Turbulence

Turbulent three-dimensional dielectric electrohydrodynamic convection between two plates

by Agissilaos Kourmatzis

Fast chemical reaction in two-dimensional Navier-Stokes flow: Initial regime

by Farid Ait Chaalal

Also available on the arXiv: http://arxiv.org/abs/1111.1483

    This paper studies an infinitely fast bimolecular chemical reaction in a two-dimensional bi-periodic... more     This paper studies an infinitely fast bimolecular chemical reaction in a two-dimensional bi-periodic Navier-Stokes flow. The reactants in stoichiometric quantities are initially segregated by infinite gradients. The focus is placed on the initial stage of the reaction characterized by a well-defined one dimensional material contact line between the reactants. Particular attention is given to the effect of the diffusion of the reactants. This study is an idealized framework for isentropic mixing in the lower stratosphere and is motivated by the need to better understand the effect of resolutionon stratospheric chemistry in Climate-Chemistry Models.
    Adopting a Lagrangian stretching theory approach, we relate theoretically the ensemble mean of the length of the contact line, of the gradients along it and of the modulus of the rate of decrease of the space averaged reactant concentrations (here called the chemical speed) to the joint statistics of the finite time Lyapunov exponent with two equivalent times. The inverse of the Lyapunov exponent measures the stretching time scale of a Lagrangian parcel on a chaotic orbit up to a finite time t, while the first equivalent time measures it in the recent past before t and the second equivalent time in the early part of the trajectory. We show that the chemical speed scales like the square root of the diffusion and that its time evolution is determined by rare large events in the finite time Lyapunov exponent distribution. The case of smooth initial gradients is also discussed. The theoretical results are tested with an ensemble of direct numerical simulations (DNS) using a pseudospectral model.

Turbulent Structure functions in Geophysical Flows

by Jose M. Redondo

TURBULENT STRUCTURE FUNCTIONS IN GEOPHYSICAL FLOWS
O. Ben Mahjoub*, J. M. Redondo and R. Alami
Dept. Fisica Aplicada, (*Estadistica) Universitat Politècnica de Catalunya. Campus Nord B5, 08034 Barcelona, Spain

In geophysical flows, there are many instances, where turbulence is originated locally. such as in surface wave... more In geophysical flows, there are many instances, where turbulence is originated locally. such as in surface wave breaking at the surfzone or
by internai wave breaking in the lee of a mountain. The use of velocity structure functions and their moments may give an indication of
the spatial and time delay from the source of turbulence. The variation of the structure functions and the scaling exponents in decaying
non-homogeneous turbulence flows produced by a grid is investigated by means of sonic velocimeter SONTEK3-D. In the analysis we
invoke the concept of the Extended Self Similarity (ESS) and find that there are changes in the structure functions related to the
intermittency.
Keywords:JTurbulence

Turbulence structure of the atmospheric boundary layer in stable conditions

by Jose M. Redondo

TURBULENCE STRUCTURE OF THE ATMOSPHERIC
BOUNDARY LAYER IN STABLE CONDITIONS
J. M. Redondo (1), I. R. Cantalapiedra (1), O.B. Mahjoub (1),
Vindel (2) and C. Yagüe (2).
(1) Univ. Politecnica de Cataluña, Barcelona, Spain.
(2) Univ. Complutense de Madrid, Spain.


Conference proceedings

A partir de las mediciones de velocidad del aire, temperatura y humedad realizadas en la atmósfera en horario nocturno, se ha analizado la influencia de la estabilidad atmosférica en los flujos de momento y temperatura.
978-84-9747-185-5
http://hdl.handle.net/2117/1418

