tailieunhanh - MICRO-ELECTRO-MECHANICAL- SYSTEMS (MEMS) AND FLUID FLOWS

The type of soil strength characteristic (. the variation of soil strength with soil water content) favourable to crop growth depends on both the amount and the distribution of the annual rainfall, and on the nature of the crop. The soil must have sufficient mechanical strength to provide adequate anchorage for the plant throughout its development, and to prevent the collapse of soil water and air pathways by soil overburden pressure and the weight of vehicle and animal traffic. Dense regions of high strength may limit root growth and crop. | Annu. Rev. FluidMech. 1998. 30 579-612 Copyright 1998 by Annual Reviews Inc. All rights reserved Annu. Rev. Fluid. Mech. 579-612. Downloaded from by CALIFORNIA INSTITUTE OF TECHNOLOGY on 09 08 05. For personal use only. MICRO-ELECTRO-MECHANICAL- SYSTEMS MEMS AND FLUID FLOWS Chih-Ming Ho Mechanical and Aerospace Engineering Department University of California at Los Angeles Los Angeles California 90095 e-mail chihming@ Yu-Chong Tai Electrical Engineering Department California Institute of Technology Pasadena California 91125 e-mail yctai@ KEY WORDS flow control MEMS micro transducers size effect surface force Abstract The micromachining technology that emerged in the late 1980s can provide micron-sized sensors and actuators. These micro transducers are able to be integrated with signal conditioning and processing circuitry to form micro-electromechanical-systems MEMS that can perform real-time distributed control. This capability opens up a new territory for flow control research. On the other hand surface effects dominate the fluid flowing through these miniature mechanical devices because of the large surface-to-volume ratio in micron-scale configurations. We need to reexamine the surface forces in the momentum equation. Owing to their smallness gas flows experience large Knudsen numbers and therefore boundary conditions need to be modified. Besides being an enabling technology MEMS also provide many challenges for fundamental flow-science research. 1. INTRODUCTION During the past decade micromachining technology has become available to fabricate micron-sized mechanical parts. Micromachines have had a major impact on many disciplines . biology medicine optics aerospace and mechanical and electrical engineering . In this article we limit our discussion to transport phenomena specifically emphasizing fluid-dynamics issues. This 0066-4189 98 0115-0579 579 580 HO TAI Annu. Rev. Fluid. Mech. .