The seasonal mean flow and distribution of large-scale weather systems in the southern hemisphere: The effects of moisture transports

Abstract A hybrid coupled ocean–atmosphere model is used to investigate the stability of the thermohaline circulation (THC) to an increase in the surface freshwater forcing in the presence of interactive meridional transports in the atmosphere. The ocean component is the idealized global general circulation model used in Part I. The atmospheric model assumes fixed latitudinal structure of the heat and moisture transports, and the amplitudes are calculated separately for each hemisphere from the large-scale sea surface temperature (SST) and SST gradient, using parameterizations based on baroclinic stability theory. The ocean–atmosphere heat and freshwater exchanges are calculated as residuals of the steady-state atmospheric budgets. Owing to the ocean component’s weak heat transport, the model has too strong a meridional SST gradient when driven with observed atmospheric meridional transports. When the latter are made interactive, the conveyor belt circulation collapses. A flux adjustment is introduced in which the efficiency of the atmospheric transports is lowered to match the too low efficiency of the ocean component. The feedbacks between the THC and both the atmospheric heat and moisture transports are positive, whether atmospheric transports are interactive in the Northern Hemisphere, the Southern Hemisphere, or both. However, the feedbacks operate differently in the Northern and Southern Hemispheres, because the Pacific THC dominates in the Southern Hemisphere, and deep water formation in the two hemispheres is negatively correlated. The feedbacks in the two hemispheres do not necessarily reinforce each other because they have opposite effects on low-latitude temperatures. The model is qualitatively similar in stability to one with conventional “additive” flux adjustment, but quantitatively more stable.

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P/Halley: The Disconnection Event of 1986 April 11

International Astronomical Union Colloquium ◽

10.1017/s0252921100092617 ◽

1988 ◽

Vol 98 ◽

pp. 155-157 ◽

Cited By ~ 1

Author(s):

P.M. Bergé ◽

G. Mahoux ◽

A.C. Levasseur-Regourd

Keyword(s):

Southern Hemisphere ◽

Large Scale ◽

Focal Length ◽

La Réunion ◽

Structure Dynamics ◽

Plasma Tail

This work was carried out by an expedition organized by the S.A.F. in 1986 April to La Réunion. The aim was to observe and photograph P/Halley, within the framework of IHW, as part of the Island Network in the southern hemisphere. To be more precise, our work consisted of studying large-scale phenomena: the structure, dynamics and possible disconnection events in the plasma tail. We were lucky enough to observe one of the latter on the night of April 11/12, and describe it here.For the Island Network, IHW had a number of Schmidt telescopes (Celestron 8). One was lent to the S.A.F. and this is what we used. This telescope has a focal ratio of 1.5, with a 200-mm (8-inch) objective and 300-mm focal length. We used only Kodak TP2415 film, hypersensitized in forming gas (24h at 60°C).

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Instability wave models for the near-field fluctuations of turbulent jets

Journal of Fluid Mechanics ◽

10.1017/jfm.2011.401 ◽

2011 ◽

Vol 689 ◽

pp. 97-128 ◽

Cited By ~ 172

Author(s):

K. Gudmundsson ◽

Tim Colonius

Keyword(s):

