Manufacturing processes that can create extremely small machines have been developed in recent years. Microelectromechanical systems (MEMS) refer to devices that have characteristic length of less than 1 mm but more than 1 micron, that combine electrical and mechanical components and that are fabricated using integrated circuit batch-processing techniques. Electrostatic, magnetic, pneumatic and thermal actuators, motors, valves, gears, and tweezers of less than 100-μm size have been fabricated. These have been used as sensors for pressure, temperature, mass flow, velocity and sound, as actuators for linear and angular motion and as simple components for complex systems such as micro-heat-engines and micro-heat-pumps The technology is progressing at a rate that fa r exceeds that of our understanding of the unconventional physics involved in the operation as well as the manufacturing of those minute devices. The primary objective of this article is to critically review the status of our understanding of fluid flow phenomena particular to microdevices. In terms of applications, the paper emphasizes the use of MEMS as sensors and actuators for flow diagnosis and control.

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The Fluid Mechanics of Microdevices—The Freeman Scholar Lecture

▪ Abstract The principles of and procedures for implementing direct simulation Monte Carlo (DSMC) are described. Guidelines to inherent and external errors common in DSMC applications are provided. Three applications of DSMC to transitional and nonequilibrium flows are considered: rarefied atmospheric flows, growth of thin films, and microsystems. Selected new, potentially important advances in DSMC capabilities are described: Lagrangian DSMC, optimization on parallel computers, and hybrid algorithms for computations in mixed flow regimes. Finally, the limitations of current computer technology for using DSMC to compute low-speed, high–Knudsen-number flows are outlined as future challenges.

Direct Simulation Monte Carlo: Recent Advances and Applications

The challenges in understanding hypersonic flight are discussed and critical hypersonic aerothermodynamics issues are reviewed. The ability of current analytical methods, numerical methods, ground testing capabilities, and flight testing approaches to predict hypersonic flow are evaluated. The areas where aerothermodynamic shortcomings restrict our ability to design and analyze hypersonic vehicles are discussed, and prospects for future capabilities are reviewed. Considerable work still needs to be done before our understanding of hypersonic flow will allow for the accurate prediction of vehicle flight characteristics throughout the flight envelope from launch to orbital insertion.

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Critical Hypersonic Aerothermodynamic Phenomena

▪ Abstract Recent considerable progress in the field of rarefied hypersonic computational fluid dynamics (CFD) gives reason to address its evolution to an independent CFD branch that covers many fundamental and closely related applied problems of high-altitude aerothermodynamics of space vehicles. The primary purpose of this review is to describe the main numerical methods and real gas models for investigation of problems of rarefied hypersonic flows, and to review results that we believe demonstrate most clearly the achievements and capabilities of the field of rarefied hypersonic CFD in the last years.

Computational hypersonic rarefied flows

1. Introduction 2. Theoretical background 3. High temperature gas dynamics and hypersonic effects 4. Cycle analyses and energy management 5. Inlets and nozzles 6. Supersonic combustion processes and characteristics 7. Testing methods and wind-tunnels 8. Computational fluid dynamics methods and solutions for high speed reacting flows.

The Scramjet Engine: Processes and Characteristics

Introduction O as well as new hypersonic flow programs continue to challenge the aerospace engineer. These programs are often of consuming difficulty, involving the synthesis of chemical kinetics, quantum mechanics, and radiation physics with fluid dynamics. To further complicate matters, the flowfield is often rarefied; thus the Knudsen number requires independent consideration along with the Mach and Reynolds numbers. Although these parameters are related, different regions of the flow, such as the interior of a bow shock, depend differently on them as compared with other regions. Because of its complexity, recent research has focused on the modeling of more realistic, complex hypersonic flows with the intensive use of computational fluid dynamics (CFD). Of necessity, the large codes used in this effort must resort to various empiricisms and approximations. Our objective in this Survey is to discuss some of these empiricisms and approximations within the context of hypersonic nonequilibrium flow. The issues and advances examined are primarily chosen for their relevance to theoretical gasdynamics; however, this Review makes no claim to being comprehensive. The books by Anderson and Park provide background material for the discussion of current issues, including lessons learned from the design and operation of the Space Shuttle. Our presentation is partly based on a previous report, some of which was published elsewhere. An alternative discussion of some of the same material can be found in an article by Tirsky whose discussion is especially valuable for emphasizing hypersonic research by Russian workers, with greater emphasis given to engineering heat transfer predictions. Some of this work is not as well known in the western community as it should be. The importance of finite rate chemistry and the problem of fuel-air mixing to hypersonic airbreathing propulsion cannot be overemphasized. These topics, however, are omitted from our discussion in as much as they fall under combustion gasdynamics and ought to be examined in this context.' The treatise on hydrogen combustion presented in Ref. 8 should be of interest to scramjet propulsion researchers. A broad perspective on hypersonic airbreathing vehicle design may be found in a survey (to be three volumes) edited by Bertin et al. In the next section, nonequilibrium aerothermodynamics is discussed. Much of this section is concerned with energy exchange that involves the vibrational modes of diatomic molecules. The subsequent section reviews the modeling of rarefied hypersonic flows and their continuum extension. There is no shortage of approaches that are available, as well as open issues. These approaches encompass the direct simulation Monte Carlo (DSMC) method, NavierStokes (NS) equations, Burnett, or augmented Burnett equations, and the 13-moment equations. The review concludes with a few brief remarks.

Perspective on hypersonic nonequilibrium flow

On utilise un algorithme de methode des caracteristiques precis au second ordre pour determiner le champ d'ecoulement et le contour de paroi d'une tuyere supersonique oxisymetrique de longueur minimum avec une ligne sonique rectiligne

Comparison of minimum length nozzles

Bulk viscosity in the Navier–Stokes equations

Currently, the bulk viscosity μb of a gas can only be obtained with an acoustic absorption experiment. A method is proposed that is primarily applicable to a dense polyatomic gas. In this circumstance, the density‐based thickness Λρ is large, consisting of many thousands of mean‐free paths. Moreover, the ratio μb/μ (μ is the shear viscosity) is linear with this thickness. Consequently, shock measurements of Λρ can be used to evaluate μb. The approach is illustrated using sulfur hexaflouride.

George Emanuel

Papers

Perspective on hypersonic nonequilibrium flow

Comparison of minimum length nozzles

Bulk viscosity in the Navier–Stokes equations

Linear dependence of the bulk viscosity on shock wave thickness

Assessment of the Martin-Hou Equation for Modeling a Nonclassical Fluid