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1.
Scalar transport from a point source in flows over wavy walls 总被引:1,自引:0,他引:1
Simultaneous measurements of the velocity and concentration field in fully developed turbulent flows over a wavy wall are
described. The concentration field originates from a low-momentum plume of a passive tracer. PLIF and digital particle image
velocimetry are used to make spatially resolved measurements of the structure of the scalar distribution and the velocity.
The measurements are performed at three different Reynolds numbers of Re
b = 5,600, Re
b = 11,200 and Re
b = 22,400, respectively, based on the bulk velocity u
b and the total channel height 2h. The velocity field and the scalar field are investigated in a water channel with an aspect ratio of 12:1, where the bottom
wall of the test section consists of a train of sinusoidal waves. The wavy wall is characterized by the amplitude to wavelength
ratio α = 0.05 and the ratio β between the wave amplitude and the half channel height where β = 0.1. The scalar is released
from a point source at the wave crest. For the concentration measurements, Rhodamine B is used as tracer dye. At low to moderate
Reynolds number, the flow field is characterized through a recirculation zone which develops after the wave crest. The recirculation
zone induces high intensities of the fluctuations of the streamwise velocity and wall-normal velocity. Furthermore, large-scale
structures are apparent in the flow field. In previous investigations it has been shown that these large-scale structures
meander laterally in flows over wavy bottom walls. The investigations show a strong effect of the wavy bottom wall on the
scalar mixing. In the vicinity of the source, the scalar is transported by packets of fluid with a high scalar concentration.
As they move downstream, these packets disintegrate into filament-like structures which are subject to strong gradients between
the filaments and the surrounding fluid. The lateral scale of the turbulent plume is smaller than the lateral scale of the
large-scale structures in the flow field and the plume dispersion is dominated by the structures in the flow field. Due to
the lateral meandering of the large-scale structures of the flow field, also the scalar plume meanders laterally. Compared
to turbulent plumes in plane channel flows, the wavy bottom wall enhances the mixing effect of the turbulent flow and the
spreading rate of the scalar plume is increased. 相似文献
2.
A new method is proposed to obtain a turbulent scalar spectrum and the energy dissipation rate in turbulent flow from ultrasonic
frequency scanning data. A scanning sonar with frequency varying from 0.5 MHz to 5 MHz has been used to directly probe the
energy dissipation rate ɛ and the three-dimensional scalar spectrum E
θ(k). Experiments were conducted in a laboratory open-channel flow in clear water with Reynolds numbers varying from 1.2×105 to 6.9×105. Good agreement is found between measured spectra and those predicted by the Batchelor theory. The energy dissipation rates
compare favourably with those obtained from acoustic Doppler velocimeter measurements.
Received: 20 March 1997/Accepted: 27 September 1997 相似文献
3.
Well-resolved 3D Large Eddy Simulations (LES) are presented for open channel flow at a Reynolds number Re
τ
= 590 based on friction velocity u
τ
and water depth h. The results are depth-averaged and thereby information is obtained on the 2D horizontal fluctuations in the channel. The
total turbulence is decomposed into 2D and 3D fluctuations and the energy content of these as well as their spectral distribution
is studied. It is found that only 15% of the fluctuating energy is contained in the 2D fluctuations and that these are mostly
of scales larger than the water depth while the 3D fluctuations are restricted by the limited vertical extent of the water
body and have scales smaller than the water depth. Information is obtained on the dispersion terms arising from the depth-averaging
procedure, and scalar transport due to a vertical line source of tracer is studied thereby investigating the contribution
of the 2D and 3D fluctuations to the transverse mixing of the scalar. 相似文献
4.
Quantitative passive scalar measurements were performed in an incompressible planar mixing layer at Re
δ
up to 104 using planar laser-induced fluorescence of acetone seeded into one side of the layer. Probability density functions compiled
from sets of images showed a preferred mixture composition, favoring the high-speed fluid, which extended across the layer.
This preferred composition produced non-marching PDFs and an inflection in the average mixture fraction profile. The spatial
resolution of the experiment was found to be sufficient to accurately measure the fraction of mixed fluid within the layer.