Flow Field ◽

Large Scale ◽

Microphone Array ◽

Near Field ◽

Turbulent Jets ◽

Instability Wave ◽

Mean Flow ◽

Velocity Fluctuations ◽

The Mean ◽

Scale Structures

AbstractPrevious work has shown that aspects of the evolution of large-scale structures, particularly in forced and transitional mixing layers and jets, can be described by linear and nonlinear stability theories. However, questions persist as to the choice of the basic (steady) flow field to perturb, and the extent to which disturbances in natural (unforced), initially turbulent jets may be modelled with the theory. For unforced jets, identification is made difficult by the lack of a phase reference that would permit a portion of the signal associated with the instability wave to be isolated from other, uncorrelated fluctuations. In this paper, we investigate the extent to which pressure and velocity fluctuations in subsonic, turbulent round jets can be described aslinearperturbations to the mean flow field. The disturbances are expanded about the experimentally measured jet mean flow field, and evolved using linear parabolized stability equations (PSE) that account, in an approximate way, for the weakly non-parallel jet mean flow field. We utilize data from an extensive microphone array that measures pressure fluctuations just outside the jet shear layer to show that, up to an unknown initial disturbance spectrum, the phase, wavelength, and amplitude envelope of convecting wavepackets agree well with PSE solutions at frequencies and azimuthal wavenumbers that can be accurately measured with the array. We next apply the proper orthogonal decomposition to near-field velocity fluctuations measured with particle image velocimetry, and show that the structure of the most energetic modes is also similar to eigenfunctions from the linear theory. Importantly, the amplitudes of the modes inferred from the velocity fluctuations are in reasonable agreement with those identified from the microphone array. The results therefore suggest that, to predict, with reasonable accuracy, the evolution of the largest-scale structures that comprise the most energetic portion of the turbulent spectrum of natural jets, nonlinear effects need only be indirectly accounted for by considering perturbations to the mean turbulent flow field, while neglecting any non-zero frequency disturbance interactions.

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The Kinetic Energy of Large-Scale Atmospheric Motion in Wavenumber-Frequency Space: 11. Mid-Troposphere of the Southern Hemisphere

Journal of the Atmospheric Sciences ◽

10.1175/1520-0469(1970)027<1008:tkeols>2.0.co;2 ◽

1970 ◽

Vol 27 (7) ◽

pp. 1008-1020 ◽

Cited By ~ 11

Author(s):

S. K. Kao ◽

R. L. Jenne ◽

J. F. Sagendorf

Keyword(s):

Kinetic Energy ◽

Southern Hemisphere ◽

Large Scale ◽

Frequency Space ◽

Atmospheric Motion

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Study of the Standard k-ε Model for Tip Leakage Flow in an Axial Compressor Rotor

International Journal of Turbo and Jet Engines ◽

10.1515/tjj-2015-0039 ◽

2016 ◽

Vol 33 (4) ◽

Cited By ~ 7

Author(s):

Yanfei Gao ◽

Yangwei Liu ◽

Luyang Zhong ◽

Jiexuan Hou ◽

Lipeng Lu

Keyword(s):

Flow Field ◽

Large Scale ◽

Turbulent Viscosity ◽

Axial Compressor ◽

Leakage Flow ◽

Reynolds Stresses ◽

Mean Flow ◽

Tip Leakage Flow ◽

Tip Leakage ◽

Compressor Rotor

AbstractThe standard k-ε model (SKE) and the Reynolds stress model (RSM) are employed to predict the tip leakage flow (TLF) in a low-speed large-scale axial compressor rotor. Then, a new research method is adopted to “freeze” the turbulent kinetic energy and dissipation rate of the flow field derived from the RSM, and obtain the turbulent viscosity using the Boussinesq hypothesis. The Reynolds stresses and mean flow field computed on the basis of the frozen viscosity are compared with the results of the SKE and the RSM. The flow field in the tip region based on the frozen viscosity is more similar to the results of the RSM than those of the SKE, although certain differences can be observed. This finding indicates that the non-equilibrium turbulence transport nature plays an important role in predicting the TLF, as well as the turbulence anisotropy.

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Turbulent flow between counter-rotating concentric cylinders: a direct numerical simulation study

Journal of Fluid Mechanics ◽

10.1017/s0022112008003716 ◽

2008 ◽

Vol 615 ◽

pp. 371-399 ◽

Cited By ~ 32

Author(s):

S. DONG

Keyword(s):

Turbulent Flow ◽

Large Scale ◽

Outer Cylinder ◽

Three Dimensional ◽

Reynolds Numbers ◽

Taylor Vortex ◽

Small Scale ◽

Mean Flow ◽

Concentric Cylinders ◽

High Reynolds

We report three-dimensional direct numerical simulations of the turbulent flow between counter-rotating concentric cylinders with a radius ratio 0.5. The inner- and outer-cylinder Reynolds numbers have the same magnitude, which ranges from 500 to 4000 in the simulations. We show that with the increase of Reynolds number, the prevailing structures in the flow are azimuthal vortices with scales much smaller than the cylinder gap. At high Reynolds numbers, while the instantaneous small-scale vortices permeate the entire domain, the large-scale Taylor vortex motions manifested by the time-averaged field do not penetrate a layer of fluid near the outer cylinder. Comparisons between the standard Taylor–Couette system (rotating inner cylinder, fixed outer cylinder) and the counter-rotating system demonstrate the profound effects of the Coriolis force on the mean flow and other statistical quantities. The dynamical and statistical features of the flow have been investigated in detail.

