranged within 1.5 ≤ X
c/D ≤ 7.0. Here X
c denotes the center-to-center distance between two tandem cylinders. For all experiments studied herein, quantitative velocity
measurements were performed using hot-film anemometer and the LDV system. The laser sheet technique was employed for qualitative
flow visualization. The wavelet transform was applied to elucidate the temporal variation and phase difference between two
spectral components of the velocity signals detected in the flow field. The remarkable finding was that when two tandem circular
cylinders were spaced at a distance within 4.5 ≤ X
c/D ≤ 5.5, two symmetrical unstable shear layers with a certain wavelength were observed to impinge onto the downstream cylinder.
The responding frequency (f
u
), measured between these two cylinders, was much higher than the natural shedding frequency behind a single isolated cylinder
at the same Re
D
. This responding frequency decreased as the distance X
c/D increased. Not until X
c/D ≥ 6.0, did it recover to the natural shedding frequency behind a single isolated cylinder. Between two tandem cylinders,
the Strouhal numbers (St
c = f
u
X
c/Uc) maintained a nearly constant value of 3, indicating the self-sustained oscillating flow characteristics with a wavelength X
c/3. Here U
c is the convection speed of the unstable shear layers between two tandem cylinders. At Re
D
= 1,000, the self-sustained oscillating characteristics between two tandem circular cylinders were proven to exhibit a sustained
flow pattern, not just a sporadic phenomenon. 相似文献
17.
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
18.
Mixed convection flow in a two-sided lid-driven cavity filled with heat-generating porous medium is numerically investigated.
The top and bottom walls are moving in opposite directions at different temperatures, while the side vertical walls are considered
adiabatic. The governing equations are solved using the finite-volume method with the SIMPLE algorithm. The numerical procedure
adopted in this study yields a consistent performance over a wide range of parameters that were 10−4 ≤ Da ≤ 10−1 and 0 ≤ Ra
I
≤ 104. The effects of the parameters involved on the heat transfer characteristics are studied in detail. It is found that the
variation of the average Nusselt number is non-linear for increasing values of the Darcy number with uniform or non-uniform
heating condition. 相似文献
19.
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
20.
An exact solution is presented for the hydromagnetic natural convection boundary layer flow past an infinite vertical flat
plate under the influence of a transverse magnetic field with magnetic induction effects included. The transformed ordinary
differential equations are solved exactly, under physically appropriate boundary conditions. Closed-form expressions are obtained
for the non-dimensional velocity (u), non-dimensional induced magnetic field component (B
x
) and wall frictional shearing stress i.e. skin friction function (τ
x
) as functions of dimensionless transverse coordinate (η), Grashof free convection number (G
r
) and the Hartmann number (M). The bulk temperature in the boundary layer (Θ) is also evaluated and shown to be purely a function of M. The Rayleigh flow distribution (R) is derived and found to be a function of both Hartmann number (M) and the buoyant diffusivity parameter (ϑ
*). The influence of Grashof number on velocity, induced magnetic field and wall shear stress profiles is computed. The response
of Rayleigh flow distribution to Grashof numbers ranging from 2 to 200 is also discussed as is the influence of Hartmann number
on the bulk temperature. Rayleigh flow is demonstrated to become stable with respect to the width of the boundary layer region
and intensifies with greater magnetic field i.e. larger Hartman number M, for constant buoyant diffusivity parameter ϑ
*. The induced magnetic field (B
x
), is elevated in the vicinity of the plate surface with a rise in free convection (buoyancy) parameter G
r
, but is reduced over the central zone of the boundary layer regime. Applications of the study include laminar magneto-aerodynamics,
materials processing and MHD propulsion thermo-fluid dynamics. 相似文献