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1.
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
2.
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
3.
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
4.
The adverse pressure gradient induced by a surface-mounted obstacle in a turbulent boundary layer causes the approaching flow
to separate and form a dynamically rich horseshoe vortex system (HSV) in the junction of the obstacle with the wall. The Reynolds
number of the flow (Re) is one of the important parameters that control the rich coherent dynamics of the vortex, which are known to give rise to
low-frequency, bimodal fluctuations of the velocity field (Devenport and Simpson, J Fluid Mech 210:23–55, 1990; Paik et al., Phys Fluids 19:045107, 2007). We carry out detached eddy simulations (DES) of the flow past a circular cylinder mounted on a rectangular channel for
Re = 2.0 × 104 and 3.9 × 104 (Dargahi, Exp Fluids 8:1–12, 1989) in order to systematically investigate the effect of the Reynolds number on the HSV dynamics. The computed results are compared
with each other and with previous experimental and computational results for a related junction flow at a much higher Reynolds
number (Re = 1.15 × 105) (Devenport and Simpson, J Fluid Mech 210:23–55, 1990; Paik et al., Phys Fluids 19:045107, 2007). The computed results reveal significant variations with Re in terms of the mean-flow quantities, turbulence statistics, and the coherent dynamics of the turbulent HSV. For Re = 2.0 × 104 the HSV system consists of a large number of necklace-type vortices that are shed periodically at higher frequencies than
those observed in the Re = 3.9 × 104 case. For this latter case the number of large-scale vortical structures that comprise the instantaneous HSV system is reduced
significantly and the flow dynamics becomes quasi-periodic. For both cases, we show that the instantaneous flowfields are
dominated by eruptions of wall-generated vorticity associated with the growth of hairpin vortices that wrap around and disorganize
the primary HSV system. The intensity and frequency of these eruptions, however, appears to diminish rapidly with decreasing
Re. In the high Re case the HSV system consists of a single, highly energetic, large-scale necklace vortex that is aperiodically disorganized
by the growth of the hairpin mode. Regardless of the Re, we find pockets in the junction region within which the histograms of velocity fluctuations are bimodal as has also been
observed in several previous experimental studies. 相似文献
5.
Recently, the tube diameter relaxation time in the evolution equation of the molecular stress function (MSF) model (Wagner
et al., J Rheol 49: 1317–1327, 2005) with the interchain pressure effect (Marrucci and Ianniruberto, Macromolecules 37:3934–3942, 2004) included was shown to be equal to three times the Rouse time in the limit of small chain stretch. From this result, an advanced
version of the MSF model was proposed, allowing modeling of the transient and steady-state elongational viscosity data of
monodisperse polystyrene melts without using any nonlinear parameter, i.e., solely based on the linear viscoelastic characterization
of the melts (Wagner and Rolón-Garrido 2009a, b). In this work, the same approach is extended to model experimental data in shear flow. The shear viscosity of two polybutadiene
solutions (Ravindranath and Wang, J Rheol 52(3):681–695, 2008), of four styrene-butadiene random copolymer melts (Boukany et al., J Rheol 53(3):617–629, 2009), and of four polyisoprene melts (Auhl et al., J Rheol 52(3):801–835, 2008) as well as the shear viscosity and the first and second normal stress differences of a polystyrene melt (Schweizer et al.,
J Rheol 48(6):1345–1363, 2004), are analyzed. The capability of the MSF model with the interchain pressure effect included in the evolution equation of
the chain stretch to model shear rheology on the basis of linear viscoelastic data alone is confirmed. 相似文献
6.
The full energy dissipation rate and enstrophy are measured simultaneously using a probe consisting of four X-wires in the
intermediate region of a cylinder wake for Taylor microscale Reynolds number in the range of 120–320. Longitudinal and transverse
velocity increments are also obtained temporally using Taylor’s hypothesis. The inertial range scaling exponents indicate
that the full enstrophy field has a stronger intermittency than does the full dissipation field for all the Reynolds numbers
considered. The approximations of the energy dissipation rate and enstrophy based on isotropy are more intermittent than their
corresponding true values. While the scaling exponents of the full energy dissipation rate remain approximately constant for
different Reynolds numbers, those of the enstrophy decrease slightly and consistently with the increase of Reynolds number.
