Mohamed Fayed

Strongly Swirling Turbulent Sink Flow Between Two Stationary Disks

The steady, incompressible turbulent sink flow developing between two stationary disks under the influence of strong swirl was numerically investigated. The simulations were made using the FLUENT 6.2 software. The purpose of the study is to first validate the method and then proceed in identifying the origin of some key known flow features, such as the appearance of a toroidal recirculation zone in the central flow region, the saddlelike behavior of the tangential velocity, and the two radial velocity kinks near the two end plates. The simulations are found to be in good accord with earlier experiments. Centrifugal force is shown to be the prime culprit for all of the previous flow manifestations. The overpowering centrifugal force compels the fluid to drain mainly through the end-plate boundary layers where its value is the minimum. The buildup of the recirculation zone is a natural response by the fluid to account for the local flow inactivity. This buildup is also responsible for the evolution of a weak reversed flow in the midchannel plane that, along with turbulence and the nonslip condition on the disk surface, produces the until-now unexplained saddlelike shape of the tangential velocity profile. The growth of the radial velocity spikes near the disks is due to the synergetic action of the boundary-layer development and the reduction of the local flow area. Finally, the simulations have also unveiled a previously unknown tangential velocity undulation inside the vortex core triggered by vortex transition from laminar to turbulent conditions.

FRACTAL GEOMETRY OF THE WAKE SHED BY A FLAPPING FILAMENT IN FLOWING SOAP-FILM

In this study, we illustrate the fractal nature of the wake shed by a periodically flapping filament. Such wake structure is a combination of primary vortex shedding resulting in the von Karman vortex street, a series of concentrated vortex dipoles formed when the trailing edges of filaments reach their maximum amplitudes and small eddies form along the shear layer connected with the concentrated vortices due to the shear layer instability. The vortex dynamics of the flapping filament are visualized and imaged experimentally using a soap-film flow tunnel with a high-speed camera and a low pressure sodium lamp as a light source. The wake fractal geometry is measured using the standard box-counting method and it is shown that the fractal dimension of the soap pattern boundaries in the wake is D = 1.38 ± 0.05, which agrees well with those measured for fully developed turbulences and other shear flow phenomena. The invariant of the fractality in the wake induced by the flapping filament thus provides another illustration of the geometrical self-similarity and nonlinear dynamics of chaotic fluid flows.

Flow visualization and numerical simulation of a two-dimensional fluid flow over a foil

This paper deals with a simple, fast and economical visualization method to validate two-dimensional large eddy simulations (LES) of the flow over a foil. This technique exploits the optical properties of soap film and relies on the wake patterns and the frequency at which these are shed at the trailing edge of the foil.Graphical Abstract

Numerical simulation and flow visualization using soap film of the self-organized vortex structure in the wake of an array of cylinders

With the aid of computational fluid dynamics (CFD) and simple flow visualization technique using flowing soap-film, we present here the wake structures behind an array of cylinders for Reynolds numbers corresponding to both laminar and turbulent flow regimes. The image results illustrate interesting vortex interactions past these equally spaced cylinders; for low Reynolds number flow, well-organized wake pattern persists and manifests unsteadily to different symmetry states. An increase of free stream flow velocity causes the wake transition, resulting in the formation of asymmetric flow wake with chaotic mixing at the far wake. Observations from both the numerical simulations and soap-film are in good agreement at least qualitatively.Graphical Abstract