Correlations between sunspots and their moat flows

Context. The presence of the moat flow around sunspots is intimately linked to the mere existence of sunspots. Aims. We characterize the moat flow (MF) and Evershed flow (EF) in sunspots to enhance our knowledge of sunspot structures and photospheric flow properties. Methods. We calibrated HMI synoptic Doppler maps and used them to analyze 3h time averages of 31 circular, stable, and fully developed sunspots at heliocentric angles of some 50$^{\circ}$. Assuming axially symmetrical flow fields, we infer the azimuthally averaged horizontal velocity component of the MF and EF from 51 velocity maps. We studied the MF properties (velocity and extension) and elaborate on how these components depend on sunspot parameters (sunspot size and EF velocity). To explore the weekly and monthly evolution of MFs, we compare spots rotating from the eastern to western limbs and spots that reappear on the eastern limb. Results. Our calibration procedure of HMI Doppler maps yields reliable and consistent results. In 3h averages, we find the MF decreases on average from some 1000 $\pm$ 200m/s just outside the spot boundary to 500m/s after an additional 4 Mm. The average MF extension lies at 9.2 $\pm$ 5 Mm, where the velocity drops below some 180m/s. Neither the MF velocity nor its extension depend significantly on the sunspot size or EF velocity. But, the EF velocity does show a tendency to be enhanced with sunspot size. On a time scale of a week and a month, we find decreasing MF extensions and a tendency for the MF velocity to increase for strongly decaying sunspots, whereas the changing EF velocity has no impact on the MF. Conclusions. On 3h averages, the EF velocity scales with the size of sunspots, while the MF properties show no significant correlation with the EF or with the sunspot size. This we interpret as a hint that the physical origins of EF and MF are distinct.Comment: 10 pages, 10 figures, 1 table; appendix: 3 pages, 2 figures, 1 table ; accepted by A&

CFD Solvers with Minimal Memory Access

Many state of the art CFD codes that exhibit low computational intensity (flops per RAM access) "saturate" the memory bandwidth of modern chips after only a few cores, thus minimizing any benefits from going to a higher number of available cores. This bottleneck is expected to become even more pronounced for future manycore systems. This has led to the quest for CFD solvers with minimal memory access. We report on recent developments and results for Finite Difference and Edge-Based Finite Element solvers. The best of these implementations yield one residual for only 6 fetches and 4 stores, regardless of the size of the stencil (and therefore the discretization order). This means that in terms of memory access they are competitive even with finite difference stencils as low as 2 (typical of CFD codes with 2nd order spatial discretization of fluxes and 4th order damping). Timings for a low Mach number finite difference code using a 6th order spatial discretization show competitive timings as compared to conventional loops. This bodes well for future HPC architectures.Publicado en: Mecánica Computacional vol. XXXV, no. 1.Facultad de Ingenierí

CFD Solvers with Minimal Memory Access

Many state of the art CFD codes that exhibit low computational intensity (flops per RAM access) "saturate" the memory bandwidth of modern chips after only a few cores, thus minimizing any benefits from going to a higher number of available cores. This bottleneck is expected to become even more pronounced for future manycore systems. This has led to the quest for CFD solvers with minimal memory access. We report on recent developments and results for Finite Difference and Edge-Based Finite Element solvers. The best of these implementations yield one residual for only 6 fetches and 4 stores, regardless of the size of the stencil (and therefore the discretization order). This means that in terms of memory access they are competitive even with finite difference stencils as low as 2 (typical of CFD codes with 2nd order spatial discretization of fluxes and 4th order damping). Timings for a low Mach number finite difference code using a 6th order spatial discretization show competitive timings as compared to conventional loops. This bodes well for future HPC architectures.Publicado en: Mecánica Computacional vol. XXXV, no. 1.Facultad de Ingenierí

Simulation of flows with violent free surface motion and moving objects using unstructured grids

This is the peer reviewed version of the following article: [Löhner, R. , Yang, C. and Oñate, E. (2007), Simulation of flows with violent free surface motion and moving objects using unstructured grids. Int. J. Numer. Meth. Fluids, 53: 1315-1338. doi:10.1002/fld.1244], which has been published in final form at https://doi.org/10.1002/fld.1244. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Self-Archiving.A volume of fluid (VOF) technique has been developed and coupled with an incompressible Euler/Navier–Stokes solver operating on adaptive, unstructured grids to simulate the interactions of extreme waves and three-dimensional structures. The present implementation follows the classic VOF implementation for the liquid–gas system, considering only the liquid phase. Extrapolation algorithms are used to obtain velocities and pressure in the gas region near the free surface. The VOF technique is validated against the classic dam-break problem, as well as series of 2D sloshing experiments and results from SPH calculations. These and a series of other examples demonstrate that the ability of the present approach to simulate violent free surface flows with strong nonlinear behaviour.Peer ReviewedPostprint (author's final draft

