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2021 ◽  
Vol 933 ◽  
Author(s):  
T.J. Madison ◽  
X. Xiang ◽  
G.R. Spedding

The flow around and behind a sphere in a linear density gradient has served as a model problem for both body-generated wakes in atmospheres and oceans, and as a means of generating a patch of turbulence that then decays in a stratified ambient. Here, experiments and numerical simulations are conducted for 20 values of Reynolds number, $Re$ , and internal Froude number, $Fr$ , where each is varied independently. In all cases, the early wake is affected by the background density gradient, notably in the form of the body-generated lee waves. Mean and fluctuating quantities do not reach similar states, and their subsequent evolution would not be collapsible under any universal scaling. There are five distinguishable flow regimes, which mostly overlap with previous literature based on qualitative visualisations and, in this parameter space, they maintain their distinguishing features up to and including buoyancy times of 20. The possible relation of the low $\{Re, Fr\}$ flows to their higher $\{Re, Fr\}$ counterparts is discussed.


2021 ◽  
Vol 928 ◽  
Author(s):  
S.F. Lewin ◽  
C.P. Caulfield

We compare the properties of the turbulence induced by the breakdown of Kelvin–Helmholtz instability (KHI) at high Reynolds number in two classes of stratified shear flows where the background density profile is given by either a linear function or a hyperbolic tangent function, at different values of the minimum initial gradient Richardson number ${{Ri}}_0$ . Considering global and local measures of mixing defined in terms of either the irreversible mixing rate $\mathscr {M}$ associated with the time evolution of the background potential energy, or an appropriately defined density variance dissipation rate $\chi$ , we find that the proliferation of secondary instabilities strongly affects the efficiency of mixing early in the flow evolution, and also that these secondary instabilities are highly sensitive to flow perturbations that are added at the point of maximal (two-dimensional) billow amplitude. Nevertheless, mixing efficiency does not appear to depend strongly on the far field density structure, a feature supported by the evolution of local horizontally averaged values of the buoyancy Reynolds number ${Re}_b$ and gradient Richardson number ${Ri}_g$ . We investigate the applicability of various proposed scaling laws for flux coefficients $\varGamma$ in terms of characteristic length scales, in particular discussing the relevance of the overturning ‘Thorpe scale’ in stratified turbulent flows. Finally, we compare a variety of empirical model parameterizations used to compute diapycnal diffusivity in an oceanographic context, arguing that for transient flows such as KHI-induced turbulence, simple models that relate the ‘age’ of a turbulent event to its mixing efficiency can produce reasonably robust mixing estimates.


2021 ◽  
Vol 926 ◽  
Author(s):  
A.F. Wienkers ◽  
L.N. Thomas ◽  
J.R. Taylor

In Part 1 (Wienkers, Thomas & Taylor, J. Fluid Mech., vol. 926, 2021, A6), we described the theory for linear growth and weakly nonlinear saturation of symmetric instability (SI) in the Eady model representing a broad frontal zone. There, we found that both the fraction of the balanced thermal wind mixed down by SI and the primary source of energy are strongly dependent on the front strength, defined as the ratio of the horizontal buoyancy gradient to the square of the Coriolis frequency. Strong fronts with steep isopycnals develop a flavour of SI we call ‘slantwise inertial instability’ by extracting kinetic energy from the background flow and rapidly mixing down the thermal wind profile. In contrast, weak fronts extract more potential energy from the background density profile, which results in ‘slantwise convection.’ Here, we extend the theory from Part 1 using nonlinear numerical simulations to focus on the adjustment of the front following saturation of SI. We find that the details of adjustment and amplitude of the induced inertial oscillations depend on the front strength. While weak fronts develop narrow frontlets and excite small-amplitude vertically sheared inertial oscillations, stronger fronts generate large inertial oscillations and produce bore-like gravity currents that propagate along the top and bottom boundaries. The turbulent dissipation rate in these strong fronts is large, highly intermittent and intensifies during periods of weak stratification. We describe each of these mechanisms and energy pathways as the front evolves towards the final adjusted state, and in particular focus on the effect of varying the dimensionless front strength.


