Plume Simulations Research Articles

Temporal Resolution of Wind Forcing Required for River Plume Simulations

AbstractWind is a primary forcing agent for river plume variability. Consequently, the temporal resolution of wind forcing is an important factor to consider for river plume simulations. This study evaluates river plume simulation errors caused by temporal subsampling of wind forcing data. We use an idealized model of a river plume over a continental shelf and force the model with temporally filtered winds to quantify the effect of temporal subsampling on simulation accuracy. The simulation error is proportional to the fraction of energy missing in the high‐frequency wind absent from the forcing. These results set requirements for temporal wind resolution in realistic simulations of river plumes. Spectral analysis of observed wind records at the Mississippi River, Columbia River, and Merrimack River regions indicates that, for simulation errors due to insufficient temporal wind resolution to be smaller than 5% of the variance, 3‐hourly or 4‐hourly wind data are reasonable. Though horizontal variations in wind forcing are lost, analyzing fast Fourier transformation spectrum of a single‐point wind measurement in the simulation region is helpful for estimating simulation errors due to temporal resolution and hence aid in properly selecting temporal resolutions.

An Investigation of Plume Response to Soil Vapor Extraction and Hypothetical Drum Failure

Core Ideas Soil vapor extraction (SVE) wells are an effective interim remediation technique. Flow and transport modeling assists in evaluation of vapor plume behavior. Plume simulations assuming drum failure and SVE provide a framework for remediation planning. A small borehole subset could be monitored for detection of VOC releases due to drum failure. Soil vapor extraction (SVE) has been used at sites across the Department of Energy complex, including sites where legacy subsurface wastes represent a potential source of groundwater contamination. At Los Alamos National Laboratory (LANL), leakage from waste drums buried at an inactive chemical waste site has created a subsurface vapor plume of volatile organic compounds (VOCs). Soil vapor extraction operation in 2015 and rebound testing through 2017 were successful in reducing the plume's mass and mitigating VOC migration toward the water table. However, the possibility that waste drums could fail and release VOCs could pose a challenge in the future. To explore the impacts of drum failure, as well as the capabilities of SVE remediation, we simulated hypothetical contaminant release scenarios and subsequent SVE remediation. Three‐dimensional subsurface VOC behavior, including advection, diffusion, and plume interactions with topography, were simulated using the porous flow simulator Finite Element Heat and Mass Transfer. Simulations of future site conditions have allowed identification of “sentry” boreholes that can be monitored for early detection in case of drum failure. Sentry boreholes can also be used to set concentration thresholds above which SVE should be initiated. For the LANL site, simulations show that SVE can be started 3 yr following drum failure and remain a viable remediation tool. More broadly, the principles outlined in this work can be used to support remediation planning at other subsurface waste sites. Predictive models of future releases can be analyzed to set concentration threshold values, guide selection of sentry boreholes, and increase operational efficiency.

Machine learning-based rapid response tools for regional air pollution modelling

A parameterised non-intrusive reduced order model (P-NIROM) based on proper orthogonal decomposition (POD) and machine learning methods has been firstly developed for model reduction of pollutant transport equations. Our motivation is to provide rapid response urban air pollution predictions and controls. The varying parameters in the P-NIROM are pollutant sources. The training data sets are obtained from the high fidelity modelling solutions (called snapshots) for selected parameters (pollutant sources, here) over the parameter space RP. From these training data sets, the machine learning method is used to generate the relationship between the reduced solutions and inputs (pollutant sources) over RP. Furthermore a set of hyper-surface functions associated with each POD basis function is constructed for representing the fluid dynamics over the reduced space. The accuracy of the P-NIROM is highly dependent on the quality of the training set, here obtained from the high fidelity model. Over existing machine learning methods, the P-NIROM algorithm proposed here has the advantages that (1) it is combined with NIROM, thus providing rapid and reasonably accurate solutions; and (2) it is a robust and efficient approach for representation of any parametrised partial differential equations as the model parameters/inputs vary. In this study, we demonstrate the way how to implement the P-NIROM for the pollutant transport equation (but not limited to due to its robustness). Its predictive capability is illustrated in a three-dimensional (3-D) simulation of power plant plumes over a large region in China, where the varying parameters are the emission intensity at three locations. Results indicate that in comparison to the high fidelity model, the CPU cost is reduced by factor up to five orders of magnitude while reasonable accuracy remains.

