共查询到20条相似文献,搜索用时 15 毫秒
1.
Philipp Geipel K. H. Henry Goh R. Peter Lindstedt 《Flow, Turbulence and Combustion》2010,85(3-4):397-419
The opposed jet configuration presents a canonical geometry suitable for the evaluation of calculation methods seeking to reproduce the impact of strain and re-distribution on turbulent transport in reacting and non-reacting flows. The geometry has the advantage of good optical access and, in principle, an absence of complex boundary conditions. Disadvantages include low frequency flow motion at high nozzle separations and comparatively low turbulence levels causing bulk strain to exceed the turbulent contribution at small nozzle separations. In the current work, fractal generated turbulence has been used to increase the turbulent strain and velocity measurements for isothermal flows are reported with an emphasis on the axis, stagnation plane and the distribution of mean and instantaneous strain rates. Energy spectra were also determined. The instrumentation comprised hot-wire anemometry and particle image velocimetry with the flows to both nozzles seeded with 1 $\upmu$ m silicon oil droplets providing a relaxation time of ? 3 $\upmu$ s. It is shown that fractal grids increase the turbulent Reynolds number range from 48–125 to 109–220 for bulk velocities from 4 to 8 m/s as compared to conventional perforated plate turbulence generators. Low frequency motion of the order 10 Hz could not be completely eliminated and probability density functions were determined for the location of the stagnation plane. Results show that the fluctuation in the position of the stagnation plane is of the order of the integral length scale, which was determined to be 3.1±0.1 mm at the nozzle exits through the use of hot-wire anemometry. Flow statistics close to the fractal plate located upstream of the nozzle exit were also determined using a transparent glass nozzle. 相似文献
2.
The effects of mean flame radius and turbulence on self-sustained combustion of turbulent premixed spherical flames in decaying
turbulence have been investigated using three-dimensional direct numerical simulations (DNS) with single step Arrhenius chemistry.
Several flame kernels with different initial radius or initial turbulent field have been studied for identical conditions
of thermo-chemistry. It has been found that for very small kernel radius the mean displacement speed may become negative leading
ultimately to extinction of the flame kernel. A mean negative displacement speed is shown to signify a physical situation
where heat transfer from the kernel overcomes the heat release due to combustion. This mechanism is further enhanced by turbulent
transport and, based on simulations with different initial turbulent velocity fields, it has been found that self-sustained
combustion is adversely affected by higher turbulent velocity fluctuation magnitude and integral length scale. A scaling analysis
is performed to estimate the critical radius for self-sustained combustion in premixed flame kernels in a turbulent environment.
The scaling analysis is found to be in good agreement with the results of the simulations. 相似文献
3.
Carl H. Gibson 《Flow, Turbulence and Combustion》2004,72(2-4):161-179
A model is proposed connecting turbulence, fossil turbulence and the big-bang origin of the universe. While details are incomplete, the model is consistent with our knowledge of these processes and is supported by observations. Turbulence arises in a hot big-bang quantum gravitational dynamics scenario at Planck scales. Chaotic, eddy-like motions produce an exothermic Planck particle cascade from 10?35 m at 1032 K to 108 larger, 104 cooler, quark-gluon scales. A Planck-Kerr instability gives high Reynolds number (Re ~ 106) turbulent combustion, space-time-energy-entropy and turbulent mixing. Batchelor-Obukhov-Corrsin turbulent-temperature fluctuations are preserved as the first fossil turbulence by inflation stretching the patterns beyond the horizon ct of causal connection faster than light speed c in time t~ 10?33 sec. Fossil big-bang temperature turbulence reenters the horizon and imprints nucleosynthesis of H-He densities that seed fragmentation by gravity at 1012 s in the low Reynolds number plasma before its transition to gas at t~ 1013 s and T~ 3000 K. Multiscaling coefficients of the cosmic microwave background (CMB) temperature anisotropies closely match those for high Reynolds number turbulence, Bershadskii, A. and Sreenivasan, K.R., Phys. Lett. A 299 (2002) 149-152; Bershadskii, A. and Sreenivasan, K.R., Phys. Lett. A 319 (2003) 21-23. CMB spectra support the interpretation that big-bang turbulence fossils triggered fragmentation of the viscous plasma at supercluster to galaxy mass scales from 1046 to 1042 kg, Gibson, C.H., Appl. Mech. Rev. 49 (5) (1996) 299-315; Gibson, C.H., J. Fluids Eng. 122 (2000) 830-835; Gibson, C.H., Combust. Sci. Technol. (2004, to be published). 相似文献
4.
A. N. Sharkovsky 《Nonlinear dynamics》2006,44(1-4):15-27
Ideal turbulence is a mathematical phenomenon which occurs in certain infinite-dimensional deterministic dynamical systems
and implies that the attractor of a system lies off the phase space and among the attractor points there are fractal or even
random functions. A mathematically rigorous definition of ideal turbulence is based on standard notions of dynamical systems
theory and chaos theory. Ideal turbulence is observed in various idealized models of real distributed systems of electrodynamics,
acoustics, radiophysics, etc. In systems without internal resistance, cascade processes are capable to birth structures of
arbitrarily small scale and even to cause stochastization of the systems. Just these phenomena are inherent in ideal turbulence
and they help to understand the mathematical scenarios for many features of real turbulence. 相似文献
5.
