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1.
The mixed convection flow over a continuous moving vertical slender cylinder under the combined buoyancy effect of thermal
and mass diffusion has been studied. Both uniform wall temperature (concentration) and uniform heat (mass) flux cases are
included in the analysis. The problem is formulated in such a manner that when the ratio λ(= u
w/(u
w + u
∞), where u
w and u
∞ are the wall and free stream velocities, is zero, the problem reduces to the flow over a stationary cylinder, and when λ = 1
it reduces to the flow over a moving cylinder in an ambient fluid. The partial differential equations governing the flow have
been solved numerically using an implicit finite-difference scheme. We have also obtained the solution using a perturbation
technique with Shanks transformation. This transformation has been used to increase the range of the validity of the solution.
For some particular cases closed form solutions are obtained. The surface skin friction, heat transfer and mass transfer increase
with the buoyancy forces. The buoyancy forces cause considerable overshoot in the velocity profiles. The Prandtl number and
the Schmidt number strongly affect the surface heat transfer and the mass transfer, respectively. The surface skin friction
decreases as the relative velocity between the surface and free stream decreases.
Received on 17 May 1999 相似文献
2.
E. Kaiser 《Experiments in fluids》2001,30(6):603-612
Experimental measurement techniques such as naphthalene sublimation, liquid crystal thermography and real-time holographic
interferometry are standard. Their application in narrow channels causes problems and is therefore limited. The channel width
must not change too much because the naphthalene sublimation and the liquid crystal coating necessary for the thermography
may cause non-negotiable variations. The interferometry fails in turbulent flow area. The diffraction along the channel edges
is an additional difficulty. A comparison of the results obtained from the application of all three techniques, which has
not been considered in earlier publications, is made here. The methods were used to measure and visualize the heat transfer
characteristics of an array of 1.2 mm diameter impinging jets in an enclosed channel (≥2.2 mm) with single-sided flow-off
at Reynolds numbers of about Re
z
≈ 20,000. Scale-up ratios as low as 2.4 have been used in order to maintain similarity as it has not been previously reported.
The naphthalene technique provided a high spatially resolved measurement of the Sherwood number along a downstream line. The
liquid crystal thermography technique provided 2D contours of the Nusselt number. The temperature distribution within dead
water zones was visualized with holographic interferometry. The cross-flow effects caused a shift in the stagnation point
and a monotone decrease in the Nusselt number in the downstream direction.
Received: 21 April 2000/Accepted: 6 July 2000 相似文献
3.
A Direct Numerical Simulation (DNS) of flow in the V103 Low-Pressure (LP) compressor cascade with incoming wakes was performed.
The computational geometry was chosen largely in accordance with the setup of the experiments performed by Hilgenfeld and
Pfitzner (J Turbomach 126:493–500, 2004) at the University of the Armed Forces in Munich. The computations were carried out on the NEC-SX8 in Stuttgart using 64
processors and 85 million grid points. The incoming wakes stemmed from a separate DNS of incompressible flow around a circular
cylinder with a Reynolds number of Re
d
= 3300 (based on mean inflow velocity and cylinder diameter). The boundary layer along the suction surface of the blade was
found to separate and roll up due to a Kelvin–Helmholtz instability triggered by the periodically passing wakes. Inside the
rolls further transition to turbulence was found to occur. The boundary-layer flow along the pressure surface did not separate,
instead it underwent by-pass transition. 相似文献
4.
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
5.
This investigation explores the mass/heat transfer from a wall-mounted block in a rectangular fully developed channel flow.
The naphthalene sublimation scheme was used to measure the level of local mass transfer from the block’s surfaces. The heat
transfer coefficient can be obtained by analogy between heat and mass transfer. The effects of the Reynolds number on the
local mass transfer from the block’s surfaces have been widely discussed. Results showed that, owing to the flow complexity
induced by vortices around the block, the block’s surfaces appeared four different spatial Sherwood number distributions,
termed “Wave type”, “U type”, “Slant type”, and “Pit type”. A change in the Reynolds number significantly altered the spatial Sherwood number distributions on the block’s surfaces.
