Molecular Tagging Velocimetry (MTV)
The understanding of the flow behavior in many fluid systems requires global information about the fluid velocities. Our group has developed a technique for the quantitative non-intrusive mapping of fluid flow velocities simultaneously at many points over a plane. In this technique, which we call Molecular Tagging Velocimetry (MTV), the flowing medium is premixed with molecules that can be turned into long-lifetime tracers upon excitation by photons. Typically a pulsed laser is used to ``tag'' small regions of interest. The tagged regions are then imaged at two successive instants within the lifetime of the tracer. The measured displacement vectors provide estimates of the average velocity for each small region during the brief interval between exposures.
An example of a typical MTV experiment is shown at the right, applied to the flow of a vortex ring impinging on a wall. The fluid is water in this case, premixed with a water-soluble phosphorescent triplex compound developed by Nocera's group in the Chemistry Department at MSU. The upper left image shows the regions in the flow initially tagged by a grid of laser lines. The upper right figure is the image of the same molecules after being convected by the flow for 8 ms. Note the distortion of the fluid element due to the vortex ring's vorticity field. The corresponding velocity field is determined using a spatial correlation technique. A more detailed map of the vortex ring interacting with the wall is shown below at a later instant in time. Click on either image to see an enlarged view.
The MTV technique has also been extended to gas-phase flows using phosphorescent gases (for example, biacetyl). The fundamental flow studies in the internal combustion engine geometry form the primary impetus for these developments. An example of this work is the mapping of the velocity and vorticity fields for the induction flow into an engine geometry.
One advantge of the MTV technique over other commonly used whole-field velocimetry techniques which rely on light scattering from particles is, of course, its molecular nature. The issues related to the accurate tracking of the flow by the seed particles, particularly important to gas flows, is eliminated. The molecular nature of MTV also makes it suitable for obtaining other flow variables, such as pressure, temperature, and concentration, along with the velocity field.
B. Stier and M. M. Koochesfahani (1997) Molecular Tagging Velocimetry (MTV) Developments in Gas Phase Flows. Submitted to Exp. in Fluids , August 26, 1997.
B. Stier and M. M. Koochesfahani (1997) Molecular Tagging Velocimetry in Gas Phase and its Application to Jet Flows. FEDSM97-3687, presented at the ASME Fluids Engineering Division Summer Meeting, June 22-26, 1997.
C. P. Gendrich and M. M. Koochesfahani (1996) A Spatial Correlation Technique for Estimating Velocity Fields Using Molecular Tagging Velocimetry (MTV). Experiments in Fluids, 22(1): 67-77.
M. M. Koochesfahani, R. K. Cohn, C. P. Gendrich, and D. G. Nocera (1996) Molecular Tagging Diagnostics for the Study of Kinematics and Mixing in Liquid-Phase Flows. Plenary presentation at the Eighth International Symposium on Applications of Laser Techniques to Fluid Mechanics, Lisbon, Portugal, July 8-11, 1996: pp. 1.2.1 - 1.2.12.
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