Aeroelastic simulations using gridless boundary condition and small perturbation techniques

This paper examines the use of stationary Cartesian mesh for non-linear flutter computations involving complex geometries. The surface boundary conditions are implemented using reflected points which are determined via a gridless approach. The method uses a cloud of nodes in the vicinity of the surface to get a weighted-average of the flow properties using radial basis functions. To ensure computational efficiency and for local grid refinements, multigrid computations within an embedded grids framework are used. As the displacements of moving surfaces from their original position are typically small for flutter problems, a small perturbation boundary condition method is used to account for the moving surfaces. The method therefore does not require repeated grid re-generation for the deforming surfaces. The overall method is both accurate and robust. Computations of the well-known Onera M6 wing, RAE wing-body configuration, the AGARD 445.6 wing flutter test case show good accuracy and efficiencies. Simulations of the aeroelastic behavior of a complete fighter-type aircraft with wing tip missiles at high transonic speeds further demonstrate the practical usefulness of the present boundary conditions technique.     

Improving stability of stabilized and multiscale formulations in flow simulations at small time steps

The objective of this paper is to show that use of the element-vector-based definition of stabilization parameters, introduced in [T.E. Tezduyar, Computation of moving boundaries and interfaces and stabilization parameters, Int. J. Numer. Methods Fluids 43 (2003) 555–575; T.E. Tezduyar, Y. Osawa, Finite element stabilization parameters computed from element matrices and vectors, Comput. Methods Appl. Mech. Engrg. 190 (2000) 411–430], circumvents the well-known instability associated with conventional stabilized formulations at small time steps. We describe formulations for linear advection–diffusion and incompressible Navier–Stokes equations and test them on three benchmark problems: advection of an L-shaped discontinuity, laminar flow in a square domain at low Reynolds number, and turbulent channel flow at friction-velocity Reynolds number of 395.     

LIGSITE: automatic and efficient detection of potential small molecule-binding sites in proteins

LIGSITE is a new program for the automatic and time-efficient detection of pockets on the surface of proteins that may act as binding sites for small molecule ligands. Pockets are identified with a series of simple operations on a cubic grid. Using a set of receptor–ligand complexes we show that LIGSITE is able to identify the binding sites of small molecule ligands with high precision. The main advantage of LIGSITE is its speed. Typical search times are in the range of 5 to 20 s for medium-sized proteins. LIGSITE is therefore well suited for identification of pockets in large sets of proteins (e.g., protein families) for comparative studies. For graphical display LIGSITE produces VRML representations of the protein–ligand complex and the binding site for display with a VRML viewer such as WebSpace from SGI.     

Parallel simulations of multiprocessors

This paper describes the implementation and the performance of a synchronous, parallel discrete event simulation (SPaDES) method on two shared memory multiprocessors. The presented method aims at the efficient simulation of architectural designs for which the asynchronous PDES methods are less effective. A multiprocessor machine is simulated, and the performance achieved is compared to the performance of a parallel version of the centralized-time synchronous simulation method. The results show that the SPaDES method alleviates bottlenecks usually attributed to synchronous methods, and thus we are able to efficiently exploit most of the parallelism available in the simulation of synchronous architectural designs.     

Visualization of wildfire simulations

The paper considers how Los Alamos researchers are partnering with Los Angeles County Fire Department, United States Forest Service, and Kennedy Space Center personnel for wildfire simulation studies. Fire behavior is highly dependent upon winds, temperatures and moisture. It is crucial to predict these weather parameters over the small regions where they directly affect a fire. Weather conditions in these small regions are driven by dynamic weather patterns such as cold fronts and high-pressure systems that develop over much larger geographic areas. The Regional Atmospheric Modeling System (RAMS), originally developed at Colorado State University, predicts these variable weather patterns. The RAMS model uses measurements from weather stations all over the United States to predict winds, temperatures, and moisture into the near future. To model the interactions between winds and fire, Higrad has been combined with a simple fire behavior model (Behave) from the US Forest Service. This combined modeling system (Higrad/Behave) is the first step in predicting the actual progress and heat release of a wildfire. Two fires have been simulated using this combined model     

Programming mechanical simulations

This paper examines the control of complex physical objects in simulation. We introduce a programming paradigm that allows a simulation to be treated as a multi-level constraint solver. The control programmer is given the ability to specify constraints on the controlled response of mechanisms and to conditionally change these constraints dependent on the state of system. The approach facilitates the development of model-based, event-driven control programs. The usefulness of the paradigm is demonstrated through the simulation of a hopping robot.     

