Flow visualization via laser-induced reflection from bubble sheets

Abstract

THE structure of steady and unsteady aerodynamic flows can be characterized by generating fluid markers at desired locations in the flow and tracking them in threedimensional space. The hydrogen bubble technique allows localized injection of both continuous and interrupted fluid markers. Its advantages and limitations are described in the works of Schraub et al. 1 and Lusseyran and Rockwell.2 The technique described herein employs multiple sheets of hydrogen bubbles in conjunction with laser sheet illumination. Arbitrary cross sections of the unsteady flow past an oscillating delta wing can be characterized. By using a phase-referencing technique, it is possible to relate the visualization at various cross sections along the wing at a given value of instantaneous angle of attack. Contents Experiments were performed in a transparent (Plexiglas) water channel having a cross section of 914 mm (wide) by 604 mm (deep). The sweep angle of the delta wing was X = 75 deg; its chord was C= 143 mm, and the corresponding Reynolds number was (700C/y = 3.8 x 10 4. The flow structure of the upper, flat surface of the wing was visualized; the lower surface of the wing was machined at a bevel angle of 15 deg. In order to minimize spurious reflections of the laser sheet, the surface of the wing was machined to a finish of one micron. The wing system was forced in the pitching mode about its trailing edge using an integrated active control system, which allows simultaneous control of the wing position, generation of marker bubbles, and movement of the laser sheet, as described by Magness3 and Utsch.4 Also described therein are the laser system, related optics, and the scanning mirror used to generate the laser sheet. In essence, a 2-W argon-ion laser beam was swept at a frequency of 500 cycles/s across the flow in order to generate the laser sheet. Figure 1 shows the conceptual diagram of the experimental system. A grid of intersecting hydrogen bubble sheets, or in this case an array of horizontal bubble sheets, is generated upstream of the delta wing in the undisturbed freestream. As the bubble sheets pass the wing, they are distorted in accord with the three-dimensional flow structure. The distorted cross section of the flow is visualized by the scanning lasersheet, which is transmitted through the optically transparent wing. An end view of the visualized cross section of the flow is obtained by reflecting the image from a mirror; it is located sufficiently far downstream such that there is no interference with the upstream flow structure. Flow visualization images are recorded on a high-speed (120 frames/s) Instar video system by Videologic Corporation.