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An O(N) and parallel approach to integral problems by a kernel-independent fast multipole method: Application to polarization and magnetization of interacting particles
https://cds.cern.ch/record/2151040
Large classes of materials systems in physics and engineering are governed by magnetic and electrostatic interactions. Continuum or mesoscale descriptions of such systems can be cast in terms of integral equations, whose direct computational evaluation requires O(N^2) operations, where N is the number of unknowns. Such a scaling, which arises from the many-body nature of the relevant Green's function, has precluded wide-spread adoption of integral methods for solution of large-scale scientific and engineering problems. In this work, a parallel computational approach is presented that relies on using scalable open source libraries and utilizes a kernel-independent Fast Multipole Method to evaluate the integrals in O(N) operations, with O(N) memory cost, thereby substantially improving the scalability and efficiency of computational integral methods. We demonstrate the accuracy, efficiency, and scalability of our approach in the contest of two examples. In the first, we solve a boundary value problem for a ferroelectric/ferromagnetic volume in free space. In the second, we solve an electrostatic problem involving polarizable dielectric bodies in an unbounded dielectric medium. The results from these test cases show that our parallel approach can enable highly efficient and accurate simulations in a broad range of applications.Jiang, XikaiLi, JiyuanZhao, XujunQin, JianKarpeev, DmitryHernandez-Ortiz, Juande Pablo, JuanHeinonen, OlleFri, 06 May 2016 06:30:00 GMT05 May 2016arXiv:1605.01715https://cds.cern.ch/record/2151040['arXiv:1605.01715']arXiv:1605.01715Single-molecule imaging of Nav1.6 on the somatic surface of hippocampal neurons reveals unique nanoclusters
https://cds.cern.ch/record/2151039
Voltage-gated sodium (Na$_v$) channels are responsible for the depolarizing phase of the action potential in most nerve cells and Na$_v$ channel localization to the axon initial segment is vital to action potential initiation. Na$_v$ channels in the soma play a role in the transfer of axonal output information to the rest of the neuron and in synaptic plasticity, although little is known about Na$_v$ channel localization and dynamics within this neuronal compartment. The present study uses single-particle tracking and photoactivation localization microscopy to analyze cell-surface Na$_v$1.6 within the soma of cultured hippocampal neurons. Mean square displacement analysis of individual trajectories indicated half of the somatic Na$_v$1.6 channels localized to stable nanoclusters ~230 nm in diameter. Strikingly, these domains were stabilized at specific sites on the cell membrane for greater than 30 min, notably via an ankyrin-independent mechanism, indicating the mechanism by which Na$_v$1.6 nanoclusters are maintained in the soma is biologically different from axonal localization. Non-clustered Na$_v$1.6 channels showed anomalous diffusion, as determined by mean squared displacement analysis. High-density single-particle tracking of Na$_v$ channels labeled with photoactivatable fluorophores in combination with Bayesian inference analysis was employed to characterize the surface nanoclusters. A subpopulation of mobile Na$_v$1.6 was observed to be transiently trapped into the nanoclusters. Somatic Na$_v$1.6 nanoclusters represent a new type of Na$_v$ channel localization and are hypothesized to be sites of localized channel regulation.Akin, Elizabeth JSolé, LauraJohnson, BenBeheiry, Mohamed elMasson, Jean-BaptisteKrapf, DiegoTamkun, Michael MFri, 06 May 2016 06:30:00 GMT05 May 2016arXiv:1605.01699https://cds.cern.ch/record/2151039['arXiv:1605.01699']arXiv:1605.01699Fractional Brownian motion, the Matern process, and stochastic modeling of turbulent dispersion
https://cds.cern.ch/record/2151038
Stochastic process exhibiting power-law slopes in the frequency domain are frequently well modeled by fractional Brownian motion (fBm). In particular, the spectral slope at high frequencies is associated with the degree of small-scale roughness or fractal dimension. However, a broad class of real-world signals have a high-frequency slope, like fBm, but a plateau in the vicinity of zero frequency. This low-frequency plateau, it is shown, implies that the temporal integral of the process exhibits diffusive behavior, dispersing from its initial location at a constant rate. Such processes are not well modeled by fBm, which has a singularity at zero frequency corresponding to an unbounded rate of dispersion. A more appropriate stochastic model is a much lesser-known random process called the Matern process, which is shown herein to be a damped version of fractional Brownian motion. This article first provides a thorough introduction to fractional Brownian motion, then examines the details of the Matern process and its relationship to fBm. An algorithm for the simulation of the Matern process in O(N log N) operations is given. Unlike fBm, the Matern process is found to provide an excellent match to modeling velocities from particle trajectories in an application to two-dimensional fluid turbulence.Lilly, J MSykulski, A MEarly, J JOlhede, S CFri, 06 May 2016 06:30:00 GMT05 May 2016arXiv:1605.01684https://cds.cern.ch/record/2151038['arXiv:1605.01684']arXiv:1605.01684Linear-scaling generation of potential energy surfaces using a double incremental expansion
https://cds.cern.ch/record/2151037
We present a combination of the incremental expansion of potential energy surfaces (PESs), known as n-mode expansion, with the incremental evaluation of the electronic energy in a many-body approach. The application of semi-local coordinates in this context allows the generation of PESs in a very cost-efficient way. For this, we employ the recently introduced FALCON (Flexible Adaptation of Local COordinates of Nuclei) coordinates. By introducing an additional transformation step, concerning only a fraction of the vibrational degrees of freedom, we can achieve linear scaling of the accumulated cost of the single point calculations required in the PES generation. Numerical examples of these double incremental approaches for oligo-phenyl examples show fast convergence with respect to the maximum number of simultaneously treated fragments and only a modest error introduced by the additional transformation step. The approach, presented here, represents a major step towards the applicability of vibrational wave function methods to sizable, covalently bound systems.König, CarolinChristiansen, OveFri, 06 May 2016 06:30:00 GMT05 May 2016arXiv:1605.01674https://cds.cern.ch/record/2151037['arXiv:1605.01674']arXiv:1605.01674Intracycle Angular Velocity Control of Cross-Flow Turbines
https://cds.cern.ch/record/2151036
Cross-flow turbines, also known as vertical-axis turbines, have numerous features that make them attractive for wind and marine renewable energy. To maximize power output, the turbine blade kinematics may be controlled during the course of the blade revolution, thus optimizing the unsteady fluid dynamic forces. Dynamically pitching the blades, similar to blade control in a helicopter, is an established method. However, this technique adds undesirable mechanical complexity to the turbine, increasing cost and reducing durability. Here we introduce a novel alternative requiring no additional moving parts: we optimize the turbine rotation rate as a function of blade position resulting in motion (including changes in the effective angle of attack) that is precisely timed to exploit unsteady fluid effects. We demonstrate experimentally that this approach results in a 79% increase in power output over industry standard control methods. Analysis of the fluid forcing and blade kinematics show that maximal power is achieved through alignment of fluid force and rotation rate extrema. In addition, the optimized controller excites a well-timed dynamic stall vortex, as is found in many examples of biological propulsion. This control strategy allows a structurally robust turbine operating at relatively low angular velocity to achieve high efficiency and could enable a new generation of environmentally-benign turbines for wind and water current power generation.Strom, BenjaminBrunton, Steven LPolagye, BrianFri, 06 May 2016 06:30:00 GMT05 May 2016arXiv:1605.01671https://cds.cern.ch/record/2151036['arXiv:1605.01671']arXiv:1605.01671Geometric integrator for simulations in the canonical ensemble
https://cds.cern.ch/record/2151035
In this work we introduce a geometric integrator for molecular dynamics simulations of physical systems in the canonical ensemble. In particular, we consider the equations arising from the so-called density dynamics algorithm with any possible type of thermostat and provide an integrator that preserves the invariant distribution. Our integrator thus constitutes a unified framework that allows the study and comparison of different thermostats and of their influence on the equilibrium and non-equilibrium (thermo-)dynamic properties of the system. To show the validity and the generality of the integrator, we implement it with a second-order, time-reversible method and apply it to the simulation of a Lennard-Jones system with three different thermostats, obtaining good conservation of the geometrical properties and recovering the expected thermodynamic results.Tapias, DiegoSanders, David PBravetti, AlessandroFri, 06 May 2016 06:29:59 GMT05 May 2016arXiv:1605.01654https://cds.cern.ch/record/2151035['arXiv:1605.01654']arXiv:1605.01654Small-angle neutron scattering and Molecular Dynamics structural study of gelling DNA nanostars
https://cds.cern.ch/record/2151034
DNA oligomers with properly designed sequences self-assemble into well defined constructs. Here, we exploit this methodology to produce bulk quantities of tetravalent DNA nanostars (each one composed by 196 nucleotides) and to explore the structural signatures of their aggregation process. We report small-angle neutron scattering experiments focused on the evaluation of both the form factor and the temperature evolution of the scattered intensity at a nano star concentration where the system forms a tetravalent equilibrium gel. We also perform molecular dynamics simulations of one isolated tetramer to evaluate the form factor theoretically, without resorting to any approximate shape. The numerical form factor is found to be in very good agreement with the experimental one. Simulations predict an essentially temperature independent form factor, offering the possibility to extract the effective structure factor and its evolution during the equilibrium gelation.Fernandez-Castanon, JavierBomboi, FrancescaRovigatti, LorenzoZanatta, MarcoPaciaroni, AlessandroComez, LuciaPorcar, LionelJafta, Charl JFadda, Giulia CBellini, TommasoSciortino, FrancescoFri, 06 May 2016 06:29:58 GMT05 May 2016arXiv:1605.01650https://cds.cern.ch/record/2151034['arXiv:1605.01650']arXiv:1605.01650Context-dependent metabolic networks
https://cds.cern.ch/record/2151033
Cells adapt their metabolism to survive changes in their environment. We present a framework for the construction and analysis of metabolic reaction networks that can be tailored to reflect different environmental conditions. Using context-dependent flux distributions from Flux Balance Analysis (FBA), we produce directed networks with weighted links representing the amount of metabolite flowing from a source reaction to a target reaction per unit time. Such networks are analyzed with tools from network theory to reveal salient features of metabolite flows in each biological context. We illustrate our approach with the directed network of the central carbon metabolism of Escherichia coli, and study its properties in four relevant biological scenarios. Our results show that both flow and network structure depend drastically on the environment: networks produced from the same metabolic model in different contexts have different edges, components, and flow communities, capturing the biological re-routing of metabolic flows inside the cell. By integrating FBA-based analysis with tools from network science, our results provide a framework to interrogate cellular metabolism beyond standard pathway descriptions that are blind to the environmental context.Beguerisse-Díaz, MarianoBosque, GabrielOyarzún, DiegoPicó, JesúsBarahona, MauricioFri, 06 May 2016 06:29:58 GMT05 May 2016arXiv:1605.01639https://cds.cern.ch/record/2151033['arXiv:1605.01639']arXiv:1605.01639Immersed Boundary Simulations of Active Fluid Droplets
https://cds.cern.ch/record/2151032
We present numerical simulations of active fluid droplets immersed in an external fluid in 2-dimensions. We use an Immersed Boundary method to simulate the fluid droplet interface as a Lagrangian mesh. We present results from two example systems, firstly a droplet filled with an active polar fluid with polar anchoring at the droplet interface. Secondly, an active isotropic fluid consisting of particles that can bind and unbind from the interface and generate surface tension gradients through active contractility. These two systems demonstrate spontaneous symmetry breaking and steady state dynamics resembling cell motility and division and show complex feedback mechanisms with minimal degrees of freedom. The simulations outlined here will be useful for quantifying the wide range of dynamics observable in these active systems and modelling the effects of confinement in a consistent and adaptable way.Whitfield, Carl AHawkins, Rhoda JFri, 06 May 2016 06:29:58 GMT05 May 2016arXiv:1605.01621https://cds.cern.ch/record/2151032['arXiv:1605.01621']arXiv:1605.01621Flat Bunches with a Hollow Distribution for Space Charge Mitigation
https://cds.cern.ch/record/2151031
Longitudinally hollow bunches provide one means to mitigate the impact of transverse space charge. The hollow distributions are created via dipolar parametric excitation during acceleration in CERN's Proton Synchrotron Booster. We present simulation work and beam measurements. Particular emphasis is given to the alleviation of space charge effects on the long injection plateau of the downstream Proton Synchrotron machine, which is the main goal of this study.Oeftiger, AdrianBartosik, HannesFindlay, Alan JamesHancock, StevenRumolo, GiovanniFri, 06 May 2016 06:29:58 GMT05 May 2016arXiv:1605.01616https://cds.cern.ch/record/2151031['arXiv:1605.01616']arXiv:1605.01616Manufacturing and the LLRF Tests of the SANAEM RFQ
https://cds.cern.ch/record/2151030
The Turkish Atomic Energy Authority has been working on building an experimental proton beamline using local resources at the Saraykoy Nuclear Research and Training Center (SANAEM). The radio frequency quadrupole (RFQ) was manufactured after the completion of beam dynamics and 3D electromagnetic simulation studies. The vanes were machined using a three axis CNC machine. A CMM was used for both the acceptance tests of the vanes and their assembly. Production and assembly errors were found acceptable for this cavity, the very first one developed in Turkey. The aluminum vanes were copper coated by electroplating. The coated vanes were bolted and bonded with eight screws, eight pins and two different adhesives. A silver paste was used for RF sealing and a low vapor pressure epoxy was used for vacuum isolation. First LLRF tests of the RFQ were performed using the phase shift method with a bead-pull setup, a VNA, an N-type RF coupler and a pick-up loop. Cavity quality factor was measured with 3dB method for different RF sealing stages. This study summarizes the machining, assembling and the first LLRF tests of the SANAEM RFQ.Turemen, GAlacakir, AAkgun, YBolukdemir, SKilic, IYasatekin, BYildiz, HUnel, GFri, 06 May 2016 06:29:58 GMT05 May 2016arXiv:1605.01601https://cds.cern.ch/record/2151030['arXiv:1605.01601']arXiv:1605.01601The Roles of Substrate vs Nonlocal Optical Nonlinearities in the Excitation of Surface Plasmons in Graphene
https://cds.cern.ch/record/2151029
It has recently been demonstrated that difference frequency mixing (DFM) can generate surface plasmons in graphene [1]. Here, we present detailed calculations comparing the contributions to this effect from substrate and from graphene nonlinearities. Our calculations show that the substrate (quartz) nonlinearity gives rise to a surface plasmon intensity that is around twelve orders of magnitude smaller than that arising from the intrinsic graphene response. This surprisingly efficient intrinsic process, given the centrosymmetric structure of graphene, arises almost entirely due to non-local contributions to the second order optical nonlinearity of graphene.Constant, Thomas JTollerton, Craig JHendry, EuanChang, Darrick EFri, 06 May 2016 06:29:58 GMT05 May 2016arXiv:1605.01542https://cds.cern.ch/record/2151029['arXiv:1605.01542']arXiv:1605.01542Exact solutions of the Grad-Shafranov equation via similarity reductions with applications to magnetically confined plasmas
https://cds.cern.ch/record/2151028
We derive exact solutions of the general linear form of the Grad-Shafranov (GS) equation, including incompressible equilbrium flow, using similarity reduction ansatzes motivated by the ansatz-based methods of group foliation and direct reduction. The linearity of the equilibrium equation allows linear combinations of solutions in order to obtain axisymmetric MHD equilibria with closed and nested magnetic surfaces which are favorable for the effective confinenment of laboratory plasmas. Employing the afforementioned ansatzes we also obtain analytical solutions for several non-linear forms of the GS equation.Kaltsas, Dimitrios AThroumoulopoulos, George NFri, 06 May 2016 06:29:58 GMT05 May 2016arXiv:1605.01538https://cds.cern.ch/record/2151028['arXiv:1605.01538']arXiv:1605.01538On the Vortex Dynamics in Fractal Fourier Turbulence
https://cds.cern.ch/record/2151027
Incompressible, homogeneous and isotropic turbulence is studied by solving the Navier-Stokes equations on a reduced set of Fourier modes, belonging to a fractal set of dimension $D$. By tuning the fractal dimension parameter, we study the dynamical effects of Fourier decimation on the vortex stretching mechanism and on the statistics of the velocity and the velocity gradient tensor. In particular, we show that as we move from $D=3$ to $D \sim 2.8$, the statistics gradually turns into a purely Gaussian one. This result suggests that even a mild fractal mode reduction strongly depletes the stretching properties of the non-linear term of the Navier-Stokes equations and suppresses anomalous fluctuations.Lanotte, Alessandra SMalapaka, Shiva KumarBiferale, LucaFri, 06 May 2016 06:29:57 GMT05 May 2016arXiv:1605.01527https://cds.cern.ch/record/2151027['arXiv:1605.01527']arXiv:1605.01527Single beam optical conveyor belt for chiral particles
https://cds.cern.ch/record/2151026
We propose a novel paradigm to selectively manipulate and transport small engineered chiral particles and discriminate different enantiomers using unstructured chiral light. It is theoretically shown that the response of a chiral metamaterial particle may be tailored to enable an optical conveyor belt operation with no optical traps, such that for a fixed incident light helicity and independent of the nanoparticle location, it is either steadily pushed towards the direction of the photon flow or steadily pulled against the photon flow. Our findings create new opportunities for unconventional optical manipulations of tailored nanoparticles and may have applications in sorting racemic mixtures of artificial chiral molecules and in particle delivery.Fernandes, David ESilveirinha, Mário GFri, 06 May 2016 06:29:57 GMT05 May 2016arXiv:1605.01525https://cds.cern.ch/record/2151026['arXiv:1605.01525']arXiv:1605.01525Preliminary study of 10Be/7Be in rainwater from Xi'an by Accelerator Mass Spectrometry
https://cds.cern.ch/record/2151025
The 10Be/7Be ratio is a sensitive tracer for the study of atmospheric transport, particularly with regard to stratosphere-troposphere exchange. Measurements with high accuracy and efficiency are crucial to 7Be and 10Be tracer studies. This article describes sample preparation procedures and analytical benchmarks for 7Be and 10Be measurements at the Xian Accelerator Mass Spectrometry (Xian-AMS) laboratory for the study of rainwater samples. We describe a sample preparation procedure to fabricate beryllium oxide (BeO) AMS targets that includes co-precipitation, anion exchange column separation and purification. We then provide details for the AMS measurement of 7Be and 10Be following the sequence BeO- -> Be2+ -> Be4+ in the Xian- AMS. The 10Be/7Be ratio of rainwater collected in Xian is shown to be about 1.3 at the time of rainfall. The virtue of the method described here is that both 7Be and 10Be are measured in the same sample, and is suitable for routine analysis of large numbers of rainwater samples by AMS.Zhang, LiFu, Yun-ChongFri, 06 May 2016 06:29:57 GMT05 May 2016arXiv:1605.01520https://cds.cern.ch/record/2151025['arXiv:1605.01520']arXiv:1605.015203D printing of gas jet nozzles for laser-plasma accelerators
https://cds.cern.ch/record/2151024
Recent results on laser wakefield acceleration in tailored plasma channels have underlined the importance of controlling the density profile of the gas target. In particular it was reported that appropriate density tailoring can result in improved injection, acceleration and collimation of laser-accelerated electron beams. To achieve such profiles innovative target designs are required. For this purpose we have reviewed the usage of additive layer manufacturing, commonly known as 3D printing, in order to produce gas jet nozzles. Notably we have compared the performance of two industry standard techniques, namely selective laser sintering (SLS) and stereolithography (SLA). Furthermore we have used the common fused deposition modeling (FDM) to reproduce basic gas jet designs and used SLA and SLS for more sophisticated nozzle designs. The nozzles are characterized interferometrically and used for electron acceleration experiments with the Salle Jaune terawatt laser at Laboratoire d'Optique Appliqu\'ee.Döpp, AGuillaume, EThaury, CGautier, JPhuoc, K TaMalka, VFri, 06 May 2016 06:29:57 GMT05 May 2016arXiv:1605.01518https://cds.cern.ch/record/2151024['arXiv:1605.01518']arXiv:1605.01518Controlling the Numerical Cerenkov Instability in PIC simulations using a customized finite difference Maxwell solver and a local FFT based current correction
https://cds.cern.ch/record/2151023
In this paper we present a customized finite-difference-time-domain (FDTD) Maxwell solver for the particle-in-cell (PIC) algorithm. The solver is customized to effectively eliminate the numerical Cerenkov instability (NCI) which arises when a plasma (neutral or non-neutral) relativistically drifts on a grid when using the PIC algorithm. We control the EM dispersion curve in the direction of the plasma drift of a FDTD Maxwell solver by using a customized higher order finite difference operator for the spatial derivative along the direction of the drift ($\hat 1$ direction). We show that this eliminates the main NCI modes with moderate $\vert k_1 \vert$, while keeps additional main NCI modes well outside the range of physical interest with higher $\vert k_1 \vert$. These main NCI modes can be easily filtered out along with first spatial aliasing NCI modes which are also at the edge of the fundamental Brillouin zone. The customized solver has the possible advantage of improved parallel scalability because it can be easily partitioned along $\hat 1$ which typically has many more cells than other directions for the problems of interest. We show that FFTs can be performed locally to current on each partition to filter out the main and first spatial aliasing NCI modes, and to correct the current so that it satisfies the continuity equation for the customized spatial derivative. This ensures that Gauss' Law is satisfied. We present simulation examples of one relativistically drifting plasmas, of two colliding relativistically drifting plasmas, and of nonlinear laser wakefield acceleration (LWFA) in a Lorentz boosted frame that show no evidence of the NCI can be observed when using this customized Maxwell solver together with its NCI elimination scheme.Li, FeiYu, PeichengXu, XinluFiuza, FredericoDecyk, Viktor KDalichaouch, ThamineDavidson, AsherTableman, AdamAn, WeimingTsung, Frank SFonseca, Ricardo ALu, WeiMori, Warren BFri, 06 May 2016 06:29:57 GMT05 May 2016arXiv:1605.01496https://cds.cern.ch/record/2151023['arXiv:1605.01496']arXiv:1605.01496SCOUT: simultaneous time segmentation and community detection in dynamic networks
https://cds.cern.ch/record/2151022
Many evolving complex systems can be modeled via dynamic networks. An important problem in dynamic network research is community detection, which identifies groups of topologically related nodes. Typically, this problem is approached by assuming either that each time point has a distinct community organization or that all time points share one community organization. In reality, the truth likely lies between these two extremes, since some time periods can have community organization that evolves while others can have community organization that stays the same. To find the compromise, we consider community detection in the context of the problem of segment detection, which identifies contiguous time periods with consistent network structure. Consequently, we formulate a combined problem of segment community detection (SCD), which simultaneously partitions the network into contiguous time segments with consistent community organization and finds this community organization for each segment. To solve SCD, we introduce SCOUT, an optimization framework that explicitly considers both segmentation quality and partition quality. SCOUT addresses limitations of existing methods that can be adapted to solve SCD, which typically consider only one of segmentation quality or partition quality. In a thorough evaluation, SCOUT outperforms the existing methods in terms of both accuracy and computational complexity.Hulovatyy, YuriyMilenkovic, TijanaFri, 06 May 2016 06:29:57 GMT05 May 2016arXiv:1605.01491https://cds.cern.ch/record/2151022['arXiv:1605.01491']arXiv:1605.01491A diabatic definition of geometric phase effects
https://cds.cern.ch/record/2151021
Electronic wave-functions in the adiabatic representation acquire nontrivial geometric phases (GPs) when corresponding potential energy surfaces undergo conical intersection (CI). To define dynamical effects arising from the GP presence in the nuclear quantum dynamics we explore a removal of the GP via modification of the underlying diabatic representation. Using an absolute value function of diabatic couplings we remove the GP while preserving adiabatic potential energy surfaces and CI. We assess GP effects in dynamics of a two-dimensional linear vibronic coupling model both for ground and excited state dynamics. Results are compared with those obtained with a conventional removal of the GP by ignoring double-valued boundary conditions of the real electronic wave-functions. Interestingly, GP effects appear similar in two approaches only for the low energy dynamics, while the new approach does not have substantial GP effects in the ultra-fast excited state dynamics.Izmaylov, Artur FLi, JiaruJoubert-Doriol, LoicFri, 06 May 2016 06:29:57 GMT05 May 2016arXiv:1605.01487https://cds.cern.ch/record/2151021['arXiv:1605.01487']arXiv:1605.01487The existence of stable BGK waves
https://cds.cern.ch/record/2151020
1D Vlasov-Poisson system is the simplest kinetic model for describing an electrostatic collisonless plasma, and the BGK waves are its famous exact steady solutions. They play an important role on the long time dynamics of a collisionless plasma as potential "final states" or "attractors", thanks to many numerical simulations and observations. Despite their importance, the existence of stable BGK waves has been an open problem since their discovery in 1958. In this paper, first linearly stable BGK waves are constructed near homogeneous states.Guo, YanLin, ZhiwuFri, 06 May 2016 06:29:57 GMT04 May 2016arXiv:1605.01463https://cds.cern.ch/record/2151020['arXiv:1605.01463']arXiv:1605.01463Fluid-structure interaction simulation of floating structures interacting with complex, large-scale ocean waves and atmospheric turbulence
https://cds.cern.ch/record/2151019
We develop a numerical method for simulating coupled interactions of complex floating structures with large-scale ocean waves and atmospheric turbulence. We employ an efficient large-scale model to develop offshore wind and wave environmental conditions, which are then incorporated into a high resolution two-phase flow solver with fluid-structure interaction (FSI). The large-scale wind-wave interaction model is based on the two-fluid dynamically-coupled approach of Yang and Shen (2011), which employs a high-order spectral method for simulating the water motion and a viscous solver with undulatory boundaries for the air motion. The two-phase flow FSI solver, developed by Calderer, Kang, and Sotiropoulos (2014), is based on the level set method and is capable of simulating the coupled dynamic interaction of arbitrarily complex bodies with airflow and waves. The large-scale wave field solver is coupled with the near-field FSI solver by feeding into the latter waves via the pressure-forcing method of Guo and Shen (2009), which has been extended herein for the level set method. We validate the model for both simple wave trains and three-dimensional directional waves and compare the results with experimental and theoretical solutions. Finally, we demonstrate the capabilities of the new computational framework by carrying out large-eddy simulation of a floating offshore wind turbine interacting with realistic ocean wind and waves.Calderer, AntoniGuo, XinShen, LianSotiropoulos, FotisFri, 06 May 2016 06:29:57 GMT04 May 2016arXiv:1605.01460https://cds.cern.ch/record/2151019['arXiv:1605.01460']arXiv:1605.01460Explicit high-order symplectic integrators for charged particles in general electromagnetic fields
https://cds.cern.ch/record/2151018
This article considers non-relativistic charged particle dynamics in both static and non-static electromagnetic fields, which are governed by nonseparable, possibly time-dependent Hamiltonians. For the first time, explicit symplectic integrators of arbitrary high-orders are constructed for accurate and efficient simulations of such mechanical systems. Performances superior to the standard non-symplectic method of Runge-Kutta are demonstrated on two examples: the first is on the confined motion of a particle in a static toroidal magnetic field used in tokamak; the second is on how time-periodic perturbations to a magnetic field inject energy into a particle via parametric resonance at a specific frequency.Tao, MoleiFri, 06 May 2016 06:29:57 GMT04 May 2016arXiv:1605.01458https://cds.cern.ch/record/2151018['arXiv:1605.01458']arXiv:1605.01458Actuator line modeling of vertical-axis turbines
https://cds.cern.ch/record/2151017
To bridge the gap between high and low fidelity numerical modeling tools for vertical-axis (or cross-flow) turbines (VATs or CFTs), an actuator line model (ALM) was developed and validated for both a high and a medium solidity vertical-axis turbine at rotor diameter Reynolds numbers $Re_D \sim 10^6$. The ALM is a hybridization of classical blade element theory with Navier--Stokes based flow models, and in this study both $k$--$\epsilon$ Reynolds-averaged Navier--Stokes (RANS) and Smagorinsky large eddy simulation (LES) turbulence models were tested. The RANS models were able to be run on coarse grids while still providing good convergence behavior in terms of the mean power coefficient, and also approximately four orders of magnitude reduction in computational expense compared with 3-D blade-resolved RANS simulations. Submodels for dynamic stall, end effects, added mass, and flow curvature were implemented, resulting in reasonable performance predictions for the high solidity rotor, more discrepancies for the medium solidity rotor, and overprediction for both cases at high tip speed ratio. The wake results showed that the ALM was able to capture some of the important flow features that contribute to VAT's relatively fast wake recovery---a large improvement over the conventional actuator disk model. The mean flow field was better realized with the LES, which still represented a computational savings of two orders of magnitude compared with 3-D blade-resolved RANS, though vortex breakdown and subsequent turbulence generation appeared to be underpredicted, which necessitates further investigation of optimal subgrid scale modeling.Bachant, PeterGoude, AndersWosnik, MartinFri, 06 May 2016 06:29:57 GMT04 May 2016arXiv:1605.01449https://cds.cern.ch/record/2151017['arXiv:1605.01449']arXiv:1605.01449High efficiency energy extraction from a relativistic electron beam in a strongly tapered undulator
https://cds.cern.ch/record/2151016
We present results of an experiment where, using a 200 GW CO2 laser seed, a 65 MeV electron beam was decelerated down to 35 MeV in a 54 cm long strongly tapered helical magnetic undulator, extracting over 30$\%$ of the initial electron beam energy to coherent radiation. These results demonstrate unparalleled electro-optical conversion efficiencies for a relativistic beam in an undulator field and represent an important step in the development of high peak and average power coherent radiation sources.Sudar, NicholasMusumeci, PietroDuris, JoeGadjev, IvanPolyaniy, MikhailPogorelsky, IgorFedurin, MikhailSwinson, ChristinaBabzien, MarcusKusche, KarlGover, AviFri, 06 May 2016 06:29:57 GMT04 May 2016arXiv:1605.01448https://cds.cern.ch/record/2151016['arXiv:1605.01448']arXiv:1605.01448