Electrokinetic microactuator arrays for active sublayer control of turbulent boundary layers.

Abstract

The present study has been the first to examine the electrokinetic principle as the basis for a new class of microscale actuator arrays for active sublayer control on full scale aeronautical and hydronautical vehicles under realistic operating conditions. The Helmholtz-Smoluchowski scalings that govern such electrokinetic actuator arrays show significant performance advantages from their miniaturization to the microscale. The electrokinetic microactuator arrays that are the subject of this study seek to interrupt the bursting process associated with naturally-occurring streamwise sublayer vortices in the turbulent boundary layer. Specific performance requirements for microactuator spacing, flow rate, and frequency response for active sublayer control have been determined from fundamental scaling laws for the streamwise vortical structures in the sublayer of turbulent boundary layers. In view of the inherently local nature of the sublayer dynamics, a general system architecture for microactuator arrays appropriate for active sublayer control has been developed based on the concept of relatively small and independent unit cells, each with their own sensing, processing, and actuation capability, that greatly simplifies the sensing and processing requirements needed to achieve practical sublayer control. A fundamental three-layer design has been developed for such electrokinetic microactuator arrays, in which electrokinetic flow is induced by an impulsively applied electric field across a center layer, with a bottom layer containing an electrolyte reservoir and a common electrode, and a top layer that containing individual electrodes and lead-outs for each microactuator in the unit cell. Microfabrication techniques have been developed that permit mass production of large numbers of individual electrokinetic microactuators in unit cells on comparatively large-area tiles. Several generations of such electrokinetic microactuator arrays have been built leading to the MEKA-5 full-scale hydronautical array, composed of 25,600 individual electrokinetic microactuators with 350 mum center-to-center spacings, arranged in a 40 x 40 pattern of unit cells, each composed of a 4 x 4 matrix of actuators. MEMS design and fabrication processes were used to produce a top layer for the MEKA-5 hydronautical-scale array. Stereo-PIV measurements successfully demonstrated lateral displacement of synthetically-generated streamwise vortical structures by volumetric pumping from a wall actuator in a set of large-scale wind tunnel tests.