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1.
Chun-Lang Yeh 《Heat and Mass Transfer》2008,44(3):275-280
The performances of three linear eddy viscosity models (LEVM) and one algebraic Reynolds stress model (ARSM) for the simulation
of turbulent flow inside and outside pressure-swirl atomizer are evaluated by comparing the interface position with available
experimental data and by comparing the turbulence intensity profiles at the atomizer exit. It is found that the turbulence
models investigated exhibit zonal behaviors, i.e. none of the models investigated performs well throughout the entire flow
field. The turbulence intensity has a significant influence on the global characteristics of the flow field. The turbulence
models with better predictions of the turbulence intensity, such as Gatski-Speziale’s ARSM model, can yield better predictions
of the global characteristics of the flow field, e.g. the reattachment lengths for the backward-facing step flow and the sudden
expansion pipe flow, or the discharge coefficient, film thickness and the liquid sheet outer surface position for the atomizer
flows. The standard k–ε model predicts stronger turbulence intensity as compared to the other models and therefore yields smaller film thickness
and larger liquid sheet outer surface position. In average, the ARSM model gives both quantitatively and qualitatively better
results as compared to the standard k–ε model and the low Reynolds number models. 相似文献
2.
Philip Schaefer Markus Gampert Jens Henrik Goebbert Lipo Wang Norbert Peters 《Flow, Turbulence and Combustion》2010,85(2):225-243
Direct Numerical Simulations (DNS) of Kolmogorov flows are performed at three different Reynolds numbers Re
λ
between 110 and 190 by imposing a mean velocity profile in y-direction of the form U(y) = F sin(y) in a periodic box of volume (2π)3. After a few integral times the turbulent flow turns out to be statistically steady. Profiles of mean quantities are then
obtained by averaging over planes at constant y. Based on these profiles two different model equations for the mean dissipation
ε in the context of two-equation RANS (Reynolds Averaged Navier–Stokes) modelling of turbulence are compared to each other.
The high Reynolds number version of the k-ε-model (Jones and Launder, Int J Heat Mass Transfer 15:301–314, 1972), to be called the standard model and a new model by Menter et al. (2006), to be called the Menter–Egorov model, are tested against the DNS results. Both models are solved numerically and it is
found that the standard model does not provide a steady solution for the present case, while the Menter–Egorov model does.
In addition a fairly good quantitative agreement of the model solution and the DNS data is found for the averaged profiles
of the kinetic energy k and the dissipation ε. Furthermore, an analysis based on flow-inherent geometries, called dissipation elements (Wang and Peters, J Fluid Mech 608:113–138,
2008), is used to examine the Menter–Egorov ε model equation. An expression for the evolution of ε is derived by taking appropriate moments of the equation for the evolution of the probability density function (pdf) of the
length of dissipation elements. A term-by-term comparison with the model equation allows a prediction of the constants, which
with increasing Reynolds number approach the empirical values. 相似文献
3.
Four turbulence models, namely, the basic and nonlinear stress-transport models and the basic and anisotropick-ε models, have been tested in the case of interaction between a longitudinal vortex pair and a flat-plate boundary layer.
The results of their predictions were compared with Mehta and Bradshaw's measurements. In this paper, part of the results
involving those of the nonlinear stress-transport model and anisotropick-ε model are presented and discussed.
The project supported by the National Natural Science Foundation of China under Contract No. 19132012 相似文献
4.
A. P. Kuryachii 《Fluid Dynamics》1998,33(6):876-881
A solution of the coupled nonstationary boundary-value problem of turbulent flow around a flat heat-conducting plate of finite
thickness having local regions with volume heat sources is given. For modeling the heat transfer in the boundary layer, thek-ε turbulence model is used. It is shown that the thermal conductivity of the plate material significantly affects the surface
distributions of both temperature and local friction.
Moscow. Translated from Izvestiya Rossiiskoi Akademii Nauk, Mekhanika Zhidkosti i Gaza, No. 6, pp. 79–86, November–December,
1998.
The work received financial support from the International Scientific and Engineering Center (project No.199). 相似文献
5.
