摘要:

Correlation functions of non‐scalar fields in isotropic hydrodynamic turbulence are characterized by a set of universal exponents. These exponents also characterize the rate of decay of the effects of anisotropic forcing in developed turbulence. These exponents are important for the general theory of turbulence, and for modeling anisotropic flows. We propose methods for measuring these exponents by designing new laboratory experiments. ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869065

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

A numerical investigation is presented of axisymmetric, static and elongating, viscous Newtonian liquid bridges confined between identical circular disks. The time‐dependent interface shapes and applied forces on the end plates, which separate at a constant prescribed velocity, are calculated as functions of the capillary number, the viscosity ratio between the inner and outer fluids, and an initial bridge configuration characterized by the aspect ratio. The numerical simulations are in excellent agreement with available experimental data and provide useful insight into the different dynamical responses of extending liquid bridge configurations. In particular, liquid bridges surrounded by fluids of a relatively small viscosity deform in a fore‐aft symmetrical manner and undergo breakup sooner than in the case of relatively viscous outer fluids, which also require a greater applied force on the end plates to maintain the desired motion. Decreasing the capillary number (increasing interfacial tension) and the initial aspect ratio result in shorter bridge lengths prior to breakup and an increase in the applied forces on the end plates. ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869044

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

In previous numerical studies [Lee and Wang, J. Fluid Mech.188, 411 (1988); Pelekasisetal., J. Fluid Mech.230, 541 (1991)], it has been shown that when an inviscid and nonconcentric liquid shell undergoes a finite‐amplitude capillary oscillation, its enclosed bubble undertakes a slow translational oscillation relative to the shell. In the present work, we study the effects of viscosity on the slow motion, in both free and forced capillary oscillations. It is found that in a free oscillation, the shell cannot become concentric because the oscillations are damped by viscosity before centering occurs. In a forced oscillation which is sustained by an external source such as a modulated acoustic radiation pressure, centering does occur when the slow oscillations are damped. The predicted centering of the shell takes place more slowly than that observed in experiments [Wangetal., J. Colloid Interface Sci.165, 19 (1994)]. However, it is noted that a comparison with experiments is not appropriate at this time, since the shell in the experiments had an uncontrolled rotation in the acoustic potential well. ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869045

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The motion of dense particles in a turbulent gas flow has been studied by means of numerical simulations. The single‐phase turbulent pipe flow was modelled using Direct Numerical Simulation and Large Eddy Simulation. At tube Reynolds numbers of 5300, 18300 and 42000 particles with dimensionless relaxation times ranging from 5 to 104were released. Assuming the system to be dilute, the characteristics of particle dispersion, deposition and concentration distribution were studied under various conditions of gravity and lift. This study shows that for small particles the deposition process is governed by the properties of the near‐wall layer where the wall‐normal turbulence intensity is low, while for large inertial particles turbulent dispersion determines the chances for particles to hit the tube wall. The motion of the latter particles appears to scale properly with the Lagrangian integral time scale of the turbulence. Furthermore we demonstrated the segregation of particles towards the wall, as a result of particle‐turbulence interaction. ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869046

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Boundary layers on stationary and rotating disks have received much attention since von Ka´rma´n’s [Z. Angew. Math. Mech.1, 233 (1921)] and Bo¨dewadt’s [Z. Angew. Math. Mech.20, 241 (1940)] studies of the cases with disks of infinite radius. Theoretical treatments have focused on similarity treatments leading to conflicting ideas about existence and uniqueness, and where self‐similar solutions exist, whether they are physically realizable. The coupling between the boundary layer flows and the interior flow between them, while being of practical importance in a variety of situations such as turbomachinery and ocean circulations, is not well understood. Here, a numerical treatment of the axisymmetric Navier–Stokes equations, together with some experiments for the case of finite stationary and rotating disks bounded by a co‐rotating sidewall is presented. We show that in the long time limit, solutions are steady and essentially self‐similar. Yet the transients are not. In particular, axisymmetric waves propagate in the stationary disk boundary layer when the vortex lines entering the boundary layer develop inflection points, and there are subsequent eruptions of vortical flow out of the boundary layer deep into the interior at large Reynolds numbers. ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869047