Data from SABLES98 experimental campaign have been used in order to study the
influence of stability (from weak... more Data from SABLES98 experimental campaign have been used in order to study the
influence of stability (from weak to strong stratification) on the flux-profile
relationships for momentum and heat: fm and fh. Standard instrumentation, 14
thermocouples and 3 sonic anemometers at three levels (5.8, 13.5 and 32 m) were
available from 10 to 28 September 1998 and calculations are done in order to extract
structure functions and intermittency. The results show increasing values of fm and fh
with increasing stability parameter and then values remain constant. As a consequence
of these constant, but different values of fm and fh for strong stability, when linear
similarity functions (Businger et al., 1971) are used to calculate surface fluxes of
momentum and heat an important underestimation of the turbulent mixing is produced.
The differences in structure and higher order moments between stable and neutral
turbulence is studied in terms of turbulent intermittent mixing and velocity PDF’s

Multiscale simulations of Langmuir cells and submesoscale eddies using XSEDE resources

by Baylor Fox-Kemper

L. P. Van Roekel, P. E. Hamlington, and B. Fox-Kemper. Multiscale simulations of Langmuir cells and submesoscale eddies using XSEDE resources. Extreme Science and Engineering Discovery Environment Conference 2012 (XSEDE12), 2012.

A proper treatment of upper ocean mixing is an essential part of accurate climate modeling. This problem is diffi cult... more A proper treatment of upper ocean mixing is an essential part of accurate climate modeling. This problem is difficult because the upper ocean is home to many competing processes. Vertical turbulent mixing acts to unstratify the water column, while lateral submesoscale eddies attempt to stratify the column. Langmuir turbulence, which often dominates the vertical mixing, is driven by an interaction of the wind stress and surface wave (Stokes) drift, while the submesoscale eddies are driven by lateral density and velocity changes. Taken together, these processes span a large range of spatial and temporal scales. They have been studied separately via theory and modeling. It has been demonstrated that the way these scales are represented in climate models has a nontrivial impact on the global climate system. The largest impact is on upper ocean processes, which lter air-sea interactions. This interaction is especially interesting, because it is the interface between nonhydrostatic and hydrostatic, quasigeostrophic and ageostrophic, and small-scale and large-scale ocean dynamics. Previous studies have resulted in parameterizations for Langmuir turbulence and submesoscale fluxes, but these parameterizations assume that there is no interaction between these important processes. In this work we have utilized a large XSEDE allocation (9 million SUs) to perform multiscale simulations that encompass the Langmuir scale (O(10-100m)) and submesoscale eddies (O(1-10km)). One simulation includes a Stokes drift, and hence Langmuir turbulence, while the other does not.

To adequately represent such disparate spatial scales is a challenge in numerous regards. Numerical prediction algorithms must balance eciency, scalability, and accuracy. These simulations also present a large challenge for data storage and transfer. However, the results of these simulations will influence climate modeling for decades.

The form and orientation of Langmuir cells for misaligned winds and waves

by Baylor Fox-Kemper

L. Van Roekel, B. Fox-Kemper, P. P. Sullivan, P. E. Hamlington, and S. R. Haney. The form and orientation of Langmuir cells for misaligned winds and waves. Journal of Geophysical Research-Oceans, 117:C05001, 22pp, 2012.

Large eddy simulations of the Craik-Leibovich equations are used to assess the effect of misaligned Stokes drift and... more Large eddy simulations of the Craik-Leibovich equations are used to assess the effect of misaligned Stokes drift and wind direction on Langmuir cells in the ocean mixed layer. Misalignments from 0{degree} to 135{degree} are examined and Langmuir turbulence structures are evident in all cases. The Stokes drift is modeled using a broadband empirical spectrum, and cases with and without the Coriolis effect, wind waves, and an initial mixed layer are examined. The expected scaling for the vertical velocity variance is recovered in the aligned simulations and is adapted here to the misaligned cases. The adjusted scaling projects the friction velocity (aligned with the wind stress) into the dominant axial direction of the Langmuir cells. The turbulent Langmuir number is generalized through a similar projection into the axial direction of the Langmuir cells, which reduces its value in realistic conditions. For known Langmuir cell orientations, the strength of Langmuir turbulence for misaligned cases can be estimated using the projected Langmuir number. A prediction for the angle between the wind stress and cell direction is obtained using the law of the wall; this prediction only requires the wind stress, Stokes drift, and boundary layer depth. Conditional analyses show that, with increasing misalignment, the typically antisymmetric Langmuir cell pairs become asymmetric. This asymmetry is due, in part, to the advection by cross cell flow of vorticity from one vortex tube onto the other, and in part due to an asymmetry induced by the stretching of vertical vorticity into cross cell vorticity.

