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1.
S. Bottin H. Chaté 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,6(1):143-155
We argue on general grounds that the transition to turbulence in plane Couette flow is best studied experimentally at a statistical
level. We present such a statistical analysis of experimental data guided by a parallel investigation of a simple coupled
map lattice model for spatiotemporal intermittency. We confirm that this generic type of spatiotemporal chaos is relevant
in the context of plane Couette flow, where the linear stability of the laminar regime at all Reynolds numbers insures the
necessary local subcriticality. Using large ensembles of similar experiments, we show the existence of a well-defined threshold
Reynolds number above which a unique, turbulent, intermittent attractor coexists with the laminar flow. Furthermore, our data
reveals that this transition to spatiotemporal intermittency is discontinuous, i.e. akin to a first-order phase transition.
Received: 10 April 1998 / Revised: 22 June 1998 / Accepted: 24 June 1998 相似文献
2.
E. Nino C. Serio 《The European Physical Journal B - Condensed Matter and Complex Systems》2000,14(1):191-200
The objective of this study has been to experimentally analyze the correlation structure of the strong temporal intermittency
which characterizes pipe flow close to the transition to turbulence. In doing so transitional pipe flow has been analyzed
by Laser Doppler velocimetry and the Reynolds number dependence of the covariance function has been studied. The range which
has been analyzed covers the transition to turbulence and moderately developed turbulence (Reynolds number from 1 500 to 5
000). The correlation structure which has been evidenced is generally in agreement with the deterministic, dynamical, interpretation
of temporal intermittency which explains the intermittent behavior as a result of a saddle node bifurcation. However, the
analysis has evidenced fluctuations even before the onset of turbulence. The structure of these fluctuations is perfectly
autoregressive which leads us to conclude that the transition to turbulence can be viewed as a transition from linear randomness
to (non-linear) homogeneity.
Received 29 March 1999 and Received in final form 6 September 1999 相似文献
3.
B. Dubrulle F. Hersant 《The European Physical Journal B - Condensed Matter and Complex Systems》2002,26(3):379-386
We generalize an analogy between rotating and stratified shear flows. This analogy is summarized in Table 1. We use this analogy
in the unstable case (centrifugally unstable flow vs. convection) to compute the torque in Taylor-Couette configuration, as a function of the Reynolds number. At low Reynolds
numbers, when most of the dissipation comes from the mean flow, we predict that the non-dimensional torque G = T/ν2
L, where L is the cylinder length, scales with Reynolds number R and gap width η, G = 1.46η3/2(1 - η)-7/4
R
3/2. At larger Reynolds number, velocity fluctuations become non-negligible in the dissipation. In these regimes, there is no
exact power law dependence the torque versus Reynolds. Instead, we obtain logarithmic corrections to the classical ultra-hard (exponent 2) regimes: G = 0.50
. These predictions are found to be in excellent agreement with avail-able experimental data. Predictions for scaling of velocity
fluctuations are also provided.
Received 7 June 2001 and Received in final form 7 December 2001 相似文献
4.
The influence of Reynolds number and blockage ratio on the vortex dynamics of a trapezoidal bluff body placed inside a circular pipe is studied experimentally and numerically. Low aspect ratio, high blockage ratio, curved end conditions (junction of pipe and bluff body), axisymmetric upstream flow with shear and turbulence are some of the intrinsic features of this class of bluff body flows which have been scarcely addressed in the literature. A large range (200:200,000) of Reynolds number (ReD) is covered in this study, encompassing all the three pipe flow regimes (laminar, transition and turbulent). Four different flow regimes are defined based on the distinct features of Strouhal number (St)–ReD relation: steady, laminar irregular, transition and turbulent. The wake in the steady regime is stationary with no oscillations in the shear layer. The laminar regime is termed as irregular owing to irregular vortex shedding. The vortex shedding in this regime is observed to be symmetric. The emergence of separation bubble downstream of the bluff body on either side is another interesting feature of this regime, which is further observed to be symmetric. Two pairs of mean streamwise vortices are noticed in the near-wake regime, which are termed as reverse dipole-type wake topology. Beyond the irregular laminar regime, the Strouhal number falls gradually and vortex shedding becomes more periodic. This regime is named transition and occurs close to the Reynolds number at which transition to turbulence takes place in a fully developed pipe. The turbulent regime is characterised by a nearly constant Strouhal number. Typical Karman-type vortex shedding is noticed in this regime. The convection velocity, wake width formation length and irrecoverable pressure loss are quantified to highlight the influence of blockage ratio. These results will be useful to develop basic understanding of vortex dynamics of confined bluff body flow for several practical applications. 相似文献
5.