Long-range transport of volcanic aerosol from the 2010 Merapi tropical eruption to Antarctica

Abstract. Volcanic sulfate aerosol is an important source of sulfur for Antarctica, where other local sources of sulfur are rare. Midlatitude and high-latitude volcanic eruptions can directly influence the aerosol budget of the polar stratosphere. However, tropical eruptions can also enhance polar aerosol load following long-range transport. In the present work, we analyze the volcanic plume of a tropical eruption, Mount Merapi in 2010, and investigate the transport pathway of the volcanic aerosol from the tropical tropopause layer (TTL) to the lower stratosphere over Antarctica. We use the Lagrangian particle dispersion model Massive-Parallel Trajectory Calculations (MPTRAC) and Atmospheric Infrared Sounder (AIRS) SO2 measurements to reconstruct the altitude-resolved SO2 injection time series during the explosive eruption period and simulate the transport of the volcanic plume using the MPTRAC model. AIRS SO2 and aerosol measurements, the aerosol cloud index values provided by Michelson Interferometer for Passive Atmospheric Sounding (MIPAS), are used to verify and complement the simulations. The Lagrangian transport simulation of the volcanic plume is compared with MIPAS aerosol measurements and shows good agreement. Both the simulations and the observations presented in this study suggest that volcanic plumes from the Merapi eruption were transported to the south of 60∘ S 1 month after the eruption and even further to Antarctica in the following months. This relatively fast meridional transport of volcanic aerosol was mainly driven by quasi-horizontal mixing from the TTL to the extratropical lower stratosphere, and most of the quasi-horizontal mixing occurred between the isentropic surfaces of 360 to 430 K. When the plume went to Southern Hemisphere high latitudes, the polar vortex was displaced from the South Pole, so that the volcanic plume was carried to the South Pole without penetrating the polar vortex. Although only 4 % of the sulfur injected by the Merapi eruption was transported into the lower stratosphere south of 60∘ S, the Merapi eruption contributed up to 8800 t of sulfur to the Antarctic lower stratosphere. This indicates that the long-range transport under favorable meteorological conditions enables a moderate tropical volcanic eruption to be an important remote source of sulfur for the Antarctic stratosphere.

Surface deposition of the Enceladus plume and the zenith angle of emissions

Since the discovery of an ice particle plume erupting from the south polar terrain on Saturn’s moon Enceladus, the geophysical mechanisms driving its activity have been the focus of substantial scientific research. The pattern and deposition rate of plume material on Enceladus’ surface is of interest because it provides valuable information about the dynamics of the ice particle ejection as well as the surface erosion. Surface deposition maps derived from numerical plume simulations by Kempf et al. (2010) have been used by various researchers to interpret data obtained by various Cassini instruments. Here, an updated and detailed set of deposition maps is provided based on a deep-source plume model (Schmidt et al., 2008), for the eight ice-particle jets identified in Spitale and Porco (2007), the updated set of jets proposed in Porco et al. (2014), and a contrasting curtain-style plume proposed in Spitale et al. (2015). Methods for computing the surface deposition are detailed, and the structure of surface deposition patterns is shown to be consistent across changes in the production rate and size distribution of the plume. Maps are also provided of the surface deposition structure originating in each of the four Tiger Stripes. Finally, the differing approaches used in Porco et al. (2014) and Spitale et al. (2015) have given rise to a jets vs. curtains controversy regarding the emission structure of the Enceladus plume. Here we simulate each, leading to new insight that, over time, most emissions must be directed relatively orthogonal to the surface because jets “tilted” significantly away from orthogonal lead to surface deposition patterns inconsistent with surface images.Data for maps are available in HDF5 format for a variety of particle sizes at http://impact.colorado.edu/southworth_data.

