Eray Uzgoren

Multigrid computations with the immersed boundary technique for multiphase flows

Multiphase flow computations involve coupled momentum, mass and energy transfer between moving and irregularly shaped boundaries, large property jumps between materials, and computational stiffness. In this study, we focus on the immersed boundary technique, which is a combined Eulerian‐Lagrangian method, to investigate the performance improvement using the multigrid technique in the context of the projection method. The main emphasis is on the interplay between the multigrid computation and the effect of the density and viscosity ratios between phases. Two problems, namely, a rising bubble in a liquid medium and impact dynamics between a liquid drop and a solid surface are adopted. As the density ratio increases, the single grid computation becomes substantially more time‐consuming; with the present problems, an increase of factor 10 in density ratio results in approximately a three‐fold increase in CPU time. Overall, the multigrid technique speeds up the computation and furthermore, the impact of the density ratio on the CPU time required is substantially reduced. On the other hand, the impact of the viscosity ratio does not play a major role on the convergence rates.

Three-Dimensional Adaptive, Cartesian Grid Method for Multiphase Flow Computations

Multiphase flows are marked by the presence of interfacial dynamics, steep jump in fluid-properties across the interface and moving boundaries between different phases and materials. Presence of moving boundaries and nonlinear coupling of phases across interface makes three-dimensional numerical simulations a difficult and expensive task. The presented work uses a continuous interface combining marker points on a stationary Eulerian grid. For an efficient computation an adaptive Cartesian grid is used to resolve local length scales of the flow. The interface location and flow solution are used as the adaptation criteria. The multiphase-interface is tracked using unstructured surface grid representation. A volume preserving interface reorganization procedure has been developed to maintain the desired interface surface grid resolution. Also, a momentum-source computation technique improving the performance of immersed boundary treatment in the governing field equation has been investigated.

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.

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.

Photovoltaic power plant design considering multiple uncertainties and risk

The objective of this study was to develop stochastic optimization tools for determining the best strategy of photovoltaic installations in a campus environment with consideration of uncertainties in load, power generation and system performance. In addition to a risk neutral approach, we used Conditional Value-at-Risk to estimate the risk in our problem. The resulting Mixed Integer Programming models were formulated using a scenario-based approach. To minimize the mismatch between supply and demand, hourly solar resource and electricity demand levels were characterized via refined models. A sample-average approximation (SAA) method was proposed to provide high-quality solutions efficiently. The SAA problems were solved using exact and heuristic methods. A complete numerical study was conducted to examine the performance of the proposed solution methods, identify optimal selection strategies and consider the sensitivity of the solution to varying levels of risk.

Pushing the performance envelop through secondary design enhancements in thermally limited compact notebooks

Thermal design enhancements in a thermally-limited, fixed-size compact notebook system are investigated in this paper. System temperature, power, and fan speed are characterized under a range of activity levels. A finite element model is developed, and validated against measurements. Design enhancements improve cooling with minimum intrusion to the existing mechanical design, and with no growth in size. A passive secondary heat pipe in the system reduces the CPU temperature by 5 °C, and improves the system performance through increased CPU + Graphics and Memory Controller Hub (GMCH) thermal design power (TDP) by 6.4%. When such a secondary heat pipe is considered with an integrated off-the shelf Peltier cooler module, the CPU temperature is only reduced by 2.3 °C and CPU+GMCH TDP is improved only by 4.9%. The results demonstrate a successful methodology to claim performance and/or energy efficiency improvement opportunity in an existing notebook box with minimum redesign effort.

NUMERICAL ANALYSIS OF A SOLAR ORGANIC RANKINE CYCLE (ORC) UNIT WITH R245FA AS WORKING FLUID

This paper presents a solar Organic Rankine Cycle (ORC) for electricity generation; where a regression based approach is used for the working fluid. Models of the unit’s sub components (pump, evaporator, expander and condenser) are also presented. Heat supplied by the solar field can heat the water up to 80-95 0 C at mass flow rates of 2-12 kg/s and deliver energy to the ORC’s heat exchanger unitresults of steady state operation using the developed model shows a maximum power output of around 40 kWe. Both refrigerant and hot water mass flow rates in the system are identified as critical parameters to optimize the power production and the cycle efficiency.

Parallelized domain decomposition techniques for multiphase flows

Direct simulation of multiphase flows is a challenging task due to the moving interface and property variations between phases. In this study, a parallel domain decomposition method is implemented for such flows to lower the computing cost. Specifically, the approach consists of the additive Schwarz method for domain decomposition, the projection method for the Navier-Stokes equations, the immersed boundary method for treating the interfacial dynamics, and the multigrid method to expedite the solution of the pressure Poisson equation. The issues related to load balancing, communication and computation, scalability in regard to grid size and the number of processors, and interface shape deformation, are studied using both SGI Altix and Linux-based Beowulf systems. As the number of processors increases, as expected, the domain decomposition technique results in modest decrease in convergence rate, while the multigrid technique is effective in reducing the computational cost. The additional computational cost incurred by the immersed boundary method for tracking the interface is not significant.Copyright