Abstract In this article, mixed convection in an open cavity with a heated wall bounded by a horizontal unheated plate is investigated experimentally. The heated wall is on the opposite side of the forced inflow. The results are reported in terms of wall temperature profiles of the heated wall and flow visualization. The range of pertinent parameters used in this experiment are Reynolds numbers (Re) from 100 to 2,000 and Richardson numbers (Ri) from 4.3 to 6,400. Also, the ratio between the length and the height of cavity (L/D) ranges from 0.5–2.0, and the ratio between the channel and cavity height (H/D) is equal to 1.0. The lack of experimental results on mixed convection in a channel with an open cavity below was an impetus for investigating this configuration when one cavity vertical wall is heated at uniform heat flux. The present results show that at the lowest investigated Reynolds number, the surface temperatures are lower than the corresponding surface temperatures for Re = 2,000 at the same ohmic heat flux. The flow visualization shows that for Re = 1,000, there are two nearly distinct fluid motions: a parallel forced flow in the channel and a recirculation flow inside the cavity. For Re = 100, the effect of a stronger buoyancy determines a penetration of thermal plumes from the heated plate wall into the upper channel. Moreover, the flow visualization shows that for lower Reynolds numbers, the forced motion penetrates inside the cavity, and a vortex structure is adjacent to the unheated vertical plate. At higher Reynolds numbers, the vortex structure has a larger extension while L/D is held constant. 相似文献
6.
E.S.C. Ching K.M. Pang 《The European Physical Journal B - Condensed Matter and Complex Systems》2002,27(4):559-564
We study numerically the dependence of heat transport on the maximum velocity and shear rate of physical circulating flows,
which are prescribed to have the key characteristics of the large-scale mean flow observed in turbulent convection. When the
side-boundary thermal layer is thinner than the viscous boundary layer, the Nusselt number (Nu), which measures the heat transport, scales with the normalized shear rate to an exponent 1/3. On the other hand, when the
side-boundary thermal layer is thicker, the dependence of Nu on the Peclet number, which measures the maximum velocity, or the normalized shear rate when the viscous boundary layer thickness
is fixed, is generally not a power law. Scaling behavior is obtained only in an asymptotic regime. The relevance of our results
to the problem of heat transport in turbulent convection is also discussed.
Received 28 November 2001 Published online 25 June 2002 相似文献
7.
Paul Manneville 《Pramana》2008,70(6):1009-1021
In contrast with free shear flows presenting velocity profiles with inflection points which cascade to turbulence in a relatively
mild way, wall bounded flows are deprived of (inertial) instability modes at low Reynolds numbers and become turbulent in
a much wilder way, most often marked by the coexistence of laminar and turbulent domains at intermediate Reynolds numbers,
well below the range where (viscous) instabilities can show up. There can even be no unstable mode at all, as for plane Couette
flow (pCf) or for Poiseuille pipe flow (Ppf) that are currently the subject of intense research. Though the mechanisms involved
in the transition to turbulence in wall flows are now better understood, statistical properties of the transition itself are
yet unsatisfactorily assessed. A widely accepted interpretation rests on non-trivial solutions of the Navier-Stokes equations
in the form of unstable travelling waves and on transient chaotic states associated to chaotic repellors. Whether these concepts
typical of the theory of temporal chaos are really appropriate is yet unclear owing to the fact that, strictly speaking, they apply when confinement in physical
space is effective while the physical systems considered are rather extended in at least one space direction, so that spatiotemporal
behaviour cannot be ruled out in the transitional regime. The case of pCf will be examined in this perspective through numerical
simulations of a model with reduced cross-stream (y) dependence, focusing on the in-plane (x, z) space dependence of a few velocity amplitudes. In the large aspect-ratio limit, the transition to turbulence takes place
via spatiotemporal intermittency and we shall attempt to make a connection with the theory of first-order (thermodynamic)
phase transitions, as suggested long ago by Pomeau.
相似文献
8.
O. Morita 《Phase Transitions》2013,86(1-4):213-244
Baroclinic flow in a rotating annulus of fluid shows remarkable transitions of flow patterns as do Rayleigh–Benard convection and Taylor vortices. There are four flow regimes in two nondimensional parameter space, called a symmetric regime (Hadley regime), a steady wave regime (Rossby regime), a vacillating wave regime and a geostrophic turbulence regime. Laminar flow in a symmetric regime is formed between the balance of a horizontal pressure gradient force and a Coriolis torque (geostrophic balance), and this flow becomes unstable when one of the nondimensional parameters, the thermal Rossby number, becomes less than the critical value. In this paper, the characteristic features of the four flow regimes are reviewed including recent findings about the behavior of geostrophic turbulence. 相似文献
9.