Numerical study of Jiulongjiang river plume in the wet season 2015

Numerical simulation of the buoyant river plume in the Jiulongjiang River (JR) estuary, China, and its shelf in a wet season, April–September 2015, is constructed and applied to explore the evolution of the JR plume. The model is forced with realistic forcing, including river discharge, tides, wind stress, surface heat flux and freshwater flux, and ocean open boundary conditions. The simulation results agree well with in-situ and satellite observations. A Lagrangian particle tracking method is used to identify the freshwater pathway. It is found that the JR freshwater tends to escape the estuary-shelf system to the north from April to mid-August but to the south since late August. Based on a self-organizing map (SOM) analysis, four major JR patterns are identified. A northeastward plume pattern has the highest frequency of occurrence (44.8%), while a southwestward plume pattern has the second highest frequency of occurrence (33.9%). The other two patterns are transitional. The analysis of the alongshore and cross-shore momentum balances indicates that the JR plume is largely shaped by wind and background alongshore current. Particularly, the background coastal current plays an important role for the JR freshwater escape to the north in the Taiwan Strait.

A 3D particle model for the plume CEX simulation

ABSTRACTCharge Exchange (CEX) ion is the main factor causing the plume pollution. The distribution of CEX ions is determined by the distribution of beam ions and neutral atoms. Hence, the primary problem in the study of the plume is how to accurately simulate the distribution of beam ions and neutral atoms. At present, the most commonly used model utilised for the plume simulation is the analytical model proposed by Roy for the plume simulation of the NASA Solar Technology Application Readiness (NSTAR) ion thruster. However, this analytical model can only be applied to the ion beam with small divergence angles. In addition, the analytical model is no longer applicable to the simulation for the plume of a new type of ion thruster that appeared recently, which is called the annular ion thruster. In this paper, a 3D particle model is proposed for the plume simulation of ion thrusters consisting of the particle model for beam ions, the Direct Simulation Monte Carlo (DSMC) model for neutral atoms and the Immersed Finite Element-Particle In Cell-Monte Carlo Collision (IFE-PIC-MCC) model for CEX ions. Then, the plume of the NSTAR ion thruster is simulated by both Roy's model and the 3D particle model. The simulation results of both models are then compared with the experimental results. It is shown that the numerical results of the 3D particle model agree well with those of the analytical model and the experimental data. And this 3D particle model can also be used for other electric thrusters.

CHAOS: An octree-based PIC-DSMC code for modeling of electron kinetic properties in a plasma plume using MPI-CUDA parallelization

A new computational framework for a coupled PIC-DSMC tool using multiple GPUs to model the kinetic behavior of electrons in plasma plumes is presented in this work. The disparate length scales of the Debye length and the collisional mean free path are resolved by using separate, independent linearized Morton Z-ordered forest of trees. A 2:1 restraint is imposed for the PIC module to solve partial differential equations in the context of an AMR/Octree framework. The MPI-CUDA parallelization strategies used to implement a preconditioned conjugate gradient method for solving the electrostatic Poisson's equation on the 2:1 octree are discussed and the scaling of the code to near ideal speedup as a function of the number of GPUs is demonstrated. The PIC method is validated using analytical test cases, and the octree-based PIC simulations are found to be ten times more efficient compared to the uniform grid method especially for plume simulations which have large density variations. The computational strategies are then demonstrated with the simulation of collisionless, mesothermal plasma plumes using a kinetic approach for both ions and electrons. The effect of ion mass and electron source location are analyzed by comparing plume dynamics and electron velocity distribution functions. It is shown that a more confined mesothermal plume is observed for the heavier xenon ions, present in electric propulsion devices, compared to protons. The confinement of the xenon plume traps the electrons resulting in higher electron temperatures compared to the proton plasma case. In both the simulations, however, the electron temperature is found to be anisotropic. Finally, when a shifted electron source location case is considered the electron velocity distributions in all three directions are found to be unequal and non-Maxwellian, contrary to the co-located case. The ion beam is observed to attract the electrons, which initially oscillate between the radial edges of the ion beam as confirmed by the bi-modal velocity distribution at early times. As the plume evolves, it electrostatically traps these electrons within the ion beam, allowing them to thermalize, as observed from the single-peak electron velocity distribution functions at later times. These results demonstrate that GPUs can efficiently accelerate computations of a fully kinetic approach to enable the study of electron kinetics and neutralization with the highest physical fidelity.

