Showing papers by "Mohamed Gad-el-Hak published in 1997"

Navier-Stokes Simulations of a Novel Viscous Pump

Abstract: A numerical study of flow in a novel viscous-based pumping device appropriate for microscale applications is described. The device, essentially consisting of a rotating cylinder eccentrically placed in a channel, is shown to be capable of generating a net flow against an externally imposed pressure gradient. Navier-Stokes simulations at low Reynolds numbers are carried out using a finite-volume approach to study the influence of various geometric parameters. Slip effects for gas flows are also briefly investigated. The numerical results indicate that the generated flow rate is a maximum when the cylinder is in contact with a channel wall and that an optimum plate spacing exists. These observations are in excellent agreement, both qualitatively and quantitatively, with a previous experimental study. Furthermore, it is shown that effective pumping is obtained even for considerably higher Reynolds numbers, thereby extending the performance envelope of the proposed device to non-microscale applications as well. Finally, slip-flow effects appear to be significant only for Knudsen numbers greater than 0.1, which is important from the point of view of microscale applications.

Flow and load characteristics of microbearings with slip

Abstract: The fluid mechanics and operating characteristics of bearings that support rotating surfaces in micromachines are different from their larger cousins. The present study analyzes microbearings represented as an eccentric cylinder rotating in a stationary housing. The flow Reynolds number is assumed small, the clearance between shaft and housing is not small relative to the overall bearing dimensions, and there is slip at the walls due to non-continuum effects. The two-dimensional governing equations are written in terms of the streamfunction in bipolar coordinates and an infinite-series solution is obtained. For high values of the eccentricity and low slip factors the flow may develop a recirculation region. The force and torque on the load-bearing inner cylinder increase with increasing eccentricity and decrease with increasing slip.

Analysis of Viscous Micropumps and Microturbines

Abstract: A numerical study of the three-dimensional viscous fluid flow in a novel pump/turbine device appropriate for microscale applications is performed. The device essentially consists of a rotating or free-to-rotate cylinder eccentrically placed in a channel, and is shown to be capable of generating a net flow against an externally imposed pressure gradient, or, conversely, generating a net torque in the presence of an externally imposed bulk flow. Full Navier-Stokes, finite-element simulations are carried out to study the influence of the width and other geometric as well as dynamic parameters, and the results are compared to previous two-dimensional numerical and physical experiments. The three-dimensional simulations indicate a gradual decrease of the bulk velocity and pump performance as the two side walls become closer providing increased viscous resistance to the flow. However, effective pumping is still observed with extremely narrow channels. The utility of the device as a microturbine is also demonstrated for the first time in the present simulations. Particularly, the angular velocity of the rotor and the viscous torque are determined when a bulk velocity is imposed.