Computational Fluid Mechanics

Computational methods for flow analysis and turbulence

Science and engineering have undergone a major transformation at the research as well as at the development and technology level. The modern scientist and engineer spend more and more time in front of a laptop, a workstation, or a parallel supercomputer and less and less time in the physical laboratory or in the workshop. The virtual wind tunnel and the virtual biology lab are not a thing of the future, they are here! The old approach of “cutand- try” has been replaced by “simulate-and-analyze” in several key technological areas such as aerospace applications, synthesis of new materials, design of new drugs, chip processing and microfabrication, etc. The new discipline of nanotechnology will be based primarily on large-scale computations and numerical experiments. The methods of scientific analysis and engineering design are changing continuously, affecting both our approach to the phenomena that we study as well as the range of applications that we address. While there is a lot of software available to be used as almost a “black-box,” working in new application areas requires good knowledge of fundamentals and mastering of effective new tools.
In the classical scientific approach, the physical system is first simplified and set in a form that suggests what type of phenomena and processes may be important, and correspondingly what experiments are to be conducted. In the absence of any known-type governing equations, dimensional inter-dependence between physical parameters can guide laboratory experiments in identifying key parametric studies. The database produced in the laboratory is then used to construct a simplified “engineering” model which after field-test validation will be used in other research, product development, design, and possibly lead to new technological applications. This approach has been used almost invariably in every scientific discipline, i.e., engineering, physics, chemistry, biology, etc.
The simulation approach follows a parallel path but with some significant differences. First, the phase of the physical model analysis is more elaborate: The physical system is cast in a form governed by a set of partial differential equations, which represent continuum approximations to microscopic models. Such approximations are not possible for all systems, and sometimes the microscopic model should be used directly. Second, the laboratory experiment is replaced by simulation, i.e., a numerical experiment based on a discrete model. Such a model may represent a discrete approximation of the continuum partial differential equations, or it may simply represent a statistical representation of the microscopic model. Finite difference approximations on a grid are examples of the first case, and Monte Carlo methods are examples of the second case. In either case, these algorithms have to be converted to software using an appropriate computer language, debugged, and run on a workstation or a parallel supercomputer.
The output is usually a large number of files of a few Megabytes to hundreds of Gigabytes, being especially large for simulations of time-dependent phenomena. To be useful, this numerical database needs to be put into graphical form using various visualization tools, which may not always be suited for the particular application considered. Visualization can be especially useful during simulations where interactivity is required as the grid may be changing or the number of molecules may be increasing. The simulation approach has already been followed by the majority of researchers across disciplines in the last few decades. The question is if this is a new science, and how one could formally obtain such skills. Moreover, does this constitute fundamental new knowledge or is it a “mechanical procedure,” an ordinary skill that a chemist, a biologist or an engineer will acquire easily as part of “training on the job” without specific formal education. It seems that the time has arrived where we need to reconsider boundaries between disciplines and reformulate the education of the future simulation scientist, an inter-disciplinary scientist.
Let us re-examine some of the requirements following the various steps in the simulation approach. The first task is to select the right representation of the physical system by making consistent assumptions in order to derive the governing equations and the associated boundary conditions. The conservation laws should be satisfied; the entropy condition should not be violated; the uncertainty principle should be honored. The second task is to develop the right algorithmic procedure to discretize the continuum model or represent the dynamics of the atomistic model. The choices are many, but which algorithm is the most accurate one, or the simplest one, or the most efficient one? These algorithms do not belong to a discipline! Finite elements, first developed by the famous mathematician Courant and rediscovered by civil engineers, have found their way into every engineering discipline, physics, geology, etc. Molecular dynamics simulations are practiced by chemists, biologists, material scientists, and others. The third task is to compute efficiently in the ever-changing world of supercomputing. How efficient the computation is translates to how realistic of a problem is solved, and therefore how useful the results can be to applications. The fourth task is to assess the accuracy of the results in cases where no direct confirmation from physical experiments is possible such as in nanotechnology or in biosystems or in astrophysics, etc. Reliability of the predicted numerical answer is an important issue in the simulation approach as some of the answers may lead to new physics or false physics contained in the discrete model or induced by the algorithm but not derived from the physical problem. Finally, visualizing the simulated phenomenon, in most cases in three-dimensional space and in time, by employing proper computer graphics (a separate specialty on its own) completes the full simulation cycle. The rest of the steps followed are similar to the classical scientific approach.
In classical science we are dealing with matter and therefore atoms but in simulation we are dealing with information and therefore bits, so it is atoms versus bits! We should, therefore, recognize the simulation scientist as a separate scientist, the same way we recognized just a few decades ago the computer scientist as different than the electrical engineer or the applied mathematician. The new scientist is certainly not a computer scientist although she should be computer literate in both software and hardware. She is not a physicist although she needs a sound physics background. She is not an applied mathematician although she needs expertise of mathematical analysis and approximation theory.
With the rapid and simultaneous advances in software and computer technology, especially commodity computing, the so-called soupercomputing, every scientist and engineer will have on her desk an advanced simulation kit of tools consisting of a software library and multi-processor computers that will make analysis, product development, and design more optimal and cost-effective. But what the future scientists and engineers will need, first and foremost, is a solid inter-disciplinary education. Scientific computing is the heart of simulation science, and this is the subject of this book. The emphasis is on a balance between classical and modern elements of numerical mathematics and of computer science, but we have selected the topics based on broad modeling concepts encountered in physico-chemical and biological sciences, or even economics.