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

In this study we present a numerical investigation of steady states, onset of oscillatory instability, and slightly supercritical oscillatory states of an axisymmetric swirling flow of a Newtonian incompressible fluid in a cylinder, with independently rotating top and bottom. The first part of the study is devoted to the influence of co‐ and counter‐rotation of the bottom on the steady vortex breakdown, which takes place in the well‐known problem of flow in a cylinder with a rotating top. It is shown that weak counter‐rotation of the bottom may suppress the vortex breakdown. Stronger counter‐rotation may induce a stable steady vortex breakdown at relatively large Reynolds numbers for which a vortex breakdown does not appear in the case of the stationary bottom. Weak corotation may promote the vortex breakdown at lower Reynolds numbers than in the cylinder with the stationary bottom. Stronger corotation leads to the detachment of the recirculation zone from the axis and the formation of an additional vortex ring. The second part of the study is devoted to the investigation of the onset of oscillatory instability of steady flows. It is shown that the oscillatory instability sets in due to a Hopf bifurcation. The critical Reynolds number and the critical frequency of oscillations were calculated as a function of the rotation ratio (&xgr;=&OHgr;bottom/&OHgr;top) for a fixed value of the aspect ratio &ggr; (height/radius) of the cylinder &ggr;=1.5. The stability analysis showed that there are several most unstable linear modes of the perturbation that become successively dominant with a continuous change of &xgr;. It is shown that the oscillatory instability may lead to an appearance and coexistence of more than one oscillating separation vortex bubble. ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869064

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Coherent structures and the dynamics of a neutrally stratified planetary boundary layer flow are studied through a large eddy simulation, which includes surface roughness, Coriolis force, and a capping inversion. Quadrant analysis and flow visualization show that low‐speed negative momentum flux (ejection) is the dominant feature throughout most of the boundary layer. The initiation of vortical structures is observed to be associated with vorticity sheets and pressure maxima, which are formed dynamically when low‐speed negative momentum flux collides with either high‐speed negative momentum flux (sweep) or the mean flow. Four dimensional conditional averages are used to study the statistical behavior of ejections and sweeps. The shape, strength, lifetime, and origin of the conditionally sampled structures at three different heights are discussed. Near the surface, sweeps are observed to induce ejections when colliding with the surface. The evolution of sweep‐induced ejections near the wall is discussed. ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869048

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

A joint experimental and computational methodology is developed and applied to investigate a vortex ring impinging normally on a wall. The method uses digital particle image velocimetry to make planar flow measurements, which are then used to initialize a second‐order finite difference calculation. The experiment and the simulation are compared at later times and agree extremely well. The ring undergoes two rebounds from the wall and continues to expand. During the approach to the wall, peak vorticity grows by 50% due to vortex stretching. Peak vorticity strengths of the secondary and tertiary vortices formed from the shedding boundary layer are 40% and 20% of the primary. In addition, a ring with a Gaussian core is simulated and compared to demonstrate the benefits of using realistic initial conditions. ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869049

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

We examine numerically the issue of convergence for initial‐value solutions and similarity solutions of the compressible Euler equations in two dimensions in the presence of vortex sheets (slip lines). We consider the problem of a normal shock wave impacting an inclined density discontinuity in the presence of a solid boundary. Two solution techniques are examined: the first solves the Euler equations by a Godunov method as an initial‐value problem and the second as a boundary value problem, after invoking self‐similarity. Our results indicate nonconvergence of the initial‐value calculation at fixed time, with increasing spatial‐temporal resolution. The similarity solution appears to converge to the weak ‘zero‐temperature’ solution of the Euler equations in the presence of the slip line. Some speculations on the geometric character of solutions of the initial‐value problem are presented. ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869050

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

An attempt is made to construct numerically equilibrium measures for the Euler equations by first examining measures for discretized approximate systems and then searching on the computer for the limit of vanishing discretization. First the partition function is evaluated for two‐dimensional discretized incompressible fields with a hydrodynamical energy function and an infinite number of invariants; the behavior of the partition functions is examined as the discretization is refined. The results are contrasted with those of recent mean‐field theories, which are seen to be reasonable approximations only at moderate temperatures. The two‐dimensional vortex system has no phase transitions and no states invariant under refinement of the discretization, except at zero temperature. Finite‐temperature equilibrium measures may appear if a simple representation of vortex stretching is added to the system, in agreement with recent work on three‐dimensional turbulence, where these equilibrium measures are used as key building blocks. ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869051

出版商:AIP    

年代:1996

数据来源: AIP