Besides, four correlations between the Reynolds number and the surface-averaged Sherwood number were presented for the front,
top, side, and rear surfaces of the block at a given block’s height, for the purpose of practical applications. 相似文献
6.
Mean and fluctuating pressure measurements over a circular cylinder in cross flow using plastic tubing 总被引:1,自引:0,他引:1
Not only can mean pressures on a surface over which a fluid flows be accurately measured using a plastic tube which connects
the surface tapping points to a remote pressure transducer, but the fluctuating pressures can also, provided that the transfer
function, which relates the fluctuating pressures at the opposite ends of the tubing, is known. This technique was used here
to measure the mean and fluctuating pressures on the surface of a circular cylinder subject to a cross-flowing airstream in
the Reynolds number range from 6.8 × 104 to 9.6 × 104 based on cylinder diameter. Good agreement with published results gave confidence in the technique.
Received: 15 April 1998/Accepted: 19 January 2000 相似文献
7.
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
8.
A. A. Tawfek 《Heat and Mass Transfer》1999,35(4):327-333
An experimental investigations of heat transfer for a stationary isothermal circular cylinder exposed normal to an impinging
round air-jet has been reported. The circumferential heat transfer distributions as well as axial Nusselt number is measured.
The measurements are taken as a function of the Reynolds number ranging from 3.8 × 103 to 4 × 104, the cylinder separation distance to the nozzle diameter (z/d) varying from 7 to 30, and the nozzle to cylinder diameter ratio (d/D) changing from 0.06 to 0.14. The output results indicated that the axial and radial distributions of the local heat transfer
peaked at the impingement point. The heat transfer rate increases as the values of z decreases, for the same d and Re. The drop-off of the Nusselt number with increasing axial distance or radial angle from the impingement point was
more pronounced for smaller z and d. The peripheral and surface average Nusselt numbers were determined by integration. The experimental data was used to produce
correlations for both average and stagnation point heat transfer.
Received on 4 January 1999 相似文献
9.
Alvaro Valencia 《Heat and Mass Transfer》1998,33(5-6):465-470
A numerical investigation was conducted into channel flows with a tandem of transverse vortex generators in the form of rectangular
cylinders. The oscillatory behavior of the flow is studied. Data for heat transfer and flow losses are presented for 100≤Re≤400
and cylinder separation distances 1≤S/H≤4. The results are obtained by numerical solution of the full Navier-Stokes equations and the energy equation. Self-sustained
flow oscillations are found for Re>100. Alternate and dynamic shedding of large vortex structures from the cylinders is observed
by visualization of the numerically determined flow field. A heat transfer enhancement up to a factor 1.78 compared to plane
channel flow is observed.
Received on 16 July 1997 相似文献
10.
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
11.
The wake of a surface-mounted finite-height circular cylinder and the associated vortex patterns are strongly dependent on the cylinder aspect ratio and the thickness of the boundary layer on the ground plane relative to the dimensions of the cylinder. Above a critical aspect ratio, the mean wake is characterized by streamwise tip vortex structures and Kármán vortex shedding from the sides of the cylinder. Below a critical aspect ratio, a unique mean wake structure is observed. Recent experimental studies in the literature that used phase-averaged techniques, as well as recent numerical simulations, have led to an improved physical understanding of the near-wake vortex flow patterns. However, the flow above the free end of the finite circular cylinder, and its relationship to the near wake, has not been systematically studied. The effects of aspect ratio and boundary layer thickness on the free-end flow field are also not completely understood, nor has the influence of Reynolds number on the free-end flow field been fully explored. Common features associated with the free end include separation from the leading edge, a mean recirculation zone containing a prominent cross-stream arch (or mushroom) vortex, and reattachment onto the free-surface. Other flow features that remain to be clarified include a separation bubble near the leading edge, one or two cross-stream vortices within this separation bubble, the origins of the streamwise tip or trailing vortices, and various critical points in the near-surface flow topology. This paper reviews the current understanding of the flow above the free end of a surface-mounted finite-height circular cylinder, with a focus on models of the flow field, surface oil flow visualization studies, pressure and heat flux distributions on the free-end surface, measurements of the local velocity field, and numerical simulations, found in the literature. 相似文献
12.