Turbulence dynamics near a turbulent/nonturbulent interface

by Miguel Teixeira

published in Journal of Fluid Mechanics

A purely non-linear route to transition approaching the edge of chaos in a boundary layer

by stefania cherubini

S. Cherubini, P. De Palma, J.Ch Robinet, A. Bottaro, to appear on the special BIFD issue of Fluid Dynamics Research, 2012

The understanding of transition in shear flows has recently progressed along new paradigms based on the central role... more The understanding of transition in shear flows has recently progressed along new paradigms based on the central role of coherent flow structures and their nonlinear interactions. We follow such paradigms to identify, by means of a non-linear
optimization of the energy growth at short time, the initial perturbation which most easily induces transition in a boundary layer. Moreover, a bisection procedure has been used to identify localized flow structures living on the edge of chaos, found to be
populated by hairpin vortices and streaks. Such an edge structure appears to act as a relative attractor for the trajectory of the laminar base state perturbed by the initial finite amplitude disturbances, mediating the route to turbulence of the flow, via the triggering of a regeneration cycle of  and hairpin structures at different space and time scales. These findings introduce a new, purely non-linear scenario of transition in a boundary-layer flow.

Spectral structure of stratified turbulence: Direct numerical simulations and predictions by large eddy simulation

by Sebastian Remmler

published in "Theoretical and Computational Fluid Dynamics"

Density stratification has a strong impact on turbulence in geophysical flows. Stratification changes the spatial... more Density stratification has a strong impact on turbulence in geophysical flows. Stratification changes the spatial turbulence spectrum and the energy transport and conversion within the spectrum. We analyze these effects based on a series of direct numerical simulations (DNS) of stratified turbulence. To facilitate
simulations of real-world problems, which are usually beyond the reach of DNS, we propose a subgrid-scale turbulence model for large eddy simulations of stratified flows based on the Adaptive Local Deconvolution Method (ALDM). Flow spectra and integral quantities predicted by ALDM are in excellent agreement with
direct numerical simulation. ALDM automatically adapts to strongly anisotropic turbulence and is thus a suitable tool for studying turbulent flow phenomena in atmosphere and ocean.

Identification of Kinetic Alfven Wave Turbulence in the Solar Wind

by Christopher Chen

C. S. Salem, G. G. Howes, D. Sundkvist, S. D. Bale, C. C. Chaston, C. H. K. Chen, F. S. Mozer
Astrophys. J. Lett. 745 L9 (2012)

Anisotropy in Space Plasma Turbulence: Solar Wind Observations

by Christopher Chen

T. S. Horbury, R. T. Wicks, C. H. K. Chen
Space Sci. Rev. (online first), (2011)

The decay of turbulence generated by a class of multi-scale grids

by Pedro Valente

Co-authored with Christos Vassilicos
J. Fluid Mech., 687, pp 300-340
doi:10.1017/jfm.2011.353

