摘要:

The response of the sectional side force coefficient to localized suction has been studied on a slender body placed at 55° angle of attack. A strong dependence in the behavior of the asymmetric vortex system on Reynolds number is shown, which resolves discrepancies observed by different investigators.

ISSN:0899-8213    

DOI:10.1063/1.858693

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Natural, or intrinsic, coordinates are examined for an unsteady, compressible, viscous, Newtonian fluid in two‐dimensional or axisymmetric flow. The corresponding Navier–Stokes equations are obtained along with Crocco’s equation, an entropy production equation, and an equation for the rate of change of vorticity following a fluid particle. The extension to a three‐dimensional flow is discussed.

ISSN:0899-8213    

DOI:10.1063/1.858694

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The sedimentation of monodisperse suspensions of rigid spheres has been studied by dynamical simulation; computational techniques are described and numerical results are reported. It has been found that there is a slow relaxation of the suspension microstructure during sedimentation, so that compared with the initial equilibrium distribution, there is an increased number of pairs of particles near contact; this leads to a 5%–10% increase in the average sedimentation velocity. Individual particle velocities fluctuate about the mean fall speed; these fluctuations are large and persist for long times. The resulting hydrodynamically induced dispersion of the particles can be characterized by strongly anisotropic diffusion coefficients; however, the dispersion process is non‐Fickian at high solids concentrations.  

ISSN:0899-8213    

DOI:10.1063/1.858695

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The flow of a periodic suspension of two‐dimensional viscous drops between two parallel plane walls is considered, and a framework for performing dynamic simulations with suspensions of increasingly complex structure is developed. The computations are based on an improved implementation of the boundary integral method which is coined the method of interfacial dynamics. An elementary configuration is considered in detail in which the suspension is composed of a periodic array of suspended drops arranged on a single file, and the flow is driven by the relative motion of the two walls. The motion is studied as a function of the capillary number, the viscosities of the fluids, and the drop size. The transition from the small‐drop behavior, where the effects of the walls are insignificant, to the large‐drop behavior, where the drops occupy almost the whole of the clearance between the walls, is illustrated. It is found that, in all cases, the suspension exhibits a shear‐thinning behavior and some type of elasticity. The single‐file arrangement is unstable to subharmonic perturbations in which the drops are displaced in an alternating fashion across the centerline of the channel. The evolution of the perturbed array may lead to oscillatory motions including orbiting and bypassing, or to formation of a double file of nearly stationary drops. The effects of drop interactions on the effective properties of the suspension are discussed.

ISSN:0899-8213    

DOI:10.1063/1.858893

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Previous studies have assumed that the propagation of shear fronts, or internal undular bores, through slowly varying changes in depth and width are described by unidirectional, unimodal models. The propagation of shear fronts is considered in the linear, nondispersive limit to show that this assumption is only valid for small changes in width, while for a continuously stratified fluid it is not a valid approximation for changes in depth. It is also shown for waves of finite extent propagating over changes in depth, that unidirectional, unimodal models will only describe the evolution of an incident first mode wave.

ISSN:0899-8213    

DOI:10.1063/1.858696

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The instability of a two‐dimensional triangular array of vortices with respect to inviscid perturbations is investigated. A numerical treatment in the framework of Floquet theory provides critical parameters for instability onset and the spatial structure of unstable oscillatory modes which are in agreement with experiments of Sommeriaetal. [MechanicsofFluidsandTransportProcesses(Kluwer Academic, Dordrecht, The Netherlands, 1989), Vol. 10] in electromagnetically driven flows of a liquid metal layer. A point vortex model allows one to relate the dynamics of infinitesimal displacements of vortices to the concept of waves in a lattice of linearly coupled oscillators.

