共查询到20条相似文献,搜索用时 31 毫秒
1.
Bart Beulen Nathalie Bijnens Marcel Rutten Peter Brands Frans van de Vosse 《Experiments in fluids》2010,49(5):1177-1186
An ultrasound velocity assessment technique was validated, which allows the estimation of velocity components perpendicular
to the ultrasound beam, using a commercially available ultrasound scanner equipped with a linear array probe. This enables
the simultaneous measurement of axial blood velocity and vessel wall position, rendering a viable and accurate flow assessment.
The validation was performed by comparing axial velocity profiles as measured in an experimental setup to analytical and computational
fluid dynamics calculations. Physiologically relevant pulsating flows were considered, employing a blood analog fluid, which
mimics both the acoustic and rheological properties of blood. In the core region (|r|/a < 0.9), an accuracy of 3 cm s−1 was reached. For an accurate estimation of flow, no averaging in time was required, making a beat to beat analysis of pulsating
flows possible. 相似文献
2.
A. Alper Ozalp 《Heat and Mass Transfer》2008,45(1):31-46
Variable fluid property continuity, Navier–Stokes and energy equations are solved for roughness induced forced convective
laminar-transitional flow in a micropipe. Influences of Reynolds number, heat flux and surface roughness, on the momentum-energy
transport mechanisms and second-law of thermodynamics, are investigated for the ranges of Re = 1–2,000, Q = 5–100 W/m2 and ε = 1–50 μm. Numerical investigations put forward that surface roughness accelerates transition with flatter velocity profiles
and increased intermittency values (γ); such that a high roughness of ε = 50 μm resulted in transitional character at Re
tra = 450 with γ = 0.136. Normalized friction coefficient (C
f*) values showed augmentation with Re, as the evaluated C
f* are 1.006, 1.028 and 1.088 for Re = 100, 500 and 1,500, respectively, at ε = 1 μm, the corresponding values rise to C
f* = 1.021, 1.116 and 1.350 at ε = 50 μm. Heat transfer rates are also recorded to rise with Re and ε; moreover the growing influence of ε on Nusselt number with Re is determined by the Nu
ε=50 μm/Nu
ε=1 μm ratios of 1.086, 1.168 and 1.259 at Re = 500, 1,000 and 1,500. Thermal volumetric entropy generation values decrease with Re and ε in heating; however the contrary is recorded for frictional volumetric entropy generation data, where the augmentations in are more considerable when compared with the decrease rates of 相似文献
3.
M. J. Giles 《Applied Scientific Research》1996,57(3-4):223-234
The exact expression for the probability distribution function (pdf),P(Δur), of a velocity difference Δur, over a distancer, in incompressible fluid turbulence, obtained from the Navier-Stokes equations, is used as a basis for deriving approximate
profiles forP(Δur). These approximate forms are deduced from an approximate factorisation of the underlying functional probability distribution
of the flow field, in which the individual factors capture different physical effects.P(Δur) is represented as the integral, with respect to the spatially averaged dissipation rateε
r, of the product of the conditionalpdf of Δur givenε
r, and thepdf ofε
r. The approximation yields the latter as a log-Poissonpdf, while the conditionalpdf is found to be a Gaussian for a transverse increment, and the product of a Gaussian and a cubic polynomial for a longitudinal
increment. This approximation is equivalent to the refined similarity hypothesis coupled with the log-Poisson distribution,
and it possesses the characteristic features ofP(Δur), including symmetric profiles for transverse increments, asymmetric profiles for longitudinal increments, and the development
of pronounced non-Gaussian features at small separations. The associated scaling exponents for longitudinal and transverse
structure functions are shown to be identical, in this approximation, and to assume the log-Poisson form. 相似文献
4.