Understanding turbulence kinetic energy (TKE) budget in gas–liquid two-phase bubbly flows is indispensable to develop and
improve turbulence models for the bubbly flows. In this study, a molecular tagging velocimetry based on photobleaching reaction
was applied to turbulent bubbly flows with sub-millimeter bubbles in a vertical square duct to examine the applicability of
the k–ε models to the bubbly flows. Effects of bubbles on TKE budget are discussed and a priori tests of the standard and low Reynolds
number k–ε models are carried out to examine the applicability of these models to the bubbly flows. The conclusions obtained are as
follows: (1) The photobleaching molecular tagging velocimetry is of use for validating turbulence models. (2) The bubbles
increase the liquid velocity gradient in the near wall region, and therefore, enhance the production and dissipation rates
of TKE. (3) The k–ε models can reasonably evaluate the production rate of TKE in the bubbly flows. (4) The modulations of diffusion due to the
bubbles have different characteristics from the diffusion enhancement due to shear-induced turbulence. Hence, the k–ε models fail in evaluating the diffusion rate in the near wall region in the bubbly flows. (5) The k–ε models represent the trends of the production, dissipation, and diffusion rates of ε in the bubbly flow, although more accurate
experimental data are required for quantitative validation of the ε equation. 相似文献
6.
The present paper focuses on the analysis of unsteady flow and heat transfer regarding an axisymmetric impinging synthetic
jet on a constant heat flux disc. Synthetic jet is a zero net mass flux jet that provides an unsteady flow without any external
source of fluid. Present results are validated against the available experimental data showing that the SST/k − ω turbulence model is more accurate and reliable than the standard and low-Re k − ε models for predicting heat transfer from an impinging synthetic jet. It is found that the time-averaged Nusselt number
enhances as the nozzle-to-plate distance is increased. As the oscillation frequency in the range of 16–400 Hz is increased,
the heat transfer is enhanced. It is shown that the instantaneous Nu distribution along the wall is influenced mainly by the interaction of produced vortex ring and wall boundary layer. Also,
the fluctuation level of Nu decreases as the frequency is raised. 相似文献
7.
The paper reports on the application of the Time-dependent Reynolds-Averaged Navier–Stokes (T-RANS) approach to analysing
the effects of magnetic force and bottom-wall configuration on the reorganisation of a large coherent structure and its role
in the transport processes in Rayleigh–Bénard convection. The large-scale deterministic motion is fully resolved in time and
space, whereas the unresolved stochastic motion is modelled by a `subscale' model for which the conventional algebraic stress/flux
expressions were used, closed with the low-Re number (k)-(ε)-(θ2) three-equation model. The applied method reproduces long-term averaged mean flow properties, turbulence second moments,
and all major features of the coherent roll/cell structure in classic Rayleigh–Bénard convection in excellent agreement with
the available DNS and experimental results. Application of the T-RANS approach to Rayleigh–Bénard convection with wavy bottom
walls and a superimposed magnetic field yielded the expected effects on there organisation of the eddy structure and consequent
modifications of the mean and turbulence parameters and wall heat transfer.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
8.
Two-dimensional numerical studies of flow and temperature fields for turbulent natural convection and surface radiation in
inclined differentially heated enclosures are performed. Investigations are carried out over a wide range of Rayleigh numbers
from 108 to 1012, with the angle of inclination varying between 0° and 90°. Turbulence is modeled with a novel variant of the k–ε closure model. The predicted results are validated against experimental and numerical results reported in literature. The
effect of the inclination of the enclosure on pure turbulent natural convection and the latter’s interaction with surface
radiation are brought out. Profiles of turbulent kinetic energy and effective viscosity are studied to observe the net effect
on the intensity of turbulence caused by the interaction of natural convection and surface radiation. The variations of local
Nusselt number and average Nusselt number are presented for various inclination angles. Marked change in the convective Nusselt
number is found with the orientation of enclosure. Also analyzed is the influence of change in emissivity on the flow and
heat transfer. A correlation relevant to practical applications in the form of average Nusselt number, as a function of Rayleigh
number, Ra, radiation convection parameter, N
RC and inclination angle of the enclosure, φ is proposed. 相似文献
9.
