In the last 30 days

• Open/close Chaos and collapse of a system of point vortices

  Yosifumi Kimura  1988 Fluid Dyn. Res. 3 98 Tag this article

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  A system of point vortices, which is governed by the Hamiltonian form, permits various kinds of singular motions, such as chaos and collapse. In this article, the phenomenon of collapse is analysed from the viewpoint of singularities in the complex time plane.

• Open/close Molecular dynamics simulations of nanodroplet spreading on solid surfaces, effect of droplet size

  Nahid Sedighi et al  2010 Fluid Dyn. Res. 42 035501 Tag this article

  Article References Full text PDF (1.81 MB)

  Molecular dynamics simulations were performed to study the spreading characteristics of nano-sized droplets on solid surfaces. The spreading behavior was analyzed in terms of the temporal evolution of the dynamic contact angle and spreading diameter for wettable, partially wettable and non-wettable surfaces. The computational model was validated through qualitative comparison with the measurements of Bayer and Megaridis, and through comparison with existing correlations. The comparison based on the ratio of relevant time scales indicated that for the conditions investigated, the spreading dynamics is governed by inertial and surface forces, with negligible influence of viscous forces. In addition, the simulation results indicated that the dynamic contact angle and spreading diameter, as well as the advancing and receding time periods, exhibit strong dependence on droplet size. These results were further analyzed to obtain correlations for the effect of droplet size on these spreading parameters. The correlations indicated that the normalized spreading diameter and contact angle scale with drop diameter as D _m / D ₀ ∝ D ₀ ^0.5 and θ _R ∝ D ₀ ^0.5, while the advancing and receding time periods scale as t∝ D ₀ ^2/3. Global kinetic energy and surface energy considerations were used to provide a physical basis for these correlations. The correlations were also found to be generally consistent with the experimentally observed spreading behavior of macroscopic droplets.

• Open/close Three-dimensional vortex structures and vorticity topology

  U Dallmann  1988 Fluid Dyn. Res. 3 183 Tag this article

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  In this paper we describe the formation of steady, three-dimensional flow structures as changes of flow topology. Possible changes are analyzed in the space of divergence-free. Navier-Stokes vector fields. Changes occur at structurally unstable flow states, for instance at incipient separation, incipient vortex formation and vortex breakdown, etc. and whenever the flow around a nominally two-dimensional or axisymmetric body becomes three-dimensional. Vortices may be defined as helical flow of fluid particles around a common axis, hut we have to distinguish between the topological structure of the velocity field and the vorticity field of such vortices (vortical flows). At first we review our results on near-wall velocity-field structures and then we discuss the topology of the associated vorticity fields. Finally we develop a geometric description which allows to give a local definition for a three-dimensional vortex based on the invariants of the Jacobian matrix of such Navier-Stokes vector fields.

• Open/close Traveling wave instability in a diverging–converging channel

  J M Floryan and C Floryan  2010 Fluid Dyn. Res. 42 025509 Tag this article

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  The temporal linear stability of a pressure-driven flow in a diverging–converging channel with respect to the traveling wave disturbances is considered. The flow may develop under either the fixed mass flowrate constraint or the fixed pressure gradient constraint. It is shown that the variations in the channel geometry lead to the flow destabilization, as compared with the straight channel case. It is demonstrated that the critical disturbances have the form of two-dimensional waves. The global critical conditions describing the minimum critical Reynolds number required to create the instability for the specified geometry of the channel are given. It is shown that the flow developed under the fixed mass flowrate constraint is slightly more unstable than the flow developed under the fixed pressure gradient constraint. This difference increases with an increase of the amplitude of the channel divergence–convergence.

• Open/close On the von Neumann paradox of weak Mach reflection

  Akira Sakurai et al  1989 Fluid Dyn. Res. 4 333 Tag this article

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  Recent experimental and numerical studies of weak Mach reflections are examined. It is shown that the fundamental reason for the von Neumann paradox is that his theory of Mach reflection is based on the assumption that the flow downstream of the reflected wave and the Mach shock near the wave triple point is uniform. The assumption is shown to be valid for strong Mach reflection which agrees with experiment, but invalid for weak Mach reflection which does not agree with experiment. It is also shown that viscous effects are dominant when the incident shock is within about 100 mean free path lengths of the corner, but not otherwise. The analytical theory of the entire subsonic region supports these conclusions.

