Numerical Particles Research Articles

Mechanisms underlying the regional morphological differences between the northern and southern radial sand ridges along the Jiangsu Coast, China

Radial sand ridges (RSRs) spread in a fan-shaped pattern over the seabed of the southern Yellow Sea along the Jiangsu Coast (China) with pronounced differences between the northern and southern channel-shoals (indicated as “NCS” and “SCS”, respectively). A depth-averaged nested numerical model is employed to analyze the tidal hydrodynamics and sediment dynamics of the RSRs. Model results show considerable regional differences in terms of tidal current vector ellipses: “eight-shaped” ellipses are typical in the NCS where rotational tidal waves dominate, while the SCS are characterized by “egg-shaped” ellipses because of the dominance of progressive tidal waves. Sand ridges tend to be elongated and straight in the NCS while short and dincontinuous in the SCS. Numerical Lagrange particles released near the ridges in the NCS tend to travel across the crest of the ridges rather than move around them as observed in the SCS. Consistently, the patterns of sediment convergence and divergence indicate that suspended sediments converge over the whole Xiaoyinsha Ridge located in the NCS, while converge to the two sides of the Hetunsha Ridge located in the SCS. Model results also highlight the importance of initial sediment compositions in determining patterns of the net sediment fluxes. The suspended sediment concentration (SSC) time series during one tidal cycle may show single or double peaks depending on the grain size and the related lag effect. Bed load transport is much smaller than suspended load. So sediment compositions should be carefully considered when simulating the SSC field, and the hydrodynamic of the morphological evolution of the RSRs at the regional scale. Overall, this study suggests that the NCS and SCS may be treated as two relatively independent geomorphological systems characterized by different tidal flows and sediment compositions.

Comparing isopycnal eddy diffusivities in the Southern Ocean with predictions from linear theory

We show that the potential vorticity diffusivity predicted by linear stability analysis (LSA), is the same as a linearized version of Lagrangian cross-stream isopycnal diffusivity. Both can be written in terms of the same expression − the product of the eddy kinetic energy (EKE) and the integral time scale that involves the Lagrangian decay scale γ or the growth rate ωi of the most unstable wave, and a frequency that is related to the difference of the mean flow speed and real part of the phase speed of the unstable waves.Diffusivities from LSA are compared to Lagrangian isopycnal eddy diffusivities estimated from more than 700,000 numerical particles in the Southern Ocean of an eddying model. They show different spatial dependency. LSA predicts eddy diffusivities that are enhanced at the steering level where the mean flow speed equals the phase speed of the unstable waves. In contrast, Lagrangian diffusivities exhibit no clear steering level maxima, but are instead surface intensified in many places. The differences between the Lagrangian and diffusivities from LSA can be understood because EKE predicted from LSA differs from the simulated one, and because the estimated decay scale γ is on average about 4 times larger than the largest linear growth rate. The diagnosed Lagrangian integral time scale has maxima at the depth where the mean flow speed equals the phase speed of the most unstable wave, but the diffusivity maxima are shifted towards the surface because the simulated EKE decreases rapidly with depth. Possibilities for a simple parameterization for the diffusivity are discussed.

Aggregate representation for mesostructure of stone based materials using a sphere growth model based on realistic aggregate shapes

The objectives of this study are: (1) to create aggregate representations based on their realistic shapes and establish an aggregate shape library by storing aggregate numerical representations of various shapes and angularities; and (2) to prepare mesostructure models for stone based materials using aggregate representations from the library. The numerical representations based on realistic shapes are created through three steps: (1) scan individual aggregate particles using an X-ray CT machine to obtain a series of cross sectional images at different heights; (2) process the sectional images and stack them to obtain the 3D aggregate boundary; and (3) apply a sphere growth model to fill the interior space of the boundary. The sphere growth includes a first growth in 26 divergent directions and a second growth in 8 divergent directions which start from each sphere in the first growth. Four aggregate particles were selected for the numerical representation construction using the sphere growth model. The modeling results showed that the four aggregates had filling ratios of 97.8, 95.3, 91.3 and 98.5 %, respectively. Afterward, the four numerical representations were stored as aggregate templates to establish a preliminary representation library, which can be invoked to generate numerical aggregate particles in the model reconstruction of stone based materials. A discrete element model and a finite element model of an asphalt concrete beam were reconstructed as examples of application of the numerical representation library. The study overall showed that: (1) the sphere growth model is an effective approach to construct aggregate numerical representations with a high accuracy; (2) the sphere growth model can significantly reduce the sphere amount of the numerical representations compared to previous related studies; and (3) these numerical representations can be successfully used to reconstruct the mesostructure models of stone based materials, which can potentially bring time and cost savings while keeping the model precision.