The mixed fluid fraction was found to increase to an asymptotic value of 0.5 by Re
δ
≈ 5,000, the approximate location of turbulent transition, in contrast to high Schmidt number experiments which show minimal
mixing before the transition point.
Received: 5 October 1999 / Accepted: 9 February 2001 相似文献
5.
We have experimentally and analytically studied transport of a passive scalar in the wake of a thin flat plate located at
the centerline of a planar contraction with flat walls. The constant Launder parameter in the contraction, K = 6.25 ×10 − 6, was twice the value required for a turbulent boundary layer to relaminarize. In addition to the mixing analysis inside the
contraction, layer mixing is also investigated downstream, where the flow continues inside a constant cross-section channel.
In order to generate the passive scalar, the airflow above the plate was heated and the temperature stratification in the
wake was traced by measuring the temperature field using constant current anemometry. Using different plate lengths, we found
that the degree of mixing, obtained at a given position in the straight channel, is a function of the distance from the plate
trailing edge to the contraction outlet. For a plate which does not protrude into the straight channel, we demonstrate the
existence of an optimal trailing edge-contraction outlet distance that results in the lowest possible degree of mixing at
a given downstream position in the straight channel. This finding is also supported by a semi-empirical relationship based
on our developed self-similar solution for mixing layers in planar contractions. 相似文献
6.
Venkata Bharathi L. Boppana Zheng-Tong Xie Ian P. Castro 《Flow, Turbulence and Combustion》2012,88(3):311-342
Large-eddy simulations of the dispersion from scalar line sources at various locations within a fully developed turbulent
channel flow at Re = uh/ν = 10,400 are presented. Both mean and fluctuating scalar quantities are compared with those from the single available set
of experimental data (Lavertu and Mydlarski, J Fluid Mech 528:135–172, 2005) and differences are highlighted and discussed. The results are also discussed in the context of scalar dispersion in other
kinds of turbulent flows, e.g. homogeneous shear-flow. Initial computations at a much lower Reynolds number are also reported
and compared with the two available direct numerical simulation data sets. 相似文献
7.
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
8.
The concentration fluctuation c of diluted fluorescein dye, a high-Schmidt-number passive scalar (Sc=ν/D ≈ 2000, ν and D are the fluid momentum and dye diffusivities, respectively), is measured in the wake of a circular cylinder using a single-point
laser-induced fluorescence (SPLIF) technique. The streamwise decay rate of the mean and rms values of c is slow in comparison to that of θ, the temperature fluctuation for which the molecular Prandtl number Pr=ν/κ is about 0.7 (κ is the thermal diffusivity). The comparison between mean and rms distributions of c and θ highlights the combined role the Reynolds and Schmidt numbers play in terms of dispersing the scalar. The streamwise
evolution of the probability density functions (pdfs) of c and θ suggest that while p(θ) is approximately Gaussian in the intermediate wake (x/d ≈ 80), p(c) is strongly non-Gaussian, and depends on both x/d and Re. The skewness of c is larger than that of θ along the wake centreline. Arguably, the asymmetry of p(c) reflects the relatively strong organisation of the large-scale motion in the far-wake.
Received: 27 July 2000/Accepted: 22 December 2000 相似文献
9.
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
10.
Role of the smallest diffusive scales of a passive scalar field in the near-wall turbulent flow was examined with pseudo-spectral
numerical simulations. Temperature fields were analyzed at friction Reynolds number Re
τ=171 and at Prandtl numbers, Pr=1 and Pr=5.4. Results of direct numerical simulations (DNS) were compared with the under-resolved
simulations where the velocity field was still resolved with the DNS accuracy, while a coarser grid was used to describe the
temperature fields. Since the smallest temperature scales remained unresolved in these simulations, an appropriate spectral
turbulent thermal diffusivity was applied to avoid pile-up at the higher wave numbers. In spite of coarser numerical grids,
the temperature fields are still highly correlated with the DNS results, including instantaneous temperature fields. Results
point to practically negligible role of the diffusive temperature scales on the macroscopic behavior of the turbulent heat
transfer. 相似文献
11.