2021 ◽  
Vol 81 (9) ◽  
Author(s):  
Pratik K. Das ◽  
Sovan Sau ◽  
Abhisek Saha ◽  
Soma Sanyal

AbstractBaryon inhomogeneities are generated early in the universe. These inhomogeneities affect the phase transition dynamics of subsequent phase transitions, they also affect the nucleosynthesis calculations. We study the decay of the inhomogeneities in the early universe using the diffusion equation in the Friedmann–Lemaître–Robertson–Walker metric. We calculate the interaction cross section of the quarks with the neutrinos, the electrons and the muons and obtain the diffusion coefficients. The diffusion coefficients are temperature dependent. We find that the expansion of the universe causes the inhomogeneities to decay at a faster rate. We find that the baryon inhomogeneities generated at the electroweak epoch have low amplitudes at the time of the quark hadron transition and hence will not affect the phase transition dynamics unless they are generated with a amplitude greater than $$10^{5}$$ 10 5 times the background density. After the quark hadron transition, we include the interaction of the muons with the hadrons till 100 MeV. We find that large density inhomogeneities generated during the quark hadron transition with sizes of the order of 1 km must have amplitudes greater than $$10^{5} $$ 10 5 times the background density to survive upto the nucleosynthesis epoch. This puts constraints on any models that generate these inhomogeneities


2021 ◽  
Vol 918 ◽  
Author(s):  
Sorush Omidvar ◽  
Mohammadreza Davoodi ◽  
C. Brock Woodson

Abstract


2021 ◽  
Vol 503 (3) ◽  
pp. 4250-4263
Author(s):  
Matthew Fong ◽  
Jiaxin Han

ABSTRACT We explore the boundary of dark matter haloes through their bias and velocity profiles. Using cosmological N-body simulations, we show that the bias profile exhibits a ubiquitous trough that can be interpreted as created by halo accretion that depletes material around the boundary. The inner edge of the active depletion region is marked by the location of the maximum mass inflow rate that separates a growing halo from the draining environment. This inner depletion radius can also be interpreted as the radius enclosing a highly complete population of splashback orbits, and matches the optimal exclusion radius in a halo model of the large-scale structure. The minimum of the bias trough defines a characteristic depletion radius, which is located within the infall region bounded by the inner depletion radius and the turnaround radius, while approaching the turnaround radius in low-mass haloes that have stopped mass accretion. The characteristic depletion radius depends the most on halo mass and environment. It is approximately 2.5 times the virial radius and encloses an average density of ∼40 times the background density of the universe, independent on halo mass but dependent on other halo properties. The inner depletion radius is smaller by 10–20 per cent and encloses an average density of ∼63 times the background density. These radii open a new window for studying the properties of haloes.


2021 ◽  
Author(s):  
Gaetan Gauthier ◽  
Thomas Chust ◽  
Olivier Le Contel ◽  
Philippe Savoini