Masaru Sumida 《Experiments in fluids》2007,43(6):949-958
This paper presents the results of an experimental study on the developing pulsatile flow in curved pipes with a long, straight
pipe upstream. In order to examine the dependence of flow-field development on the governing parameters, LDV measurements
were conducted systematically for six cases of flow, where the Womersley number α was varied from 5.5 to 18, the mean Dean number D
m
was 200 and 300, the flow rate ratio η was 0.5 and 1, and the curvature radius ratio Rc was 10 and 30. Peculiar flow phenomena, such as flow reversal for all values of α and a depression in the axial velocity profile for α = 10, were analyzed by decomposing the axial velocity into a time-mean and a varying component, as well as by obtaining the
bias of their profiles. The velocity distributions abruptly change with the phase at turn angles Ω of 15–30°, corresponding to the nondimensional axial length z′ ≅ 1–2 from the bend entrance, and their development along the pipe axis is the most complicated for the flow at a moderate
α of 10 and large η of 1. The entrance length in the case of pulsatile flow is shorter than that for steady flow with the same flow rate as the
maximum pulsatile flow rate. 相似文献
5.
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
6.
A pulsating flow is typical of inlet and exhaust pipes of internal combustion engines and piston compressors. Unsteady flow phenomena are especially important in the case of turbocharged engines, because dynamic effects occurring in the exhaust pipe can affect turbine operation conditions and performance.One of the basic parameters describing the unsteady flow is a transient mass flow rate related to the instantaneous flow velocity, which is usually measured by means of hot-wire anemometers. For the flowing gas, it is more appropriate to analyze the specific mass flow rate φm = ρv, which takes into account also variations in the gas density. In order to minimize the volume occupied by measuring devices in the control section, special double-wire sensors for the specific mass flow rate (CTA) and temperature (CCT) measurement were applied. The article describes procedures of their calibration and measurement. Different forms of calibration curves are analyzed as well in order to match the approximation function to calibration points. Special attention is paid to dynamic phenomena related to the resonance occurring in a pipe for characteristic frequencies depending on the pipe length. One of these phenomena is a reverse flow, which makes it difficult to interpret properly the recorded CTA signal. Procedures of signal correction are described in detail. To verify the measurements, a flow field investigation was carried out by displacing probes radially and determining the profiles of the specific mass flow rate under the conditions of a steady and pulsating flow. The presence and general features of a reverse flow, which was identified experimentally, were confirmed by 1-D unsteady flow calculations. 相似文献
7.
Andrew J. Onstad Christopher J. Elkins Frank Medina Ryan B. Wicker John K. Eaton 《Experiments in fluids》2011,50(6):1571-1585
Open-celled foam geometries show great promise in heat/mass transfer, chemical treatment, and enhanced mixing applications.
Flow measurements on these geometries have consisted primarily of observations of the upstream and downstream effects the
foam has on the velocity field. Unfortunately, these observations give little insight into the flow inside the foam. We have
performed quantitative flow measurements inside a scaled replica of a metal foam, ϕ = 0.921, D
Cell = 2.5 mm, by Magnetic Resonance Velocimetry to better understand the fluid motion inside the foam and give an alternative
method to determine the foam cell and pore sizes. Through these 3-D, spatially resolved measurements of the flow field for
a cell Reynolds number of 840, we have shown that the transverse motion of the fluid has velocities 20–30% of the superficial
velocity passing through the foam. This strong transverse motion creates and dissipates streamwise jets with peak velocities
2–3 times the superficial velocity and whose coherence length is strongly correlated to the cell size of the foam. This complex
fluid motion is described as “mechanical mixing” and is attributed to the geometry of the foam. A mechanical dispersion coefficient,
D
M, was formed which demonstrates the transverse dispersion of this geometry to be 14 times the kinematic viscosity and 10 times
the thermal diffusivity of air at 20°C and 1 atm. 相似文献
8.
The near-field flow structure of a tip vortex behind a sweptback and tapered NACA 0015 wing was investigated and compared
with a rectangular wing at the same lift force and Re=1.81×105. The tangential velocity decreased with the downstream distance while increased with the airfoil incidence. The core radius
was about 3% of the root chord c
r, regardless of the downstream distance and α for α<8°. The core axial velocity was always wake-like. The core Γc and total Γo circulation of the tip vortex remained nearly constant up to x/c
r=3.5 and had a Γc/Γo ratio of 0.63. The total circulation of the tip vortex accounted for only about 40% of the bound root circulation Γb. For a rectangular wing, the axial flow exhibited islands of wake- and jet-like velocity distributions with Γc/Γo=0.75 and Γo/Γb=0.70. For the sweptback and tapered wing tested, the inner region of the tip vortex flow exhibited a self-similar behavior
for x/c
r≥1.0. The lift force computed from the spanwise circulation distributions agreed well with the force-balance data. A large
difference in the lift-induced drag was, however, observed between the wake integral method and the inviscid lifting-line
theory. 相似文献
9.