The detailed flow structure behind an impulsively started circular cylinder has been investigated experimentally. The Reynolds
number based on the steady state velocity and the diameter of the cylinder was 500 to 3,000. This work is unique in that unsteady
spatial velocities were measured simultaneously by a quantitative visualization technique — Laser Induced Photochemical Anemometry
(LIPA). The surface vorticity at g/q = π/2 and vorticity distribution behind the cylinder in the Lagrangian coordinates (i.e. coordinates fixed on the cylinder)
were calculated from the measured velocities. The surface vorticity shows in the early stage of flow development a close agreement
with the previous results obtained by analytical and numerical approaches. The large-field velocity and vorticity information
provides an insight into the formation process of the vortices downstream of the cylinder. In addition to the quantitative
information, the results of visualized flow pattern obtained by LIPA technique are also presented.
A preliminary version of this paper was presented at the Twelfth Symposium on Turbulence, University of Missouri-Rolla, Sept.
24–26, 1990 相似文献
13.
The paper presents the results of an investigation concerning heat transfer and pressure loss in air crossflow of diagonally
shaped membrane heating surfaces. The heat and mass transfer analogy by means of naphthalene sublimation technique is used
in order to evaluate mean Nusselt number values in such tube banks. The effect of tube bank arrangement on heat transfer coefficients
and flow resistance is discussed. Convective heat transfer and pressure loss characteristics of diagonally shaped membrane
tube banks, plain tube banks and ordinary membrane tube banks are compared.
Received on 15 June 2000 / Published online: 29 November 2001 相似文献
14.
Sang Woo Lee Hyun Suk Moon Seong Eun Lee 《International Journal of Heat and Fluid Flow》2009,30(2):198-210
The effects of tip gap height-to-chord ratio, h/c, on the flow structure and heat/mass transfer over the plane tip surface of a large-scale high-turning turbine rotor blade have been investigated for h/c = 1.0%, 2.0%, 3.0% and 4.0%. For near-wall tip gap flow visualizations, a high-resolution oil film method is employed, and the naphthalene sublimation technique is used for local heat/mass transfer rate measurements. From the tip surface visualizations, a pair of vortices named “tip gap vortices” is identified in the leading edge region within the tip gap. The overall tip gap flow is characterized not only by the tip gap vortices but also by the flow separation/re-attachment process along the pressure-side tip edge. Within the separation bubble, there exist complicated near-wall flows moving toward a mid-chord flow converging area. With increasing h/c, the tip gap vortices, the flow separation/re-attachment, and the converging flows within the separation bubble tend to be intensified. In general, higher thermal load is found along the loci of the tip gap vortices and along the re-attachment line, while lower thermal load is observed behind the tip gap vortex system and near the mid-chord flow converging area. Heat/mass transfer characteristics with the variation of h/c are discussed in detail in conjunction with the tip gap flow features. Based on the flow visualizations and heat/mass transfer data, new realistic tip gap flow models have been proposed for h/c = 1.0 and 4.0%. 相似文献
15.
