Cohesive particulate flows play an important role in environmental fluid dynamics, as well as in a wide variety of civil and process engineering applications. However, the scaling laws, constitutive equations and continuum field descriptions governing such flows are currently less well understood than for their non-cohesive counterparts. Grain-resolved simulations represent an essential tool for addressing this shortcoming, along with theoretical investigations, laboratory experiments and field studies. Here we provide a tutorial introduction to simulations of fine-grained sediments in viscous fluids, along with an overview of some representative insights that this approach has yielded to date. After a brief review of the key physical concepts governing van der Waals forces as the main cohesive effect for subaqueous sediment suspensions, we discuss their incorporation into particle-resolved simulations based on the immersed boundary method. We subsequently describe simulations of cohesive particles in several model turbulent flows, which demonstrate the emergence of a statistical equilibrium between the growth and break-up of aggregates. As a next step, we review the influence of cohesive forces on the settling behaviour of dense suspensions, before moving on to submerged granular collapses. Throughout the article, we highlight open research questions in the field of cohesive particulate flows whose investigation may benefit from grain-resolved simulations.

The history of modern Iraq has been marked by violence, oppression, and foreign interventions to a degree that stands out even among other war-torn countries. On the occasion of the 20th anniversary of the US-led invasion of Iraq, many retrospectives were still dominated by a US-centric navel gazing of the chattering classes inside the beltway, but more Iraqi voices and alternative viewpoints were present in op-eds and articles than a decade earlier. In this spirit this roundtable section reflects on recent Iraqi history and contemporary developments with an eye toward memory politics in the context of transforming governance mechanisms and evolving civil society actors. It builds on a conference held at the German Institute for Global and Area Studies (GIGA) in Hamburg in March 2023 and portrays emerging avenues for research as well as new perspectives on long running debates.

We conduct three-dimensional direct numerical simulations to investigate the mixing, entrainment and energy budgets of gravity currents emerging from two-layer stratified locks. Depending on the density and layer thickness ratios, we find that either the upper layer or lower layer fluid can propagate faster, and that the density structure of the overall gravity current can range from strongly stratified to near-complete mixing. We furthermore observe that intermediate values of the density ratio can maximise mixing between the gravity current layers. Based on the vorticity budget, we propose a theoretical model for predicting the overall gravity current height, along with the front velocity of the two layers, for situations in which the lower layer moves faster than the upper layer. The model identifies the role of the height and thickness ratios in determining the velocity structure of the current, and it clarifies the dynamics of the ambient counter-current. A detailed analysis of the energy budget quantifies the conversion of potential into kinetic energy as a function of the governing parameters, along with the energy transfer between the different layers of the gravity current and the ambient fluid. Depending on the values of the density and layer thickness ratios, we find that the lower lock layer can gain or lose energy, whereas the upper layer always loses energy.

We investigate the submerged collapse of weakly polydisperse, loosely packed cohesive granular columns, as a function of aspect ratio and cohesive force strength, via grain-resolving direct numerical simulations. The cohesive forces act to prevent the detachment of individual particles from the main body of the collapsing column, reduce its front velocity, and yield a shorter and thicker final deposit. All of these effects can be captured accurately across a broad range of parameters by piecewise power-law relationships. The cohesive forces reduce significantly the amount of available potential energy released by the particles. For shallow columns, the particle and fluid kinetic energy decreases for stronger cohesion. For tall columns, on the other hand, moderate cohesive forces increase the maximum particle kinetic energy, since they accelerate the initial free-fall of the upper column section. Only for larger cohesive forces does the peak kinetic energy of the particles decrease. Computational particle tracking indicates that the cohesive forces reduce the mixing of particles within the collapsing column, and it identifies the regions of origin of those particles that travel the farthest. The simulations demonstrate that cohesion promotes aggregation and the formation of aggregates. Furthermore, they provide complete information on the temporally and spatially evolving network of cohesive and direct contact force bonds. While the normal contact forces are aligned primarily in the vertical direction, the cohesive bonds adjust their preferred spatial orientation throughout the collapse. They result in a net macroscopic stress that counteracts deformation and slows the spreading of the advancing particle front.

Introduction

The reforms initiated by Reza Shah's urban modernization drive led to changes not only in cultural patterns of urban life in Iran and the economic structure of the country, but even in spatial organization. The full effect of these initiatives only started to be felt under Mohammad Reza Shah, who continued to pursue the policy of his predecessor. For our specific purpose—urban change under Reza Shah—we will focus on an analysis of the development of the Iranian city, specifically the following three aspects: spatial reorganization; economic changes; and consequences of urban change.

Our basic assumption is that the city, unlike any other phenomenon, became the symbol of political and socioeconomic transformation in Iran and that, to this day, it has preserved this character of Pahlavi modernization and change like no other institution or phenomenon.

