8,574 research outputs found
Intermittency in the transition to turbulence
It is commonly known that the intermittent transition from laminar to turbulent flow in pipes occurs because, at intermediate values of a prescribed pressure drop, a purely laminar flow offers too little resistance, but a fully turbulent one offers too much. We propose a phenomenological model of the flow, which is able to explain this in a quantitative way through a hysteretic transition between laminar and turbulent states, characterized by a disturbance amplitude variable that satisfies a natural type of evolution equation. The form of this equation is motivated by physical observations and derived by an averaging procedure, and we show that it naturally predicts disturbances having the characteristics of slugs and puffs. The model predicts oscillations similar to those which occur in intermittency in pipe flow, but it also predicts that stationary biphasic states can occur in sufficiently short pipes
Intermittency and structure functions in channel flow turbulence
We present a study of intermittency in a turbulent channel flow. Scaling
exponents of longitudinal streamwise structure functions, ,
are used as quantitative indicators of intermittency.
We find that, near the center of the channel the values of
up to are consistent with the assumption of homogeneous/isotropic
turbulence. Moving towards the boundaries, we observe a growth of intermittency
which appears to be related to an intensified presence of ordered vortical
structures. In fact, the behaviour along the normal-to-wall direction of
suitably normalized scaling exponents shows a remarkable correlation with the
local strength of the Reynolds stress and with the \rms value of helicity
density fluctuations. We argue that the clear transition in the nature of
intermittency appearing in the region close to the wall, is related to a new
length scale which becomes the relevant one for scaling in high shear flows.Comment: 4 pages, 6 eps figure
Intermittency and emergence of coherent structures in wave turbulence of a vibrating plate
We report numerical investigations of wave turbulence in a vibrating plate.
The possibility to implement advanced measurement techniques and long time
numerical simulations makes this system extremely valuable for wave turbulence
studies. The purely 2D character of dynamics of the elastic plate makes it much
simpler to handle compared to much more complex 3D physical systems that are
typical of geo- and astrophysical issues (ocean surface or internal waves,
magnetized plasmas or strongly rotating and/or stratified flows). When the
forcing is small the observed wave turbulence is consistent with the
predictions of the Weak Turbulent Theory. Here we focus on the case of stronger
forcing for which coherent structures can be observed. These structures look
similar to the folds and D-cones that are commonly observed for strongly
deformed static thin elastic sheets (crumpled paper) except that they evolve
dynamically in our forced system. We describe their evolution and show that
their emergence is associated with statistical intermittency (lack of self
similarity) of strongly nonlinear wave turbulence. This behavior is reminiscent
of intermittency in Navier-Stokes turbulence. Experimental data show hints of
the weak to strong turbulence transition. However, due to technical limitations
and dissipation, the strong nonlinear regime remains out of reach of
experiments and therefore has been explored numerically.Comment: accepted for publication in Phys. Rev.
Statistical anisotropy of magnetohydrodynamic turbulence
Direct numerical simulations of decaying and forced magnetohydrodynamic (MHD)
turbulence without and with mean magnetic field are analyzed by higher-order
two-point statistics. The turbulence exhibits statistical anisotropy with
respect to the direction of the local magnetic field even in the case of global
isotropy. A mean magnetic field reduces the parallel-field dynamics while in
the perpendicular direction a gradual transition towards two-dimensional MHD
turbulence is observed with inertial-range scaling of the
perpendicular energy spectrum. An intermittency model based on the Log-Poisson
approach, , is able to describe the observed
structure function scalings.Comment: 4 pages, 3 figures. To appear in Phys.Rev.
Transition Modeling for Low to High Speed Boundary Layer Flows with CFD Applications
Transition modeling as applied to CFD methods has followed certain line of evolution starting from simple linear stability methods to almost or fully predictive methods such as LES and DNS. One pragmatic approach among these methods, such as the local correlation-based transition modeling approach, is gaining more popularity due to its straightforward incorporation into RANS solvers. Such models are based on blending the laminar and turbulent regions of the flow field by introducing intermittency equations into the turbulence equations. Menter et al. pioneered this approach by their two-equation γ-Reθ intermittency equation model that was incorporated into the k-ω SST turbulence model that results in a total of four equations. Later, a range of various three-equation models was developed for super-/hypersonic flow applications. However, striking the idea that the Reθ-equation was rather redundant, Menter produced a novel one-equation intermittency transport γ-equation model. In this report, yet another recently introduced transition model called as the Bas-Cakmakcioglu (B-C) algebraic model is elaborated. In this model, an algebraic γ-function, rather than the intermittency transport γ-equation, is incorporated into the one-equation Spalart-Allmaras turbulence model. Using the present B-C model, a number of two-dimensional test cases and three-dimensional test cases were simulated with quite successful results
Investigation of intermittency in superfluid turbulence
International audienceThis paper reports new experimental and simulation velocity data for superfluid steady turbulence above 1 K. We present values for the scaling exponent of the absolute value of velocity-increment structure functions. In both experiments and simulations, they evidence that intermittency occurs in superfluid flows in a quite comparable way to classical turbulence. In particular, the deviation from Kolmogorov 1941 keeps the same strength as we cross the superfluid transition. To the best of our knowledge, this is the first confirmation of the superfluid 4He experimental results from Maurer et al. EPL 1998 and the first numerical evidence of intermittency in superfluid turbulence
Transition models for turbomachinery boundary layer flows : a review
Current models for transition in turbomachinery boundary layer flows are reviewed. The basic physical mechanisms of transition processes and the way these processes are expressed by model ingredients are discussed. The fundamentals of models are described as far as possible, with a common structure of the equations and with emphasis on the similarities between the models. Tests of models reported in the literature are summarized and our own test is added. A conclusion on the performance of models is formulated
A novel type of intermittency in a nonlinear dynamo in a compressible flow
The transition to intermittent mean--field dynamos is studied using numerical
simulations of isotropic magnetohydrodynamic turbulence driven by a helical
flow. The low-Prandtl number regime is investigated by keeping the kinematic
viscosity fixed while the magnetic diffusivity is varied. Just below the
critical parameter value for the onset of dynamo action, a transient
mean--field with low magnetic energy is observed. After the transition to a
sustained dynamo, the system is shown to evolve through different types of
intermittency until a large--scale coherent field with small--scale turbulent
fluctuations is formed. Prior to this coherent field stage, a new type of
intermittency is detected, where the magnetic field randomly alternates between
phases of coherent and incoherent large--scale spatial structures. The
relevance of these findings to the understanding of the physics of mean--field
dynamo and the physical mechanisms behind intermittent behavior observed in
stellar magnetic field variability are discussed.Comment: 19 pages, 13 figure
The signature of laminar instabilities in the zone of transition to turbulence
We demonstrate that the space-time statistics of the birth of turbulent spots
in boundary layers can be reconstructed qualitatively from the average behavior
of macroscopic measures in the transition zone. The conclusion in \cite{vg04}
that there exists a connection between the patterns in laminar instability and
the birth of turbulent spots is strengthened. We examine why the relationship
between instability and transition to turbulence is manifest in some cases and
appears to be totally absent in others. Novel cellular automaton type
simulations of the transition zone are conducted, and the pattern of spot birth
is obtained from secondary instability analysis. The validity of the hypothesis
of concentrated breakdown, according to which most turbulent spots originate at
a particular streamwise location, is assessed. The predictions made lend
themselves to straightforward experimental verification.Comment: 12 pages, 25 figures, submitted to PR
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