Kaushik Das

Numerical Simulations of Tsunami Wave Generation by Submarine and Aerial Landslides Using RANS and SPH Models

Tsunami wave generation by submarine and aerial landslides is examined in this paper. Two different two-dimensional numerical methods have been used to simulate the time histories of fluid motion, free surface deformation, shoreline movement, and wave runup from tsunami waves generated by aerial and submarine landslides. The first approach is based on the Navier-Stokes equation and the volume of fluid (VOF) method: the Reynolds Averaged Navier-Stokes (RANS)-based turbulence model simulates turbulence, and the VOF method tracks the free surface locations. The second method uses Smoothed Particle Hydrodynamics (SPH)—a numerical model based on a fully Lagrangian approach. In the current work, two-dimensional numerical simulations are carried out for a freely falling wedge representing the landslide and subsequent wave generations. Numerical simulations for the landslide-driven tsunami waves have been performed with different values of landslide material densities. Numerical results obtained from both approaches are compared with experimental data. Simulated results for both aerial and submerged landslides show the complex flow patterns in terms of the velocity field, shoreline evolution, and free-surface profiles. Flows are found to be strongly transient, rotational, and turbulent. Predicted numerical results for time histories of free-surface fluctuations and the runup/rundown at various locations are in good agreement with the available experimental data. The similarity and discrepancy between the solutions obtained by the two approaches are explored and discussed.Copyright

Comparison of Eulerian-Granular and Discrete Element Models for Simulation of Proppant Flows in Fractured Reservoirs

Numerical simulations provide an effective technique to understand the proppant behavior within hydraulic fractures and determine fracking efficiency. Numerical techniques currently available for simulating particulate flow include a range of methods, from resolved direct numerical simulation (DNS) to Eulerian-Eulerian models. Employing high fidelity techniques, such as DNS, that fully resolve the physics are most often impractical for regular engineering applications due to exorbitant computational resource requirements. Hence, alternative simplified methods with reasonable computing power are generally used in regular engineering practice and design. In the present study, the Eulerian-Granular method, which is based on a simplified continuum approximation of the particulate phase, is compared with a relatively more detailed discrete element method (DEM), where individual particles are tracked in a Lagrangian sense. Numerical simulation of proppant flow through a representative fracture has been carried out to understand the relative suitability of these two different multiphase flow simulation techniques. Simulations are carried out for an idealized fracture geometry with a specified leak-off rate along the fracture wall. Computed results for the spreading rate of the proppant obtained from the Eulerian-Granular method are found to be marginally higher than the spreading rate of the proppant obtained from the DEM simulations. As DEM explicitly simulates particle movement; it is expected to provide results that are closer to the actual physical processes. The computational time required to perform the DEM simulations, however, is almost an order of magnitude greater than for the Eulerian-Eulerian technique. Hence, the efficiency of the Eulerian-Granular method probably offsets some modeling shortcomings in resolving particle setting characteristics for regular engineering applications.Copyright

Numerical Simulations of Non-Newtonian Geophysical Flows Using Smoothed Particle Hydrodynamics (SPH) Method: A Rheological Analysis

This paper presents computational results for two-dimensional (2-D) simulations of geophysical flows using the Smoothed Particle Hydrodynamics (SPH) method. The basic equations solved are the incompressible mass conservation and Navier-Stokes equations, and the discretization is carried out using the SPH method. The simulations are carried out for two problems. The first problem involved a 2-D dam-break problem with mud flow. The second problem involved non-Newtonian flow of deformable landslide on a mild slope. In both the simulations, the flow is assumed to be incompressible. In the present study, the mud flow materials are represented as non-Newtonian fluids with a Bingham model. The effects of the rheological formulation are assessed for the predicted mudflow shape. The simulation results are compared with the experimental data available in open literature. The velocity profiles and the free surface shape are in good agreement with the experimental data. To distinguish between the non-Newtonian model simulations and the Newtonian model, the dam-break simulations were also carried out using water and Newtonian models. The simulations reveal several distinctive flow features between the Newtonian and non-Newtonian approaches. The results of the simulations are of engineering interest in mitigation of natural hazards such as debris flows.Copyright

Navier-Stokes Simulations of Surface Waves Generated by Submarine Landslides: Effect of Slide Geometry and Turbulence

Surface waves generated by submarine landslides are studied using a computational model based on Navier-Stokes equations. The volume of fluid (VOF) method is used to track the free surface and shoreline movements. A Renormalization Group (RNG) turbulence model and Detached Eddy Simulation (DES) multiscale model are used to simulate turbulence dissipation. The submarine landslide is simulated using a sliding mass. The three-dimensional numerical simulations are carried out for a freely falling wedge representing the landslide and subsequent wave generations. Simulation results are compared with the experimental data. Modeled illustrate the effect of slide geometry, computational grid, turbulence model parameters, and slide material density on the predicted wave characteristics and runup/rundown at various locations. Computed results also show the complex three-dimensional flow patterns in terms of the velocity field, shoreline evolution, and free-surface profiles. Predicted numerical results for time histories of free-surface fluctuations and the runup/rundown at various locations are in good agreement with the available experimental data.

