Flow over a Distribution of Obliquely Aligned Elements: Part I - Exerimental Investigation

Abstract

Flows over surface elements with a directional orientation different from the free stream flow are common. Some examples are conduit flows with helical ribs and flow in gas-turbine internal cooling passages with angled-rib turbulators. While internal flows over obliquely aligned elements are common and have been studied extensively in the literature, the production of secondary flows in boundary layers over obliquely aligned three-dimensional elements is not well understood. Obliquely aligned elements affect the flow-direction boundary layer development, create bulk or localized circulation, and induce turbulence in the resulting three-dimensional boundary layers. MEMS devices, such as micro-balloon actuators, have been studied in recent literature as a means for active and passive control of flow separation and of flow transition to turbulence. By elongating MEMS type devices and orienting them so that the major axis of the elements is at an oblique angle to the primary flow direction, arrays of these devices may be used to control the magnitude of secondary or transverse flows induced as well as controlling the separation and transition of the mainstream flow. Systems naturally exhibiting three-dimensional boundary layers would benefit from directional flow control, or flow tailoring. The purpose of this investigation was to measure the secondary flow over a flat plate induced by an array of obliquelyaligned, right frustums of elliptical cross-section. The boundary-layer flow directional components were measured upstream and downstream of the coupon using miniature endflow X-array hotwire anemometer probes mounted on an automated traversing system. The boundary-layer measurements were performed for test coupons with arrays of elements aligned at 0°, -5°, -10°, and -15° to the free stream flow direction under nominal free stream velocities of 2, 5, and 10 m/s. The resulting boundary-layer measurements demonstrate the ability of obliquely aligned elements to produce controlled (or tailored) secondary flows. Since the test coupons used were of a finite span, the results demonstrate that controlled vortices can also be generated using the arrays.