Simulating Fine-Scale Marine Pollution Plumes for Autonomous Robotic Environmental Monitoring

Marine plumes exhibit characteristics such as intermittency, sinuous structure, shape and flow field coherency, and a time varying concentration profile. Due to the lack of experimental quantification of these characteristics for marine plumes, existing work often assumes marine plumes exhibit behavior similar to aerial plumes and are commonly modeled by filament based Lagrangian models. Our previous field experiments with Rhodamine dye plumes at Makai Research Pier at Oahu, Hawaii revealed that marine plumes show similar characteristics to aerial plumes qualitatively, but quantitatively they are disparate. Based on the field data collected, this paper presents a calibrated Eulerian plume model that exhibits the qualitative and quantitative characteristics exhibited by experimentally generated marine plumes. We propose a modified model with an intermittent source, and implement it in a Robot Operating System (ROS) based simulator. Concentration time series of stationary sampling points and dynamic sampling points across cross-sections and plume fronts are collected and analyzed for statistical parameters of the simulated plume. These parameters are then compared with statistical parameters from experimentally generated plumes. The comparison validates that the simulated plumes exhibit fine-scale qualitative and quantitative characteristics similar to experimental plumes. The ROS plume simulator facilitates future evaluations of environmental monitoring strategies by marine robots, and is made available for community use.

The importance of vertical resolution in the free troposphere for modeling intercontinental plumes

Abstract. Chemical plumes in the free troposphere can preserve their identity for more than a week as they are transported on intercontinental scales. Current global models cannot reproduce this transport. The plumes dilute far too rapidly due to numerical diffusion in sheared flow. We show how model accuracy can be limited by either horizontal resolution (Δx) or vertical resolution (Δz). Balancing horizontal and vertical numerical diffusion, and weighing computational cost, implies an optimal grid resolution ratio (Δx ∕ Δz)opt ∼ 1000 for simulating the plumes. This is considerably higher than current global models (Δx ∕ Δz ∼ 20) and explains the rapid plume dilution in the models as caused by insufficient vertical resolution. Plume simulations with the Geophysical Fluid Dynamics Laboratory Finite-Volume Cubed-Sphere Dynamical Core (GFDL-FV3) over a range of horizontal and vertical grid resolutions confirm this limiting behavior. Our highest-resolution simulation (Δx ≈ 25 km, Δz ≈ 80 m) preserves the maximum mixing ratio in the plume to within 35 % after 8 days in strongly sheared flow, a drastic improvement over current models. Adding free tropospheric vertical levels in global models is computationally inexpensive and would also improve the simulation of water vapor.

Nonhydrostatic simulation of hyperpycnal river plumes on sloping continental shelves: Flow structures and nonhydrostatic effect

A three-dimensional nonhydrostatic coastal model SUNTANS is used to study hyperpycnal plumes on sloping continental shelves with idealized domain setup. The study aims to examine the nonhydrostatic effect of the plunging hyperpycnal plume and the associated flow structures on different shelf slopes. The unstructured triangular grid in SUNTANS allows for local refinement of the grid size for regions in which the flow varies abruptly, while retaining low-cost computation using the coarse grid resolution for regions in which the flow is more uniform. These nonhydrostatic simulations reveal detailed three-dimensional flow structures in both transient and steady states. Via comparison with the hydrostatic simulation, we show that the nonhydrostatic effect is particularly important before plunging, when the plume is subject to significant changes in both the along-shore and vertical directions. After plunging, where the plume becomes an undercurrent that is more spatially uniform, little difference is found between the hydrostatic and nonhydrostatic simulations in the present gentle- and mild-slope cases. A grid-dependence study shows that the nonhydrostatic effect can be seen only when the grid resolution is sufficiently fine that the calculation is not overly diffusive. A depth-integrated momentum budget analysis is then conducted to show that the flow convergence due to plunging is an important factor in the three-dimensional flow structures. Moreover, it shows that the nonhydrostatic effect becomes more important as the slope increases, and in the steep-slope case, neglect of transport of the vertical momentum during plunging in the hydrostatic case further leads to an erroneous prediction for the undercurrent.