A new experimental investigation of decaying turbulence generated by a low-blockage space-filling fractal square grid... more A new experimental investigation of decaying turbulence generated by a low-blockage space-filling fractal square grid is presented. We find agreement with previous works by Seoud & Vassilicos (Phys. Fluids, vol. 19, 2007, 105108) and Mazellier & Vassilicos (Phys. Fluids, vol. 22, 2010, 075101) but also extend the length of the assessed decay region and consolidate the results by repeating the experiments with different probes of increased spatial resolution. It is confirmed that this moderately high Reynolds number $Re_{\lambda}$ turbulence (up to $Re_{\lambda} \approx 350$ here) does not follow the classical high Reynolds number scaling of the dissipation rate $\varepsilon \sim u'^3/L$ and does not obey the equivalent proportionality between the Taylor-based Reynolds number $Re_{\lambda}$ and the ratio of integral scale $L$ to the Taylor microscale $\lambda$. Instead we observe an approximate proportionality between $L$ and $\lambda$ during decay. This non-classical behaviour is investigated by studying how the energy spectra evolve during decay and examining how well they can be described by self-preserving single-length-scale forms. A detailed study of homogeneity and isotropy is also presented which reveals the presence of transverse energy transport and pressure transport in the part of the turbulence decay region where we take data (even though previous studies found mean flow and turbulence intensity profiles to be approximately homogeneous in much of the decay region). The exceptionally fast turbulence decay observed in the part of the decay region where we take data is consistent with the non-classical behaviour of the dissipation rate. Measurements with a regular square mesh grid as well as comparisons with active-grid experiments by Mydlarski & Warhaft (J. Fluid Mech., vol. 320, 1996, pp. 331–368) and Kang, Chester & Meveneau (J. Fluid Mech., vol. 480, 2003, pp. 129–160) are also presented to highlight the similarities and differences between these turbulent flows and the turbulence generated by our fractal square grid.

Dependence of decaying homogeneous turbulence on initial/boundary conditions

by Pedro Valente

Co-authored with Christos Vassilicos
Phys. Lett. A, Vol 376, Iss 4, 510-514,
10.1016/j.physleta.2011.11.040.

A careful data analysis of far downstream turbulent flows generated by conventional and multiscale grids shows that... more A careful data analysis of far downstream turbulent flows generated by conventional and multiscale grids shows that these decaying flows are very clearly different from both Saffman and Loitsyansky turbulence. The analysis also shows that there are marked differences between the far downstream turbulence behaviours generated by different types of grid. There is an inflow condition dependence on both the normalised energy dissipation and the conserved large-scale invariant.

Layer integrated modeling of three dimensional recirculating flows in model tidal basins

by Roger Falconer - Cardiff University

Paper 104:  Hakimzadeh, H. and Falconer, R. A. 2007. Layer integrated modeling of three dimensional recirculating flows in model tidal basins. Journal of Waterway, Port, Coastal and Ocean Engineering, ASCE. 133(5), 324-333.

Abstract
Details are given of a numerical model study to refine a layer integrated model, applied to scaled... more Abstract
Details are given of a numerical model study to refine a layer integrated model, applied to scaled hydraulic model rectangular tidal basins with large aspect ratios (i.e., L/B = 1/4–4/1), and using two- and zero-equation turbulence models. For the zero-equation turbulence model the mixing length model was deployed to calculate the horizontal eddy viscosity coefficient, whereas for the two-equation model, the depth integrated k-ε turbulence model was used. Likewise, the layer integrated mixing length and k-ε turbulence models were used to determine the vertical eddy viscosity coefficient. The model was first applied to idealized channel flows, with the agreement between the predicted values and experimental data being satisfactory. The model was then tested by applying it to an idealized set of data for wind driven currents in a closed rectangular basin, with good agreement again being obtained between the numerical model results and published experimental data. The model was finally applied to a set of hydraulic model studies, undertaken by the writers for a number of different model rectangular tidal basin configurations (or aspect ratios), each with an asymmetric entrance, a flat bed, and vertical sidewalls. The numerical model results obtained using the k-ε and mixing length turbulence models in the horizontal plane were compared graphically with the experimental results to indicate the best model setup for these model basin configurations. Model simulations were also undertaken to investigate the sensitivity of the eddy circulation structure to the closed boundary representation, within the finite difference scheme, and three different boundary conditions were considered including: the no-slip, semislip, and partial-slip representations. The numerical model results showed that the horizontal current structure obtained using the k-ε model was similar to the results obtained using the mixing length turbulence model. The velocity distributions for the different layers were similar, except near the bed, and the horizontal velocity distribution only showed small variations through much of the water column. The three different representations of the closed boundary condition gave very different tidal circulation patterns within the basins, showing that the representation of the closed boundary was crucial for modeling vorticity in such basins. The model results for the k-ε and mixing length turbulence models were found to be in closest agreement with the experimental results when the no-slip and partial-slip conditions were used, respectively.

