Annual Review of Fluid Mechanics - Volume 30, 1998

Aircraft laminar flow control (LFC) from the 1930s through the 1990s is reviewed and the current status of the technology is assessed. Examples are provided to demonstrate the benefits of LFC for subsonic and supersonic aircraft. Early studies related to the laminar boundary-layer flow physics, manufacturing tolerances for laminar flow, and insect-contamination avoidance are discussed. LFC concept studies in wind-tunnel and flight experiments are the major focus of the paper. LFC design tools are briefly outlined for completeness.

We survey attempts to construct vortex models of the inertial-range and fine-scale range of high Reynolds number turbulence. An emphasis is placed on models capable of quantitative predictions or postdictions.

This article reviews the use of Darcy's law for analyzing waves moving past a porous structure. The engineering applications of these analyses are emph asized. The first part of the article studies theories on the effect of a porous structure on incoming wave trains. It also reviews the movement of waves past a plate with regular gaps in it in an attempt to compare the results of potential theory with those of Darcy's law. The second part reviews the use of a porous structure as a wavemaker when the structure is subject to horizontal oscillation. The third part extends the theories to the use of a porous structure as a breakwater in a two-dimensional harbor. The effect of the structure on reducing waves and suppressing harbor resonance is investigated.

A liquid jet emanating from a nozzle into an ambient gas is inherently unstable. It may break up into drops of diameters comparable to the jet diameter or into droplets of diameters several orders of magnitude smaller. The sizes of the drops formed from a liquid jet without external control are in general not uniform. The sizes as well as the size distribution depend on the range of flow parameters in which the jet is produced. The jet breakup exhibits different characteristics in different regimes of the relevant flow parameters because of the different physical mechanisms involved. Some recent works based on linear stability theories aimed at the delineation of the different regimes and elucidation of the associated physical mechanisms are reviewed, with the intention of presenting current scientific knowledge on the subject. The unresolved scientific issues are pointed out.

This review surveys the formation, motion, and persistence of trailing vortices as relevant to the safety and productivity of air travel. It highlights findings or shifts made since Widnall's (1975) review in this series. This review also examines the predictability of the vortices (particularly in terms of lifespan), the durability of multiple vortex pairs, the controversy between expectations of vortex decay and of vortex collapse, the many types of turbulence that may influence the flow, the rich interplay between the rotational and the axial velocity fields in the vortex, the various atmospheric and ground-related factors that dominate the late behavior, a few instances of wakes rising back, and the still unexplained bursting of the vortices. The article also briefly covers prospects for detection and control.

We review the development of diffuse-interface models of hydrodynamics and their application to a wide variety of interfacial phenomena. These models have been applied successfully to situations in which the physical phenomena of interest have a length scale commensurate with the thickness of the interfacial region (e.g. near-critical interfacial phenomena or small-scale flows such as those occurring near contact lines) and fluid flows involving large interface deformations and/or topological changes (e.g. breakup and coalescence events associated with fluid jets, droplets, and large-deformation waves). We discuss the issues involved in formulating diffuse-interface models for single-component and binary fluids. Recent applications and computations using these models are discussed in each case. Further, we address issues including sharp-interface analyses that relate these models to the classical free-boundary problem, computational approaches to describe interfacial phenomena, and models of fully miscible fluids.

Turbulence is ubiquitous in astrophysics, ranging from cosmology, interstellar medium to stars, supernovae, accretion disks, etc. Large scales and small viscosities combine to form large Reynolds numbers. Because it is not possible in a single article to review all the above scenarios, we limit ourselves to stars, in which thermal instabilities give rise to turbulent convection as the dominant heat transport mechanism. (Accretion disks, where shear instabilities dominate the outward transport of angular momentum, will be the subject of a second article, planned for Volume 31.) Because of the lack of a satisfactory theory, turbulence constitutes a bottleneck that prevents astrophysical models from being fully predictive. Because continued use of phenomenological turbulence expressions would make astrophysical models perennially unpredictive, a way must be found to make astrophysical models as prognostic as possible. In addition to the difficulties brought about by turbulence, astrophysical settings introduce “malicious conditions,” of which the most refractory to a satisfactory quantification are compressibility (caused by the large density excursions that characterize convective zones in stars) and rotation. Basic understanding of how they affect turbulence in general is still rather sketchy. Reasons for the choice of stars and accretion disks as prototype examples are the following: The underlying instabilities are very basic; laboratory and direct numerical simulations data help constrain theoretical models; and new observational data, especially from helioseismology, help discriminate among different models with unprecedented accuracy.

