Numerical Simulations of 2D Hydraulic Jumps by a Parallel SPH Model

Simulating large-scale systems usually entails exhaustive computational powers and lengthy execution times. The goal of this research is to reduce execution time of large-scale simulations without sacrificing their accuracy by partitioning a monolithic model into multiple pieces automatically and executing them in a distributed computing environment. While this partitioning allows us to distribute required computational power to multiple computers, it creates a new challenge of synchronizing the partitioned models. In this article, a partitioning methodology based on a modified Prim’s algorithm is proposed to minimize the overall simulation execution time considering 1) internal computation in each of the partitioned models and 2) time synchronization between them. In addition, the authors seek to find the most advantageous number of partitioned models from the monolithic model by evaluating the tradeoff between reduced computations vs. increased time synchronization requirements. In this article, epoch- based synchronization is employed to synchronize logical times of the partitioned simulations, where an appropriate time interval is determined based on the off-line simulation analyses. A computational grid framework is employed for execution of the simulations partitioned by the proposed methodology. The experimental results reveal that the proposed approach reduces simulation execution time significantly while maintaining the accuracy as compared with the monolithic simulation execution approach.

Download Full-text

Hydrodynamics of pedestrians' instability in floodwaters

Hydrology and Earth System Sciences ◽

10.5194/hess-21-515-2017 ◽

2017 ◽

Vol 21 (1) ◽

pp. 515-531 ◽

Cited By ~ 25

Author(s):

Chiara Arrighi ◽

Hocine Oumeraci ◽

Fabio Castelli

Keyword(s):

Experimental Data ◽

Froude Number ◽

Flood Risk ◽

Risk Mitigation ◽

Three Dimensional ◽

Numerical Results ◽

Mitigation Measures ◽

Dimensionless Numbers ◽

Large Variability ◽

Mobility Parameter

Abstract. People's safety is the first objective to be fulfilled by flood risk mitigation measures, and according to existing reports on the causes of casualties, most of the fatalities are due to inappropriate behaviour such as walking or driving in floodwaters. Currently available experimental data on people instability in floodwaters suffer from a large dispersion primarily depending on the large variability of the physical characteristics of the subjects. This paper introduces a dimensionless mobility parameter θP for people partly immersed in flood flows, which accounts for both flood and subject characteristics. The parameter θP is capable of identifying a unique threshold of instability depending on a Froude number, thus reducing the scatter of existing experimental data. Moreover, a three-dimensional (3-D) numerical model describing the detailed geometry of a human body and reproducing a selection of critical pairs of water depth and velocity is presented. The numerical results in terms of hydrodynamic forces and force coefficients are analysed and discussed. Both the mobility parameter θP and the numerical results hint at the crucial role of the Froude number and relative submergence as the most relevant dimensionless numbers to interpret the loss of stability. Finally, the mobility parameter θP is compared with an analogous dimensionless parameter for vehicles' instability in floodwaters, providing a new contribution to support flood risk management and educating people.

Download Full-text

Particle Dispersion in Fine Scales of Homogeneous Isotropic Turbulence

Volume 1: Symposia, Parts A and B ◽

10.1115/fedsm2007-37234 ◽

2007 ◽

Author(s):

M. Sato ◽

M. Tanahashi ◽

T. Miyauchi

Keyword(s):

Isotropic Turbulence ◽

Major Axis ◽

Stokes Number ◽

High Energy ◽

Energy Dissipation Rate ◽

Particle Dispersion ◽

Fine Scale ◽

Number Density ◽

Homogeneous Isotropic Turbulence ◽

Preferential Concentration

Direct numerical simulations of homogeneous isotropic turbulence laden with particles have been conducted to clarify the relationship between particle dispersion and coherent fine scale eddies in turbulence. Dispersion of 106 particles are analyzed for several particle Stokes numbers. The spatial distributions of particles depend on their Stokes number, and the Stokes number that causes preferential concentration of particles is closely related to the time scale of coherent fine scale eddies in turbulence. On the plane perpendicular to the rotating axes of fine scale eddies, number density of particle with particular Stokes number is low at the center of the fine scale eddy, and high in the regions with high energy dissipation rate around the eddy. The maximum number density can be observed at about 1.5 to 2.0 times the eddy radius on the major axis of the fine scale eddy.

Download Full-text

Automatic Partitioning of Large Scale Simulation in Grid Computing for Run Time Reduction

International Journal of Operations Research and Information Systems ◽

10.4018/joris.2010040105 ◽

2010 ◽

Vol 1 (2) ◽

pp. 64-90 ◽

Cited By ~ 6

Author(s):

Nurcin Celik ◽

Esfandyar Mazhari ◽

John Canby ◽

Omid Kazemi ◽

Parag Sarfare ◽

...