Communications Challenges in Crisis and Transition.

Over the last 5 y, the federal community has made significant progress in preparing for coordinated and efficient public communication efforts during a radiological response. Preparations include the development of pre-scripted messages and plume simulations in the event that there is detonation of an improvised nuclear device or a radiological release from a nuclear power plant. However, challenges remain for improving crisis communications across federal agencies.Interagency language barriers, as well as variances in federal-to-local vernacular, lead to communications challenges that in times of calm can be confusing but in times of crisis could cause major disruptions. In addition to dealing with language barriers, the federal community continues to work on overcoming the "stay in your lane" mentality that could impact the ability to identify a lead voice during a crisis. Federal exercises over the past 2 y have identified the lack of a "lead" federal voice during an incident as a major challenge. They have attributed the problem to the nature of the federal agencies' response structures and the desire of state and local leaders to maintain authority of an incident in their communities. In a transition year, it will be key to review current guidelines based on law and the new administration's communications procedures.

Outdoor air thermal plume simulation of layer-based VRF air conditioners in high-rise buildings

Variable refrigerant flow (VRF) air conditioning system is widely used in commercial buildings for space cooling and heating. However, some VRF systems used in high-rise buildings cannot work efficiently or even stop working because of the relatively high ambient air temperature, caused by the thermal plume effect of exhaust heat from outdoor units. In this paper, the thermal plume air flow of the layer-based VRF systems is investigated through computational fluids dynamics (CFD) simulation. Moreover, an illustrative example of practical VRF system in a 30-storey office building in Shenzhen is analyzed to optimize the layout of the outdoor units. Preliminary results show that the exhaust heat of outdoor units can cause ascending thermal plume flow, leading to higher inlet temperatures for VRF air conditioners on upper floors, even exceeding the warning upper threshold value. It also indicates that enlarging the distance between outdoor units on different floors is an effective way to impair the thermal plume effect for VRF outdoor units and improve the thermal performance of the whole system. For the studied case, the average inlet temperatures can be decreased by 22% for VRF outdoor units with floor interval. This work can provide guidance for the optimization layout design of practical VRF air conditioning systems used in high-rise buildings.

Performance evaluation of dispersion parameterization schemes in the plume simulation of FFT-07 diffusion experiment

Application of atmospheric dispersion models in air quality analysis requires a proper representation of the vertical and horizontal growth of the plume. For this purpose, various schemes for the parameterization of dispersion parameters σ′s are described in both stable and unstable conditions. These schemes differ on the use of (i) extent of availability of on-site measurements (ii) formulations developed for other sites and (iii) empirical relations. The performance of these schemes is evaluated in an earlier developed IIT (Indian Institute of Technology) dispersion model with the data set in single and multiple releases conducted at Fusion Field Trials, Dugway Proving Ground, Utah 2007. Qualitative and quantitative evaluation of the relative performance of all the schemes is carried out in both stable and unstable conditions in the light of (i) peak/maximum concentrations, and (ii) overall concentration distribution. The blocked bootstrap resampling technique is adopted to investigate the statistical significance of the differences in performances of each of the schemes by computing 95% confidence limits on the parameters FB and NMSE. The various analysis based on some selected statistical measures indicated consistency in the qualitative and quantitative performances of σ schemes. The scheme which is based on standard deviation of wind velocity fluctuations and Lagrangian time scales exhibits a relatively better performance in predicting the peak as well as the lateral spread.

