Vakhtang Makarashvili

A performance analysis of ensemble averaging for high fidelity turbulence simulations at the strong scaling limit

Abstract We analyze the potential performance benefits of estimating expected quantities in large eddy simulations of turbulent flows using true ensembles rather than ergodic time averaging. Multiple realizations of the same flow are simulated in parallel, using slightly perturbed initial conditions to create unique instantaneous evolutions of the flow field. Each realization is then used to calculate statistical quantities. Provided each instance is sufficiently de-correlated, this approach potentially allows considerable reduction in the time to solution beyond the strong scaling limit for a given accuracy. This paper focuses on the theory and implementation of the methodology in Nek5000, a massively parallel open-source spectral element code.

Accelerating the High-Fidelity Simulation of Turbulence: Ensemble Averaging

Computational fluid dynamics (CFD) is increasingly used to simulate complex industrial systems. Most CFD analysis relies on the Reynolds-averaged Navier-Stokes (RANS) approach and traditional two-equation turbulence models. Higher-fidelity approaches to the simulation of turbulence such as wallresolved large eddy simulation (LES) and direct numerical simulation (DNS) remain limited to smaller applications or to large supercomputing platforms. Nonetheless, continued advances in supercomputing are enabling the simulation of physical systems of increasing size and complexity. These simulations can be used to gain unprecedented insight into the physics of turbulence in complex flows and will become more widespread as petascale architectures become more accessible. As the scale and size of LES and DNS simulations increase, however, the limitations of current algorithms become apparent. For larger systems, more temporal and spatial scale must be resolved, thus increasing the time-scale separation. While the smaller time scales dictate the size and the computational cost associated with each time step, the larger time scales dictate the length of the transient. An increased time-scale separation leads to smaller time steps and longer transients, eventually leading to simulations that are impractical or infeasible. In practice the presence of multiple and strongly separated time scales limits the effectiveness of CFD algorithms for LES and DNS applied to large industrial systems. Moreover, the situation is likely to become worse on future computer architectures, as even larger systems will be simulated, thus increasing the size and length of transients. At the same time transients currently simulated on petascale architectures are unlikely to become any faster on exascale architectures. In this paper we consider a technique to accelerate current transient simulations aimed at collect averaged turbulent statistics. The focus is on ergodic flows and simulations. This technique is ensemble averaging, commonplace in machine learning and artificial neural networks. Ensemble averaging is the process of creating multiple models and combining them to produce a desired output. It is also at the basis of RANS/URANS turbulence modeling. In the proposed approach, multiple instances of the same ergodic flows are simulated in parallel for a short timeframe and summed to create an ensemble. Provided each instance is sufficiently statistically decorrelated, this allows considerable reduction in the time to solution. This paper focuses on the theory and implementation of the methodology in Nek5000, a massively parallel open-source spectral element code. Moreover, we present the application of the method to the DNS and LES simulation of channel flow and pipe flow. INTRODUCTION Computational fluid dynamics (CFD) is increasingly used to simulate turbulent flows. Most CFD analysis, especially in industry, relies on the Reynolds-averaged Navier-Stokes (RANS) approach and traditional two-equation turbulence models. Higher-fidelity approaches to the simulation of turbulence, such as wall-resolved large eddy simulation (LES) and direct numerical simulation (DNS), remain limited to smaller applications or to large supercomputing platforms. In fact, since the Reynolds number dictates the local resolution, large machines are currently necessary to simulate engineering systems with turbulence-resolving techniques. Nonetheless, continued advances in supercomputing are enabling the simulation of physical systems of increasing size and complexity. Current supercomputers can accommodate grids that reach tens to hundreds of billions of points, enabling the simulation of entire rod bundles with wall-resolved LES using CFD algorithms with good scalability properties. As supercomputer become more powerful and larger, LES and DNS simulations become possible. With traditional algorithms, however, little can be done to make the solution of traditional problems run faster once the strong-scaling limit is reached. Therefore, the time to solution of traditional algorithms is

Optimization of the Processing of Mo Disks

The objective of this work is to decrease the processing time for irradiated disks of enriched Mo for the production of 99Mo. Results are given for the dissolution of nonirradiated Mo disks, optimization of the process for large-scale dissolution of sintered disks, optimization of the removal of the main side products (Zr and Nb) from dissolved targets, and dissolution of irradiated Mo disks.

Assessment of candidates for target window material in accelerator-driven molybdenum-99 production

NorthStar Medical Technologies is pursuing production of an important medical isotope, Mo-99, through a photo-nuclear reaction of a Mo-100 target using a high-power electron accelerator. The current target utilizes an Inconel 718 window. The purpose of this study was to evaluate other candidate materials for the target window, which separates the high-pressure helium gas inside the target from the vacuum inside the accelerator beamline and is subjected to significant stress. Our initial analysis assessed the properties (density, thermal conductivity, maximum stress, minimum window thickness, maximum temperature, and figure of merit) for a range of materials, from which the three most promising were chosen: Inconel 718, 250 maraging steel, and standard-grade beryllium. These materials were subjected to further analysis to determine the effects of thermal and mechanical strain versus beam power at varying thicknesses. Both beryllium and the maraging steel were calculated to withstand more than twice as high beam power than Inconel 718.

Experimental Results for Direct Electron Irradiation of a Uranyl Sulfate Solution: Bubble Formation and Thermal Hydraulics Studies

In support of the development of accelerator-driven production of fission product Mo-99 as proposed by SHINE Medical Technologies, a 35 MeV electron linac was used to irradiate depleted-uranium (DU) uranyl sulfate dissolved in pH 1 sulfuric acid at average power densities of 6 kW, 12 kW, and 15 kW. During these irradiations, gas bubbles were generated in the solution due to the radiolytic decomposition of water molecules in the solution. Multiple video cameras were used to record the behavior of bubble generation and transport in the solution. Seven six-channel thermocouples were used to record temperature gradients in the solution from self-heating. Measurements of hydrogen and oxygen concentrations in a helium sweep gas were recorded by a gas chromatograph to estimate production rates during irradiation. These data are being used to validate a computational fluid dynamics (CFD) model of the experiment that includes multiphase flow and a custom bubble injection model for the solution region.

Cu‐67 Photonuclear Production

Cu‐67 is considered as one of the most promising radioisotopes for cancer therapy with monoclonal antibodies. Current production schemes using high‐flux reactors and cyclotrons do not meet potential market need. In this paper we discuss Cu‐67 photonuclear production through the reaction Zn‐68(γ,p)Cu‐67. Computer simulations were done together with experiments to study and optimize Cu‐67 yield in natural Zn target. The data confirms that the photonuclear method has potential to produce large quantities of the isotope with sufficient purity to be used in medical field.