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Laboratory experiments and direct numerical simulations are employed to investigate lock-exchange gravity currents propagating over close-packed, fixed porous beds of monodisperse spherical particles, and to quantify the mass and momentum transfer between the currents and the bed. The simulations show that the mass exchange of the current with the bed involves two separate steps that operate on different time scales. In a first step, the dense current front rapidly sweeps away the resident fluid in the exposed pore spaces between the top layer of spheres, while in a second step, a buoyancy-driven vertical exchange flow between the current and the deeper pores is set up that takes significantly longer to develop. This process depends on the permeability of the bed, which in turn is a function of the particle diameter. The momentum exchange between the current and the bed strongly depends on the ratio of the particle size to the viscous sublayer of the current. The bottom friction is moderate when the particle size is smaller than or comparable to the thickness of the viscous sublayer, but it jumps for particles that strongly protrude from the sublayer, leading to a more rapid deceleration of the flow.
When the density of a gravitationally stable fluid depends on a fast diffusing scalar and a slowly diffusing scalar of opposite contribution to the stability, ‘double diffusive’ instabilities may develop and drive convection. When the slow diffuser settles under gravity, as is for instance the case for small sediment particles in water, settling-driven double-diffusive instabilities can additionally occur. Such instabilities are relevant in a variety of naturally occurring settings, such as particle-laden river discharges, or underground inflows in lakes. Inspired by the dynamics of the more traditional thermohaline double-diffusive instabilities, we ask whether large-scale ‘mean-field’ instabilities can develop as a result of sedimentary double-diffusive convection. We first apply the mean-field instability theory of Traxler et al. (J. Fluid Mech., vol. 677, 2011, pp. 530–553) to high-Prandtl-number fluids, and find that these are unstable to Radko's layering instability, yet collectively stable. We then extend the theory of Traxler et al. (2011) to include settling and study its impact on the development of the collective instability. We find that two distinct regimes exist. At low settling velocities, the double-diffusive turbulence in the fingering regime is relatively unaffected by settling and remains stable to the classical collective instability. It is, however, unstable to a new instability in which large-scale gravity waves are excited by the phase shift between the salinity and particle concentration fields. At higher settling velocities, the double-diffusive turbulence is substantially affected by settling, and becomes unstable to the classic collective instability. Our findings, validated by direct numerical simulations, reveal new opportunities to observe settling-driven layering in laboratory and field experiments.
We show how settling and phase change can combine to drive an instability, as a simple model for the formation of mammatus clouds. Our idealised system consists of a layer (an ‘anvil’) of air mixed with saturated water vapour and monodisperse water droplets, sitting atop dry air. The water droplets in the anvil settle under gravity due to their finite size, evaporating as they enter dry air and cooling the layer of air just below the anvil. The colder air just below the anvil thus becomes denser than the dry air below it, forming a density ‘overhang’, which is unstable. The strength of the instability depends on the density difference between the density overhang and the dry ambient, and the depth of the overhang. Using linear stability analysis and nonlinear simulations in one, two and three dimensions, we study how the amplitude and depth of the density layer depend on the initial conditions, finding that their variations can be explained in terms only of the size of the droplets making up the liquid content of the anvil and by the total amount of liquid water contained in the anvil. We find that the size of the water droplets is the controlling factor in the structure of the clouds: mammatus-like lobes form for large droplet sizes; and small droplet sizes lead to a ‘leaky’ instability resulting in a stringy cloud structure resembling the newly designated asperitas.
A series of well-preserved specimens of the little-known Palaeogene species Biselenaria placentula (Reuss, 1867) warrant the designation of a new family of free-living cheilostome bryozoans, Biselenariidae n. fam. In contrast to the structural organization of all other free-living, lunulitiform cheilostomes, all zooids are enclosed almost completely by cuticular, exterior walls. Yet, like other lunulitiforms, the colony must be regarded as a highly integrated functional unit, as revealed through the unique, highly complex interior architecture. In creating the new monotypic family, the type species Biselenaria placentula is described in detail, while the ambiguous taxonomic status of the only other taxon, B. offa Gregory, 1893, is discussed.
– Earlier studies on the influence of pregnancy and postpartum period on the course of panic disorder have been inconsistent. The present study aims to quantify panic manifestations in these periods in large sample of women.
– Panic manifestations, including exacerbations and new manifestations of panic disorder, were assessed retrospectively in a sample of 128 women with panic disorder with or without agoraphobia, 93 of whom had had 195 pregnancies.
– Panic manifestations were fewer during pregnancy and more frequent in the postpartum period when compared with the control period. Women who had never been pregnant had significantly more panic manifestations than women with prior pregnancies. Breastfeeding and miscarriages did not have a significant effect. Women with postpartum panic reported more psychosocial stress events during this period.
– Possible reasons for postpartum panic and the protective effects of pregnancy are discussed, including psychosocial or hormonal factors and other neurobiological changes. Postpartum panic coincides with a sudden drop of hormones after delivery.