Soil Structure and Fluid Interaction Assessment of New Modular Reactor: Part 2 — Numerical Study of Soil Reactor Structure Interaction

To meet the growing demand of affordable power, several designs of Small Modular Reactors (SMRs), which will be installed below-grade, have been proposed by the nuclear industry. The containment vessels of these reactors will be under water. During a seismic event, these reactors will experience a complex soil (ground)-structure (SMR)-fluid (water) interaction that can affect the integrity of the system. Each of these reactors uses a seismic damping or isolation system to protect its important to safety structures, systems, and components from a design-basis earthquake. Designers of these damping/isolation systems need to have a thorough understanding of the complex soil-structure-fluid interactions to ensure the adequacy of the isolation system. In addition, regulators need to understand these interactions to evaluate the safety of such installations and systems.This study was initiated to understand the complexities in modeling facility responses that may accompany a design-basis earthquake. The ability to model these complexities is important to designers and regulators. It was recognized that a three-way coupled approach that can satisfactorily model the unique dynamic characteristics of soil surrounding the reactor, the reactor structure, and fluid contained within the reactor is not available. As a first step in understanding the complex interaction phenomena, a sequential coupling approach was adopted in this study. It was assumed that the feedback loop (such as structural deformation affecting ground motion and sloshing) has limited influence because of the high inertia of the massive structure. The general-purpose geological continuum package FLAC was used to simulate the propagation of earthquake-generated ground motion. The fluid analysis was conducted using the commercial computational fluid dynamics (CFD) package ANSYS-FLUENT. This paper briefly discusses the modeling techniques used in soil-structure and structure-fluid interaction analyses. Using a strong motion earthquake record, the ground acceleration at the base of the SMR was calculated and used as input to the CFD analysis of fluid motion inside the structure.Copyright

Comparative Assessment of Turbulence Models for Prediction of Flow-Induced Corrosion Damages

The present study makes a comparative assessment of different turbulence models in simulating the flow-assisted corrosion (FAC) process for pipes with noncircular cross sections and bends, features regularly encountered in heat exchangers and other pipeline networks. The case study investigates material damage due to corrosion caused by dissolved oxygen (O2 ) in a stainless steel pipe carrying an aqueous solution. A discrete solid phase is also present in the solution, but the transport of the solid particles is not explicitly modeled. It is assumed that the volume fraction of the solid phase is low, so it does not affect the continuous phase. Traditional two-equation models are compared, such as isotropic eddy viscosity, standard k-e and k-ω models, shear stress transport (SST) k-ω models, and the anisotropic Reynolds Stress Model (RSM). Computed axial and radial velocities, and turbulent kinetic energy profiles predicted by the turbulence models are compared with available experimental data. Results show that all the turbulence models provide comparable results, though the RSM model provided better predictions in certain locations. The convective and diffusive motion of dissolved O2 is calculated by solving the species transport equations. The study assumes that solid particle impingement on the pipe wall will completely remove the protective film formed by corrosion products. It is also assumed that the rate of corrosion is controlled by diffusion of O2 through the mass transfer boundary layer. Based on these assumptions, corrosion rate is calculated at the internal pipe walls. Results indicate that the predicted O2 corrosion rate along the walls varies for different turbulence models but show the same general trend and pattern.Copyright

Numerical Simulation of Surface Waves Generated by a Subaerial Landslide at Lituya Bay, Alaska