Computerized clinical simulations

The technique involved in designing a clinical simulation problem for the allied health field of Respiratory Therapy is described. The structure, content and scoring categories are discussed. A sample program is provided illustrating a programming technique in BASIC (Beginners' All Purpose Symbolic Instruction Code) which includes a sample flowchart.     

Multidimensional simulations of pair-instability supernovae

We present preliminary results from multidimensional numerical studies of pair-instability supernova (PSN), studying the fluid instabilities that occur in multiple spatial dimensions. We use the new radiation-hydrodynamics code, CASTRO, and introduce a new mapping procedure that defines the initial conditions for the multidimensional runs in such a way that conservation of physical quantities is guaranteed at any level of resolution.     

Multiscale simulations of primary atomization

A liquid jet upon atomization breaks up into small droplets that are orders of magnitude smaller than its diameter. Direct numerical simulations of atomization are exceedingly expensive computationally. Thus, the need to perform multiscale simulations. In the present study, we performed multiscale simulations of primary atomization using a Volume-of-Fluid (VOF) algorithm coupled with a two-way coupling Lagrangian particle-tracking model to simulate the motion and influence of the smallest droplets. Collisions between two particles are efficiently predicted using a spatial-hashing algorithm. The code is validated by comparing the numerical simulations for the motion of particles in several vortical structures with analytical solutions. We present simulations of the atomization of a liquid jet into droplets which are modeled as particles when away from the primary jet. We also present the probability density function of the droplets thus obtained and show the evolution of the PDF in space.     

Efficient visualization of crash-worthiness simulations

Software tightly integrated with simulation packages has dominated finite element postprocessing. Many of these packages have not kept up with the state-of-the-art developments in graphics technology and visualization techniques. Large and time-dependent data sets resulting from crash-worthiness simulations in the automotive development process demand new visualization tools that allow interactive manipulation of complex geometries and meaningful mapping of physical properties. This article demonstrates that careful scene-graph structure design and extensive use of texture mapping can improve rendering performance and visual appearance for postprocessing tasks. These tasks include inspecting finite element (FE) discretization and analyzing intrusion depth or vector quantities     

Meshfree simulations of spall fracture

Shock wave induced spall fracture is a complex multiscale phenomenon, and it is a challenge to build a constitutive and computational model that can capture essential features of the spall fracture. In this work, we present a computational micro-mechanics model to simulate spall fracture by utilizing the multiscale micro-mechanics theory proposed by Wright and Ramesh [36] and a RKPM meshfree method. The focus of this work is to develop and demonstrate a simulation tool that is capable of simulations of spall fracture in engineering application. First, based on a well-known empirical formula, we relate the macroscale spall strength to the kinematics of micro void growth in a Representative Volume Element (RVE). The connection between micro void growth and overall kinematics of the RVE is made through the conservation of mass in the micro to macro transition process. Second, we develop a set of meshfree void growth algorithms that is tailored to represent kinematics of void nucleation, growth and coalescence, and these algorithms retain the conservation of mass, momentum, and energy during simulations of ductile spall fracture. Third, based on the Johnson–Cook model, we developed a meshfree computational formulation, and we have carried out simulations of the spall fracture of a Ti–6Al plate under impact loads to validate the model. From the simulation, we find that the interaction between the first two inelastic wave pulses plays an important role in the mechanism of spall fracture. The numerical results show that the proposed method can capture some features of the spall fracture, and it may be used to simulate the spall fracture in engineering applications.     

