Nonintrusive Flow Visualization to Locate Vortices and Laminar-to-Turbulent Transition

We propose to demonstrate a new optical measurement of velocity gradient based on direct observation of the differential displacement of a marked fluid element in ordinary, unseeded air. The method utilizes a pulsed laser beam to "write" an arbitrary pattern, such as a line or a cross, into air by means of stimulated Raman vibrational excitation of oxygen. After suitable time delay, during which the velocity gradient causes the pattern to distort, the new pattern is determined via fluorescence induced by a second laser beam. An image of the new pattern is captured with a video camera. Distortion of a line will determine point of laminar-to- turbulent transition, whereas rotation of a cross will yield vorticity. By writing a grid, multiple spatial location can be probed simultaneously. The use of a pair of cameras, imaging orthogonal planes, results in a full, three-dimensional vector. The accuracy of the method is limited only by the ability to determine time and displacement, and can be performed at vector collection rates of 30 Hz using commercially available equipment. In this effort we propose a detailed study, including appropriate laboratory proof-of-concept measurements, of the applicability of the technique to advanced aircraft testing. The results of this effort will be a prototype system designed for aircraft flight testing applications.