A high-intensity swirling flow in a model combustor subjected to large density variations has been examined computationally.
The focus is on the Favre-averaged Navier–Stokes computations of the momentum and scalar transport employing turbulence models
based on the differential second-moment closure (SMC) strategy. An updated version of the basic, high-Reynolds number SMC
model accounting for a quadratic expansion of both the pressure–strain and dissipation tensors and a near-wall SMC model were
used for predicting the mean velocity and turbulence fields. The accompanied mixing between the annular swirling air flow
and the central non-swirling helium jet was studied by applying three scalar flux models differing mainly in the model formulation
for the pressure-scalar gradient correlation. The computed axial and circumferential velocities agree fairly well with the
reference experiment [So et al., NASA Contractor Report 3832, 1984; Ahmed and So, Exp. Fluids 4 (1986) 107], reproducing important features of such a weakly supercritical flow configuration (tendency of the flow core
to separate). Although the length at which the mixing was completed was reproduced in reasonable agreement with the experimental
results, the mixing activity in terms of the spreading rate of the shear/mixing layer, that is its thickness, was somewhat
more intensive. Prior to these investigations, the model applied was validated by computing the transport of the passive scalar
in the non-swirling (Johnson and Bennet, Report NASA CR-165574, UTRC Report R81-915540-9, 1981) and swirling (Roback and Johnson, NASA Contractor Report 168252, 1983) flow in a model combustor. 相似文献
12.
A calibration scheme for quantitative concentration measurements using simultaneous PIV and PLIF 总被引:3,自引:0,他引:3
A new approach for calibration of planar laser-induced fluorescence (PLIF) measurements is presented. The calibration scheme
is based on the fact that there is a constant concentration flux through each cross-section of a fluorescent plume in a given
flow field and makes use of simultaneous measurements of particle image velocimetry (PIV) and PLIF. The following are the
advantages of the current technique: (1) it is experimentally less demanding and (2) it does not require in situ calibration
for generating the calibration curves. The technique can be implemented in many experimental setups (both in water and gaseous
flows) provided the geometry of the time-averaged scalar field is known. Using the calibration scheme, an analysis is carried
out on the measurements of concentration fields in grid turbulence to validate the proposed technique. To demonstrate the
feasibility of the scheme, the distributed second-order moments (μ
2), and concentration and velocity correlations (
á u¢c¢
ñ \left\langle {u^{\prime}c^{\prime}} \right\rangle and
á v¢c¢
ñ \left\langle {v^{\prime}c^{\prime}} \right\rangle ) are computed. Good agreement is found with previous studies. In addition, a quantitative appraisal of a simple closure approximation
of the moment-based transport equation is also presented using simultaneous PIV and PLIF. 相似文献
13.
Krishna K. Venkatesan Galen B. King Normand M. Laurendeau Michael W. Renfro Benjamin Böhm 《Flow, Turbulence and Combustion》2009,83(1):131-152
Simultaneous high repetition-rate, two-point hydroxyl (OH) time-series measurements with associated PLIF/PIV measurements
are employed to investigate spatio–temporal scales and flame-velocity interactions in turbulent opposed jets sustaining methane-air
double flames. For a fuel-side equivalence ratio, ϕ
B
= 1.2, a rich premixed flame exists on the fuel side while a diffusion flame exists on the air side of the stagnation plane.
The bulk Reynolds number (Re) and strain rate (SR) can be adjusted to generate flames at ϕ
B
= 1.2 with both well separated and completely merged flame fronts. Simultaneous PLIF/PIV measurements highlight distinct
spatial OH structures of the premixed and diffusive fronts corresponding to variations in the flow field. The self-propagating
tendency of the rich premixed front causes large-scale wrinkling, thereby enhancing the OH contour length by 15% as compared
to the diffusive front. Two-point OH time-series measurements are implemented to quantify both spatial and temporal fluctuations
via study of radial length and time scales. In general, these integral length and time scales follow similar trends and reach
a minimum at the axial location of peak [OH]. In comparison to merged double flames having higher Re and SR, greater OH fluctuations
are observed in the rich-premixed front as compared to the diffusive front for a well separated double flame. Because of the
developing turbulence, the OH length scales exhibit reduced axial gradients across the reaction zone for higher Re in comparison
to lower Re. A stochastic time-series simulation, using a state relationship based on a joint mixture fraction and progress
variable, is utilized to extract estimated scalar time scales from those of measured OH. The simulations indicate that the
hydroxyl fluctuations in double flames are only twice those of the underlying conserved scalar.