The flow of a two-dimensional plane turbulent jet impinging on a porous screen has been studied experimentally. It is shown
how the overall flow structure depends on the porosity of the surface. For low screen porosity (β < 0.41, say), transverse wall jets can be formed on both sides of the screen and in extreme cases the axial momentum flux some way downstream of the screen falls to zero, so that
the screen has the same drag as would a solid wall. For high screen porosity (β > 0.57, say) the axial volume flux is largely preserved through the screen, but the dominant eddy structures present in the
upstream jet are largely destroyed, so that entrainment rates downstream of the screen can be very low. The relatively small,
intermediate range of porosities (0.41 < β < 0.57, where β is the screen open area ratio) is associated with dramatic changes in flow pattern and recirculating regions can exist on
the upstream side of the screen. These flows, although all geometrically very simple, provide a serious challenge for computational
modelling.
Received: 25 May 2000 / Accepted: 22 February 2001 相似文献
16.
康顺 《应用数学和力学(英文版)》1990,11(5):489-495
Based on the working of Lighthill and Hunt et al., in the present paper the author has established the topological rules adapting to analysing the skin-friction lines and the section streamlines in cascades. These rules are (1) for a rotor cascade without shroud band, the total number of nodal points equals that the saddle points on the skin-friction line vector fields in each pitch range; (2) for an annular or straight cascade with no clearances at blade ends, the total number of saddle points is two more than that of nodal points on the skin-friction line fields in a pitch; (3) the total number of saddles in the secondary flow fields on cross-sections in cascade is one less than that of nodes; (4) in the section streamline vector fields on a meridian surface penetrating a flow passage, and on leading and trailing edge sections, the total number of nodes is equal to that of saddles; (5) on the streamline vector fields of a blade-to-blade surface, the total number of nodes is one less than that of saddles. 相似文献
17.
Experiments are carried out in the wake of a cylinder of d
c
= 10 mm diameter placed symmetrically between two parallel walls with a blockage ratio r = 1/3 and a Reynolds number varying between 75 ≤ Re ≤ 277. Particle image velocimetry is exerted to obtain the instantaneous velocity components in the cylinder wake. A snapshot
proper orthogonal decomposition (POD) is also applied to these PIV results in order to extract the dominant modes through
the implementation of an inhomogeneous filtering of these different snapshots, apart from an interpolation to estimate the
wall shear rate at the lower wall downstream the cylinder. Mass transfer circular probes are placed at the lower wall downstream
this obstacle so as to further determine the time evolution of the wall shear rate, by bringing the inverse method to bear
on the convective-diffusion equation. Comparisons between the two synchronized techniques demonstrate that electrochemical
method can give more accurate information about the coherent structures present in the flow and about the interaction of the
von Kármán vortices with the walls of the tunnel as well. The comparison between the two measurement techniques in the flow
regions concerns the spatiotemporal evolutions of the wall shear rate obtained from PIV measurements and the wall shear rate
using mass transfer probes. Discrepancy between the PIV measurements and the electrochemical ones near the wall, where the
secondary vortices P
1′ are generated at wall, are caused by a PIV bias and a limitations of the singular mass transfer probes. 相似文献
18.
Edge fracture is an instability of cone-plate and parallel plate flows of viscoelastic liquids and suspensions, characterised
by the formation of a `crack' or indentation at a critical shear rate on the free surface of the liquid. A study is undertaken
of the theoretical, experimental and computational aspects of edge fracture. The Tanner-Keentok theory of edge fracture in
second-order liquids is re-examined and is approximately extended to cover the Criminale-Ericksen-Filbey (CEF) model. The
second-order theory shows that the stress distribution on the semi-circular crack is not constant, requiring an average to
be taken of the stress; this affects the proportionality constant, K in the edge fracture equation −N
2c = KΓ/a, where N
2c is the critical second normal stress difference, Γ is the surface tension coefficient and a is the fracture diameter. When the minimum stress is used, K = 2/3 as found by Tanner and Keentok (1983). Consideration is given to the sources of experimental error, including secondary
flow and slip (wall effect). The effect of inertia on edge fracture is derived. A video camera was used to record the inception
and development of edge fracture in four viscoelastic liquids and two suspensions. The recorded image was then measured to
obtain the fracture diameter. The edge fracture phenomenon was examined to find its dependence on the physical dimensions
of the flow (i.e. parallel plate gap or cone angle), on the surface tension coefficient, on the critical shear rate and on
the critical second normal stress difference. The critical second normal stress difference was found to depend on the surface
tension coefficient and the fracture diameter, as shown by the theory of Tanner and Keentok (1983); however, the experimental
data were best fitted by the equation −N
2c = 1.095Γ/a. It was found that edge fracture in viscoelastic liquids depends on the Reynolds number, which is in good agreement with
the inertial theory of edge fracture. Edge fracture in lubricating grease and toothpaste is broadly consistent with the CEF
model of edge fracture. A finite volume method program was used to simulate the flow of a viscoelastic liquid, obeying the
modified Phan-Thien-Tanner model, to obtain the velocity and stress distribution in parallel plate flow in three dimensions.