A Molecular Theory of the Onset of Turbulence

by Perry Esguerra

Amador Muriel, Sergei Novopashin, Jose Perico Esguerra, Edgardo Gutierrez,
Science Diliman, Vol 11, No 2 (1999), 3-7

We review experimental data which shows the dependence of the critical Reynolds number on molecular composition. We... more We review experimental data which shows the dependence of the critical Reynolds number on molecular composition. We then use the cell model of a gas to explain the onset of turbulence by the excitation of the internal degrees of freedom of molecules. Two sources of internal energy states are identified: quantum confinement for monoatomic molecules and rotational states of diatoms.

Kinetic evolution of the perpendicular turbulent cascade in the solar wind

by luca marradi

First author

We investigate the solar-wind dynamics at typical kinetic scales resulting from a turbulent cascade along the... more We investigate the solar-wind dynamics at typical kinetic scales resulting from a turbulent cascade along the direction strictly perpendicular to a background magnetic field. We use a hybrid Vlasov-Maxwell numerical model that solves Vlasov equation for the proton distribution function. Electrons are assumed as an isothermal fluid. We find, over a range of about four decades of wavenumbers, a Kolmogorov slope at wavelengths larger than the proton inertial scale and an abrupt change in the scaling law across the proton inertial length. The energy is carried along the cascade in the form of magneto-sonic fluctuations while the short-scale termination of the spectra is dominated by a significant level of electrostatic activity. As a result of the turbulent energy cascade, we observe the generation of shock structures where the proton distribution function departs from the Maxwellian equilibrium configuration displaying the presence of beams of accelerated protons. Finally, quasi-static magnetic-field structures are produced in the early stage of the system dynamics and remain stable up to the end of the simulation. The numerical results discussed in the present paper qualitatively reproduce a complex phenomenology frequently recovered by spacecraft observations.

Dispersive MHD turbulence in one dimension

by luca marradi

Co-author

Numerical simulations of dispersive turbulence in magnetized plasmas based on the Hall-MHD description are presented,... more Numerical simulations of dispersive turbulence in magnetized plasmas based on the Hall-MHD description are presented, assuming spatial variations along a unique direction making a prescribed angle with the ambient magnetic field. Main observations concern the energy transfers among the different scales and the various types of MHD waves, together with the conditions for the establishment of pressure-balanced structures. For parallel propagation, Alfvén-wave transfer to small scales is strongly inhibited and rather feeds magnetosonic modes, unless the effect of dispersion is strong enough at the energy injection scale. In oblique directions, the dominantly compressible character of the turbulence is pointed out with, for quasi-transverse propagation, the presence of conspicuous kinetic Alfvén waves. Preliminary simulations of a Landau fluid model incorporating relevant linear kinetic effects reveal the development of a significant plasma temperature anisotropy leading to recurrent instabilities.

Fluid simulations of mirror constraints on proton temperature anisotropy in solar wind turbulence

by luca marradi

Non-resonant ion perpendicular heating by low-frequency kinetic Alfvén wave turbulence, together with the constraining... more Non-resonant ion perpendicular heating by low-frequency kinetic Alfvén wave turbulence, together with the constraining effect of the mirror instability on the developing temperature anisotropy observed in the solar wind, are simulated for the first time in a self-consistent way using a fluid model retaining low-frequency kinetic effects. This model which does not include solar wind expansion, concentrates on the influence of small-scale turbulence. It provides a sufficiently refined description of Landau damping and finite Larmor corrections to accurately capture the transverse dynamics at sub-ionic scales, including the self-regulating influence of the developing mirror modes. A fit of the simulation results with the usual mirror-instability threshold is obtained, reproducing the frontier of the slow solar wind WIND/SWE data in the (T$\perp$i/T$\parallel$i, β$\parallel$i) diagram. The quality of the fit is improved in the presence of a small amount of collisions, which suggests that the deviations from bi-Maxwellianity in the slow solar wind are weak enough not to significantly affect the mirror threshold.