Applying broad notions of adaptation, this chapter seeks to bring “recombinant adaptation” – mashups and remixes on digital platforms – in dialogue with Gerard Genette's idea of the paratext as a text's “relations with the public.” It takes four steps towards investigating how literary publishing houses such as Quirk Books respond to recombinant adaptation. Firstly, it delineates the paratexts of mashup novels as performative zones of transaction. Secondly, it examines the question of how paratexts regulate the quasi-religious textuality of fandom participation. Thirdly, it looks at the role of paratextual canonization within this textuality. And finally, it argues that printed products within the field attempt to perform a nostalgic authorization and re-materialization of literature, highlighting the haptic and material qualities of the book. Adapting the term “polytext,” the chapter calls these multifarious paratextual transactions the “polyprocess.”
This essay seeks to bring the field of “recombinant adaptation” – mashups and remixes on digital platforms – in dialogue with the Genettian idea of the paratext. Genette held that paratexts shape a given text's “relations with the public.” More recently, Jonathan Gray has applied the notion of paratext to media franchises, highlighting the active role of paratexts in creating and continuing franchise texts. Dorothee Birke and Birte Christ have elaborated Genette's ideas for a situation of convergence culture and transmedia storytelling, examining how paratexts fulfill interpretive, commercial, or navigational functions in determining contemporary readers’ transmedia experience of narratives.
This chapter takes four steps towards investigating how literary publishing houses respond to the ubiquitous remixes and mashups to be found on lowthreshold digital platforms of participation. It will, first, delineate paratexts as zones of transaction, shifting research emphases from textual towards performative concerns and highlighting the way cultures negotiate textual distribution and circulation. Secondly, it will examine the question of how paratexts regulate the quasi-religious textuality of fandom participation; thirdly, the role of paratextual canonization will be a special focus within this textuality. Finally, the chapter argues that printed products within the field attempt to perform a nostalgic authorization and re-materialization of literature, highlighting the haptic and material qualities of “bookishness.”
Ibn Raḥīq is an 11th century scholar who compiled a book on popular astronomy. This work included a section in which he summarizes basic knowledge of the Milky Way as it was wide spread in the first centuries after the hejira. Ibn Raḥīq gives a comprehensive overview of the perception of the Milky Way that reaches from its use as a test for knowledge of the religious tradition and for agricultural purposes on the one hand to an exact astronomical description of its shape in the sky during the year and to theories of its nature and composition on the other hand. We use a comparison of his text to those of Ibn al-Haytham and others to investigate the role the Milky Way played in early Islamic civilization from its beginning until the 15th century.
We investigate the interaction of a downslope gravity current with an internal wave propagating along a two-layer density jump. Direct numerical simulations confirm earlier experimental findings of a reduced gravity current mass flux, as well as the partial removal of the gravity current head from its body by large-amplitude waves (Hogg et al., Environ. Fluid Mech., vol. 18 (2), 2018, pp. 383–394). The current is observed to split into an intrusion of diluted fluid that propagates along the interface and a hyperpycnal current that continues to move downslope. The simulations provide detailed quantitative information on the energy budget components and the mixing dynamics of the current–wave interaction, which demonstrates the existence of two distinct parameter regimes. Small-amplitude waves affect the current in a largely transient fashion, so that the post-interaction properties of the current approach those in the absence of a wave. Large-amplitude waves, on the other hand, perform a sufficiently large amount of work on the gravity current fluid so as to modify its properties over the long term. The ‘decapitation’ of the current by large waves, along with the associated formation of an upslope current, enhance both viscous dissipation and irreversible mixing, thereby strongly reducing the available potential energy of the flow.
As a historical model of how to end an extended period of international conflict and to establish a stable and peaceful international order, the Vienna Congress has claimed the attention of academics and politicians ever since 1815. Against this background the chapter will deal with the question of how the Congress of Vienna and the Vienna system were regarded by various actors and under changing political circumstances. Rather than merely collecting views and interpretations of the Congress and the international system taking shape in 1814/15, the chapter will ask how the varying interpretations of Vienna and the Vienna system reflected changing ideas and visions of international order and what they can tell us about national and international security cultures in the nineteenth and twentieth centuries.
Miscible liquids often come into contact with one another in natural and technological situations, commonly as a drop of one liquid present in a second, miscible liquid. The shape of a liquid droplet present in a miscible environment evolves spontaneously in time, in a distinctly different fashion than drops present in immiscible environments, which have been reported previously. We consider drops of two classical types, pendant and sessile, in building upon our prior work with miscible systems. Here we present experimental findings of the shape evolution of pendant drops along with an expanded study of the spreading of sessile drops in miscible environments. We develop scalings considering the diffusion of mass to group volumetric data of the evolving pendant drops and the diffusion of momentum to group leading-edge radial data of the spreading sessile drops. These treatments are effective in obtaining single responses for the measurements of each type of droplet, where the volume of a pendant drop diminishes exponentially in time and the leading-edge radius of a sessile drop grows following a power law of
at long times. A complementary numerical approach to compute the concentration and velocity fields of these systems using a simplified set of governing equations is paired with our experimental findings.