We present experimental results of heat transfer processes in mixed-convective flow from a ducted vertical hot-plate thermal flow sensor for aiding (upward) and opposing (downward) flows. The results are obtained for three different Grashof numbers, Gr = 289, 411, and 456, using air, in the Reynolds number range from 0 to 120. The Nusselt number for aiding flows can be adequately described by Nu tot – Nu 0 = (Nu n forced + Nu n free ) 1/n , with Nu 0 = 0.5, as originally proposed by Churchill for a free flat plate. For n, a value of 1.5–1.7 is found. For opposing flows in the mixed-flow region (0.1 h Gr/Re 2 S 10), flow visualization shows an oscillating buoyant plume around the flat plate. In the transition from free to mixed and to essentially forced convection, distinct sequences of instabilities of this plume are observed, leading to several local minima and maxima in the heat transfer from the plate. The results are summarized in a bifurcation diagram. Here, several windows with instabilities are found, both nonperiodic, with strong indications of chaotic behavior, and (quasi-) periodic. Typical fundamental frequencies of the instabilities range from 0.15 to 1 Hz. 相似文献
10.
A mean field approach is used to estimate the energy dissipation during the homogeneous sedimentation or the particulate fluidization
of non Brownian hard spheres in a concentrated suspension of infinite extent. Depending on inertial screening and the range
of the hydrodynamic interactions, the effective buoyancy force is determined either from the average suspension density in
a Stokes flow or from the fluid density in the turbulent flow regime. An energy balance then yields a settling or fluidization
law depending on the particle Reynolds number in reasonable agreement with the Richardson and Zaki correlation and recent
experimental results for particle settling or fluidization. We further estimate the energy dissipation in the turbulent boundary
layers around the particles to precise the Reynolds number dependence of the hindered settling function in the intermediate
flow regime.
Received 22 February 1999 and Received in final form 14 June 1999 相似文献
11.
S.E. Guidoni H.O. Mártin C.M. Aldao 《The European physical journal. E, Soft matter》2002,7(3):291-295
We present a one-dimensional Monte Carlo simulation for the diffusion motion of a chain of N beads. We found that the scaling exponent for the viscosity can be smaller or greater than 3. This anomalous behavior cannot be attributed to the diffusivity scaling or the length fluctuations but is due to the chain dynamics details
during diffusion in which the end beads play the key role. The viscosity exponent 3 and its expected relation with the diffusivity
exponent are recovered in the asymptotic regime (N ↦∞).
Received 24 September 2001 and Received in final form 28 January 2002 相似文献
12.
The results of mathematical modeling of convection of a viscous incompressible liquid in a rectangular domain with sources
of mass input and output are presented. A conjugate statement within the framework of the Boussinesq approximation is used.
The regimes of forced and mixed convection in a domain have been investigated. The domain has two vertical walls and one horizontal
wall of finite thickness, two zones of liquid input and output, and a free surface. A plane nonstationary problem within the
framework of the Navier-Stokes model for the liquid phase and the heat conduction equation for the solid phase are considered.
The distributions of the hydrodynamic parameters and temperatures characterizing the main regularities of the processes under
investigation have been obtained. Circulation flows have been identified. The vortex formation mechanism and the temperature
distribution in the solution domain under the regimes of forced and mixed convection and different locations of mass input
and output zones have been analyzed. It has been found that natural convection should be taken into account when modeling
convective heat transfer with Gr number values from 105 and higher. 相似文献
13.
B. Dubrulle 《The European Physical Journal B - Condensed Matter and Complex Systems》2001,21(2):295-304
We use a simplified model of turbulent convection to compute analytically heat transport in a horizontal layer heated from below, as a function of the Rayleigh and the Prandtl number. At low Reynolds numbers, when most of the dissipation comes from the mean flow, we recover power classical scaling regimes of the Nusselt versus Rayleigh number, with exponent 1/3 or 1/4. At larger Reynolds number, velocity and temperature fluctuations become non-negligible in the dissipation. In these regimes, there is no exact power law dependence the Nusselt versus Rayleigh or Prandtl. Instead, we obtain logarithmic corrections to the classical soft (exponent 1/3) or ultra-hard (exponent 1/2) regimes, in a way consistent with the most accurate experimental measurements available nowadays. This sets a need for the search of new measurable quantities that are less prone to dimensional theories. 相似文献
14.
15.
The silo discharge process is studied by molecular
dynamics simulations. The development of the velocity profile and
the probability density function for the displacements in the
horizontal and vertical axis are obtained. The PDFs obtained at
the beginning of the discharge reveal non-Gaussian statistics and
superdiffusive behaviors. When the stationary flow is developed,
the PDFs at shorter temporal scales are non-Gaussian too. For big
orifices a well-defined transition between ballistic and diffusive
regime is observed. In the case of a small outlet orifice, no
well-defined transition is observed. We use a nonlinear diffusion
equation introduced in the framework of non-extensive
thermodynamics in order to describe the movements of the grains.