ISSN:0899-8213    

DOI:10.1063/1.858858

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Theory and calculations are presented for the evolution of Richtmyer–Meshkov instability in two‐dimensional continuously stratified fluid layers. The initial acceleration and subsequent instability of the fluid layer are induced by means of an impulsive pressure distribution. The subsequent dynamics of the fluid layer are then calculated numerically using the incompressible equations of motion. Initial conditions representing single‐scale perturbations and multiple‐scale random perturbations are considered. It is found that the growth rates for Richtmyer–Meshkov instability of stratified fluid layers are substantially lower than those predicted by Richtmyer for a sharp fluid interface with an equivalent jump in density. A frozen field approximation for the early‐time dynamics of the instability is proposed, and shown to approximate the initial behavior of the layer over a time equivalent to the traversal of several layer thicknesses. It is observed that the nonlinear development of the instability results in the formation of plumes of penetrating fluid. Late in the process, the initial momentum deposited by the impulse is primarily used in the internal mixing of the layer rather than in the overall growth of the stratified layer. At intermediate times, some evidence for the existence of scaling behavior in the width of the mixing layer of the instability is observed for the multiple‐scale random perturbations, but not for the single‐scale perturbations. The time variation of the layer thickness differs from the scaling derived using ideas of self‐similarity due to Barenblatt [Non‐LinearDynamicsandTurbulence, edited by G. I. Barenblatt, G. Ioos, and D. D. Joseph (Pitman, Boston, 1983), p. 48] even at low Atwood ratio, presumably because of the inhomogeneity and anisotropy due to the excitation of vortical plumes.

ISSN:0899-8213    

DOI:10.1063/1.858859

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Simplified asymptotic equations describing the nonlinear dynamics of perturbed pairs of parallel vortex filaments are derived and analyzed here. The derivations are general enough to allow for vortices of unequal strength, but emphasis here is on the antiparallel vortex pair. The simplified asymptotic equations account for both the internal effects of self‐induction and self‐stretching for each filament and also the external effects of mutual induction that lead to a nontrivial coupling of the perturbations of the two filaments. When these nonlinear equations are linearized at the unperturbed filament pair, the linearized stability theory of Crow [AIAA J.8, 2172 (1970)] is recovered in a systematic fashion. The asymptotic equations are derived in a novel singular limit at high Reynolds numbers through assumptions similar to the authors’ recent theories [Physica D49, 323 (1991);ibid.53, 267 (1991); Phys. Fluids A4, 2271 (1992)] for the dynamics of a single perturbed vortex filament. Through the Hasimoto transform [J. Fluid Mech.51, 477 (1972)], these equations become two coupled perturbed nonlinear Schro¨dinger equations for a pair of filament functions. A series of numerical solutions of the asymptotic equations exhibits several new phenomena in the nonlinear instability of pairs of antiparallel vortex filaments.

ISSN:0899-8213    

DOI:10.1063/1.858860

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The reflection of linear surface waves by sinusoidal bottom undulations is considered in the case where the incident wave is not necessarily close to the resonant frequency. For finite detuning away from the resonant frequency, two previous solutions are shown to give results which are inconsistent with direct numerical solutions, especially when the results are extended to oblique incidence. The correction to the methods is given, and various consequences of the new results are examined.

ISSN:0899-8213    

DOI:10.1063/1.858861

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Flow‐visualization methods were used to explore the structure of streamwise vortices and their interactions with the von Ka´rma´n vortices in the immediate wakes of cylinders. The experiments were conducted in a water tunnel at Reynolds numbers from 330–21 000 based on cylinder diameter. Over this entire range of Reynolds number, pairs of counter‐rotating streamwise vortices were observed immediately behind the cylinders, with a mean spanwise spacing of approximately one pair per diameter. The streamwise vortices significantly distorted the von Ka´rma´n vortices, but only on their upstream‐facing sides. The cylinder near‐surface‐flow topology was found to include a secondary separation line containing ‘‘dominant‐foci structures’’ whose spanwise locations correlated with those of the streamwise vortices.

ISSN:0899-8213    

DOI:10.1063/1.858862

出版商:AIP    

年代:1993

数据来源: AIP