Keyword(s):

Execution Time ◽

Large Scale ◽

Time Synchronization ◽

Computational Grid ◽

Time Interval ◽

Computational Power ◽

Computing Environment ◽

Large Scale Systems ◽

Large Scale Simulations ◽

Reduce Execution Time

Simulating large-scale systems usually entails exhaustive computational powers and lengthy execution times. The goal of this research is to reduce execution time of large-scale simulations without sacrificing their accuracy by partitioning a monolithic model into multiple pieces automatically and executing them in a distributed computing environment. While this partitioning allows us to distribute required computational power to multiple computers, it creates a new challenge of synchronizing the partitioned models. In this article, a partitioning methodology based on a modified Prim’s algorithm is proposed to minimize the overall simulation execution time considering 1) internal computation in each of the partitioned models and 2) time synchronization between them. In addition, the authors seek to find the most advantageous number of partitioned models from the monolithic model by evaluating the tradeoff between reduced computations vs. increased time synchronization requirements. In this article, epoch- based synchronization is employed to synchronize logical times of the partitioned simulations, where an appropriate time interval is determined based on the off-line simulation analyses. A computational grid framework is employed for execution of the simulations partitioned by the proposed methodology. The experimental results reveal that the proposed approach reduces simulation execution time significantly while maintaining the accuracy as compared with the monolithic simulation execution approach.

Download Full-text

Study on the Best Depth of Stilling Basin with Shallow-Water Cushion

Water ◽

10.3390/w10121801 ◽

2018 ◽

Vol 10 (12) ◽

pp. 1801

Author(s):

Qiulin Li ◽

Lianxia Li ◽

Huasheng Liao

Keyword(s):

Energy Dissipation ◽

Shallow Water ◽

Froude Number ◽

Flow Regime ◽

Dissipation Rate ◽

Hydraulic Jump ◽

Energy Dissipation Rate ◽

Main Stream ◽

Stilling Basin ◽

Key Factor

The depth of the stilling basin with shallow-water cushion (SBSWC) is a key factor that affects the flow regime of hydraulic jump in the basin. However, the specific depth at which the water cushion is considered as ‘shallow’ has not been stated clearly by far, and only conceptual description is provided. Therefore, in order to define the best depth of SBSWC and its relationship between the Froude number at the inlet of the stilling basin, a large number of experiments were carried out to investigate SBSWC. First of all, 30 cases including five different Froude numbers and six depths were selected for which large eddy simulation (LES) was firstly verified by the experiments and then adopted to calculate the hydraulic characteristics in the stilling basin. Finally, three standards, based on the flow regime of hydraulic jump, the location of the main stream and the energy dissipation rate, were proposed to define the best depth of SBSWC. The three criteria are as follows: (1) a complete hydraulic jump occurs in the basin (2) the water cushion is about 1/10–1/3 deep of the stilling basin, and (3) the energy dissipation rate is more than 70% and the unit volume energy dissipation rate is as high as possible. It showed that the best depth ratio of SBSWC (depth to length ratio) was between 0.1 and 0.3 and it also indicated the best depth increased with the increase in Froude number. The results of the work are of significance to the design and optimizing of SBSWC.

Download Full-text

Accretion heated atmospheres of X-ray bursting neutron stars

Astronomy and Astrophysics ◽

10.1051/0004-6361/201833581 ◽

2018 ◽

Vol 619 ◽

pp. A114 ◽

Cited By ~ 7

Author(s):

V. F. Suleimanov ◽

J. Poutanen ◽

K. Werner

Keyword(s):

Neutron Star ◽

Model Atmosphere ◽

High Energy ◽

Energy Dissipation Rate ◽

Chemical Compositions ◽

Type I ◽

Coulomb Interactions ◽

Spectral Evolution ◽

X Ray ◽

High Energy Tail

Some thermonuclear (type I) X-ray bursts at the neutron star surfaces in low-mass X-ray binaries take place during hard persistent states of the systems. Spectral evolution of these bursts is well described by the atmosphere model of a passively cooling neutron star when the burst luminosity is high enough. The observed spectral evolution deviates from the model predictions when the burst luminosity drops below a critical value of 20–70% of the maximum luminosity. The amplitude of the deviations and the critical luminosity correlate with the persistent luminosity, which leads us to suggest that these deviations are induced by the additional heating of the accreted particles. We present a method for computation of the neutron star atmosphere models heated by accreted particles assuming that their energy is released via Coulomb interactions with electrons. We computed the temperature structures and the emergent spectra of the atmospheres of various chemical compositions and investigate the dependence of the results on the velocity of accreted particles, their temperature and the penetration angle. We show that the heated atmosphere develops two different regions. The upper one is the hot (20–100 keV) corona-like surface layer cooled by Compton scattering, and the deeper, almost isothermal optically thick region with a temperature of a few keV. The emergent spectra correspondingly have two components: a blackbody with the temperature close to that of the isothermal region and a hard Comptonized component (a power law with an exponential decay). Their relative contribution depends on the ratio of the energy dissipation rate of the accreted particles to the intrinsic flux from the neutron star surface. These spectra deviate strongly from those of undisturbed, passively cooling neutron star atmospheres, with the main differences being the presence of a high-energy tail and a strong excess in the low-energy part of the spectrum. They also lack the iron absorption edge, which is visible in the spectra of undisturbed low-luminosity atmospheres with solar chemical composition. Using the computed spectra, we obtained the dependences of the dilution and color-correction factors as functions of relative luminosities for pure helium and solar abundance atmospheres. We show that the helium model atmosphere heated by accretion corresponding to 5% of the Eddington luminosity describes well the late stages of the X-ray bursts in 4U 1820−30.

Download Full-text