Rotating 2d point source plume models with application to Deepwater Horizon

The 2010 Deepwater Horizon (DwH) accident in the Gulf of Mexico has renewed oceanographic interest in point source buoyant convection. The present paper applies modern numerical techniques to study this problem, focussing specifically on the DwH event. The gas/oil/seawater nature of the problem requires a ‘multiphase’ approach, which is relatively unfamiliar in physical oceanography, although applications are becoming more common. The model is cast in an Eulerian framework and includes feedbacks between the convection and the environment, unlike past oil/gas plume simulations that adopt a semi-passive, Lagrangian approach. Fully three dimensional (3d) simulations are too computationally demanding for practical multi-day use, so a two-dimensional (2d) radially symmetric model is developed from the equations and calibrated to the 3d results. Both the 2d and 3d solutions show the somewhat unexpected result that oil/bubble plumes modelled after the DwH event are strongly affected by rotation and exert a considerable dynamic feedback on the ambient. These effects are not typically included in classical oil/gas plume models.

Sonar gas flux estimation by bubble insonification: application to methane bubble flux from seep areas in the outer Laptev Sea

Abstract. Sonar surveys provide an effective mechanism for mapping seabed methane flux emissions, with Arctic submerged permafrost seepage having great potential to significantly affect climate. We created in situ engineered bubble plumes from 40 m depth with fluxes spanning 0.019 to 1.1 L s−1 to derive the in situ calibration curve (Q(σ)). These nonlinear curves related flux (Q) to sonar return (σ) for a multibeam echosounder (MBES) and a single-beam echosounder (SBES) for a range of depths. The analysis demonstrated significant multiple bubble acoustic scattering – precluding the use of a theoretical approach to derive Q(σ) from the product of the bubble σ(r) and the bubble size distribution where r is bubble radius. The bubble plume σ occurrence probability distribution function (Ψ(σ)) with respect to Q found Ψ(σ) for weak σ well described by a power law that likely correlated with small-bubble dispersion and was strongly depth dependent. Ψ(σ) for strong σ was largely depth independent, consistent with bubble plume behavior where large bubbles in a plume remain in a focused core. Ψ(σ) was bimodal for all but the weakest plumes. Q(σ) was applied to sonar observations of natural arctic Laptev Sea seepage after accounting for volumetric change with numerical bubble plume simulations. Simulations addressed different depths and gases between calibration and seep plumes. Total mass fluxes (Qm) were 5.56, 42.73, and 4.88 mmol s−1 for MBES data with good to reasonable agreement (4–37 %) between the SBES and MBES systems. The seepage flux occurrence probability distribution function (Ψ(Q)) was bimodal, with weak Ψ(Q) in each seep area well described by a power law, suggesting primarily minor bubble plumes. The seepage-mapped spatial patterns suggested subsurface geologic control attributing methane fluxes to the current state of subsea permafrost.

Oil plume simulations: Tracking oil droplet size distribution and fluorescence within high-pressure release jets

ABSTRACT Optical measurements have been used during oil spill response for more than three decades to determine oil presence in slicks and plumes. Oil surveillance approaches range from simple (human eyeball) to the sophisticated (sensors on AUVs, aircraft, satellites). In situ fluorometers and particle size analyzers were deployed during the Deepwater Horizon (DWH) Gulf of Mexico oil spill to track shallow and deep subsea plumes. Uncertainties regarding instrument specifications and capabilities during DWH necessitated performance testing of sensors exposed to simulated, dispersed oil plumes. Seventy-two wave tank experiments were conducted at the Bedford Institute of Oceanography. Simulated were oil releases with varying parameters such as oil release rate, oil temperature (reservoir temp ~ 80 °C), water temperature (<8 °C and >15 °C), oil type, dispersant type (Corexit 9500 and Finasol OSR52) and dispersant to oil ratio (DOR). Plumes of Alaskan North Slope Crude (ANS), South Louisiana Crude (SLC) and IFO-120 oils were tracked using in situ fluorescence, droplet size distribution (DSD), total petroleum hydrocarbons (TPH) and benzene-toluene-ethylbenzene-xylene (BTEX). For the lighter SLC, bimodal droplet size with mean diameter < 70 μm was achieved for 1:20 and 1:100 DOR, regardless of water temperature. Similarly, the medium ANS crude exhibited mean droplet diameter <70 μm, but was bimodal only for the 1:20 treatment. Bimodal distribution was not achieved with the heavy IFO, but droplet < 70 μm were observed for 1:20 warm waters, indicating poor dispersibility of the high viscosity oil even for jet releases. Results offer valuable information on the behavior and dispersibility of oils over a range of viscosity, DOR and environmental conditions. Findings have implications for fate and transport models, where DSD, chemistry and fluorescence are all impacted by release variables. This research was supported by the Bureau of Safety and Environmental Enforcement.