Modelling Infectious Hematopoietic Necrosis Virus Dispersion from Marine Salmon Farms in the Discovery Islands, British Columbia, Canada.

Finite volume ocean circulation and particle tracking models are used to simulate water-borne transmission of infectious hematopoietic necrosis virus (IHNV) among Atlantic salmon (Salmo salar) farms in the Discovery Islands region of British Columbia, Canada. Historical simulations for April and July 2010 are carried out to demonstrate the seasonal impact of river discharge, wind, ultra-violet (UV) radiation, and heat flux conditions on near-surface currents, viral dispersion and survival. Numerical particles released from infected farm fish in accordance with IHNV shedding rates estimated through laboratory experiments are dispersed by model oceanic flows. Viral particles are inactivated by ambient UV radiation levels and by the natural microbial community at rates derived through laboratory studies. Viral concentration maps showing temporal and spatial changes are produced and combined with lab-determined minimum infectious dosages to estimate the infective connectivity among farms. Results demonstrate that neighbouring naïve farms can become exposed to IHNV via water-borne transport from an IHNV diseased farm, with a higher risk in April than July, and that many events in the sequence of farm outbreaks in 2001-2002 are consistent with higher risks in our farm connectivity matrix. Applications to other diseases, transfers between farmed and wild fish, and the effect of vaccinations are also discussed.

Dust-enshrouded star near supermassive black hole: predictions for high-eccentricity passages near low-luminosity galactic nuclei

Supermassive black holes reside in cores of galaxies, where they are often surrounded by a nuclear cluster and a clumpy torus of gas and dust. Mutual interactions can set some stars on a plunging trajectory towards the black hole. We model the pericentre passage of a dust-enshrouded star during which the dusty envelope becomes stretched by tidal forces and is affected by the interaction with the surrounding medium. In particular, we explore under which conditions these encounters can lead to periods of enhanced accretion activity. We discuss different scenarios for such a dusty source. To this end, we employed a modification of the Swift integration package. Elements of the cloud were modelled as numerical particles that represent the dust component that interacts with the optically thin gaseous environment. We determine the fraction of the total mass of the dust component that is diverted from the original path during the passages through the pericentre at $\sim 10^3$ Schwarzschild radii and find that the main part of the dust ($\gtrsim 90\%$ of its mass) is significantly affected upon the first crossing. The fraction of mass captured at the second passage generally decreases to very low values. As an example, we show predictions for the dusty source evolution assuming the current orbital parameters of the G2 cloud (also known as Dusty S-Cluster Object, DSO) in our Galactic centre. Encounter of a core-less cloud with a supermassive black hole is, most likely, a non-repeating event: the cloud is destroyed. However, in the case of a dust-enshrouded star, part of the envelope survives the pericentre passage. We discuss an offset of $\lesssim 0.3$ arcsec between the centre of mass of the diverted part and the star along the eccentric orbit. Finally, we examine an interesting possibility of a binary star embedded within a common wind envelope that becomes dispersed at the pericentre passage.