“Turbulent Opposed-Jet Double Flames” is submitted for consideration as a full length article to Flow Turbulence and Combustion. 相似文献
14.
We study compressible turbulent flow in a circular pipe at computationally high Reynolds number. Classical related issues are addressed and discussed in light of the DNS data, including validity of compressibility transformations, velocity/temperature relations, passive scalar statistics, and size of turbulent eddies. Regarding velocity statistics, we find that Huang’s transformation yields excellent universality of the scaled Reynolds stresses distributions, whereas the transformation proposed by Trettel and Larsson (2016) yields better representation of the effects of strong variation of density and viscosity occurring in the buffer layer on the mean velocity distribution. A clear logarithmic layer is recovered in terms of transformed velocity and wall distance coordinates at the higher Reynolds number under scrutiny (Reτ ≈ 1000), whereas the core part of the flow is found to be characterized by a universal parabolic velocity profile. Based on formal similarity between the streamwise velocity and the passive scalar transport equations, we further propose an extension of the above compressibility transformations to also achieve universality of passive scalar statistics. Analysis of the velocity/temperature relationship provides evidence for quadratic dependence which is very well approximated by the thermal analogy proposed by Zhang et al. (2014). The azimuthal velocity and scalar spectra show an organization very similar to canonical incompressible flow, with a bump-shaped distribution across the flow scales, whose peak increases with the wall distance. We find that the size growth effect is well accounted for through an effective length scale accounting for the local friction velocity and for the local mean shear. 相似文献
15.
Qiang Li Philipp Schlatter Luca Brandt Dan S. Henningson 《International Journal of Heat and Fluid Flow》2009,30(5):916-929
A direct numerical simulation (DNS) of a spatially developing turbulent boundary layer over a flat plate under zero pressure gradient (ZPG) has been carried out. The evolution of several passive scalars with both isoscalar and isoflux wall boundary condition are computed during the simulation. The Navier–Stokes equations as well as the scalar transport equation are solved using a fully spectral method. The highest Reynolds number based on the free-stream velocity U∞ and momentum thickness θ is Reθ=830, and the molecular Prandtl numbers are 0.2, 0.71 and 2. To the authors’ knowledge, this Reynolds number is to date the highest with such a variety of scalars. A large number of turbulence statistics for both flow and scalar fields are obtained and compared when possible to existing experimental and numerical simulations at comparable Reynolds number. The main focus of the present paper is on the statistical behaviour of the scalars in the outer region of the boundary layer, distinctly different from the channel-flow simulations. Agreements as well as discrepancies are discussed while the influence of the molecular Prandtl number and wall boundary conditions is also highlighted. A Pr scaling for various quantities is proposed in outer scalings. In addition, spanwise two-point correlation and instantaneous fields are employed to investigate the near-wall streak spacing and the coherence between the velocity and the scalar fields. Probability density functions (PDF) and joint probability density functions (JPDF) are shown to identify the intermittency both near the wall and in the outer region of the boundary layer. The present simulation data will be available online for the research community. 相似文献
16.
Stereoscopic PIV measurements of a turbulent boundary layer with a large spatial dynamic range 总被引:1,自引:1,他引:0
The flow in a streamwise/wall-normal plane of a turbulent boundary layer at moderate Reynolds number (Re
θ = 2,200) is characterized using two stereo PIV systems just overlapping in the streamwise direction. The aim is to generate
SPIV data for near-wall turbulence with enough spatial dynamic range to resolve most of the coherent structures present in
the flow and to facilitate future comparisons with direct numerical simulations. This is made possibly through the use of
four cameras with large CCD arrays (4,008 px × 2,672 px) and through a rigorous experimental procedure designed to minimize
the impact of measurement noise on the resolution of the small scales. For the first time, both a large field of view [S
x
; S
y
] = [2.6δ; 0.75δ] and a high spatial resolution (with an interrogation window size of 13.6+) have been achieved. The quality of the data is assessed through an analysis of some of the statistical results such as the
mean velocity profile, the rms and the PDF of the fluctuations, and the power spectra. 相似文献
17.