This paper presents preliminary results of a computational study conducted to analyze the impulse waves generated by the subaerial landslide at Lituya Bay, Alaska. The volume of fluid (VOF) method is used to track the free surface and shoreline movements. The Renormalization Group (RNG) turbulence model and Detached Eddy Simulation (DES) multiscale model were used to simulate turbulence dissipation. The subaerial landslide is simulated using a sliding mass. Results from the two-dimensional (2-D) simulations are compared with results from a scaled-down experiment. The experiment is carried out at a 1:675 scale. In the experimental setup, the subaerial rockslide impact into the Gilbert Inlet, wave generation, propagation, and runup on the headland slope are considered in a geometrically undistorted Froude similarity model. The rockslide is simulated by a granular material driven by a pneumatic acceleration mechanism so that the impact characteristics can be controlled. Simulations are performed for different values of the landslide density to estimate the influence of slide deformation on the generated tsunami characteristics. Simulated results show the complex flow patterns in terms of the velocity field, shoreline evolution, and free surface profiles. The predicted wave runup height is in close agreement with both the observed wave runup height and that obtained from the scaled-down experimental model.© 2009 ASME

Effect of Slide Deformation and Geometry on Waves Generated by Submarine Landslides: A Numerical Investigation

The impact of slide deformation and geometry on surface waves generated by submarine landslides is studied using a computational model based on Navier-Stokes equations. The volume of fluid (VOF) method is used to track the free surface and shoreline movements. The Renormalization Group (RNG) turbulence model and the Detached Eddy Simulation (DES) multiscale model are used to simulate turbulence dissipation. Three-dimensional simulations are first compared with available experimental data involving the generation of waves by a rigid block sliding down an inclined plane. The role of slide deformation on the characteristics of the generated waves is evaluated. The results from the simulations are compared with the experimental data for the rigid slide. Simulated results highlight the importance and complexity of slide deformation on the generated wave characteristics and runup/rundown at various locations. Computed results also show the complex three-dimensional flow patterns in terms of the velocity field, shoreline evolution, and free-surface profiles. Predicted numerical results for time histories of free-surface fluctuations and the runup/rundown at various locations are found to be in good agreement with the available experimental data for the rigid slide. The location of the slide (whether it is fully submerged or aerial) also seems to influence the runup and wave height.

Detached Eddy Simulations and Transient RANS Simulations of Turbulent Flow in the Lower Plenum of a Gas-Cooled Reactor

This paper presents preliminary results of a computational study conducted to analyze the turbulent flow in the lower plenum of an advanced next generation gas-cooled reactor. The turbulence models used in the current simulations are the Detached Eddy Simulation (DES) model and the transient RANS (Reynolds Averaged Navier Stokes) model. The current study is limited to flow in a row of confined cylinders designed to mimic a model of a prismatic gas-cooled reactor lower plenum design. The experimental configuration consists of a finite array of short graphite posts supporting the reactor core. Five cylinders, which represent vertical support posts in the lower plenum of an advanced reactor concept, are emplaced on the cross-stream centerline. In the current work, an idealized model was used to model a region of the lower plenum for a simplified set of conditions that enabled the flow to be treated as an isothermal, incompressible fluid with constant properties. The simulated results are compared with available experimental data, which were obtained using three-dimensional Particle Image Velocimetry (PIV). The two-equation realizable k-e model is used as the baseline model for both the Unsteady Reynolds Averaged Navier Stokes Equations (URANS) as well as the DES simulations. The flow unsteadiness accounts for the fluctuations due to unsteady vortex shedding. The DES simulations predicted the flow unsteadiness more accurately than the URANS simulations. The simulated time-averaged quantities were also compared with the experimental data. The RANS simulations and the DES simulations provide almost same predictions for the time averaged quantities. The predicted results show discrepancies with the experimental results.Copyright

Analysis of Turbulence Induced Thermal Mixing Effects on T-Junction Fluid-Structure Degradation

Thermal striping generally is recognized as a significant long-term degradation mechanism in the primary cooling water circuit of nuclear power plants (NPPs). This phenomenon occurs by mixing of hot and cold water streams in the primary coolant loop. Depending on the flow configuration, the turbulent mixing process can lead to thermal striping, temperature fluctuations in the T-junction region, thermal fatigue, and crack generation in the associated structure. The objective of this study is to provide an in-depth look into the underlying physics for thermal fatigue to determine appropriate screening criteria and risk significance for the regulatory safety evaluation process. In addition, the structure of turbulence in the T-junction also is investigated. The computational method comprised of Large Eddy Simulation (LES) modeling to simulate turbulence and Proper Orthogonal Decomposition (POD) analysis to capture the coherent structures and turbulence scales. In addition, Conjugate Heat Transfer (CHT) analyses have been performed to predict the thermal field and temperature distribution in the solid piping material of the T-junction. Finally, the corresponding thermal stress in the solid pipe is estimated based on a simplified one-dimensional model to assess the thermal-structure degradation.Copyright