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On the irreversibility of internal-wave dynamics due to wave trapping by mean flow inhomogeneities. Part 1. Local analysis

Journal of Fluid Mechanics ◽

10.1017/s0022112093003325 ◽

1993 ◽

Vol 251 ◽

pp. 21-53 ◽

Cited By ~ 28

Author(s):

Sergei I. Badulin ◽

Victor I. Shrira

Keyword(s):

Shear Flow ◽

Internal Wave ◽

Wave Field ◽

Large Scale ◽

Spatial Scales ◽

Energy Concentration ◽

Local Analysis ◽

Mean Flow ◽

Wave Trapping ◽

Wave Dynamics

The propagation of guided internal waves on non-uniform large-scale flows of arbitrary geometry is studied within the framework of linear inviscid theory in the WKB-approximation. Our study is based on a set of Hamiltonian ray equations, with the Hamiltonian being determined from the Taylor-Goldstein boundary-value problem for a stratified shear flow. Attention is focused on the fundamental fact that the generic smooth non-uniformities of the large-scale flow result in specific singularities of the Hamiltonian. Interpreting wave packets as particles with momenta equal to their wave vectors moving in a certain force field, one can consider these singularities as infinitely deep potential holes acting quite similarly to the ‘black holes’ of astrophysics. It is shown that the particles fall for infinitely long time, each into its own ‘black hole‘. In terms of a particular wave packet this falling implies infinite growth with time of the wavenumber and the amplitude, as well as wave motion focusing at a certain depth. For internal-wave-field dynamics this provides a robust mechanism of a very specific conservative and moreover Hamiltonian irreversibility.This phenomenon was previously studied for the simplest model of the flow non-uniformity, parallel shear flow (Badulin, Shrira & Tsimring 1985), where the term ‘trapping’ for it was introduced and the basic features were established. In the present paper we study the case of arbitrary flow geometry. Our main conclusion is that although the wave dynamics in the general case is incomparably more complicated, the phenomenon persists and retains its most fundamental features. Qualitatively new features appear as well, namely, the possibility of three-dimensional wave focusing and of ‘non-dispersive’ focusing. In terms of the particle analogy, the latter means that a certain group of particles fall into the same hole.These results indicate a robust tendency of the wave field towards an irreversible transformation into small spatial scales, due to the presence of large-scale flows and towards considerable wave energy concentration in narrow spatial zones.

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Unravelling the large-scale circulation modes in turbulent Rayleigh–Bénard convection

EPL (Europhysics Letters) ◽

10.1209/0295-5075/ac3da2 ◽

2021 ◽

Author(s):

Susanne Horn ◽

Peter J. Schmid ◽

Jonathan M. Aurnou

Keyword(s):

Large Scale ◽

Three Dimensional ◽

Dynamic Mode Decomposition ◽

Dynamic Mode ◽

Mean Flow ◽

Large Scale Circulation ◽

Convective Systems ◽

Aspect Ratios ◽

Mode Decomposition ◽

Coherent Flow Structure

Abstract The large-scale circulation (LSC) is the most fundamental turbulent coherent flow structure in Rayleigh-B\'enard convection. Further, LSCs provide the foundation upon which superstructures, the largest observable features in convective systems, are formed. In confined cylindrical geometries with diameter-to-height aspect ratios of Γ ≅ 1, LSC dynamics are known to be governed by a quasi-two-dimensional, coupled horizontal sloshing and torsional (ST) oscillatory mode. In contrast, in Γ ≥ √2 cylinders, a three-dimensional jump rope vortex (JRV) motion dominates the LSC dynamics. Here, we use dynamic mode decomposition (DMD) on direct numerical simulation data of liquid metal to show that both types of modes co-exist in Γ = 1 and Γ = 2 cylinders but with opposite dynamical importance. Furthermore, with this analysis, we demonstrate that ST oscillations originate from a tilted elliptical mean flow superposed with a symmetric higher order mode, which is connected to the four rolls in the plane perpendicular to the LSC in Γ = 1 tanks.