Prediction of wear via DEM and phenomenological models

The Discrete Element Method (DEM) is a computational method used to describe the movement of a large number of particle of different sized and shapes, which interact through a contact model. Among other applications, in the field of mining DEM have been used extensively for predicting the trajectory of material inside Semi-Autogenous Grinding (SAG) mills and in the chutes of minerals transfer. However, no calculations that predict the wear of the enclosing walls have been performed to date. After an extensive review of the literature, a methodology to predict wear via DEM and phenomenological wear models has been developed. The decision was taken to use Archard's model (one of the simplest yet most accurate models proposed to date) in the context of DEM. Given that the wear occurs in a matter of weeks or months, and that a DEM run of even a minute can consume copious amounts of computer resources, a separation of timescales was implemented. For each stage of the overall cycle, the present configuration is run for a relatively small amount of physical time (from T0 to T1) in order to get the statistics of wear. For a mill, this could be a few rotations. For all the faces on the boundaries, the wear is updated every time step. At the end of the DEM run, the total change in volume is used to compute a `recession speed' for each face. The recession speed is then used to extrapolate the recession distance (i.e. the wear) from T0 to a much larger time T2. Once the surface is moved via the recession distance, the run is restarted and the cycle repeats. The result obtained to date show that the methodology is able to compute realistic wear patterns with CPU requirements that are acceptable in an engineering design environment.Publicado en: Mecánica Computacional vol. XXXV, no. 7.Facultad de Ingenierí

Improvements in speed and scalability of a DEM code

A number of near-optimal techniques were implemented to reduce computing times for the Discrete Element Method (DEM) code named DESOL. Among these, the following showed the largest improvements: multilevel bins, periodic rebuild, trimming and Symmetric Multiprocessor (SMP) parallelization. These improvements have led to Central Processing Unit (CPU) reduction of the order of 1:3-1:5 on scalar machines, while also showing excellent scalability up to the point of memory saturation, which on current Intel Xeon processors occurs at approximately 8 cores for double precision and 16 cores for single precision.Publicado en: Mecánica Computacional vol. XXXV, no. 10.Facultad de Ingenierí

Improvements in speed and scalability of a DEM code

A number of near-optimal techniques were implemented to reduce computing times for the Discrete Element Method (DEM) code named DESOL. Among these, the following showed the largest improvements: multilevel bins, periodic rebuild, trimming and Symmetric Multiprocessor (SMP) parallelization. These improvements have led to Central Processing Unit (CPU) reduction of the order of 1:3-1:5 on scalar machines, while also showing excellent scalability up to the point of memory saturation, which on current Intel Xeon processors occurs at approximately 8 cores for double precision and 16 cores for single precision.Publicado en: Mecánica Computacional vol. XXXV, no. 10.Facultad de Ingenierí

A general advancing front technique for filling space with arbitrary objects

An advancing front space‐filling technique for arbitrary objects has been developed. The input required consists of the specification of the desired mean point distance in space and an initial triangulation of the surface. One object at a time is removed from the active front, and, if possible, surrounded by admissible new objects. This operation is repeated until no active objects are left. Two techniques to obtain maximum packing are discussed: closest object placement (during generation) and move/enlarge (after generation). Different deposition or layering patterns can be achieved by selecting the order in which objects are eliminated from the active front. Timings show that for simple objects like spheres the scheme is considerably faster than volume mesh generators based on the advancing front technique, making it possible to generate large (> 106) yet optimal clouds of points in a matter of minutes on a PC. For more general objects, the performance may degrade depending on the complexity of the penetration checks. Several examples are included that demonstrate the capabilities of the technique.&nbsp

Advancing front techniques for filling space with arbitrary separated objects

A review is given of advancing front techniques for filling space with arbitrary separated objects. Over the last decade, these techniques have reached a considerable degree of maturity and are being used to generate clouds of points for SPH and FPM simulations, as well as spheres, ellipsoids, objects defined by a collection of spheres or polyhedral objects for DEM simulations. Algorithmic as well as implementational aspects are discussed. Techniques to obtain maximum packing, such as closest object placement (during generation) and move/enlarge (after generation) are also considered. Several examples are included that demonstrate the capabilities developed