Micromachined sensors and actuators for fluid mechanics research and high-speed flow control applications.

Abstract

Understanding the physics of turbulence and fluid dynamics is crucial to prevent unsteady and potentially dangerous flow fluctuations in aviation systems. One such condition occurs in high-speed jets when air exiting a jet nozzle at high velocity produce a high-frequency acoustic wave called jet screech. The resulting large acoustic pressure fluctuations produce high noise levels and may result in structure failure. It is postulated that by introducing perturbations into the shear layer using microactuators, it may be possible to interfere with the screech feedback loop to control and cancel screech. This research concentrates on developing new tools with Micro Electro Mechanical Systems (MEMS) technologies to control screech using microactuators placed around jet nozzle lip. The actuators can be driven by an appropriate control signal produced by measuring the flow velocity and acoustic wave pressure using micromachined hot-wire anemometers and sound detectors. The high frequency, high amplitude lateral comb drive electrostatic microactuators generate oscillations at 5kHz with amplitudes 80$\mu$m, and survive up to 320m/s when inserted 25$\mu$m into the flow. Each actuator is 1.3mm $\times$ 1.3mm and 14$\mu$m thick. Flow activity around the microactuators is monitored using on-chip silicon sound detectors and hot-wire anemometers. The detector uses ultra-thin p$\sp{++}$ piezoresistors supported on a dielectric diaphragm and provides a static sensitivity of 1.1$\mu$V/V/Pa, an acoustic sensitivity of 0.18$\mu$V/V/Pa, and tolerates overload pressures $>$50kPa. Each detector measures 310$\mu$m $\times$ 310$\mu$m to 910$\mu$m $\times$ 910$\mu$m and has a bandwidth of 10kHz. The hot-wire anemometers is fabricated from p$\sp{++}$ silicon, is $\rm200\mu m\times12\mu m\times4\mu m,$ and has a measured sensitivity of 963mV/$\rm\sqrt{m/s}.$ All these devices have survived exposure to a high-speed air jet with a maximum velocity of 320m/s. This represents the first demonstration of MEMS-based devices in such harsh conditions. A complete system consisting of two microactuators, two sound detectors, and several hot-wire anemometers has been fabricated and supported on a glass substrate with dimension of 4mm $\times$ 10mm. Many of the multi-element chips can be mounted around a 1 jet nozzle for use in experiments to be carried out later aimed at eventual control and cancellation of screech.