Stress concentrations of the second normal stress difference (N
2) were found in the plane of the crack; the velocity distribution shows a secondary flow tending to aid crack formation if
N
2 is negative, and a secondary flow tending to suppress crack formation if N
2 is positive.
Received: 4 January 1999 Accepted: 19 May 1999 相似文献
19.
Large-eddy simulation of a turbulent reactive jet with and without evaporating droplets is performed to investigate the interactions
among turbulence, combustion, heat transfer and evaporation. A hybrid Eulerian–Lagrangian approach is used for the gas–liquid
flow system. Arrhenius-type finite-rate chemistry is employed for the chemical reaction. To capture the highly local interactions,
dynamic procedures are used for all the subgrid-scale models, except that the filtered reaction rate is modelled by a scale
similarity model. Various representative cases with different initial droplet sizes (St
0) and mass loading ratios (MLR) have been simulated, along with a case without droplets. It is found that compared with the bigger, slow responding droplets
(St
0 = 16), smaller droplets (St
0 = 1) are more efficient in suppressing combustion due to their preferential concentration in the reaction zones. The peak
temperature and intensity of temperature fluctuations are found to be reduced in all the droplet cases, to a varying extent
depending on the droplet properties. Detailed analysis on the contributions of respective terms in a transport equation for
grid-scale kinetic energy (GSKE) shows that the droplet evaporation effect on GSKE is small, while the droplet momentum effect
depends on St
0. When the MLR is sufficiently high, the bigger (St
0 = 16) droplets can have profound influence on GSKE, and consequently on the formation and evolution of large-scale flow structures.
On the other hand, the turbulence level is found to be lower in the droplet cases than in the pure flame case, due to the
dissipative droplet dynamic effect. 相似文献
20.
In the present study, we perform a wind-tunnel experiment to investigate the aerodynamic performance of a gliding swallowtail-butterfly
wing model having a low aspect ratio. The drag, lift and pitching moment are directly measured using a 6-axis force/torque
sensor. The lift coefficient increases rapidly at attack angles less than 10° and then slowly at larger attack angles. The
lift coefficient does not fall off rapidly even at quite high angles of attack, showing the characteristics of low-aspect-ratio
wings. On the other hand, the drag coefficient increases more rapidly at higher angles of attack due to the increase in the
effective area responsible for the drag. The maximum lift-to-drag ratio of the present modeled swallowtail butterfly wing
is larger than those of wings of fruitfly and bumblebee, and even comparable to those of wings of birds such as the petrel
and starling. From the measurement of pitching moment, we show that the modeled swallowtail butterfly wing has a longitudinal
static stability. Flow visualization shows that the flow separated from the leading edge reattaches on the wing surface at
α < 15°, forming a small separation bubble, and full separation occurs at α ≥ 15°. On the other hand, strong wing-tip vortices are observed in the wake at α ≥ 5° and they are an important source of the lift as well as the main reason for broad stall. Finally, in the absence of
long hind-wing tails, the lift and longitudinal static stability are reduced, indicating that the hind-wing tails play an
important role in enhancing the aerodynamic performance. 相似文献