It is conjectured that the scaling of the energy dissipation rate and the enstrophy may be the same when Reynolds number is
extremely high, a trend that is consistent with that suggested by Nelkin (Phys Fluids 11:2202–2204, 1999; Am J Phys 68:310–318, 2000). 相似文献
7.
In this paper we investigate a subgrid model based on an anisotropic version of the NS-α model using a lid-driven cavity flow at a Reynolds number of 10,000. Previously the NS-α model has only been used numerically in the isotropic form. The subgrid model is developed from the Eulerian-averaged anisotropic
equations (Holm, Physica D 133:215, 1999). It was found that when α
2 was based on the mesh numerical oscillations developed which manifested themselves in the appearance of streamwise vortices
and a ‘mixing out’ of the velocity profile. This is analogous to the Craik–Leibovich mechanism, with the difference being
that the oscillations here are not physical but numerical. The problem could be traced back to the discontinuity in α
2 encountered when α
2 = 0 on the endwalls. A definition of α
2 based on velocity gradients, rather than mesh spacing, is proposed and tested. Using this definition the results with the
model show a significant improvement. The splitting of the downstream wall jet, rms and shear stress profiles are correctly
captured a coarse mesh. The model is shown to predict both positive and negative energy transfer in the jet impingement region,
in qualitative agreement with DNS results. 相似文献
8.
Incompressible 3-D DNS is performed in non-decaying turbulence with single step chemistry to validate a new analytical expression
for turbulent burning velocity. The proposed expression is given as a sum of laminar and turbulent contributions, the latter
of which is given as a product of turbulent diffusivity in unburned gas and inverse scale of wrinkling at the leading edge.
The bending behavior of U
T at higher u′ was successfully reproduced by the proposed expression. It is due to decrease in the inverse scale of wrinkling at the leading
edge, which is related with an asymmetric profile of FSD with increasing u′. Good agreement is achieved between the analytical expression and the turbulent burning velocities from DNS throughout the
wrinkled, corrugated and thin reaction zone regimes. Results show consistent behavior with most experimental correlations
in literature including those by Bradley et al. (Philos Trans R Soc Lond A 338:359–387, 1992), Peters (J Fluid Mech 384:107–132, 1999) and Lipatnikov et al. (Progr Energ Combust Sci 28:1–74, 2002). 相似文献
9.
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
10.
In the present case, the conjugate heat transfer involving a turbulent plane offset jet is considered. The bottom wall of
the solid block is maintained at an isothermal temperature higher than the jet inlet temperature. The parameters considered
are the offset ratio (OR), the conductivity ratio (K), the solid slab thickness (S) and the Prandtl number (Pr). The Reynolds number considered is 15,000 because the flow becomes fully turbulent and then it becomes independent of the
Reynolds number. The ranges of parameters considered are: OR = 3, 7 and 11, K = 1–1,000, S = 1–10 and Pr = 0.01–100. High Reynolds number two-equation model (k–ε) has been used for turbulence modeling. Results for the solid–fluid interface temperature, local Nusselt number, local
heat flux, average Nusselt number and average heat transfer have been presented and discussed. 相似文献
11.
External heat transfer prediction is performed in two-dimensional turbine blade cascades using the Reynolds-averaged Navier–Stokes
equations. For this purpose, six different turbulence models including the algebraic Baldwin–Lomax (AIAA paper 78-257, 1978), three low-Re
k−ɛ models (Chien in AIAA J 20:33–38, 1982; Launder and Sharma in Lett Heat Mass Transf 1(2):131–138, 1974; Biswas and Fukuyama in J Turbomach 116:765–773, 1994), and two k−ω models (Wilcox in AIAA J 32(2):247–255, 1994) are taken into account. The computer code developed employs a finite volume method to solve governing equations based on
an explicit time marching approach with capability to simulate subsonic, transonic and supersonic flows. The Roe method is
used to decompose the inviscid fluxes and the gradient theorem to decompose viscous fluxes. The performance of different turbulence
models in prediction of heat transfer is examined. To do so, the effect of Reynolds and Mach numbers along with the turbulent
intensity are taken into account, and the numerical results obtained are compared with the experimental data available. 相似文献
12.