Interaction of a vortex, or combinations of them, with a cylinder, blade, or foil may involve both rapid distortion of the incident vorticity field and shedding of vorticity from the surface of the body. This review focuses on the underlying flow physics, with the aim of clarifying the origin of the induced loading. In the case of near or direct encounter of the incident vortex, the relation between three-dimensional features of the flow structure and the local loading poses interesting challenges for further research. With recently developed simulation and laboratory techniques, opportunities now exist to determine the instantaneous quantitative structure of these complex distortions and to interpret them within a theoretical, vorticity-based framework. Effective control of vortex interactions appears to be attainable. It will be necessary, however, to develop creative strategies, distinct from those traditionally employed for control of unstable shear flows.

The need to develop new diagnostics for turbulent flows at supersonic and hypersonic regimes is discussed. New experimental results can be obtained in supersonic flows by using the collective light scattering method. Typical results obtained by this method in a supersonic mixing layer are illustrated. The collective light scattering method is a directional densitometer (with a new type of spectral analysis of density fluctuations), a nonparticle anemometer, a Mach-meter (or thermometer), and a directional remote microphone. Various other optical techniques that can be applied for point, line-of-sight, or imaging measurements are reviewed. For point measurements, light-scattering methods such as Raman, Rayleigh, or electron beam fluorescence are discussed, but only briefly, since they are of little use, especially when enthalpy is very high and flow naturally bright. Emphasis is placed instead on nonlinear laser spectroscopy such as coherent anti-Stokes Raman scattering, which has recently been successful in determining temperature and density in high-enthalpy shocks. A description of diode laser absorption spectroscopy follows. A high data-rate instrument now routinely gives the static temperature and the velocity of the stream in the hot shot facility F4 of ONERA, at stagnation enthalpies in excess of 15 MJ/kg. Finally, electron beam fluorescence imaging in the same facility has made it possible to perform measurements of velocity across the external boundary layer into the flow core using a high-energy–pulsed electron gun.

The renormalization-group (RG) analysis of turbulence, based primarily on KG Wilson's coarse-graining procedure, leads to suggestive results for turbulence coefficients and models. Application of the method to turbulence evolved from the contributions of many authors and received widespread attention following the 1986 work of V Yakhot and SA Orszag. The Yakhot-Orszag method involves the basic renormalization-group scale-removal procedure, as well as additional hypotheses and approximations; their analysis is reviewed here with an attempt to clarify those approximations. Discussion of some related and subsequent literature is also included. Following the work of M Avellaneda and AJ Majda, a simpler version of the method is appplied to a model passive scalar problem wherein it is seen that, in certain cases, the RG method can recover exact results.

We discuss a few applications of active control of turbulent fluid flow and their implications for the economy and the environment. We outline a conceptual basis for control, sketching sensors, actuators, and the algorithm. The control of turbulence requires an understanding of turbulent flows beyond our present capabilities, but we describe the physical basis for control of the boundary layer: coherent structures and bursts, the connection between burst frequency and friction velocity, the change of burst frequency and drag reduction possible with polymers or active control, and other effects on burst frequency (e.g. streamline curvature, pressure gradients, and extra rates of strain). Given that the state of the flow must be sensed from the surface, and that this information is necessarily incomplete and aliased, sophisticated techniques may be required to interpret the signals. A control strategy, an algorithm, is necessary, and we express the need for a model of the flow as an interpretor and a predictor. Although surface actuators are necessary, the question of their precise effect on the fluid must be considered. We present surprising results of direct numerical simulation (DNS) of a type of actuator. Before controlling the fluid, we tried to control a model. As background, we present results of attempts to control several different models.

We present an overview of the lattice Boltzmann method (LBM), a parallel and efficient algorithm for simulating single-phase and multiphase fluid flows and for incorporating additional physical complexities. The LBM is especially useful for modeling complicated boundary conditions and multiphase interfaces. Recent extensions of this method are described, including simulations of fluid turbulence, suspension flows, and reaction diffusion systems.

