Measurement of velocity fields: Molecular Tagging Velocimetry algorithms
Molecular Tagging Velocimetry (MTV) methods are an alternative to Particle Image Velocimetry (PTV) for the accurate measurement of velocities in high-speed turbulent flows. MTV methods are particularly useful when large errors in velocity determination are caused from the use of particles in high-speed flows. Additionally, the use of particles in high-speed wind tunnels can cause damage to these facilities. However, despite the potential of MTV methods for the study of high-speed flows, their application has been limited by a number of factors that include the lack of ready-to-use data analysis codes, which are largely written in-house. We have been working over the last two years on the development of a method to analyze MTV image pairs to estimate velocity with high spatial resolution. This method involves the determination and use of proper image transformations to map an entire MTV grid distorted by the flow velocity onto the first perfect grid.
Molecular Tagging Velocimetry uses tagged molecules instead of large particles as tracers of flow motion. For example, a variant of this technique employs Nitric Oxide (NO) as a molecular tracer, which upon excitation with a laser emits light as fluorescence. If a laser pulse used to excite the NO molecules present in the flow under study is shaped into a thin line, a thin fluorescence line is generated in the flow. When multiple laser lines are shaped into intersecting fluorescence lines aligned from different directions, a fluorescence grid is formed. The movement of this generated grid and its distortions in time can be monitored through a time-lapse series of images. Experimentally, the technique requires obtaining a pair of images consisting of an initial image at time zero following laser excitation, when the fluorescence lines are straight due to their recent formation, and a time-delayed image when the lines are distorted due to flow motion. The velocity is estimated by dividing the spatial displacement of the fluorescence intersections by the time delay between the two images. This time of flight velocity is directly determined using u = d/t, where u is the flow velocity, d is the flow displacement measured from the image pair at different locations, and t is the time delay between both images. An example of this principle is shown in the following figure: