Preliminary implementation of piecewise linear conformal boundaries in ICEPIC

A prediction of radiative heat transfer in a complex geometry is performed using different boundary treatments such as blocked-off, spatial-multiblock, and embedded boundary methods. The formulation of embedded boundary treatment for finite volume is derived. The finite-volume method (FVM) is used to model the radiative transfer in an absorbing and emitting medium which is maintained at an isothermal condition and enclosed by cold and black walls. While the body-fitted grid system is used for the spatial multiblock treatment, the Cartesian grid system is chosen for the others. Their results are compared and discussed for three different cases, including a trapezoidal enclosure, a semicircular enclosure with internal block, and an incinerator-shaped enclosure. The accuracy obtained by application of each treatment is shown to be highly satisfactory. Consequently, each treatment is suitable for modeling the radiative heat transfer in the complex geometry. However, the solution obtained by the blocked-off treatment yields some errors compared with the others, since the Cartesian grid used in the blocked-off treatment cannot configure exactly the complex boundaries. Especially, the radiative heat flux on the nonorthogonal wall is largely underestimated due to its stepwise description of the wall.

A simple and high-performance ldquomulti-flowrdquo cooling structure for plasma display panel television sets was developed using computational fluid dynamics with heat transfer based on Cartesian grid system. Upward flow induced by natural convection and fans in the conventional cooling structure has mainly contributed to cooling the plasma panel and boards. However, simulation results indicated that shortcut flows between the bottom and rear inlets and fans were formed along the back cover and the flows decreased the mean outlet-air temperature. The bottom and rear inlets were reduced for the multi-flow structure, and top and side inlets were added to directly induce the fresh flow toward the high-temperature regions at the upper area. In addition to the effects of the increased gaps between the base-chassis and boards, simulation and experimental results revealed that the cooling accomplished by the multi-flow structure was superior to that by the conventional structure.

The solution accuracy and computational efficiency of high order Large Eddy Simulation (LES) solvers are evaluated on two benchmark open literature blade cascade problems. The first problem concerns wake development in the T106A low pressure turbine cascade [1]. The second problem examines the effect of free-stream turbulence on heat transfer from the VKI first stage high pressure turbine vane [2]. The calculations are performed with two independently developed high order LES solvers using completely different numerical algorithms. The first solver FDL3Di [3] was originally developed at the Airforce Research Laboratory (AFRL) and employs structured overset grids. It uses a sixth order compact finite difference scheme in space along with an implicit Beam-Warming scheme for time marching. The second solver, hpMusic, is developed at the University of Kansas [4]. This is a variable order (up to sixth order) unstructured grid solver employing a discontinuous formulation known as flux reconstruction (FR) / correction procedure via reconstruction (CPR) [5]. The computational grids used are independently tuned for each application. The solvers are benchmarked against experimental data for wake development and blade heat transfer coefficient. Further physical insights in to the test cases are also obtained, filling gaps in experimental results, especially for the VKI problem.

Density functional theory (DFT) has emerged as one of the most versatile and lucrative approaches in electronic structure calculations of many-electron systems in past four decades. Here we give an account of the development of a variational DFT method for atoms and molecules completely in a Cartesian grid. The non-relativistic Kohn–Sham equation is solved by using an LCAO-MO ansatz. Atom-centered localized basis set, electron density, molecular orbitals, two-body potentials are directly constructed on the grid. We adopt a Fourier convolution method for classical Coulomb potentials by making an Ewald-type decomposition technique in terms of short- and long-range interactions. It produces quite accurate and competitive results for various properties of interest, such as component energy, total energy, ionization energy, potential energy curve, atomization energy, etc. Both local and non-local functionals are employed for pseudopotential as well as full calculations. While most results are offered in a uniform grid, initial exploratory attempts are made in a non-uniform grid, which can significantly reduce the computational overhead. This offers a practical, viable alternative to atom-centered grid-based implementations, currently exploited by the majority of programs available world-wide.

The classical Child-Langmuir (CL) law predicts the maximum current density that can be obtained from a planar diode. Recently the 1D classical CL law has been extended to multi-dimensional models, quantum models and short pulse models. In this paper, we look beyond the accelerating stage (in the diode) and focus on the electron flows in the drift space at high power microwave devices, where space charge effects. In particular, we study the space-charge-limited (SCL) electron flows in drift space in classical, relativistic and quantum regimes, including the effects of finite emission and short pulse. The calculations are verified using particle-in-cell (PIC) simulation. The results are also compared to previous calculations on the SCL flows in a diode model.

Investigation on the effect of bow configuration on motion and green water for tumblehome vessels based on cartesian grid finite difference method

This paper presents an efficient, fine-grained parallel algorithm for solving the Eikonal equation on triangular meshes. The Eikonal equation, and the broader class of Hamilton-Jacobi equations to which it belongs, have a wide range of applications from geometric optics and seismology to biological modeling and analysis of geometry and images. The ability to solve such equations accurately and efficiently provides new capabilities for exploring and visualizing parameter spaces and for solving inverse problems that rely on such equations in the forward model. Efficient solvers on state-of-the-art, parallel architectures require new algorithms that are not, in many cases, optimal, but are better suited to synchronous updates of the solution. In previous work [W. K. Jeong and R. T. Whitaker, SIAM J. Sci. Comput., 30 (2008), pp. 2512-2534], the authors proposed the fast iterative method (FIM) to efficiently solve the Eikonal equation on regular grids. In this paper we extend the fast iterative method to solve Eikonal equations efficiently on triangulated domains on the CPU and on parallel architectures, including graphics processors. We propose a new local update scheme that provides solutions of first-order accuracy for both architectures. We also propose a novel triangle-based update scheme and its corresponding data structure for efficient irregular data mapping to parallel single-instruction multiple-data (SIMD) processors. We provide detailed descriptions of the implementations on a single CPU, a multicore CPU with shared memory, and SIMD architectures with comparative results against state-of-the-art Eikonal solvers.

Summary form only given. The results from the development of a three-dimensional particle-in-cell (PIC) model investigating frequency tunability of a reflex-triode virtual cathode oscillator (vircator) are presented. These efforts have focused upon achieving stable frequency output at many different frequencies in the S, L, and C-bands, from a single tube, using the ICEPIC (Improved Concurrent Electromagnetic Particle-In-Cell) code from the Air Force Research Laboratory (AFRL). Experimental data from the hard-tube vircator at Texas Tech University (TTU), which operates at background pressures less than 10-9 Torr and utilizes a bimodal carbon fiber cathode and pyrolytic graphite anode, is used to validate simulation results. Additionally, the vircator at TTU is capable of changing the accelerating voltage, the anode-cathode (A-K) gap distance, and the distance of the cavity backwall relative to the position of the A-K gap on the fly without breaking tube vacuum, all in an effort to achieve greater frequency tunability and output power. However, this creates a very large, time-intensive experimental parameter space and makes a simulation model attractive for exploring additional output capabilities. ICEPIC results from a large combination of A-K gap distances, backwall distances, driving voltages, and cavity diameters are presented and compared to experimental results, highlighting frequency tunability of the system from a single tube.

Ongoing research at the University of New Mexico (UNM) shows significant improvement in the start and rate-of-build-up of microwave oscillations in a relativistic magnetron driven by a transparent cathode [1,2]. Nearly all simulation work on this cathode to-date has been carried out utilizing the MAGIC particle-in-cell (PIC) code. Simulations of the A6 magnetron with transparent cathode using the 3-dimensional code Improved Concurrent Electromagnetic Particle-In-Cell (ICEPIC) at the Air Force Research Lab (AFRL) were carried out to examine the performance of the transparent code and compare with results from MAGIC. Output parameters such as microwave power, microwave frequency and anode current all as function of axial magnetic field are presented and compared with the MAGIC results.

The Magnetic Insulated Line Oscillator (MILO) has gained popularity in recent decades as a useful high-power microwave device, due to its inherent property of self-insulating magnetic field. The self-generated magnetic field acts as an internal insulation barrier within the device and prevents electrical breakdown, which is important for gigawatt range operation. S-band MILO design, analysis, simulation and parameter optimization has been studied and presented in this paper. The presented MILO design consists of a three-stage choke cavity, three- cavity extraction region, and a slow wave structure. An attempt is made to increase the efficiency by using the concept of a tapered extraction region which improves wave extraction at the output a beam dump disk which acts as a tool to properly utilize the load current for self-insulating magnetic field generation. The disk is supported by four appropriately sized and positioned stubs for impedance matching, and hence improve output power. A parametric optimization run on both the structural and beam parameters provides the best values for operation. The results presented are obtained using the commercial software CST Studio, using its 3D Particle-In-Cell (PIC) simulation method. Eigenmode analysis is also performed on the slow wave structure to get dispersion curves for resonant frequency of MILO and verify its operation in n-mode. Simulation results reveal that the S-band MILO generates dominant TM01 mode at the desired frequency of 3.4 GHz. The results show a stable frequency response throughout the time of operation, as well as a high dominant mode efficiency of 98%. The generated wave has a peak power of 6.8 GW and a peak efficiency of 22.5% with the application of an input pulse voltage of 530kV and 57kA current.

Finite-difference time domain (FDTD) techniques are widely used to model the propagation of viscoelastic waves through complex and heterogeneous structures. However, in the specific case of media mixing liquid and solid, attempts to model continuous media onto a Cartesian grid produces errors when the liquid-solid interface between different media do not align precisely with the Cartesian grid. The increase in spatial resolution required to eliminate this grid staircasing effect can be computationally prohibitive. Here, a modification to the Virieux staggered-grid FDTD scheme called the superposition method is presented. This method is intended to reduce this staircasing effect while keeping a manageable computational time. The method was validated by comparing low-spatial-resolution simulations against simulations with sufficiently high resolution to provide reasonably accurate results at any incident angle. The comparison of the root-mean-square of the stress amplitude maps showed that the amplitude of artifactual waves could be reduced by several orders of magnitude when compared to the Virieux staggered-grid FDTD method and that the superposition method helped to significantly reduce the staircasing effect in FDTD simulations.

Efficient data layouts for a three-dimensional electrostatic Particle-in-Cell code

On the computer evaluation of Thiessen weights

Because of the irregular sampling pattern of raw altimeter data, many oceanographic applications rely on information from sea surface height (SSH) products gridded on regular grids where gaps have been filled with interpolation. Today, the operational SSH products are created using the simple, but robust, optimal interpolation (OI) method. If well tuned, the OI becomes computationally cheap and provides accurate results at low resolution. However, OI is not adapted to produce high-resolution and high-frequency maps of SSH. To improve the interpolation of SSH satellite observations, a data-driven approach (i.e., constructing a dynamical forecast model from the data) was recently proposed: analog data assimilation (AnDA). AnDA adaptively chooses analog situations from a catalog of SSH scenes—originating from numerical simulations or a large database of observations—which allow the temporal propagation of physical features at different scales, while each observation is assimilated. In this article, we review the AnDA and OI algorithms and compare their skills in numerical experiments. The experiments are observing system simulation experiments (OSSE) on the Lorenz-63 system and on an SSH reconstruction problem in the Gulf of Mexico. The results show that AnDA, with no necessary tuning, produces comparable reconstructions as does OI with tuned parameters. Moreover, AnDA manages to reconstruct the signals at higher frequencies than OI. Finally, an important additional feature for any interpolation method is to be able to assess the quality of its reconstruction. This study shows that the standard deviation estimated by AnDA is flow dependent, hence more informative on the reconstruction quality, than the one estimated by OI.

Algorithms used in the optimization of wireless deployments have computational time as one of their main drawbacks. This computational time is very dependent on the test points established to characterize a deployment scenario. The approach commonly used to define sample points is based on a regular grid, which leads to a remarkable number of sample points if accuracy is required. In this paper, a novel scenario-dependent sampling scheme based on mathematical morphology is presented. Its performance is evaluated and compared to regular sampling, outperforming typical schemes reducing computational time while maintaining very accurate results.