Surface water renewal and mixing mechanisms in a semi-enclosed microtidal domain. The Barcelona harbour case

Water renewal and mixing are highly related to the evolution of the ecological status in harbour areas and are crucial for environmental harbour management. For the first time, these processes have been studied in the Barcelona harbour with a high resolution 3D hydrodynamic model. This harbour is representative of a semi-enclosed domain with a complex coastline in a micro-tidal environment (like most Mediterranean harbours). The tracking of numerical Lagrangian particles deployed in the flow was used to parameterize the surface water renewal mechanisms. The use of Finite Size Lyapunov Exponents has proven to be a relatively easy and efficient way to characterize the mixing patterns. The complex geometry of the harbour and the meteo-oceanographic forcings lead to intricate hydrodynamics that define spatial heterogeneity of water renewal and mixing. The most favourable conditions for enhancing surface water renewal and mixing have been identified in idealized scenarios and realistic simulations. In general, renewal is enhanced near the harbour mouths and strongly limited in the inner areas. However, under particular forcing conditions, the residence time can decrease even in the most sheltered areas. The presence of two mouths in the harbour seems to favour water renewal in comparison to harbours with only one mouth. Mixing is mainly induced by the action of wind forcing, while external shelf currents are much less efficient. The study of two realistic simulations suggests that harbour response to real forcing cannot be deduced from the combination of simple idealized scenarios. The time evolution of the forcings and the transient response of the system play a key role in defining the residence time patterns.

Transport and retention of vertically migrating adult mysid and decapod shrimp in the tidal front on Georges Bank

Vertical profiles of the adult epibenthic shrimp Neomysis americana and Crangon septemspinosus obtained during June 1985 were used to simulate possible rates of ascent from bottom (40 to 50 m) to near surface at night and return by day, and the consequence of these rates on their horizontal distribution. Numerical particles were released at the sampling site using archived model current fields with specified vertical rates (from no swim behavior to 20 mm s −1 ) and tracked for up to 30 d. The best match between observed and modeled vertical profiles was with a vertical swimming speed of 10 mm s −1 for N. americana and 2 mm s −1 for C. septemspinosus. Whereas N. americana rapidly swims towards the surface at dusk and descends to bottom by dawn, C. septemspinosus tends to only swim up to the middle of the water column at night. After 16 d, the simulation with 10 mm s −1 swim speed showed most particles were concentrated in an area centered around the 60 m isobath, where the tidal front was located. At 2 mm s −1 swim speed particles were concentrated more shoalward onto the western end of Georges Bank. N. americana are expected to be more closely associated with the tidal front, since they spend more time near the front surface convergence, but are more likely to be transported off the bank due to the south- westward-flowing surface tidal jet, whereas C. septemspinosus would be retained primarily on the bank, since they are found deeper in the water column during both day and night.

Transit and residence times in the Adriatic Sea surface as derived from drifter data and Lagrangian numerical simulations

Abstract. Statistics of transit and residence times in the Adriatic Sea surface, a semi-enclosed basin of the Mediterranean, are estimated from drifter data and Lagrangian numerical simulations. The results obtained from the drifters are generally underestimated given their short operating lifetimes (half life of ∼40 days) compared to the transit and residence times. This bias can be removed by considering a large amount of numerical particles whose trajectories are integrated over a long time (750 days) with a statistical advection–dispersion model of the Adriatic surface circulation. Numerical particles indicate that the maximum transit time to exit the basin is about 216–260 days for particles released near the northern tip of the Adriatic, and that a particle entering on the eastern Otranto Channel will typically exit on the other side of the channel after 170–185 days. A duration of 150–168 days is estimated as the residence time in the Adriatic Basin.

Fluid moments and spectral diagnostics in global particle-in-cell simulations

The convergence properties of global electrostatic particle-in-cell gyrokinetic simulations have been studied by means of a set of new 3D diagnostics recently implemented in the code ORB5, allowing for measurements of electromagnetic potentials and relevant fluid quantities (density, temperature, vorticity) as well as turbulence spectral analysis. For all the quantities considered, non converged simulations show flatter spectra. However, not all the physically relevant quantities converge with the same rate: density fluctuation spectra can still show an unphysical flattening of the spectrum at high kθ ρs, even when electrostatic potential and heat fluxes appear to be converged. A remarkable results is that resolution in terms of number of markers, required to correctly describe the turbulence density fluctuations, is found to be size-dependent: larger systems (ITER like) require less numerical particles per active mode than smaller systems.