Two hot-wire flow diagnostics have been developed to measure a variety of turbulence statistics in the buoyancy driven, air-helium
Rayleigh–Taylor mixing layer. The first diagnostic uses a multi-position, multi-overheat (MPMO) single wire technique that
is based on evaluating the wire response function to variations in density, velocity and orientation, and gives time-averaged
statistics inside the mixing layer. The second diagnostic utilizes the concept of temperature as a fluid marker, and employs
a simultaneous three-wire/cold-wire anemometry technique (S3WCA) to measure instantaneous statistics. Both of these diagnostics
have been validated in a low Atwood number (A
t
≤ 0.04), small density difference regime, that allowed validation of the diagnostics with similar experiments done in a hot-water/cold-water
water channel facility. Good agreement is found for the measured growth parameters for the mixing layer, velocity fluctuation
anisotropy, velocity fluctuation p.d.f behavior, and measurements of molecular mixing. We describe in detail the MPMO and S3WCA diagnostics, and the validation
measurements in the low Atwood number regime (A
t
≤ 0.04). We also outline the advantages of each technique for measurement of turbulence statistics in fluid mixtures with
large density differences. 相似文献
10.
This paper presents an exact asymptotic analysis on the interfacial crack between two dissimilar elastic-plastic materials.
These two materials have identical hardening exponent (n
1=n
2) but different hardening coefficient (α1 ≠ α2). Two groups of the near-crack-tip fields have been obtained, which not only satisfy the continuity of both tractions (σθ, τrθ) and displacements (u
r
,u
θ) on the interface, but also meet the traction free conditions on the crack faces. The first group of fields have the mode
mixityM
P
quite close toM
P
=1 (MODE I) within the whole range 0 ≤ α1/α2 < ∞. As for the second group of fields, which is only obtained within the narrow range 0.9 ≤ α1/α2 ≤ 1, it is found that the mode mixity changes sharply with the ratio value α1/α2.
The project supported by National Natural Science Foundation of China 相似文献
11.
H. C. H. Ng J. P. Monty N. Hutchins M. S. Chong I. Marusic 《Experiments in fluids》2011,51(5):1261-1281
Single normal hot-wire measurements of the streamwise component of velocity were taken in fully developed turbulent channel
and pipe flows for matched friction Reynolds numbers ranging from 1,000 ≤ Re
τ ≤ 3,000. A total of 27 velocity profile measurements were taken with a systematic variation in the inner-scaled hot-wire
sensor length l
+ and the hot-wire length-to-diameter ratio (l/d). It was observed that for constant l
+ = 22 and l/d >~200l/d \gtrsim 200, the near-wall peak in turbulence intensity rises with Reynolds number in both channels and pipes. This is in contrast to
Hultmark et al. in J Fluid Mech 649:103–113, (2010), who report no growth in the near-wall peak turbulence intensity for pipe flow with l
+ = 20. Further, it was found that channel and pipe flows have very similar streamwise velocity statistics and energy spectra
over this range of Reynolds numbers, with the only difference observed in the outer region of the mean velocity profile. Measurements
where l
+ and l/d were systematically varied reveal that l
+ effects are akin to spatial filtering and that increasing sensor size will lead to attenuation of an increasingly large range
of small scales. In contrast, when l/d was insufficient, the measured energy is attenuated over a very broad range of scales. These findings are in agreement with
similar studies in boundary layer flows and highlight the need to carefully consider sensor and anemometry parameters when
comparing flows across different geometries and when drawing conclusions regarding the Reynolds number dependency of measured
turbulence statistics. With an emphasis on accuracy, measurement resolution and wall proximity, these measurements are taken
at comparable Reynolds numbers to currently available DNS data sets of turbulent channel/pipe flows and are intended to serve
as a database for comparison between physical and numerical experiments. 相似文献
12.
