QUANTITATIVE FLOW VISUALIZATION IN UNSEEDED FLOWS

The various tools for flow visualization have been significantly expan ded over the past several years through the use of molecular scatterin g and molecular laser-induced fluorescence. These approaches have adde d the capability of sampling individual small volume elements within a flow, and by using cameras for detection, they are easily extended to sample Lines and cross-sectional planes. This localized measurement c apability means that these approaches can be made quantitative even in complex and/or unsteady flow fields. If the molecular species is natu rally occurring, such as oxygen or nitrogen in air, then no seeding is required. Furthermore, in these applications, images of the flow can be frozen in time by using a short pulse laser for illumination. The d istribution of the molecules reflects the true physics of the flow, so even raw images taken in this manner give an immediate understanding of flow field properties. With proper calibration, the images can be f urther analyzed to yield quantitative information about the how. In th e case of flow tagging, the analysis gives velocity profiles when line s are written, and deformation, vorticity, and dilation with grid patt erns. Molecular scattering can be used to give quantitative values of density, temperature, and two-dimensional velocity. This paper present s three such molecular-based approaches: laser-induced fluorescence fr om oxygen, flow tagging by oxygen excitation, and Rayleigh scattering. These three approaches are chosen because all three can be used in na turally occurring air with no seeding. The raw data from each of these approaches gives an immediate appreciation of the flow structure and further analysis yields accurate values of velocity, temperature, and density. These approaches use readily available laser sources; however , they will: be greatly enhanced with new source technologies that are currently under development.