C.G. Speziale 《Theoretical and Computational Fluid Dynamics》1999,13(3):161-166
A resent extension of the nonlinear K–ε model is critically discussed from a basic theoretical standpoint. While it was said in the paper that this model was formulated
to incorporate relaxation effects, it will be shown that the model is incapable of describing one of the most basic such turbulent
flows as is obvious but is described for clarity. It will be shown in detail that this generalized nonlinear K–ε model yields erroneous results for the Reynolds stress tensor when the mean strains are set to zero in a turbulent flow
– the return-to-isotropy problem which is one of the most elementary relaxational turbulent flows. It is clear that K–ε type models cannot describe relaxation effects. While their general formalism can describe relaxation effects, the nonlinear
K–ε model – which the paper is centered on – cannot. The deviatoric part of the Reynolds stress tensor is predicted to be zero
when it actually only gradually relaxes to zero. Since this model was formulated by using the extended thermodynamics, it
too will be critically assessed. It will be argued that there is an unsubstantial physical basis for the use of extended thermodynamics
in turbulence. The role of Material Frame-Indifference and the implications for future research in turbulence modeling are
also discussed.
Received 19 February 1998 and accepted 23 October 1998 相似文献
10.
H. Watanabe Tomohiro Sato Motoyuki Hirose Kunihiro Osaki Ming-Long Yao 《Rheologica Acta》1998,37(6):519-527
Dielectric relaxation behavior was examined for 4-4′-n-pentyl-cyanobiphenyl (5CB) and 4-4′-n-heptyl-cyanobiphenyl (7CB) under flow. In quiescent states at all temperatures examined, both 5CB and 7CB exhibited dispersions
in their complex dielectric constant ε*(ω) at characteristic frequencies ω
c
above 106 rad s–1. This dispersion reflected orientational fluctuation of individual 5CB and 7CB molecules having large dipoles parallel to
their principal axis (in the direction of C≡N bond). In the isotropic state at high temperatures, these molecules exhibited no detectable changes of ε*(ω) under flow at
shear rates . In contrast, in the nematic state at lower temperatures the terminal relaxation intensity of ε*(ω) as well as the static
dielectric constant ε′(0) decreased under flow at . This rheo-dielectric change was discussed in relation to the flow effects on the nematic texture (director distribution)
and anisotropy in motion of individual molecules with respect to the director.
Received: 14 April 1998 Accepted: 29 July 1998 相似文献
11.
H. Irago 《Journal of Elasticity》1999,57(1):55-83
Let u(ε) be a rescaled 3-dimensional displacement field solution of the linear elastic model for a free prismatic rod Ωε having cross section with diameter of order ε, and let u
(0) –Bernoulli–Navier displacement – and u
(2) be the two first terms derived from the asymptotic method. We analyze the residue r(ε) = u(ε) − (u
(0) + ε2
u
(2)) and if the cross section is star-shaped, we prove such residue presents a Saint-Venant"s phenomenon near the ends of the
rod.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
12.
Alistair J. Revell Tim J. Craft Dominique R. Laurence 《Flow, Turbulence and Combustion》2011,86(1):129-151
This paper reports the application of a recently developed turbulence modelling scheme known as the C
as
model. This model was specifically developed to capture the effects of stress-strain misalignment observed in turbulent flows
with mean unsteadiness. Earlier work has reported the approach applied within a linear k-ε modelling framework, and some initial testing of it within the k-ω SST model of Menter (AIAA J 32:1598–1605, 1994). The resulting k-ε-C
as
and SST-C
as
models have been shown to result in some of the advantages of a full Reynolds Stress transport Model (RSM), whilst retaining
the computational efficiency and stability benefits of a eddy viscosity model (EVM). Here, the development of the the high-Reynolds-number
version of the C
as
model is outlined, with some example applications to steady and unsteady homogeneous shear flows. The SST-C
as
form of the model is then applied to further, more challenging cases of 2-D flow around a NACA0012 aerofoil beyond stall
and the 3-D flow around a circular cylinder in a square duct, both being flows which exhibit large, unsteady, separated flow
regions. The predictions returned by a range of other common turbulence modelling schemes are included for comparison and
the SST-C
as
scheme is shown to return generally good results, comparable in some respects to those obtainable from far more complex schemes,
for only moderate computing resource requirements. 相似文献
13.