<div> <div> <div> <p>Recent MMS observations (<em>e.g.</em> [Holmes et al, 2018, Steinvall et al., 2019]) exploring various regions of the magnetosphere have found solitary potential structures call Electron phase-space Hole (EH). These structures have kinetic scale (dozens of Debye lengths) and persist during long time (dozens of plasma frequency periods). EH are characterized by a bipolar electric field parallel to ambient magnetic field and fastly propagate along this latter (a few tenths of speed light). We have created a 3D Bernstein-Greene-Kruskal (BGK) model (as [Chen et al, 2004]) adapted to various magnetospheric ambient magnetic fields. BGK model results depend on choice of potential shape and passing distribution function at infinity (before EH potential interaction).</p> <p>2D-3V Particle-In-Cell simulations have been developed with the fully kinetic code Smilei [Derouillat et al, 2017], using real magnetosphere plasma parameters. Solitary waves in the magnetotail are three-dimensional potentials which can be generated through nonlinear evolution of an electron beam instability (or bump on tail). The simulated EH are comparable to the EH observed in the magnetosphere with the same parameters.</p> <p>We have also investigated the EH formation with density inhomogeneities using a BGK stability model we have developed. Indeed, density inhomogeneities exist notably in interplanetary plasmas. As a result taking into account the background density inhomogeneities, significantly alters the stability criteria. We have performed 2D-3V PIC simulations with realistic inhomogeneous density background (smaller than 10% of mean density) to understand such a type of EH formation.</p> <p><strong>References:</strong></p> <ul><li>Holmes et al., J. Geophys. Res. Space Phys. 123, 9963, 2018</li> <li>Steinvall et al., Phys. Rev. Lett. 123, 255101, 2019</li> <li>Chen et al., Phys. Rev. E 69, 055401, 2004</li> <li>Derouillat et al., Comput. Phys. Commun. 222, 351, 2017</li> </ul><div> <div> <div> <div> <div> <div> <div> <div> <div> </div> </div> </div> </div> </div> </div> </div> </div> </div> </div> </div> </div>


2021 ◽  
Author(s):  
Rafal Sieradzki ◽  
Jacek Paziewski

<p>The circumpolar ionosphere is recognised as one of the most disturbed region of the ionized part of the atmosphere. The reasons for that are mainly dynamic conditions in the coupled system of the magnetosphere and the ionosphere as well as feeding of the polar plasma from the mid-latitude reservoir. One of the consequences of these phenomenon is the occurrence of large-scale ionospheric structures called polar patches. These are commonly defined as the enhancement of the F-region plasma characterized with a foreground-to-background density ratio larger than 2 and a size up to several hundred kilometres.</p><p>In this work we present GNSS-based characteristics of a patch occurrence in the northern hemisphere. The study covers a period of January–May 2014 corresponding to the maximum of the solar activity. The detection of structures was performed with a relative STEC value that is defined as a difference between epoch-wise L4 data and 4<sup>th</sup> order polynomial corresponding to background variations of the ionosphere. In order to ensure a continuous monitoring of the ionosphere over the north pole, we used data from ~45 permanent stations. The results prove that ground-based GNSS data can be successfully used in the climatological investigations of polar patches. We found a strong seasonal effect in the occurrence of these structures with the maximum at the turn of February and March and the minimum in May. Such outcomes correspond to variations of a TEC gradient between subauroral and polar regions. This parameter seems to be also responsible for a subdaily pattern of patches observed for particular months. The comparison of GNSS-based results with in-situ SWARM data revealed some differences, which are probably related to different characteristics of the ionosphere provided by both techniques. Furthermore, the study confirms that most of the patches are observed for the negative values of IMF Bz,  whereas IMF By component has no significant impact on the number of analysed structures. </p>


Author(s):  
Joseph A. Veech

There are many different design and statistical issues that a researcher should consider when developing the data collection protocol or when interpreting results from a habitat analysis. One of the first considerations is simply the area to include in the study. This depends on the behavior (particularly mobility) of the focal species and logistical constraints. The amount of area also relates to the number of survey locations (plots, transects, or other) and their spatial placement. Survey data often include many instances of a species absent from a spatial sampling unit. These could be true absences or might represent very low species detection probability. There are different statistical techniques for estimating detection probability as well as analyzing data with a substantial proportion of zero-abundance values. The spatial dispersion of the species within the overall study area or region is never random. Even apart from the effect of habitat, individuals are often aggregated due to various environmental factors or species traits. This can affect count data collected from survey plots. Related to spatial dispersion, the overall background density of the species within the study area can introduce particular challenges in identifying meaningful habitat associations. Statistical issues such as normality, multicollinearity, spatial and temporal autocorrelation may be relatively common and need to be addressed prior to an analysis. None of these design and statistical issues presents insurmountable challenges to a habitat analysis.


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