By Definition, an Encyclopaedia is “A Work that Aims at Giving a comprehensive summary of all branches of knowledge… Encyclopaedias usually consist of articles on separate subjects arranged in dictionary or alphabetical order to facilitate use… .” A more differentiated view is given by the New Encyclopaedia Britannica, which describes encyclopaedias as follows: “Today most people think of an encyclopaedia as a multivolume compendium of all available knowledge, complete with maps and a detailed index, as well as numerous adjuncts such as bibliographies, illustrations, list of abbreviations and foreign expressions, gazetteers, and so on…”

Unlike the Encyclopaedia Americana or the Encyclopaedia Britannica, whose primary goals have been and still are to serve as comprehensive and always up-to-date works of reference of general knowledge, the Encyclopaedia Iranica would fall under the category of specialized encyclopaedias “that have deliberately been planned for a special purpose” or even better: encyclopaedias of countries and regions “dealing with a single country and region.”

Emerging technologies such as deep-sea mining and geoengineering pose fundamentally new questions regarding the dynamics of gravity currents. Such activities can continuously release dense sediment plumes from moving locations, thereafter propagating as gravity currents. Here, we present the results of idealized numerical simulations of this novel configuration, and investigate the propagation of a gravity current that results from a moving source of buoyancy, as a function of the ratio of source speed to buoyancy velocity. We show that above a certain value of this ratio, the flow enters a supercritical regime in which the source moves more rapidly than the generated current, resulting in a statistically steady state in the reference frame of the moving source. Once in the supercritical regime, the current goes through a second transition beyond which fluid in the head of the current moves approximately in the direction normal to the direction of motion of the source, and the time evolution of the front in the lateral direction is well described by an equivalent constant volume lock-release gravity current. We use our findings to gain insight into the propagation of sediment plumes released by deep-sea mining collector vehicles, and present proof-of-concept tow-tank laboratory experiments of a model deep-sea mining collector discharging dense dyed fluid in its wake. The experiments reveal the formation a wedge-shaped gravity current front which narrows as the ratio of collector-to-buoyancy velocity increases. The time-averaged front position shows good agreement with the results of the numerical model in the supercritical regime.

We investigate the removal of a dense bottom layer by a gravity current, via Navier–Stokes simulations in the Boussinesq limit. The problem is governed by a dimensionless thickness parameter for the bottom layer, and by the ratio of the density differences between bottom layer, gravity current and ambient fluids. A quasisteady gravity current forms that propagates along the interface and displaces some of the dense bottom fluid, which accumulates ahead of the gravity current and forms an undular bore or a series of internal gravity waves. Depending on the ratio of the gravity current front velocity to the linear shallow-water wave velocity, we observe the existence of different regimes, characterized by small-amplitude waves or by a train of steep, nonlinear internal waves. We develop a semiempirical model that provides reasonable estimates of several important flow properties. We also formulate a more sophisticated, self-contained model based on the conservation principles for mass and vorticity that does not require empirical closure assumptions. This model is able to predict such quantities as the gravity current height and the internal wave or bore velocity as a function of the governing dimensionless parameters, generally to within approximately a 10 $\%$ accuracy. An energy budget analysis provides information on the rates at which potential energy is converted into kinetic energy and then dissipated, and on the processes by which energy is transferred from the gravity current fluid to the dense and ambient fluids. We observe that the energy content of thicker and denser bottom layers grows more rapidly.

We study to what extent the Milky Way was used as an orientation tool at the beginning of the Islamic period covering the 8th to the 15th century, with a focus on the first half of that era. We compare the texts of three authors from three different periods and give detailed comments on their astronomical and traditional content. The text of al-Marzūqī summarises the information on the Milky Way put forward by the astronomer and geographer ʾAbū Ḥanīfa al-Dīnawarī. The text makes it clear that in some areas the Milky Way could be used as a geographical guide to determine the approximate direction toward a region on Earth or the direction of prayer. In the 15th century, the famous navigator Aḥmad b. Māǧid describes the Milky Way in his nautical instructions. He frequently demonstrates that the Milky Way serves as a guidance aid to find constellations and stars that are useful for precise navigation on land and at sea. On the other hand, Ibn Qutayba quotes in his description of the Milky Way a saying from the famous Bedouin poet Ḏū al-Rumma, which is also mentioned by al-Marzūqī. In this saying the Milky Way is used to indicate the hot summer times in which travelling the desert was particularly difficult. Hence, the Milky Way was useful for orientation in space and time and was used for agricultural and navigational purposes.

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.

UUID: http://zoobank.org/6ad8a795-c45e-4619-b2bb-46916585fbd9

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.