Flow characteristics in the interdisk midplane between two shrouded co-rotating disks were experimentally studied. A laser-assisted
particle-laden flow-visualization method was used to identify the qualitative flow behaviors. Particle image velocimetry was
employed to measure the instantaneous flow velocities. The flow visualization revealed rotating polygonal flow structures
(hexagon, pentagon, quadrangle, triangle, and oval) existing in the core region of the interdisk spacing. There existed a difference between the rotating frequencies of the
polygon and the disks. The rotating frequency ratio between the polygonal flow structure and the disks depended on the mode
shapes of the polygonal core flow structures—0.8 for pentagon, 0.75 for quadrangle, 0.69 for triangle, and 0.6 for oval. The
phase-resolved flow velocities relative to the bulk rotation speed of the polygonal core flow structure were calculated, and
the streamline patterns were delineated. It was found that outside the polygonal core flow structure, there existed a cluster
of vortex rings—each side of the polygon was associated with a vortex ring. The radial distributions of the time-averaged
and phase-resolved ensemble-averaged circumferential and radial velocities were presented. Five characteristic regions (solid-body
rotation region, hub-influenced region, buffer region, vortex region, and shroud-influenced region) were identified according
to the prominent physical features of the flow velocity distributions in the interdisk midplane. In the solid-body rotation
region, the fluid rotated at the angular velocity of the disks and hub. In the hub-influenced region, the circumferential
flow velocity departed slightly from the disks’ angular velocity. The circumferential velocities in the hub-influenced and
vortex regions varied linearly with variation of radial coordinates. The phase-resolved ensemble-averaged relative radial
velocity profiles in the interdisk midplane at various phase angles exhibited grouping behaviors in three ranges of polygon
phase angles (θ = 0 and α/2, 0 < θ < α/2, and α/2 < θ < α) because three-dimensional flow induced similar flow patterns to
appear in the same range of polygon phase angles. 相似文献
13.
An experimental investigation of water flow in a T-shaped channel with rectangular cross section (20 × 20 mm inlet ID and
20 × 40 mm outlet ID) has been conducted for a Reynolds number Re range of 56–422, based on inlet diameter. Dynamical conditions and the T-channel geometry of the current study are applicable
to the microscale. 2-D planar particle imaging velocimetry (PIV) and laser-induced fluorescence (LIF) were used in multiple
locations of the T-channel to investigate local dynamical behaviors. Steady symmetric and asymmetric flow regimes predicted
in the literature, which is largely numerical, are experimentally verified. Unsteady flow regimes, which are numerically predicted
to occur at higher Re but have not yet been experimentally characterized, are also examined, and real-time LIF results illuminate the evolution
of unsteady structure. Experimental data of the present resolution and scope are not presently available for unsteady flow
regimes. Time scales are presented for unsteady flow regimes, which are found to exhibit periodic behavior and to occur for
Re ≥ 195. An unsteady symmetrical regime is identified for Re ≥ 350 that is detrimental to mixing. Momentum fields and dynamical behaviors of all flow regimes are characterized in detail,
such that published mixing trends may be better understood. Results of all experimental trials were used to construct a regime
map. A symmetric topology is found to be dominant for Re from 56 to 116, when flow is steady, and 350 to 422, when flow is characterized by unsteady stagnation-point oscillation
in the T-channel junction. Asymmetric flow, which is positively indicated for mixing, is dominant for Re between 142 and 298, and the fluid interface exhibits both steady (two standing vortices) and unsteady (shear-layer type
roll-up) behaviors. This result is based on multiple experiments and suggests a practical operating range of 142 ≤ Re ≤ 298 where asymmetric flow is highly likely to experimentally occur. The identification of an upper limit on Re, beyond which mixing appears negatively impacted by a more symmetrical momentum field, is practically important as pressure
drops on the microscale are significant. 相似文献
14.
15.