A non-linear relationship of the Reynolds stresses in function of the strain rate and vorticity tensors, with terms up to
third order, is developed. Anisotropies in the normal stresses, influence from streamline curvature or rotation of the reference
frame, and swirl effects are accounted for. The relationship is linked to ak–ε model with a modified transport equation for the dissipation rate. A new low-Reynolds source term is introduced and a model
parameter is written in terms of dimensionless rate-of-strain and vorticity. The model is checked on different realizability
constraints. It is shown that practically all constraints are fulfilled. The model is numerically tested on a fully developed
channel and pipe flow, both stationary and rotating. The plane jet–round jet anomaly is addressed. Finally, the model is applied
to the flow over a backward-facing step. Results are compared with a linear low-Reynolds k–ε model and the shear stress transport model.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
13.
Artur Bartosik 《Flow, Turbulence and Combustion》2010,84(2):277-293
The paper deals with fully developed steady turbulent flow of slurry in a circular straight and smooth pipe. The Kaolin slurry
consists of very fine solid particles, so the solid particles concentration, and density, and viscosity are assumed to be
constant across the pipe. The mathematical model is based on the time averaged momentum equation. The problem of closure was
solved by the Launder and Sharma k-ε turbulence model (Launder and Sharma, Lett Heat Mass Transf 1:131–138, 1974) but with a different turbulence damping function. The turbulence damping function, used in the mathematical model in the
present paper, is that proposed by Bartosik (1997). The mathematical model uses the apparent viscosity concept and the apparent viscosity was calculated using two- and three-parameter
rheological models, namely Bingham and Herschel–Bulkley. The main aim of the paper is to compare measurements and predictions
of the frictional head loss and velocity distribution, taking into account two- and three-parameter rheological models, namely
Bingham and Herschel–Bulkley, if the Kaolin slurry possesses low, moderate, and high yield stress. Predictions compared with
measurements show an observable advantage of the Herschel–Bulkley rheological model over the Bingham model particularly if
the bulk velocity decreases. 相似文献
14.
The objective of this experimental study is to characterise the small-scale turbulence in the intermediate wake of a circular
cylinder using measured mean-squared velocity gradients. Seven of the twelve terms which feature in ε, the mean dissipation rate of the turbulent kinetic energy, were measured throughout the intermediate wake at a Reynolds
number of Re
d
≈ 3000 based on the cylinder diameter (d). Earlier measurements of the nine major terms of ε by Browne et al. (J Fluid Mech 179: 307–326 1987) at a downstream distance (x) of x = 420d and Re
d
≈ 1170 are also used. Whilst departures from local isotropy are significant at all locations in the wake, local axisymmetry
of the small-scale turbulence with respect to the mean flow direction is first satisfied approximately at x = 40d. The approach towards local axisymmetry is discussed in some detail in the context of the relative values of the mean-squared
velocity gradients. The data also indicate that axisymmetry is approximately satisfied by the large scales at x/d ≥ 40, suggesting that the characteristics of the small scales reflect to a major extent those of the large scales. Nevertheless,
the far-wake data of Browne et al. (1987) show a discernible departure from axisymmetry for both small and large scales. 相似文献
15.
Duvan Henao 《Journal of Elasticity》2009,94(1):55-68
We prove that energy minimizers for nonlinear elasticity in which cavitation is allowed only at a finite number of prescribed
flaw points can be obtained, in the limit as ε→0, by introducing micro-voids of radius ε in the domain at the prescribed locations and minimizing the energy without allowing for cavitation. This extends the result
by Sivaloganathan, Spector, and Tilakraj (SIAM J. Appl. Math. 66:736–757, 2006) to the case of multiple cavities, and constitutes a first step towards the numerical simulation of cavitation (in the nonradially-symmetric
case).
相似文献
16.
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
17.
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
18.
Buoyant flows often contain regions with unstable and stable thermal stratification from which counter gradient turbulent
fluxes are resulting, e.g. fluxes of heat or of any turbulence quantity. Basing on investigations in meteorology an improvement
in the standard gradient-diffusion model for turbulent diffusion of turbulent kinetic energy is discussed. The two closure
terms of the turbulent diffusion, the velocity-fluctuation triple correlation and the velocity-pressure fluctuation correlation,
are investigated based on Direct Numerical Simulation (DNS) data for an internally heated fluid layer and for Rayleigh–Bénard
convection. As a result it is decided to extend the standard gradient-diffusion model for the turbulent energy diffusion by
modeling its closure terms separately. Coupling of two models leads to an extended RANS model for the turbulent energy diffusion.