Ivan Delbende Maurice Rossi Olivier Daube 《Theoretical and Computational Fluid Dynamics》2012,26(1-4):141-160
Some flows such as the wakes of rotating devices often display helical symmetry. We present an original DNS code for the dynamics of such helically symmetric systems. We show that, by enforcing helical symmetry, the three-dimensional Navier–Stokes equations can be reduced to a two-dimensional unsteady problem. The numerical method is a generalisation of the vorticity/streamfunction formulation in a circular domain, with finite differences in the radial direction and spectral decomposition along the azimuth. When compared to a standard three-dimensional code, this allows us to reach larger Reynolds numbers and to compute quasi-steady patterns. We illustrate the importance of helical pitch by some physical cases: the dynamics of several helical vortices and a quasi-steady vortex flow. We also study the linear dynamics and nonlinear saturation in the Batchelor vortex basic flow, a paradigmatic example of trailing vortex instability. We retrieve the behaviour of inviscid modes and present new results concerning the saturation of viscous centre modes. 相似文献
6.
This paper attempts to give a concise overview of the turbulence research performed at the Laboratory for Aero and Hydrodynamics
of the Delft University of Technology under the guidance of Frans Nieuwstadt. Frans Nieuwstadt was appointed in 1986 as director
of the laboratory, and he held this position until his sudden death in 2005. Frans’ principal interest was to investigate
turbulence at a fundamental level, but also to consider turbulence and its role in other processes. He coined a name for this
research: turbulence plus. 相似文献
7.
A parallel finite volume method for unstructured grids is used for a direct numerical simulation of the flow around a sphere at Re = 5000 (based on the sphere diameter and undisturbed velocity). The observed flow structures are confirmed by visualization experiments. A quantitative analysis of the Reynolds averaged flow provides a data base for future model evaluations. 相似文献
8.
EDQNM and DNS calculations of homogenous axisymmetric turbulence in the absence of a mean flow are compared. Special care is taken in making sure that exactly equivalent initial conditions are used in the two methods. Three calculations with different degrees of anisotropy have been carried out. 相似文献
9.
《Fluid Dynamics Research》2006,38(2-3):145-173
In this paper, direct numerical simulations (DNS) and large eddy simulations (LES) of three engineering flows carried out in the author's research group are presented. The first example, simulated both with DNS and LES, is the flow in a low-pressure turbine cascade with wakes passing periodically through the cascade channel. In this situation, the laminar–turbulent transition of the boundary layers on the blade surfaces, which is strongly influenced by the passing wakes, is of special interest. Next, LES of the flow past the Ahmed body is presented, which is a car model with slant back. In spite of the fairly simple geometry, the flow around the model has many features of the complex, fully 3D flow around real cars. The third example, for which LES is presented, is the flow past a surface mounted circular cylinder of height-to-diameter ratio of 2.5. In this case also complex 3D flow develops with interaction of various vortices behind the cylinder. By means of these examples, the paper shows that complex turbulent flows of engineering relevance can be predicted realistically by DNS and LES, albeit at large cost. The methods are particularly suited and superior to RANS methods for situations where unsteadiness like shedding and large-scale structures dominate the flow, and DNS has evolved into an important tool for studying transition mechanisms. 相似文献
10.
A brief, superficial survey of some very personal nominations for highpoints of the last hundred years in turbulence. Some
conclusions can be dimly seen. This field does not appear to have a pyramidal structure, like the best of physics. We have
very few great hypotheses. Most of our experiments are exploratory experiments. What does this mean?
We believe it means that, even after 100 years, turbulence studies are still in their infancy. We are naturalists, observing
butterflies in the wild. We are still discovering how turbulence behaves, in many respects. We do have a crude, practical,
working understanding of many turbulence phenomena but certainly nothing approaching a comprehensive theory, and nothing that
will provide predictions of an accuracy demanded by designers.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
11.
On Pseudo Turbulence 总被引:3,自引:0,他引:3
L. van Wijngaarden 《Theoretical and Computational Fluid Dynamics》1998,10(1-4):449-458
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D. C. Besnard
F. H. Harlow
《International Journal of Multiphase Flow》1988,14(6):679-699We establish in this paper the foundations of a two-field turbulent flow model that includes two turbulent fields. The case of dispersed particles in an incompressible carrier fluid is treated here, but the very presence of these two fields allows for the generalization of the model to the instability-induced turbulent mixing of two materials. This model describes both cases of turbulent mass diffusion and small drag regime, “wave-like” interpenetration of the two components. It also includes the damping of the turbulence due to the presence of the particles. In addition, a theoretical derivation of the drag-induced decay of the large-scale turbulence kinetic energy is proposed as another mechanism specific to turbulent multiphase flow. 相似文献
16.