Understanding the radiation embrittlement of reactor pressure vessel (RPV) steels is required to be able to operate safely
a nuclear power plant or to extend its lifetime. The mechanical properties degradation is partly due to the clustering of
solute under irradiation. To gain knowledge about the clustering process, a Fe−1.1 Mn−0.7 Ni (at.%) alloy was irradiated in
a test reactor at two fluxes of 0.15 and 9 ×1017 n
E > 1MeV
.m − 2.s − 1 and at increasing doses from 0.18 to 1.3 ×1024 n
E > 1MeV
.m − 2 at 300°C. Atom probe tomography (APT) experiments revealed that the irradiation promotes the formation in the α iron matrix of Mn/Mn and/or Ni/Ni pair correlations at low dose and Mn–Ni enriched clusters at high dose. These clusters
dissolve partially after a thermal treatment at 400°C. Based on a comparison with thermodynamic calculations, we show that
the solute clustering under irradiation can just result from an induced mechanism. 相似文献
18.
Jean-Louis Auriault 《Transport in Porous Media》2009,79(2):215-223
An increasing number of articles are adopting Brinkman’s equation in place of Darcy’s law for describing flow in porous media.
That poses the question of the respective domains of validity of both laws, as well as the question of the value of the effective
viscosity μ
e
which is present in Brinkman’s equation. These two topics are addressed in this article, mainly by a priori estimates and
by recalling existing analyses. Three main classes of porous media can be distinguished: “classical” porous media with a connected
solid structure where the pore surface S
p
is a function of the characteristic pore size l
p
(such as for cylindrical pores), swarms of low concentration fixed particles where the pore surface is a function of the
characteristic particle size l
s
, and fiber-made porous media at low solid concentration where the pore surface is a function of the fiber diameter. If Brinkman’s
3D flow equation is valid to describe the flow of a Newtonian fluid through a swarm of fixed particles or fibrous media at
low concentration under very precise conditions (Lévy 1983), then we show that it cannot apply to the flow of such a fluid
through classical porous media. 相似文献
19.
I. V. Egorov A. V. Novikov A. V. Fedorov 《Journal of Applied Mechanics and Technical Physics》2007,48(2):176-183
Stability of a supersonic (M
∞ = 5.373) boundary layer with local separation in a compression corner with a passive porous coating partly absorbing flow
perturbations is considered by solving two-dimensional Navier-Stokes equations numerically. The second mode of disturbances
of a supersonic boundary layer is demonstrated to be the most important one behind the boundary-layer reattachment point.
The possibility of effective stabilization of these disturbances behind the reattachment point with the use of porous coatings
is confirmed.
__________
Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 48, No. 2, pp. 39–47, March–April, 2007. 相似文献
20.
Velocity and scalar concentration characteristics of low Reynolds number (Re) neutrally buoyant turbulent round jets were studied using coupled particle image velocimetry and laser induced fluorescence.
Experiments were conducted on a jet with a fully developed pipe exit profile at Re = 1,500 and Re = 4,000. Measurements were made in the far field (60 < x/D < 80). Results show that the centerline velocity decay constant increases with Re and the virtual origin is located far from the jet exit. The Re does not have such an influence on the scalar concentration decay constant and scalar field virtual origin. Centerline turbulent
intensities show evidence of becoming self-similar past x/D ≈ 75 but the same is not observed in the scalar fluctuations. The latter are strongly influenced by the Re, increasing as entrainment of ambient flow on the jet axis increases as the flow approaches laminar conditions. Scalar fluxes
showed a marked Re dependence as well. Results were used to estimate the turbulent Schmidt number which decreases as the Re decreases. 相似文献