Jaeheon Sim

Computations of Multiphase Fluid Flows Using Marker-Based Adaptive, Multilevel Cartesian Grid Method

Addressing the dynamics of multiphase fronts is crucial for successful practices of many engineering applications that involve both gas and liquid constituents. Numerical simulations of such flows need to resolve the location and the shape of the front, where interfacial conditions are satisfied as part of the solution. In this work, immersed boundary method on adaptively refined Cartesian grid is utilized to capture the dynamics of the multiphase front. Marker based interface representation is extended to handle the complex solid geometries that exist in the flow field. The entire computational algorithm is applied to simulate steady state and time dependent liquid plug problem and assessed against existing theoretical analyses. It is then used to simulate time dependent draining flow problems, motivated by spacecraft fuel tank operations where experimental guidance is available. Distinct interfacial characteristics and fluid physics associated with different flow regimes are highlighted.

A unified adaptive cartesian grid method for solid-multiphase fluid dynamics with moving boundaries

Numerical simulations of flows involving moving boundaries are challenging as they need to address the location and the conditions of the interface that interacts with the flow field. We have developed a unified, marker-based approach, which can treat moving solid and multiphase fluid dynamics using adaptively refined Cartesian grids. The interfaces separating the fluid phases are modeled using a continuous interface method, while the noslip condition on solid interfaces is imposed by a sharp interface method. A smoothly varying Heaviside-like function is used for handling discontinuous material properties between fluids and for identifying the solid-fluid interface location. Furthermore, a distance-based formulation is adopted to treat solid-fluid interface intersections. A domain decomposition method via Hilbert space filling curves and preconditioned multigrid solvers are incorporated into the staggered grid arrangement for scalar and velocity variables. To highlight the performance of the present approach, case studies are conducted for (i) interface shapes, residual volumes, formation of sloshes and corresponding wave periods in draining tank with different control parameters and flow regimes, (ii) fluid dynamics around a flapping airfoil, and (iii) fluid flow around complex solid geometries.

Simulation of Spacecraft Fuel Tank Self-pressurization Using Eulerian-Lagrangian Method

A 3-D adaptive Eulerian-Lagrangian method is further implemented with a phase change model to study the thermal effect in a spacecraft fuel tank, especially the self-pressurization of a cryogenic propellant tank. The stationary (Eulerian) frame is used to resolve the flow field, and the marker-based triangulated moving (Lagrangian) surface meshes are utilized to treat fluid interfaces and solid boundaries. The main focus of the present study is to improve the accuracy of the sharp solid boundary treatment and to implement a phase change model. The energy and mass transfer across the interface due to phase change is computed from Stefan condition using probe-based temperature gradient computations. Uniform flow past a cylinder, 1-D Stefan problems, and 2-D melting cases by natural convection flow are presented for validation purpose of the present approaches. The self-pressurization in a liquid hydrogen fuel tank is simulated and it is shown that the conduction-only solution underestimates the pressure rise, and the full Navier-Stokes and energy equation solution is required to study liquid fuel tank pressurization due to the influence of transport phenomena.

Marker-Based, 3-D Adaptive Cartesian Grid Method for Multiphase Flow around Irregular Geometries

Computational simulations of multiphase flow are challenging because many practical applications require adequate resolution of not only interfacial physics associated with moving boundaries with possible topological changes, but also around three-dimensional, irregular solid geometries. In this paper, we focus on the simulations of fluid/fluid dynamics around complex geometries, based on an Eulerian-Lagrangian framework. The approach uses two independent but related grid layouts to track the interfacial and solid boundary conditions, and is capable of capturing interfacial as well as multiphase dynamics. In particular, the stationary Cartesian grid with time dependent, local adaptive refinement is utilized to handle the computation of the transport equations, while the interface shape and movement are treated by marker-based triangulated surface meshes which freely move and interact with the Cartesian grid. The markers are also used to identify the location of solid boundaries and enforce the no-slip condition there. Issues related to the contact line treatment, topological changes of multiphase fronts during merger or breakup of objects, and necessary data structures and solution techniques are also highlighted. Selected test cases including spacecraft fuel tank flow management, and movement and rupture of interfaces associated with liquid plug flow are presented.