Modelling open channel flows with vegetation using a three-dimensional model.

by Roger Falconer - Cardiff University

Paper 131: Gao, G., Falconer, R. A. and Lin B. 2011. Modelling open channel flows with vegetation using a three-dimensional model. Journal of Water Resource and Protection. 3(2), 114-119.

Abstract
The effects of vegetation on the flow structure are investigated in this paper. In previous studies of... more Abstract
The effects of vegetation on the flow structure are investigated in this paper. In previous studies of modelling vegetated flows, two-equation turbulence models, such as the model, were often used. However, this approach involves a level of uncertainty since the empirical coefficients in these two equations have not yet been satisfactorily obtained for such flow conditions. In addition to this, two extra partial differential equations needing which will result in an increase in the computational cost. The main purpose of the study was therefore to try and acquire accurate velocity profiles without the more advanced two-equation turbulence models. A three-dimensional model using a simple two layer mixing length model was therefore used. The governing hydrodynamic equations were refined to include the effects of drag force induced by vegetation on the flow structure. The model was applied to an experiment flume to study the flow field with vegetations, where experiment data are available. Distributions predicted by the model were compared with laboratory measured ones, with very good agreements being obtained. The results showed that the simple mixing length model could produce accurate complex velocity profile predictions requiring fewer coefficient data and less computation.

9789400725058-p1PrefaceERCOFTAC20011

by Jose M. Redondo

New Approaches in Modeling Multiphase Flows and Dispersion in Turbulence, Fractal Methods and Synthetic Turbulence
New Approaches in Modeling Multiphase Flows and Dispersion in Turbulence, Fractal Methods and Synthetic Turbulence

Series: ERCOFTAC Series, Vol. 18

Nicolleau, F.C.G.A.; Cambon, C.; Redondo, J.-M.; Vassilicos, J.C.; Reeks, M.; Nowakowski, A.F. (Eds.)

2012, 2012, XIV, 152 p. 80 illus.

This book contains a collection of the main contributions from the first five workshops held by Ercoftac Special... more This book contains a collection of the main contributions from the first five workshops held by Ercoftac Special Interest Group on Synthetic Turbulence Models (SIG42. It is intended as an illustration of the sig’s activities and of the latest developments in the field.

This volume investigates the use of Kinematic Simulation (KS) and other synthetic turbulence models for the particular application to environmental flows.
This volume offers the best syntheses on the research status in KS, which is widely used in various domains, including Lagrangian aspects in turbulence mixing/stirring, particle dispersion/clustering, and last but not least, aeroacoustics. Flow realizations with complete spatial, and sometime spatio-temporal, dependency, are generated via superposition of random modes (mostly spatial, and sometime spatial and temporal, Fourier modes), with prescribed constraints such as: strict incompressibility (divergence-free velocity field at each point), high Reynolds energy spectrum. Recent improvements consisted in incorporating linear dynamics, for instance in rotating and/or stably-stratified flows, with possible easy generalization to MHD flows, and perhaps to plasmas. KS for channel flows have also been validated. However, the absence of "sweeping effects" in present conventional KS versions is identified as a major drawback in very different applications: inertial particle clustering as well as in aeroacoustics. Nevertheless, this issue was addressed in some reference papers, and merits to be revisited in the light of new studies in progress.

Content Level » Research

Keywords » atmospheric flows - fractal fluids - isotropic turbulence - lagrangian dispersion - multiphase flows - super fluids - synthetic turbulence models

Related subjects » Classical Continuum Physics - Computational Science & Engineering - Mechanics - Theoretical, Mathematical & Computational Physics