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Large-scale Structure and Turbulence Transport in the Young Solar Wind – Comparison of Parker Solar Probe Observations with a Global 3D Reynolds-averaged MHD Model

10.5194/egusphere-egu21-12726 ◽

2021 ◽

Author(s):

Rohit Chhiber ◽

Arcadi Usmanov ◽

William Matthaeus ◽

Melvyn Goldstein ◽

Riddhi Bandyopadhyay

Keyword(s):

Solar Wind ◽

Large Scale ◽

Transport Equations ◽

Turbulence Energy ◽

Reynolds Stresses ◽

Mean Flow ◽

Coulomb Collisions ◽

Flow Equations ◽

Simulation Results ◽

Turbulence Transport

<div>Simulation results from a global magnetohydrodynamic model of the solar corona and the solar wind are compared with Parker Solar Probe's (PSP) observations during its first several orbits. The fully three-dimensional model (Usmanov et al., 2018, ApJ, 865, 25) is based on Reynolds-averaged mean-flow equations coupled with turbulence transport equations. The model accounts for effects of electron heat conduction, Coulomb collisions, Reynolds stresses, and heating of protons and electrons via nonlinear turbulent cascade. Turbulence transport equations for turbulence energy, cross helicity, and correlation length are solved concurrently with the mean-flow equations. We specify boundary conditions at the coronal base using solar synoptic magnetograms and calculate plasma, magnetic field, and turbulence parameters along the PSP trajectory. We also accumulate data from all orbits considered, to obtain the trends observed as a function of heliocentric distance. Comparison of simulation results with PSP data show general agreement. Finally, we generate synthetic fluctuations constrained by the local rms turbulence amplitude given by the model, and compare properties of this synthetic turbulence with PSP observations.</div>

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Investigation on hydrodynamic forces leading to coarse particle entrainment using Large-Eddy Simulations

10.5194/egusphere-egu21-13221 ◽

2021 ◽

Author(s):

Gaston Latessa ◽

Angela Busse ◽

Manousos Valyrakis

Keyword(s):

Experimental Data ◽

Flow Field ◽

Large Scale ◽

Large Eddy Simulations ◽

Flow Conditions ◽

Mean Flow ◽

Protection Measures ◽

Practical Applications ◽

Particle Entrainment ◽

Large Eddy

The prediction of particle motion in a fluid flow environment presents several challenges from the quantification of the forces exerted by the fluid onto the solids -normally with fluctuating behaviour due to turbulence- and the definition of the potential particle entrainment from these actions. An accurate description of these phenomena has many practical applications in local scour definition and to the design of protection measures.In the present work, the actions of different flow conditions on sediment particles is investigated with the aim to translate these effects into particle entrainment identification through analytical solid dynamic equations.Large Eddy Simulations (LES) are an increasingly practical tool that provide an accurate representation of both the mean flow field and the large-scale turbulent fluctuations. For the present case, the forces exerted by the flow are integrated over the surface of a stationary particle in the streamwise (drag) and vertical (lift) directions, together with the torques around the particle&#8217;s centre of mass. These forces are validated against experimental data under the same bed and flow conditions.The forces are then compared against threshold values, obtained through theoretical equations of simple motions such as rolling without sliding. Thus, the frequency of entrainment is related to the different flow conditions in good agreement with results from experimental sediment entrainment research.A thorough monitoring of the velocity flow field on several locations is carried out to determine the relationships between velocity time series at several locations around the particle and the forces acting on its surface. These results a relevant to determine ideal locations for flow investigation both in numerical and physical experiments.Through numerical experiments, a large number of flow conditions were simulated obtaining a full set of actions over a fixed particle sitting on a smooth bed. These actions were translated into potential particle entrainment events and validated against experimental data. Future work will present the coupling of these LES models with Discrete Element Method (DEM) models to verify the entrainment phenomena entirely from a numerical perspective.

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