The streamwise evolution of an inclined circular cylinder wake was investigated by measuring all three velocity and vorticity
components using an eight-hotwire vorticity probe in a wind tunnel at a Reynolds number Red of 7,200 based on free stream velocity (U
∞) and cylinder diameter (d). The measurements were conducted at four different inclination angles (α), namely 0°, 15°, 30°, and 45° and at three downstream
locations, i.e., x/d = 10, 20, and 40 from the cylinder. At x/d = 10, the effects of α on the three coherent vorticity components are negligibly small for α ≤ 15°. When α increases further
to 45°, the maximum of coherent spanwise vorticity reduces by about 50%, while that of the streamwise vorticity increases
by about 70%. Similar results are found at x/d = 20, indicating the impaired spanwise vortices and the enhancement of the three-dimensionality of the wake with increasing
α. The streamwise decay rate of the coherent spanwise vorticity is smaller for a larger α. This is because the streamwise
spacing between the spanwise vortices is bigger for a larger α, resulting in a weak interaction between the vortices and hence
slower decaying rate in the streamwise direction. For all tested α, the coherent contribution to [`(v2)] \overline{{v^{2}}} is remarkable at x/d = 10 and 20 and significantly larger than that to [`(u2)] \overline{{u^{2}}} and [`(w2)]. \overline{{w^{2}}}. This contribution to all three Reynolds normal stresses becomes negligibly small at x/d = 40. The coherent contribution to [`(u2)] \overline{{u^{2}}} and [`(v2)] \overline{{v^{2}}} decays slower as moving downstream for a larger α, consistent with the slow decay of the coherent spanwise vorticity for
a larger α. 相似文献
16.
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
17.
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
18.
In relation to the development of the interfacial area transport equation, local flow measurements of vertical downward air–water flows in a pipe with an inner diameter of 50.8 mm were performed at three axial locations of z/D=6.50, 34.0, and 66.5 as well as ten radial locations from r/R=0 to r/R=0.9 using a multi-sensor probe. In the experiment, the superficial liquid velocity and the void fraction ranged from –0.620 m/s to –2.49 m/s and from 0.21% to 8.4%, respectively. The dependence of the interfacial area transport on the liquid velocity, void fraction, and bubble size is discussed in detail. 相似文献
19.
Kyung Min Kim Hyun Lee Beom Seok Kim Sangwoo Shin Dong Hyun Lee Hyung Hee Cho 《Heat and Mass Transfer》2009,45(12):1617-1625
In the present study, an optimal design for enhancement of heat transfer and thermal performance for a stationary channel
with angled rib turbulators was investigated to find the most suitable rib geometry. Among various design parameters, two
design variables, rib angle of attack (α) and pitch-to-rib height (p/e), were chosen. The ranges of two design variables were set as 30° ≤ α ≤ 80° and 3.0 ≤ p/e ≤ 15.0. Approximations for design of the best rib turbulators were obtained using the advanced response surface method with
functional variables. The second-order response surfaces (or correlations) within the ranges of two design variables were
completed by this method. As for the optimized results, maximum averaged heat transfer value was obtained at α = 53.31° and p/e = 6.50, while the highest thermal performance value was presented at α = 54.67° and p/e = 6.80. 相似文献
20.
Wassim Kriaa Kamel Abderrazak Hatem Mhiri Georges Le Palec Philippe Bournot 《Heat and Mass Transfer》2008,44(9):1051-1063
In this work, we propose to study non isothermal air–air coaxial jets with two different approaches: parabolic and elliptic
approaches. The standard k−ε model and the RSM model were applied in this study. The numerical resolution of the equations governing this flow type was
carried out for: the parabolic approach, by a “home-made” CFD code based on a finite difference method, and the elliptic approach
by an industrial code (FLUENT) based on a finite volume method. In forced convection mode (Fr = ∞), the two turbulence models are valid for the prediction of the mean flow. But for turbulent sizes, k−ε model gives results closer to those achieved in experiments compared to RSM Model. Concerning the limit of validity of the
parabolic and elliptic approaches, we showed that for velocities ratio r lower than 1, the results of the two approaches were satisfactory. On the other hand, for r > 1, the difference between the results became increasingly significant. In mixed convection mode (Fr ≅ 20), the results obtained by the two turbulence models for the mean axial velocity were very different even in the plume
region. For the temperature and the turbulent sizes the two models give satisfactory results which agree well with the correlations
suggested by the experimenters for X ≥ 20. Thus, the second order model with σ
t = 0.85 is more effective for a coaxial jet study in a mixed convection mode. 相似文献