Showing papers in "Annual Review of Fluid Mechanics in 1984"

Vortex shedding from oscillating bluff bodies

Abstract: When placed ih a fluid stream, some bodies generate separated flow over a substantial proportion of their surface and hence can be classified as bluff. On sharp-edged bluff bodies, separation is fixed at the salient edges, whereas on bluff bodies with continuous surface curvature the location of separation depends both on the shape of the body and the state of the boundary layer. At low Reynolds numbers, when separation first occurs, the flow around a bluff body remains stable, but as the Reynolds number is increased a critical value is reached beyond which instabilities develop. These instabilities can lead to organized unsteady wake motion, dis­ organized motion, or a combination of both. Regular vortex shedding, the subject of this article, is a dominant feature of two-dimensional bluff-body wakes and is present irrespective of whether the separating boundary layers are laminar or turbulent. It has been the subject of research for more than a century, and many hundreds of papers have been written. In recent years vortex shedding has been the topic of Euromech meetings reported on by Mair & Maull (1971) and Bearman & Graham (1980), and a comprehensive review has been undertaken by Berger & Wille (1972). Vortex shedding and general wake turbulence induce fluctuating pres­ sures on the surface of the generating bluff body, and if the body is flexible this can cause oscillations. Oscillations excited by vortex shedding are usually in a direction normal to that of the free stream, and amplitudes as large as 1.5 to 2 body diameters may be recorded. In addition to the generating body, any other bodies in its wake may be forced into oscillation. Broad-band force fluctuations, induced by turbulence produced in the flow around a bluff body, rarely lead to oscillations as severe as those caused by vortex shedding. Some form of aerodynamic instability, such that move-

Numerical Simulation of Turbulent Flows

Abstract: Computational models of turbulence in incompressible Newtonian fluids governed by the Navier-Stokes equations are reviewed. The governing equations are presented, and both direct and large-eddy-simulations are examined. Resolution requirements and numerical techniques of spatial representation, definition of initial and boundary conditions, and time advancement are considered. Results of simulations of homogeneous turbulence in uniform shear, the evolution of a turbulent mixing layer, and turbulent channel flow are presented graphically and discussed.

Dynamic Parameters of Gaseous Detonations

Abstract: In addition to gases, flammable liquids and solids in the form of fine droplets and dust particles also form explosive mixtures with air. An explosive mixture can, in general, support two modes of combustion. The slow laminar deflagration mode is at one extreme; here the flame propagates at typical velocities of the order 1 m s -1 relative to the unburned gases and there is negligible overpressure development when the explosion is unconfined. At the other extreme is the detonation mode, in which the detonation wave propagates at about 2000 m s -1 accompanied by an overpressure rise of about 20 bars across the wave. The propagation of laminar defiagrations is governed by the molecular diffusion of heat and mass from the reaction zone to the unburned mixture. The propagation of detonations depends on the adiabatic shock compression of the unburned mixtures to elevated temperatures to bring about autoignition. The very strong exponential temperature dependence of chemical reaction rates in general makes possible the rapid combustion in the detonation mode. Two­ phase liquid droplets or dust-air mixtures are similar, but they require more physical processes (e.g. droplet break-up, phase change, mixing, etc.) prior to combustion. Thus, characteristic time or length scales associated with the combustion front are usually much larger than those of homogeneous gaseous fuel-air mixtures. The essential mechanisms of propagation of the combustion waves, however, are similar. In between the two extremes of laminar detlagration and detonation, there is an almost continuous spectrum of burning rates where turbulence plays the dominant role in the combustion process. Due to space limitations, only homogeneous gaseous fuel-air detonations are considered in this article.

Modern Optical Techniques in Fluid Mechanics

Abstract: Optical techniques are widely used in fluid mechanics to observe and measure properties of flow fields such as velocities or densities. Many of these techniques are qualitative but of great value in guiding intuition for further research by quantitative means. Beautiful examples can be seen in the Album of Fluid Motion (Van Dyke 1982). Optical techniques are usually known for their largely nonintrusive properties as compared with methods like the Pitot tube or the hot-wire technique. The last few years, however, have seen some examples where light has been used not only to probe fluid flows but to generate them (Lauterborn 1980). This gives rise to a new classification of optical techniques in fluid mechanics (see Figure 1). Flow­ visualization techniques use light as an information carrier where the information is impressed on the light beam by the fluid flow. Flow­ generation techniques use light as an energy carrier to initiate fluid flow by radiation pressure, heating, or optical breakdown. Flow-visualization techniques may be coarsely subdivided into two categories: those that make use of light scattered by tiny particles in the fluid and those that make use of variations in refractive index. Among the methods that rely on scattered light, laser Doppler anemometry is now a standard means of obtaining fluid velocities. This method and its various refinements are well documented (Durst et al. 1976, Durrani & Greated 1977, Drain 1980, Schulz-DuBois 1983) and are not discussed here. In laser Doppler anemometry, the fluid velocity can be measured with high accuracy as a function of time but only at a single point in the fluid at any given time. The ultimate aim, of course, is the simultaneous determination of fluid velocities in a whole volume of a fluid. First steps in this direction

Aeroacoustics of Turbulent Shear Flows

Abstract: Recent analytical, numerical-simulation and experimental studies of sound generation by high-Reynolds-number turbulent shear flows are reviewed, with a focus on the application of linear rapid-distortion theory to the calculation of the unsteady flow field producing the sound. This approach is considered the most important alternative to acoustic-analogy methods. Topics surveyed include the linear theory of solid-surface interactions, the jet-noise problem, extensions to more complex turbulent flows, and supersonic flows. Graphs comparing theoretical and experimental results are shown.

Numerical solution of the nonlinear boltzmann equation for nonequilibrium gas flow problems

Abstract: This review concerns the numerical solution of the nonlinear Boltzmann equation for a gas flow under conditions far from thermal equilibrium. The rarefied-gas flow problem, which is characterized by a large global parameter, the Knudsen number, is often thought to be the orily non­ equilibrium problem. An appropriate measure of the local departure from equilibrium is the local Knudsen number, which may be defined in terms of the local property gradient. Nonequilibrium conditions characterized by large pro perty gradients do occur in certain regions in continuum-flow problems : a shock wave is a familiar example. Since equilibrium conditions exist in the upstream and downstream regions of the shock wave and since the relations between the upstream and downstream properties are known, the internal shock structure is not needed for the solution of such continuum-flow problems. Another example, which is less familiar, is the Knudsen layer next to an evaporation or a condensation interface. In contrast to the shock wave, the condition of the vapor at the inte rface is far from equilibrium. Neither this nonequilibrium condition nor its relation with the downstream equilibrium condition is known. This Knudsen-layer problem, therefore, cannot be treated by using a continuum approach, even though the flow characteristics in this layer may not be of interest. One of

Nonlinear Interactions in the Fluid Mechanics of Helium II

Abstract: Besides its practical importance in a host of technical applications, fluid mechanics retains its intrinsic interest as a physical discipline. The governing equations are nonlinear, and hence the motion of fluids demonstrates the complexities of solution of a nonlinear field theory, a fact that has been appreciated more and more in recent times. The most striking manifestations of this nonlinearity are shock waves and turbulence, corresponding to nonlinear wave and vortex interactions, respectively.