• Open/close Non-Newtonian fluid flow development in a circular pipe

  R Gupta and R P Agarwal  1993 Fluid Dyn. Res. 12 203 Tag this article

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  Laminar non-Newtonian power-law fluid flow development in a pipe has been obtained by studying a hydrodynamically equivalent Newtonian analogue of the problem. Several integral solutions of the problem have been presented and their predictions have been compared with another similar solution.

• Open/close The fundamental solution of Brinkman's equation in two dimensions

  Hideo Yano et al  1991 Fluid Dyn. Res. 7 109 Tag this article

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  The fundamental solution of Brinkman's equation for seepage flows is derived in two-dimensional cases with the use of complex-variable technique. Seepage flows past cavities and the Oseen flows past permeable bodies are solved to clarify the availability of this fundamental solution.

• Open/close New insights into large eddy simulation

  J P Boris et al  1992 Fluid Dyn. Res. 10 199 Tag this article

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  Fluid dynamic turbulence is one of the most challenging computational physics problems because of the extremely wide range of time and space scales involved, the strong nonlinearity of the governing equations, and the many practical and important applications. While most linear fluid instabilities are well understood, the nonlinear interactions among them makes even the relatively simple limit of homogeneous isotropic turbulence difficult to treat physically, mathematically, and computationally. Turbulence is modeled computationally by a two-stage bootstrap process. The first stage, direct numerical simulation, attempts to resolve the relevant physical time and space scales but its application is limited to diffusive flows with a relatively small Reynolds number (Re). Using direct numerical simulation to provide a database, in turn, allows calibration of phenomenological turbulence models for engineering applications. Large eddy simulation incorporates a form of turbulence modeling applicable when the large-scale flows of interest are intrinsically time dependent, thus throwing common statistical models into question. A promising approach to large eddy simulation involves the use of high-resolution monotone computational fluid dynamics algorithms such as flux-corrected transport or the piecewise parabolic method which have intrinsic subgrid turbulence models coupled naturally to the resolved scales in the computed flow. The physical considerations underlying and evidence supporting this monotone integrated large eddy simulation approach are discussed.

• Open/close Chaotic mixing in a helix-like pipe with periodic variations in curvature and torsion

  Bongkyun Jang and Mitsuaki Funakoshi  2010 Fluid Dyn. Res. 42 035506 Tag this article

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  Chaotic motion of fluid particles due to a steady viscous flow in a helix-like circular pipe caused by an axial pressure gradient, and the mixing efficiency of this flow are numerically examined here. The pipe is wound around a circular or elliptic cylinder with a constant pitch so that the curvature κ and torsion τ of the centerline of this pipe vary continuously and periodically. If both κ and τ are small and slowly varying, the cross-sectional motion of fluid particles is expected to be approximately governed by the sum of Dean's flow and the flow of rigid rotation. From Poincaré sections and the values of an index of the extent of mixing, it is found that there is an intermediate range of Reynolds number Re of flow within which chaotic regions in Poincaré sections are large and mixing efficiency over a short time is high. Moreover, larger chaotic regions and higher mixing efficiency are observed for pipes wound around a circular cylinder of smaller radius and for pipes wound around a thinner elliptic cylinder. These results can be explained by the variation in characteristic ratio λ=12τ/(κ Re) in one period.

• Open/close Computation of high Reynolds number flow around a circular cylinder with surface roughness

  Tetuya Kawamura et al  1986 Fluid Dyn. Res. 1 145 Tag this article

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  Incompressible high-Reynolds-number flows around a circular cylinder are analyzed by direct integration of the Navier-Stokes equations using finite-difference method. A generalized coordinate system is used so that a sufficient number of grid points are distributed in the boundary layer and the wake. A numerical scheme which suppresses non-linear instability for calculations of high-Reynolds-number flows is developed. The computation of an impulsively started flow at Re = 1200 is compared with corresponding experimental observations, and excellent agreements are obtained.

  A series of computations are carried out on the flow around a circular cylinder with surface roughness. The height of the roughness in these computations is 0.5% of the diameter. The range of Reynolds numbers is from 10 ³ to 10 ⁵; no turbulence model is employed. Sharp reduction of drag coefficient is observed near Re = 2 × 10 ⁴, which indicates that the critical Reynolds number is captured in the present computation.