Simulation of Wind Flow Around a Building with a k–ε Model 总被引:1,自引:0,他引:1
The three-dimensional numerical simulation of airflow around a building using a k–ε two-equation turbulence model is presented in this paper. Several cases of numerical simulation of airflow around a building
are carried out to estimate the influence of mesh spacing on simulated results. The accuracy of simulations is examined by
comparing the predicted results with wind-tunnel experiments. It is confirmed that numerical simulations by means of the k–ε model reproduce the velocity fields well when using fine mesh resolution. In the latter part of the paper, the simulation
method is applied to predict the flow field around a building with different width-to-height ratios, under light wind conditions.
Received 16 June 1999 and accepted 20 July 2000 相似文献
14.
S.V. Utyuzhnikov 《Flow, Turbulence and Combustion》2002,68(2):137-152
The paper is concerned with a numerical simulation of fuel cloud behaviour which follows releases of a liquid fuel. The main
aim of the work is to develop further a mathematical model to simulate such releases into the atmosphere. The model is validated
by a comparison with experimental results. The influence of boundary conditions for turbulent kinetic energy k and its dissipation rate ε on the solution is investigated. It is concluded that the solution depends mainly on the combination
of k and ε in the form k
3/2/ε rather than each of these values separately. A way to define the boundary conditions for k and ε is suggested. The KIVA-II code has been used as the base of the code used. The original code has been modified to simulate
low Mach number atmospheric flows, radiation, soot formation and turbulent combustion.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
15.
An experimental study was conducted to examine the effects of surface roughness and adverse pressure gradient (APG) on the
development of a turbulent boundary layer. Hot-wire anemometry measurements were carried out using single and X-wire probes in all regions of a developing APG flow in an open return wind tunnel test section. The same experimental conditions
(i.e., T
∞, U
ref, and C
p) were maintained for smooth, k
+ = 0, and rough, k
+ = 41–60, surfaces with Reynolds number based on momentum thickness, 3,000 < Re
θ < 40,000. The experiment was carefully designed such that the x-dependence in the flow field was known. Despite this fact, only a very small region of the boundary layer showed a balance
of the various terms in the integrated boundary layer equation. The skin friction computed from this technique showed up to
a 58% increase due to the surface roughness. Various equilibrium parameters were studied and the effect of roughness was investigated.
The generated flow was not in equilibrium according to the Clauser (J Aero Sci 21:91–108, 1954) definition due to its developing nature. After a development region, the flow reached the equilibrium condition as defined
by Castillo and George (2001), where Λ = const, is the pressure gradient parameter. Moreover, it was found that this equilibrium condition can be used
to classify developing APG flows. Furthermore, the Zagarola and Smits (J Fluid Mech 373:33–79, 1998a) scaling of the mean velocity deficit, U
∞δ*/δ, can also be used as a criteria to classify developing APG flows which supports the equilibrium condition of Castillo
and George (2001). With this information a ‘full APG region’ was defined. 相似文献
16.
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
17.
The flow initiated by a hot gas cloud (thermal) in a stratified atmosphere is calculated on the basis of theκ-ε turbulence model and the transport model for the Reynolds stresses and turbulent fluxes and the results obtained are compared
The nonlocal nature of the turbulent transport in a vortex ring and its effect on certain flow characteristics are explained
In particular, the calculations carried out using the Reynolds stress model show much slower cooling of the temperature-vortex
torus than those based calculated on theκ-ε-model Modification of theκ-ε-model to take the effect of curvature of the streamlines approximately into account makes it only partially possible to reproduce
the results obtained on the basis of the Reynolds stress model
Moscow. Translated from Izvestiya Rossiiskoi Akademii Nauk, Mekhanika Zhidkosti i Gaza, No. 1, pp. 12–20, January–February,
1999.
The research was carried out with support from the Russian Foundation for Basic Research (project No. 95-01-00544a). 相似文献
18.
