EDITORIAL

The capability of non-intrusively tagging the molecules in a flowing medium and observing their subsequent evolution offers exciting new possibilities for studying fluid dynamics. Molecular tagging methods have been used primarily for flow velocimetry, even though more recently they have also found novel uses in studying the Lagrangian evolution of passive scalar fields, entrainment and mixing in turbulent flows. Molecular Tagging Velocimetry (MTV) relies on molecules that can be turned into long lifetime tracers upon excitation by photons of an appropriate wavelength. These molecules are either naturally present in the flowing medium (i.e. unseeded applications) or need to be premixed. Typically a pulsed laser is used to `tag' the regions of interest, and those tagged regions are interrogated at two successive times within the lifetime of the tracer. The measured Lagrangian displacement vector provides the estimate of the velocity vector. This velocimetry approach offers particular advantages over particle-based techniques in flows in which the use of seed particles is not warranted or may lead to complications such as flow tracking problems and buoyancy effects.

While the earliest use of molecular tagging velocimetry can be traced back at least three decades, this technique has seen significant advances over the past ten years (sometimes under alternate titles such as laser-induced photochemical anemometry or flow tagging velocimetry). These advances have been driven by improvements in laser and imaging techniques, data analysis methods, and chemical design and synthesis of novel molecular structures. As a result, the use of this velocimetry technique has been increasing steadily. The MTV technique has been used in flows over a wide range of speeds, from liquid-phase flows with speeds below 1 mm s^-1 to gas-phase flows at supersonic speeds. The scope of the measurements covers a range from the instantaneous profile of one component of velocity vector along a tagged line to whole-field three-component velocity data over a plane which are obtained using stereo imaging. Some of the flows that have been investigated include pulsatile flow in tubes, internal circulation in droplets, unsteady boundary layer separation, convective flows in directional solidification, flows during intake and compression inside motored IC engines, free and wall-bounded turbulent flows, and highly three-dimensional vortex flows with strong out-of-plane motions. Summaries of various molecular tagging methods and an extensive reference list of papers and applications can be found in several review articles which have appeared over recent years [1-4].

The aim of this special feature is to provide the reader with an overview of the recent progress and current state-of-the-art in the instrumentation, application, and extensions of molecular tagging methods for velocimetry and mixing studies. Many thanks go to the authors and reviewers who made this possible. Thanks also go to IOP Publishing for supporting this initiative, and to the staff for their effort and hard work.

References

[1] Falco R E and Nocera D G 1993 Quantitative multipoint measurements and visualization of dense solid--liquid flows using laser induced photochemical anemometry (LIPA) Particulate Two-Phase Flow ed M C Rocco (London: Butterworth-Heinemann) pp 59-126

[2] Koochesfahani M M, Cohn R K, Gendrich C P and Nocera D G 1996 Molecular tagging diagnostics for the study of kinematics and mixing in liquid phase flows Proc. 8th Int. Symp. on Applications of Laser Techniques to Fluid Mechanics, 8-11 July, 1996, Lisbon vol I, 1.2.1-1.2.12; also in 1997 Developments in Laser Techniques and Fluid Mechanics ch 2, section 1, eds Adrian, Durao, Durst, Maeda and Whitelaw (Berlin: Springer)

[3] Koochesfahani M M 1999 Molecular tagging velocimetry (MTV): progress and applications AIAA Paper AIAA-99-3786

[4] Lempert W R 2000 Molecular Tagging Velocimetry Flow Visualization: Techniques and Examples ed A J Smits and T T Lim (London: Imperial College Press) in press