The involved closure term, the turbulent diffusion of heat flux, is studied based on its transport equation. This results
in a buoyancy-extended version of the Daly and Harlow model. The models for all closure terms and for the turbulent energy
diffusion are validated with the help of DNS data for internally heated fluid layers with Prandtl number Pr = 7 and for Rayleigh–Bénard convection with Pr = 0.71. It is found that the buoyancy-extended diffusion model which involves also a transport equation for the variance
of the vertical velocity fluctuation gives improved turbulent energy diffusion data for the combined case with local stable
and unstable stratification and that it allows for the required counter gradient energy flux. 相似文献
19.
This paper mainly concerns the mathematical justification of a viscous compressible multi-fluid model linked to the Baer-Nunziato
model used by engineers, see for instance Ishii (Thermo-fluid dynamic theory of two-phase flow, Eyrolles, Paris, 1975), under a “stratification” assumption. More precisely, we show that some approximate finite-energy weak solutions of the
isentropic compressible Navier–Stokes equations converge, on a short time interval, to the strong solution of this viscous
compressible multi-fluid model, provided the initial density sequence is uniformly bounded with corresponding Young measures
which are linear convex combinations of m Dirac measures. To the authors’ knowledge, this provides, in the multidimensional in space case, a first positive answer
to an open question, see Hillairet (J Math Fluid Mech 9:343–376, 2007), with a stratification assumption. The proof is based on the weak solutions constructed by Desjardins (Commun Partial Differ Equ 22(5–6):977–1008, 1997) and on the existence and uniqueness of a local strong solution for the multi-fluid model established by Hillairet assuming initial density to be far from vacuum. In a first step, adapting the ideas from Hoff and Santos (Arch Ration Mech Anal 188:509–543, 2008), we prove that the sequence of weak solutions built by Desjardins has extra regularity linked to the divergence of the velocity without any relation assumption between λ and μ. Coupled with the uniform bound of the density property, this allows us to use appropriate defect measures and their nice
properties introduced and proved by Hillairet (Aspects interactifs de la m’ecanique des fluides, PhD Thesis, ENS Lyon, 2005) in order to prove that the Young measure associated to the weak limit is the convex combination of m Dirac measures. Finally, under a non-degeneracy assumption of this combination (“stratification” assumption), this provides
a multi-fluid system. Using a weak–strong uniqueness argument, we prove that this convex combination is the one corresponding
to the strong solution of the multi-fluid model built by Hillairet, if initial data are equal. We will briefly discuss this assumption. To complete the paper, we also present a blow-up criterion
for this multi-fluid system following (Huang et al. in Serrin type criterion for the three-dimensional viscous compressible flows, arXiv, 2010). 相似文献
20.
A specially constructed hot-wire probe was used to obtain very near-wall velocity measurements in both a fully developed
turbulent channel flow and flat plate boundary layer flow. The near-wall hot-wire probe, having been calibrated in a specially
constructed laminar flow calibration rig, was used to measure the mean streamwise velocity profile, distributions of streamwise
and spanwise intensities of turbulence and turbulence kinetic energy k in the viscous sublayer and beyond; these distributions compare very favorably with available DNS results obtained for channel
flow. While low Reynolds number effects were clearly evident for the channel flow, these effects are much less distinct for
the boundary layer flow. By assuming the dissipating range of eddy sizes to be statistically isotropic and the validity of
Taylor's hypothesis, the dissipation rate ɛ
iso in the very near-wall viscous sublayer region and beyond was determined for both the channel and boundary layer flows. It
was found that if the convective velocity U
c in Taylor's hypothesis was assumed to be equal to the mean velocity Uˉ at the point of measurement, the value of (ɛ+
iso)1 thus obtained agrees well with that of (ɛ
+)DNS for y
+ ≥ 80 for channel flow; this suggests the validity of assuming U
c=Uˉ and local isotropy for large values of y
+. However, if U
c was assumed to be 10.6u
τ
, the value of (ɛ+
iso)2 thus obtained was found to compare reasonably well with the distribution of (ɛ+
iso)DNS for y
+≤ 15.
Received: 31 May 1999/Accepted: 20 December 1999 相似文献