LOW-REYNOLDS-NUMBER INSTABILITY OF THE LAMINAR FLOW BETWEEN WAVY WALLS
Instability of a viscous incompressible flow in a channel with wavy walls is investigated theoretically, numerically and experimentally. Linear stability analysis shows that appropriately chosen wall waviness leads to flow destabilization at surprisingly low Reynolds numbers. The unstable mode of disturbances forms a vortex array, which travels downstream. The remarkable feature is that the most destabilizing waviness does not introduce any additional flow resistance. The outcome of the stability analysis are consistent with the result of direct numerical simulation obtained using CFD finite volume package FLUENT (Ansys Inc.). Preliminary experimental data gained for a channel with appropriately corrugated wall seem to confirm these predictions.
Turbulent Flow in a Microchannel With Surface Patterned Microribs Oriented Parallel to the Flow Direction
Due to the increase of application in a number of emerging technologies, a growing amount of research has focused on the reduction of drag in microfluidic transport. A novel approach reported in the recent literature is to fabricate micro-ribs and cavities in the channel wall that are then treated with a hydrophobic coating. Such surfaces have been termed super- or ultrahydrophobic and the contact area between the flowing liquid and the solid wall is greatly reduced. Previous numerical studies have focused primarily on the laminar flow through such channels with reductions in the flow resistance as large as 87% being predicted and observed. There has been little work however, that has explored the physics and the potential drag reduction associated with turbulent flow through microchannels with ultrahydrophobic walls. This paper reports the results of a numerical investigation of the turbulent flow in a parallel plate microchannel with ultrahydrophobic walls. In this study microribs and cavities are oriented parallel to the flow direction. The channel walls are modeled in an idealized fashion, with the shape of the liquid-vapor meniscus approximated as flat. A k-ω turbulence modeling scheme is implemented for closure to the turbulent RANS equations. Results are presented for the friction factor Reynolds number product as a function of relevant governing dimensionless parameters. The Reynolds number was varied from 2,000 to 10,000. Results show, as with the laminar flow case, that as the shear-free region increases the friction factor-Reynolds number product decreases. The observed reduction, however, was found to be significantly greater under turbulent flow conditions than for the laminar flow scenarios.© 2007 ASME
Mass transfer enhancement in a sinusoidal wavy channel for pulsatile flow
Abstract We report experimental work on mass transfer enhancement through pulsatile flow in an asymmetric channel under the laminar flow condition. Mass transfer experiments were carried out by the electrochemical method. The vortices dynamics were visualized both experimentally and numerically. The present results were compared with previous ones in the case of a symmetric channel with the same sinusoidal wavy walls. The mass transport enhancement factor for the asymmetric channel is found to be larger than that for the symmetric channel, for a wide range of flow parameters. This is a consequence of the specific fluid mixing induced by vortices dynamics. It is also confirmed that the Sherwood number for pulsatile flow can be expressed in terms of the Sherwood numbers for steady and oscillatory flows at large amplitude of fluid oscillation, for both channels.Zusammenfassung Es wird über eine experimentelle Untersuchung zur Verstärkung des Stoffaustausches bei pulsierender Laminarströmung durch einen asymmetrischen Kanal berichtet, wobei die elektrochemische Methode zur Anwendung kam. Die Wirbeldynamik wurde experimentell und numerisch aufgeklärt. Beim Vergleich der Ergebnisse mit jenen aus früheren Versuchen an einem symmetrisch–sinusförmig berandeten Kanal zeigte sich, daß der Verstärkungsfaktor für den Stoffaustausch beim asymmetrischen Kanal in weiten Bereichen der Strömungsparameter größer ist als beim symmetrischen und zwar infolge der durch die Wirbeldynamik bewirkten Fluiddurchmischung. Es konnte ferner bestätigt werden, daß sich die Sherwood–Zahl für pulsierende Strömung bezüglich beider Kanalformen als Funktion der Sherwood–Zahlen für stationäre und für oszillierender Strömung bei hohen Amplituden der Fluidoszillation bestimmen läßt.