This review examines recent advances made in predicting boiling heat fluxes, including some key results from the past. The topics covered are nucleate boiling, maximum heat flux, transition boiling, and film boiling. The review focuses on pool boiling of pure liquids, but flow boiling is also discussed briefly.

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.

The exchange of inert and sparingly soluble gases—including carbon dioxide, methane, and oxygen—between the atmosphere and oceans is controlled by a thin 20- to 200-μm-thick boundary layer at the top of the ocean. The hydrodynamics in this layer are significantly different from boundary layers at rigid walls, since the orbital motion of the waves is of the same order as the velocities in the viscous boundary layer. Laboratory and field measurements show that wind waves significantly increase the gas transfer rate and that it is significantly influenced in this way by surfactants. Because of limited experimental techniques, the mechanisms for this enhancement and the structure of the turbulence in the boundary layer at a wavy water surface are still not known. A number of new imaging techniques are described that give direct insight into the processes and promise to trigger substantial theoretical progress in the near future.

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.

This is a review of the mechanisms that control neutrally stable turbulent boundary-layer flow over hills and waves, their relative magnitudes, and how they exert their greatest effects in different regions of the flow. We compare calculations based on various analytical and computational models with each other and with relevant experimental data. We discuss practical applications of these studies.

We review the direct numerical simulation (DNS) of turbulent flows. We stress that DNS is a research tool, and not a brute-force solution to the Navier-Stokes equations for engineering problems. The wide range of scales in turbulent flows requires that care be taken in their numerical solution. We discuss related numerical issues such as boundary conditions and spatial and temporal discretization. Significant insight into turbulence physics has been gained from DNS of certain idealized flows that cannot be easily attained in the laboratory. We discuss some examples. Further, we illustrate the complementary nature of experiments and computations in turbulence research. Examples are provided where DNS data has been used to evaluate measurement accuracy. Finally, we consider how DNS has impacted turbulence modeling and provided further insight into the structure of turbulent boundary layers.

The micromachining technology that emerged in the late 1980s can provide micron-sized sensors and actuators. These micro transducers are able to be integrated with signal conditioning and processing circuitry to form micro-electro-mechanical-systems (MEMS) that can perform real-time distributed control. This capability opens up a new territory for flow control research. On the other hand, surface effects dominate the fluid flowing through these miniature mechanical devices because of the large surface-to-volume ratio in micron-scale configurations. We need to reexamine the surface forces in the momentum equation. Owing to their smallness, gas flows experience large Knudsen numbers, and therefore boundary conditions need to be modified. Besides being an enabling technology, MEMS also provide many challenges for fundamental flow-science research.

The design of sailing vessels is an ancient art that places an ever-increasing reliance on modern engineering sciences. Fluid mechanics shares the forefront of this reliance along with structural mechanics. This review focuses on the application of fluid mechanics in modern sailing vessel design. It is now common practice to predict sailing performance with what are called velocity prediction computer programs. The validity of the predictions is crucially dependent on accurate modeling of the air and water forces on the vessel. This article reviews existing methods of force modeling that include theory, experimentation, and numerical fluid mechanics and aerodynamics. The accuracy and reliability of the numerical methods are considered on the basis of experimental results and full-scale performance in areas for which the information is available.

Non-premixed turbulent combustion processes control most practical applications of combustion. Studying these mechanisms has been the objective of numerous theoretical and experimental works in the last century. In the past 10 years, direct numerical simulation (DNS) has emerged as a new methodology. It has become an essential tool to understand and model turbulent combustion. DNS numerically solves the set of equations describing turbulent flames by resolving all chemical and flow scales. Because formulated assumptions can be tested accurately, the resulting data provide unique information to build turbulent combustion models. This review first discusses the fundamental properties of laminar diffusion flames. It then presents various DNS results to illustrate the different problems that can be studied using this tool. These problems include validity of the modeling hypothesis, topology of flame surfaces, and ignition and extinction mechanisms. The review also discusses the different analysis techniques employed to extract information from DNS and explores the limits of these methods.