The solution of this equation gives a well-defined relationship
(γ= 2/(3-q)) between the anomalous diffusion exponent
γ and the entropic parameter q introduced by the
non-extensive formalism to fit the PDF of the fluctuations. 相似文献
16.
Mitsuru Tanaka 《Journal of Turbulence》2017,18(12):1144-1179
Fully resolved simulations of homogeneous shear turbulence (HST) laden with sedimenting spherical particles of finite size have been performed to clarify the effects of gravity on the development of particle-laden turbulent shear flows. We consider turbulence in a horizontal flow subjected to vertical or horizontal shear. Numerical results show that the development of HST laden with finite-size particles are significantly altered by gravity. The effects of gravity lead to a slower increase in the Taylor-microscale Reynolds number, whose value is found to be well correlated with the average particle Reynolds number. The gravity also causes a slower increase in the turbulence kinetic energy (TKE) through the enhancement of energy dissipation. The change in the Reynolds shear stress (RSS) due to particles also significantly contributes to the relative change in TKE. In vertically sheared cases, RSS has high values between counter-rotating trailing vortices behind the particles, which causes a transient relative increase in TKE. In horizontally sheared cases, on the other hand, RSS is reduced in the wakes of particles, which contributes to a significant relative reduction in TKE. 相似文献
17.
E. Plaut R. Ribotta 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,5(2):265-281
We study theoretically the formation of convection patterns in a laterally extended planar nematic layer heated from below,
in the linear and weakly nonlinear regimes. By reformulating the viscous coupling terms of the basic nematohydrodynamic equations,
a simple interpretation of the flow effects on the director dynamics can be proposed. A detailed linear analysis of the problem
is presented. A systematic method to investigate nonlinear mechanisms is developed, and exemplified by the study of the nonlinear saturation in rolls. The extension of the roll amplitude equation
with the envelope formalism is used to characterize the dynamics of the roll modulations near threshold. Coupled envelope
equations are shown to describe the structure of the point defects in zig-zags observed experimentally. Finally the bifurcation
to the bimodal varicose is studied. The secondary wavevector in the bimodal appears to be selected by a rotation of the director
in the horizontal plane. Quantitative predictions concerning the amplitude of this rotation are given.
Received: 1st December 1997 / Revised: 25 May 1998 / Accepted: 2 June 1998 相似文献
18.
The transition to turbulence in plane Poiseuille flow (PPF) is connected with the presence of exact coherent structures. We here discuss a variety of different structures that are relevant for the transition, compare the critical Reynolds numbers and optimal wavelengths for their appearance, and explore the differences between flows operating at constant mass flux or at constant pressure drop. The Reynolds numbers quoted here are based on the mean flow velocity and refer to constant mass flux. Reynolds numbers based on constant pressure drop are always higher. The Tollmien–Schlichting (TS) waves bifurcate subcritically from the laminar profile at Re = 5772 at wavelength 6.16 and reach down to Re = 2610 at a different optimal wave length of 4.65. Their streamwise localised counter part bifurcates at the even lower value Re = 2334. Three-dimensional exact solutions appear at much lower Reynolds numbers. We describe one exact solutions that has a critical Reynolds number of 316. Streamwise localised versions of this state require higher Reynolds numbers, with the lowest bifurcation occurring near Re = 1018. The analysis shows that the various branches of TS-waves cannot be connected with transition observed near Re ≈ 1000 and that the exact coherent structures related to downstream vortices come in at lower Reynolds numbers and prepare for the transition. 相似文献
19.
É. Falcon S. Fauve C. Laroche 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,7(2):183-186
We report experimental results on the behavior of an ensemble of inelastically colliding particles, excited by a vibrated
piston in a vertical cylinder. When the particle number is increased, we observe a transition from a regime where the particles
have erratic motions (“granular gas”) to a collective behavior where all the particles bounce like a nearly solid body. In
the gas-like regime, we measure the density of particles as a function of the altitude and the pressure as a function of the
number N of particles. The atmosphere is found to be exponential far enough from the piston, and the “granular temperature”, T, dependence on the piston velocity, V, is of the form , where is a decreasing function of N. This may explain previous conflicting numerical results.
Received 1 February 1999 相似文献
20.
F.A. Tamarit S.A. Cannas C. Tsallis 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,1(4):545-548
We consider biological evolution as described within the Bak and Sneppen 1993 model. We exhibit, at the self-organized critical
state, a power-law sensitivity to the initial conditions, calculate the associated exponent, and relate it to the recently
introduced nonextensive thermostatistics. The scenario which here emerges without tuning strongly reminds of that of the tuned onset of chaos in say logistic-like one-dimensional maps. We also calculate the dynamical exponent z.
Received: 5 November 1997 / Received in final form: 11 November 1997 / Accepted: 19 November 1997 相似文献