Dust grain charging in a wake of other grains

The charging of dust grain in the wake of another grains in sonic and supersonic collisionless plasma flows is studied by numerical simulations. We consider two grains aligned with the flow, as well as dust chains and multiple grain arrangements. It is found that the dust charge depends significantly on the flow speed, distance between the grains, and the grain arrangement. For two and three grains aligned, the charges on downstream grains depend linearly on the flow velocity and intergrain distance. The simulations are carried out with DiP3D, a three dimensional particle-in-cell code with both electrons and ions represented as numerical particles [W. J. Miloch et al., Phys. Plasmas 17, 103703 (2010)].

Inclusion of collisional return-to-isotropy in the MP-PIC method

We document a new method for including collisional return-to-isotropy in particle velocity distributions calculated by the MP-PIC method for numerical simulation of particle/fluid flows. Mathematically, we include the new method by adding to the transport equation for the particle distribution function (PDF), a Bhatnager, Gross, and Krook (BGK) collision term that causes velocity distributions to relax to isotropic Gaussian distributions. The numerical implementation is by a splitting technique in which we randomly sample from velocity distributions obtained as solutions to the PDF transport equation with just the return-to-isotropy source. Thus, collisions cause numerical particles to scatter in MP-PIC calculations, just as physical particles do in particle/fluid flows. The method is implemented in the Barracuda© code, and two computational examples verify proper implementation of the method and show that more realistic results are obtained in calculations of impinging particle jets. In an appendix, we derive the particle continuum-flow (PCF) equations implied by the MP-PIC method in the collision dominated limit, and we obtain the collisional relaxation time for return-to-isotropy by matching the shear viscosity of the MP-PIC PCF equations, and the kinetic part of the shear viscosity used in PCF equations in the literature.

Sensitivity of advective transfer times across the North Atlantic Ocean to the temporal and spatial resolution of model velocity data: Implication for European eel larval transport

European eel (Anguilla anguilla) larvae achieve one of the longest larval migrations of the marine realm, i.e., more than 6000km from their spawning grounds in the Sargasso Sea to European continental shelves. The duration of this migration remains debated, between 7 months and 3 years. This information is, however, crucial since it determines the period over which larvae are affected by environmental conditions and hence the subsequent recruitment success.We investigate the pathways and duration of trans-Atlantic connections using 3 years of high-resolution (daily, 1/12°) velocity fields available from a Mercator-Océan model configuration without data assimilation. We study specifically the effect of spatial and temporal resolutions on our estimates by applying various filters in time (from daily to 12-day averages) and space (from 1/12° to 1° gridcell aggregation) to the nominal model outputs. Numerical particles are released in the presumed European eel spawning area and considered as passive tracers at three specific depths (around 0, 50, and 200m).We diagnose particularly the intensity of the water transfer between suitable control sections that encompass the eel larva distribution. Transit ages are also investigated, with a particular focus on the pathways that minimize the connection times between the western and eastern North Atlantic. We show that small-scale structures (eddies and filaments) contribute to faster connections though they also correspond to additional complexity in trajectories. The shortest pathways mostly follow the Gulf Stream and the North Atlantic Drift, whereas interior connections require longer transfers that prove less compatible with biological observations.

Comment on “Particle dispersal due to interplay of motions in the surface layer of a small reservoir” (by P. Okely, J. Imberger, and K. Shimizu) and “Processes affecting horizontal mixing and dispersion in Winam Gulf, Lake Victoria” (by P. Okely, J. Imberger, and J. P. Antenucci)