A thre-dimensional direct numerical simulation is combined with a laboratory study to describe the turbulent flow in an enclosed
annular rotor-stator cavity characterized by a large aspect ratio G = (b − a)/h = 18.32 and a small radius ratio a/b = 0.152, where a and b are the inner and outer radii of the rotating disk and h is the interdisk spacing. The rotation rate Ω considered is equivalent to the rotational Reynolds number Re = Ωb
2/ν= 9 .5 × 104 (ν the kinematic viscosity of water). This corresponds to a value at which experiment has revealed that the stator boundary
layer is turbulent, whereas the rotor boundary layer is still laminar. Comparisons of the computed solution with velocity
measurements have given good agreement for the mean and turbulent fields. The results enhance evidence of weak turbulence
by comparing the turbulence properties with available data in the literature (Lygren and Andersson, J Fluid Mech 426:297–326,
2001). An approximately self-similar boundary layer behavior is observed along the stator. The wall-normal variations of the structural
parameter and of characteristic angles confirm that this boundary layer is three-dimensional. A quadrant analysis (Kang et
al., Phys Fluids 10:2315–2322, 1998) of conditionally averaged velocities shows that the asymmetries obtained are dominated by Reynolds stress-producing events
in the stator boundary layer. Moreover, Case 1 vortices (with a positive wall induced velocity) are found to be the major
source of generation of special strong events, in agreement with the conclusions of Lygren and Andersson (J Fluid Mech 426:297–326,
2001). 相似文献
19.
Wind turbines operate in the surface layer of the atmospheric boundary layer, where they are subjected to strong wind shear
and relatively high turbulence levels. These incoming boundary layer flow characteristics are expected to affect the structure
of wind turbine wakes. The near-wake region is characterized by a complex coupled vortex system (including helicoidal tip
vortices), unsteadiness and strong turbulence heterogeneity. Limited information about the spatial distribution of turbulence
in the near wake, the vortex behavior and their influence on the downwind development of the far wake hinders our capability
to predict wind turbine power production and fatigue loads in wind farms. This calls for a better understanding of the spatial
distribution of the 3D flow and coherent turbulence structures in the near wake. Systematic wind-tunnel experiments were designed
and carried out to characterize the structure of the near-wake flow downwind of a model wind turbine placed in a neutral boundary
layer flow. A horizontal-axis, three-blade wind turbine model, with a rotor diameter of 13 cm and the hub height at 10.5 cm,
occupied the lowest one-third of the boundary layer. High-resolution particle image velocimetry (PIV) was used to measure
velocities in multiple vertical stream-wise planes (x–z) and vertical span-wise planes (y–z). In particular, we identified localized regions of strong vorticity and swirling strength, which are the signature of helicoidal
tip vortices. These vortices are most pronounced at the top-tip level and persist up to a distance of two to three rotor diameters
downwind. The measurements also reveal strong flow rotation and a highly non-axisymmetric distribution of the mean flow and
turbulence structure in the near wake. The results provide new insight into the physical mechanisms that govern the development
of the near wake of a wind turbine immersed in a neutral boundary layer. They also serve as important data for the development
and validation of numerical models. 相似文献
20.
N. I. Akatnov 《Fluid Dynamics》1994,29(4):479-490
Thek-kl-model of turbulence based on the equation of second two-point moments (pointsA andB) of the fluctuating velocity field is presented. The second and third moments entering into this equation are expressed using
polynomials whose terms are products of the tensor components characterizing a given turbulent motion and scalar functions
of the distanceAB. ForAB=0 the equation obtained gives thek-turbulence energy balance equation and, on being integrated overAB from 0 to ∞, the transport equation for thekl-quantity (l is the integral turbulence scale). The model is used for calculating mixing layers, plane and circular jets, the wake behind
a cylinder, tube and channel flows, and the boundary layer on a plate. The results of all the calculations agree well with
the experimental data for a single set of empirical coefficients.
St. Petersburg. Translated from Izvestiya Rossiiskoi Akademii Nauk, Mekhanika Zhidkosti i Gaza, No. 4, pp. 51–64, July–August,
1994. 相似文献