Parallel Eulerian-Lagrangian Method with Adaptive Mesh Refinement for Moving Boundary Computation

In this study, we present a parallelized adaptive moving boundary computation technique on distributed memory multi-processor systems for multi-scale multiphase flow simulations. The solver utilizes the Eulerian-Lagrangian method to track moving (Lagrangian) interfaces explicitly on the stationary (Eulerian) Cartesian grid where the flow fields are computed. Since there exists strong data and task dependency between two distinct Eulerian and Lagrangian domain, we address the decomposition strategies for each domain separately. We then propose a trade-off approach aiming for parallel scalability. Spatial domain decomposition is adopted for both Eulerian and Lagrangian domains for load balancing and data locality to minimize inter-processor communication. In addition, a cellbased unstructured parallel adaptive mesh refinement (AMR) technique is implemented for the flexible local refinement with efficient grid usage and even-distributed computational workload among processors. The parallel performance is evaluated independently for the Cartesian grid solver and sub-procedures in cell-based unstructured AMR. The capability and the overall performance of the parallel adaptive Eulerian-Lagrangian method including moving boundary and topological change is demonstrated by modeling binary droplet collisions. With the aid of the present techniques, large scale moving boundary problems can be effectively computed.

Parallel, Adaptive Grid Computing of Multiphase Flows in Spacecraft Fuel Tanks

‡A parallel adaptive Eulerian-Lagrangian method is developed for effective large scale multiphase flow computation. The intricate complexity of parallelism for the EulerianLagrangian method is discussed and spatial domain decomposition strategies are proposed. An efficient communication approach is presented with MPI standard communication and the performance of the parallel Eulerian-Lagrangian method is evaluated. The speedup on problems having million of cells shows super-linear behavior due to the effect of cache hit rate, and the parallel efficiency peaks at the trade-off point of the cache hit rate and the computation-to-communication ratio. A first-principle-based phase change model is introduced by computing the rate of phase change in each phase as well as across liquid/vapor interface. The self-pressurization in a liquid hydrogen fuel tank is simulated and it is shown that the phase change in a liquid phase has a dominant contribution comparing phase change on the liquid/vapor surface, and the rate decreases as saturation temperature increases. The effects of heat flux amount and liquid fuel fill-level are investigated, and the pressurization accelerates showing longer transition period with higher pressure rise rate as heat flux and fill level increases.

3-D Multiscale Adaptive Eulerian-Lagrangian Method for Multiphase Flows with Phase Change

A multi-scale multiphase computational model including phase change has been developed to study the moving interfacial dynamics and thermal effect in various engineering and scientific applications, including spacecraft cryogenic propellant delivery processes. A 3-D adaptive Eulerian-Lagrangian method is implemented, utilizing the stationary (Eulerian) frame to resolve the flow field, and the marker-based triangulated moving (Lagrangian) surface meshes to treat the fluid interface and solid boundaries. Other than treating the unsteady, convection, pressure, viscous/diffusion, and buoyancy terms in the governing field equations, the energy and mass transfer across interface due to phase change is handled using probe-based profile computations. Numerous test cases are presented, including liquid fuel draining, sloshing, and surface flow stability related to the interfacial dynamics, and natural convection in a cavity and Stefan problem for energy transport and phase change dynamics.

3-D Adaptive Eulerian-Lagrangian Method for Gravity- and Capillarity-Induced Flows

Delivery of cryogenic propellants from a spacecraft fuel tank is complicated due to the low and fluctuating gravity level and its interaction with the capillary, convective, and diffusive mechanisms. The present effort is aimed at developing suitable computational modeling techniques capable of offering adequate resolution of moving interfacial dynamics, topological changes due to break-up and merger of the fluid objects, and interactions between phase boundaries and complex solid boundaries. A 3-D adaptive EulerianLagrangian method is developed, utilizing the stationary (Eulerian) frame to resolve the flow field, and the marker-based triangulated moving (Lagrangian) surface meshes to treat the fluid interface. The multiphase fluid boundary is modeled using a continuous interface method, and the solid boundary is treated by a sharp interface method along with the ghost cell method. The performance of the present framework is assessed using several test cases of different challenges, including the (i) sloshing liquid motion by a sudden reduction of acceleration exhibiting substantial variations in the shape and the location of the phase boundary, and (ii) stability of the liquid-gas interface dynamics due to vertically oscillating gravitational acceleration of varying frequency and amplitudes, resulting in complex surface wave patterns.

Multiphase Fluid Dynamics for Spacecraft Applications

Multiphase flows involving moving interfaces between different fluids/phases are observed in nature as well as in a wide range of engineering applications. With the recent development of high fidelity computational techniques, a number of challenging multiphase flow problems can now be computed. We introduce the basic notion of the main categories of multiphase flow computation; Lagrangian, Eulerian, and Eulerian‐Lagrangian techniques to represent and follow interface, and sharp and continuous interface methods to model interfacial dynamics. The marker‐based adaptive Eulerian‐Lagrangian method, which is one of the most popular methods, is highlighted with microgravity and space applications including droplet collision and spacecraft liquid fuel tank surface stability.