Okely et al. (2010a,b) are concerned with horizontal dispersion and mixing in lakes and employ numerical modeling to estimate horizontal dispersion coefficients. In both papers, the dispersal of four numerical Lagrangian particles is employed to estimate dispersion coefficients, and the authors explicitly claim that ‘‘...this method measures horizontal dispersion due to large-scale horizontal shear only, and the influence of other processes, such as turbulent diffusion, is not accounted for’’ (Okely et al. 2010b, p. 591). However, we demonstrate here that without a diffusive process, e.g., molecular or turbulent diffusion, the dispersal rate estimated from Lagrangian particles as in Okely et al. (2010a,b) should be zero in principal for largescale nondivergent horizontal flow fields, independent of the horizontal shear. The particle dispersal presented by Okely et al. (2010a,b) is not a measure of horizontal dispersion due to horizontal shear but depends on the initial position of the particles and the divergence of the simulated mean flow field and may be affected by undersampling of the flow field, since only four particles were considered. Whereas the study of Okely et al. (2010b) was entirely based on this Lagrangian particle technique, Okely et al. (2010a) compared horizontal dispersion coefficients estimated from this technique with coefficients determined from the spread of numerically simulated tracer distributions. According to Okely et al. (2010a), their horizontal dispersion coefficients agree reasonably well with an estimate based on the assumption that the effects of vertical shear and vertical turbulent mixing (Kz) determine horizontal dispersion (see Table 1 and Eq. 11 in Okely et al. 2010a). Okely et al. (2010a, p. 1875) state that ‘‘The vertical shear dispersion scaling was a good approximation for the magnitude and spatial variation of the average horizontal dispersion rates.’’ However, the horizontal dispersion coefficients obtained from Eq. 11 using the observed vertical diffusivities are an order of magnitude smaller than the values presented in Table 1 of Okely et al. (2010a) and thus are substantially smaller than the horizontal dispersion coefficients obtained from the tracer simulations. Hence, the combined effect of vertical shear and vertical diffusion cannot explain the large horizontal dispersion in the simulation of Okely et al. (2010a). Finally, the Lagrangian particle technique and also the dispersion of the simulated tracer clouds appear to depend mainly on the horizontal divergence of the simulated flow field. Comparison of simulated and observed particle tracks and horizontal currents at a single location indicates that the quality of the simulated flow field is not sufficient to provide the divergence of the true flow field in the lakes studied. Hence, Okely et al. (2010a,b) may provide estimates of the spread of particles and tracers in their simulated flow field but cannot contribute information on horizontal particle dispersal or horizontal dispersion and mixing under field conditions in lakes.

In-flight measurement of the absolute energy scale of the Fermi Large Area Telescope

The Large Area Telescope (LAT) on-board the Fermi Gamma-ray Space Telescope is a pair-conversion telescope designed to survey the gamma-ray sky from 20 MeV to several hundreds of GeV. In this energy band there are no astronomical sources with sufficiently well known and sharp spectral features to allow an absolute calibration of the LAT energy scale. However, the geomagnetic cutoff in the cosmic ray electron-plus-positron (CRE) spectrum in low Earth orbit does provide such a spectral feature. The energy and spectral shape of this cutoff can be calculated with the aid of a numerical code tracing charged particles in the Earth’s magnetic field. By comparing the cutoff value with that measured by the LAT in different geomagnetic positions, we have obtained several calibration points between ∼6 and ∼13 GeV with an estimated uncertainty of ∼2%. An energy calibration with such high accuracy reduces the systematic uncertainty in LAT measurements of, for example, the spectral cutoff in the emission from gamma ray pulsars.

A numerical study of horizontal dispersion in a macro tidal basin

Tidal circulation in Cobscook Bay, a macro tidal basin, is simulated using the three-dimensional, nonlinear, finite element ocean model, QUODDY_dry. Numerical particles are released from various transects in the bay at different tidal phases and tracked for several tidal cycles. Initially, nearby particles in the main tidal channel experience a great deal of spreading and straining, and after a few tidal cycles, they are separated in different parts of the bay. The fundamental mechanism for particle dispersion is the chaotic advection that arises from long tidal excursions passing through many residual eddies. A loosely correlated, inverse relationship between the two dimensionless parameters, ν (the ratio of the residual current to the tidal current) and λ (the ratio of the tidal excursion to the main topographic scale), can be constructed for large values of ν. Several Lagrangian statistical measures are used to quantify and distinguish dispersion regimes in different parts of Cobscook Bay. It is found that the effective Lagrangian dispersion coefficient can be estimated using the product of the magnitude of residual currents and the tidal excursion.