We carry out Direct Numerical Simulation (DNS) of flows in closed straight ducts with complex peripheral shape. To perform the simulations the Navier-Stokes equations in cylindrical coordinates are discretized by a second-order finite difference scheme, and the immersed-boundary technique is used to resolve the flow close to walls of complex shape. The basic geometry is a circular pipe of radius R, with imposed sinusoidal perturbations of the type \(\eta R \sin (N_{w}\theta )\). Simulations by varying N w at fixed η were performed to investigate the effect of the perturbation wavenumber. Additional simulations by fixing N w and varying η also allow to investigate the influence of the amplitude of the wall corrugations. The modifications of the near-wall structures due to change in the shape of the walls are well depicted through contour plots of the radial component of the vorticity. The presence of geometrical disturbances anchors the structures at the locations where curvature changes, and the shape of the structures is strongly linked to the amplitude of the wall corrugation. Our interest is also in understanding the influence of the shape of the surface on wall friction. We were expecting some changes in the profile of the total stress with respect to that of the circular pipe, which instead were not found. This is a first indication that changes in the near-wall region do not affect the outer region, and that Townsend’s similarity hypothesis holds. 相似文献
17.
We investigate a sequence of low-dimensional models of turbulent channel flows. These models are based on the extraction of
the Karhunen–Loève (KL) eigenfunctions from a large-scale simulation in a wide channel with R
*=180 (based on the friction velocity). KL eigenfunctions provide an optimal coordinate system in which to represent the dynamics
of the turbulent flow. The hierarchy of KL modes identifies the most energetic independent phenomena in the system. A series
of Galerkin projections is then used to derive a dynamical approximation to flows. In order to capture essential aspects of
the flow in a low-dimensional system, a careful selection of modes is carried out. The resulting models satisfy momentum and
energy conservation as well as yielding the amount of viscous dissipation found in the exact system. Their dynamics includes
modes which characterize the flux, rolls, and propagating waves. Unlike previous treatments the instantaneous streamwise flow
is time dependent and represented by KL flux modes. The rolls correspond to the streaks observed in experiments in the viscous
sublayer. Propagating waves which first appear in the model flow at low Reynolds number (R
*∼ 60) persist through the chaotic regime that appears as the Reynolds number is increased. Statistical measures of the chaotic
flows which have been generated by the models compare favorably with those found in full-scale simulations.
Received 13 July 1998 and accepted 8 January 1999 相似文献
18.
G. G. CHERNYKH Z. L. KOROBITSINA V. A. KOSTOMAKHA 《International Journal of Computational Fluid Dynamics》2013,27(2):173-182
The Karman-Howarth closed equation is used to describe the behavior of homogeneous isotropic turbulence. A numerical solution is obtained by the collocation-grid and finite-difference methods using moving grids. The predicted results are compared with experimental data on the decay of fully developed and weak turbulence. The numerical realization of the Loitsiansky-MilHonshtchikov asymptotic solution has been made. 相似文献
19.
应用 L DV测试技术对方管内网栅后的气固两相流动 ,在颗粒平均粒径 Dp =0 .2 1 mm,颗粒质量浓度为 0 .2 4%、0 .36 % ;Dp =0 .35 mm,颗粒质量浓度为 0 .1 2 %、0 .2 1 %、0 .335 % ;Dp =0 .6 mm,颗粒质量浓度为 0 .1 6 %、0 .2 45 %、0 .34 5 % ;Dp =0 .9mm,颗粒质量浓度为 0 .2 0 5 %、0 .30 %多种工况下进行了测量 ,得出各种工况下气流脉动速度、湍动能沿流动方向的衰减规律 ,通过与纯气流条件下的实验结果比较 ,分析了颗粒浓度及颗粒尺寸对网栅后气相流动湍流特性的影响。根据测量结果 ,提出了一个在有固相颗粒存在时 ,关于湍流模型方程中模型常数 C2的修定方法。 相似文献
20.
In this paper, the waves' breaking in the lee waves in successfully simulated by the atmospheric mesoscale numerical model
with a second-order turbulent closure. It is further proved that the turbulence in the wave-breaking region plays the role
of intense mixing for the average field, which leads to the trapping of upward propagating waves and thus promotes the development
of the downslope wind. The turbulent structure in the wave-breaking region is discussed and the following conclusions are
obtained: (1) In the wave-breaking region, the turbulent heat fluxes transfer from inside to outside and the turbulent momentum
fluxes transfer from outside to inside. (2) In the wave-breaking region, the turbulent energy mainly comes from the wind shear
and the buoyancy promotes the turbulent development only in part of the region. (3) In the upper part of the wave-breaking
region, the turbulent momentum fluxes behave as a counter-gradient transfer. (4) The turbulent mixing in the wave-breaking
region is non-local.
recommended by Professor Li Jiachun. 相似文献