Numerical simulation of bubble plumes and an analysis of their seismic attributes

To study the bubble plume’s seismic response characteristics, the model of a plume water body has been built in this article using the bubble-contained medium acoustic velocity model and the stochastic medium theory based on an analysis of both the acoustic characteristics of a bubble-contained water body and the actual features of a plume. The finite difference method is used for forward modelling, and the single-shot seismic record exhibits the characteristics of a scattered wave field generated by a plume. A meaningful conclusion is obtained by extracting seismic attributes from the pre-stack shot gather record of a plume. The values of the amplitude-related seismic attributes increase greatly as the bubble content goes up, and changes in bubble radius will not cause seismic attributes to change, which is primarily observed because the bubble content has a strong impact on the plume’s acoustic velocity, while the bubble radius has a weak impact on the acoustic velocity. The above conclusion provides a theoretical reference for identifying hydrate plumes using seismic methods and contributes to further study on hydrate decomposition and migration, as well as on distribution of the methane bubble in seawater.

Numerical analysis on plume temperature properties formed above a harmonically oscillating fire source

Large eddy simulation (LES) of turbulent buoyant plumes formed above an oscillating fire source has been carried out so as to clarify their temperature properties in the offshore environment. For the sake of simplicity, we are concerned here only with a rolling environment among six-degree-of-freedom system of complex ship motion. Furthermore, under the assumption of a small rolling angle, unsteady fire source movement associated with the rolling motion can be approximated by simple harmonic oscillation in the horizontal direction. Numerical results obtained under several sets of oscillation conditions, especially under different amplitude conditions, are compared with temperature variations of conventional axisymmetric and two-dimensional fire plumes above stationary sources. Consequently, it has been clarified that in the case where the amplitude is four times longer than the side length of a square source, the present buoyant plumes exhibit almost the same feature as the two-dimensional fire plume in the lower part of the buoyant plume region. On the other hand, it has been found that for any value of amplitude, the temperature properties are close to those of axisymmetric plumes in the upper part of the buoyant plume region. Based on these findings, a simple theoretical model to estimate the variation in mean excess temperature as a function of elevation has been developed by the use of a well-recognized, semi-analytical solution of the axisymmetric plume formed above a virtual point source. It has been confirmed that this simple model can reasonably reproduce the same plume features as obtained by the LES simulations.

Data Management and Volcano Plume Simulation with Parallel SPH Method and Dynamic Halo Domains

This paper presents data management and strategies for implementing smoothed particle hydrodynamics (SPH) method to simulate volcano plumes. These simulations require a careful definition of the domain of interest and multi-phase material involved in the flow, both of which change over time and involve transport over vast distances in a short time. Computational strategies are developed to overcome these challenges by building mechanisms for efficient creation and deletion of particles for simulation, parallel processing (using the message passing interface (MPI)) and a dynamically defined halo domain (a domain that ”optimally” captures all the material involved in the flow). A background grid is adopted to reduce neighbor search costs and to decompose the domain. A Space Filing Curve (SFC) based ordering is used to assign unique identifiers to background grid entities and particles. Time-dependent SFC based indices are assigned to particles to guarantee uniqueness of the identifier. Both particles and background grids are managed by hash tables which can ensure quick and flexible access. An SFC based three dimensional (3D) domain decomposition and a dynamic load balancing strategy are implemented to ensure good load balance. Several strategies are developed to improve performance: dynamic halo domains, calibrated particle weight and optimized work load check intervals. Numerical tests show that our code has good scalability and performance. The strategies described in this paper can be further applied to many other implementations of mesh-free methods, especially those implementations that require flexibility in adding and deleting of particles.