Validating trace-driven microarchitectural simulations

Benchmark complexity and sluggish performance analysis create an ever-increasing gap between workload size and the speed of analysis. Experimental results with a new data-sampling methodology, however, show promise in closing this gap     

Animating simulations using TESS

Animation techniques provide for displaying the dynamics and operating strategies of a simulation graphically. Thus, both analysts and decision makers can see how a simulation represented the modeled system. Within its framework for performing simulation studies, TESS provides a framework for animating simulations. Animations are performed as a post-simulation presentation tool using color-raster graphics. Animations are shown on a schematic diagram of the system, driven by a list of the event occurrences in the simulation, and related to the simulation by a set of statements called a rule.An example animation illustrates the use of the TESS animation framework. A case study shows its application to an actual system.     

Large-scale rigid body simulations

For decades, rigid body dynamics has been used in several active research fields to simulate the behavior of completely undeformable, rigid bodies. Due to the focus of the simulations to either high physical accuracy or real time environments, the state-of-the-art algorithms cannot be used in excess of several thousand to several ten thousand rigid bodies. Either the complexity of the algorithms would result in infeasible runtimes, or the simulation could no longer satisfy the real time aspects.     

Object-oriented simulations in mechatronics

The artificial word 'mechatronics' represents the symbiosis of 'classical' mechanical systems and 'modern' electmnics that has opened up a wide range of new possibilities. A typical application is an anti-lock braking system (ABS), a very complex combination of sensors, microprocessor-based controllers, electromagnetic actuators and mechanical as well as hydraulic components. Increasing complexity and their mostly non-linear behaviour are the major challenges in designing, implementing and testing these systems. A very promising design methodology is based on the idea of integrating real hardware via interfaces into a system simulation ('hardware-in-the-loop'). Development may start with a completely simulated model. With hardware being designed and becoming available, it is step by step incorporated into the system. This methodology significantly reduces development time since there is early feedback on the design approaches and the final solution is proved under real operating conditions. Obviously the real-time simulation of fast mechatronical systems requires tremendous computing power. Supposing one finds a way to express the simulation problem in a parallel form, a multi-transputer system could provide the necessary computing power for a relatively low price. One solution is a modelling approach that decomposes the systems hierarchically into components with a strong relationship to the physical world. Each component is described by a set of equations operating on the component's own local state. The communication of physical interface data is modelled by sending messages via connections between the components. This approach may be attributed as 'object-oriented' since computer science defines objects as independent elements maintaining their own internal state and communicating via message-passing mechanisms. Taking a close look at a transputer, it reveals a very 'object'-like architecture: local memory to maintain its own local state and links designed to pass messages between transputers. Therefore transputers are the ideal vehicle for fast real-time simulation by following the principle of object-orientation, from system modelling all the way down to simulation.     

Multiphysics simulations of rocket engine combustion

Recently, the hybrid rocket propulsion has become attractive to the research community and has developed the trend to become an alternative to the conventional liquid and solid rockets. The hybrid rocket is a combination of both the solid and liquid systems with half of the plumbing of the liquid rocket but retaining its operational flexibility and avoiding the explosive nature of the solid rocket. Among available hybrid systems, the N₂O (Nitrous Oxide)–HTPB (Hydroxyl-Terminated PolyButadiene) hybrid propulsion represents the simplest but sufficiently efficient design. Unfortunately, even until now, research in developing hybrid N₂O–HTPB propulsion system still strongly depends on trials-and-errors, which are time-consuming and expensive. Thus, detailed understanding of the fundamental combustion processes that are involved in the N₂O–HTPB propulsion system can greatly impact the research community in this field. This may further facilitate the successful modeling of the combustion processes and help improving the design of N₂O–HTPB propulsion system in the future. A comprehensive numerical model with real-fluid properties and finite-rate chemistry was developed in this research to predict the combustion flowfield inside a N₂O–HTPB hybrid rocket system. Good numerical predictions as compared to experimental data are also presented.