Charging and dynamics of a dust grain in the wake of another grain in flowing plasmas

The charging of a dust grain in supersonic plasma flows in the wake of another grain is studied by numerical simulations. While entering the Mach cone originating from the upstream grain, the grain is discharged by scattered ions. Electrostatic forces acting on the grain in the wake will move it to the stable position in the wake at a distance close to the electron Debye length from the upstream grain. The onset for discharging can be used to estimate the ion flow speed in the system. The simulations are carried out with the DiP3D code, a three-dimensional particle-in-cell code where both electrons and ions are represented as numerical particles [W. J. Miloch et al., Nonlinear Processes Geophys. 14, 575 (2007); New J. Phys. 11, 043005 (2009)].

Identification of typical scenarios for the surface Lagrangian residual circulation in the Iroise Sea

This paper describes the surface Lagrangian residual circulation (LRC) over 2.5 day intervals in the Iroise Sea, west of France, and evaluates, for operational purposes, the influence of the different physical mechanisms that govern it. The method consists of the calculation of water displacements with a diagnostic Lagrangian tool that computes the trajectories of numerical particles in a given velocity field. The LRC is inferred from trajectories integrated over five M2 tidal cycles. The analysis is applied to both gridded genuine current measurements and ocean model outputs: the sea surface currents are derived from high‐frequency (HF) radar measurements and from MARS, a 3‐D regional ocean model used here in idealized configurations. To substantiate the analysis, the Lagrangian residual currents are also compared to genuine movements of drifters released in the Iroise Sea in 2005 and 2007. The LRC is mapped for typical scenarios identified from the Lagrangian analysis of HF radar surface currents measured in winter and summer, under weak (<7 m/s) and strong (>10 m/s) wind conditions, and in neap tide and spring tide seasons. Idealized numerical simulations that switch on and off each individual physical process are used to isolate in the LRC the patterns induced by the atmospheric forcing, tides, and density‐driven currents.

Numerical study of a stochastic particle algorithm solving a multidimensional population balance model for high shear granulation

This paper is concerned with computational aspects of a multidimensional population balance model of a wet granulation process. Wet granulation is a manufacturing method to form composite particles, granules, from small particles and binders. A detailed numerical study of a stochastic particle algorithm for the solution of a five-dimensional population balance model for wet granulation is presented. Each particle consists of two types of solids (containing pores) and of external and internal liquid (located in the pores). Several transformations of particles are considered, including coalescence, compaction and breakage. A convergence study is performed with respect to the parameter that determines the number of numerical particles. Averaged properties of the system are computed. In addition, the ensemble is subdivided into practically relevant size classes and analysed with respect to the amount of mass and the particle porosity in each class. These results illustrate the importance of the multidimensional approach. Finally, the kinetic equation corresponding to the stochastic model is discussed.

Ontogenetic development of migration: Lagrangian drift trajectories suggest a new paradigm for sea turtles

Long distance migration occurs in a wide variety of taxa including birds, insects, fishes, mammals and reptiles. Here, we provide evidence for a new paradigm for the determinants of migration destination. As adults, sea turtles show fidelity to their natal nesting areas and then at the end of the breeding season may migrate to distant foraging sites. For a major rookery in the Mediterranean, we simulated hatchling drift by releasing 288 000 numerical particles in an area close to the nesting beaches. We show that the pattern of adult dispersion from the breeding area reflects the extent of passive dispersion that would be experienced by hatchlings. Hence, the prevailing oceanography around nesting areas may be crucial to the selection of foraging sites used by adult sea turtles. This environmental forcing may allow the rapid evolution of new migration destinations if ocean currents alter with climate change.

Coalescence Characteristics of Fusible Particles in Solderable Isotropic Conductive Adhesives (ICAs)

A new hybrid soldering and conductive adhesive joining technology using a fusible lowmelting- point alloy (LMPA) have been developed. A numerical method for numerical analysis of fusible particles behavior is proposed to investigate coalescence characteristics of fusible particles in solderable isotropic conductive adhesives (ICAs). For finding out suitable conditions to obtain reliable conduction paths, the present study examines the influence of process-related parameters such as volume fraction and viscosity on coalescence characteristics of fusible particles.