Molecular Velocity-field Measurement (NAF.5663)

Research

Velocity is a key parameter of any fluid in motion. At present, however, there exists no direct method to measure velocity in gas (air) flows. Present velocimetry techniques, Laser Doppler Anemometry (LDA) and Particle Image Velocimetry (PIV), rely on elastic light scattering off particles that are supposed to faithfully follow the flow. Useful as these techniques may be, their applicability has some serious limitations that are imposed by flare light scattering off surfaces, and by the requirement that seeding particles be added to the flow of interest. Some examples where these limitations show up are vortex cores and base flows (seed particle density too low), small-scale turbulence (seed particle density too high, perturbing the flow itself), and boundary layers (flare light scattering off bounding surfaces). With this proposal we aim to develop an optical, laser-based velocity field measurement technique that does not suffer from these seeding-related limitations, simply because it does not require particle seeding of the flow and does not detect elastically scattered light. The technique, known as Molecular Tagging Velocimetry (MTV), is based on the in situ, instantaneous creation of a well defined spatial distribution of specifically tagged molecules in the flow of interest. A short time later the distribution, modified by the flow pattern, is visualised. Both creation and visualisation of the tagged molecules proceed by laser excitation. Like in PIV, the velocity field is reconstructed from the difference between the initial distribution and the modified distribution. Unlike PIV, however, MTV utilises inelastic, molecular light scattering, and does not require particle seeding of the flow. As a result, MTV is applicable to flow situations where PIV fails.
Molecular Tagging Velocimetry is applicable to a wide range of flow conditions, including sub-, super- and hypersonic air flows. A specific implementation of MTV that was recently discovered in our laboratory can be used in plain, ambient air. This method, baptised APART (Air Photolysis And Recombination Tracking), is discussed in more detail in the main text. Within the framework of this proposal we plan to focus attention to two specific applications in which existing methods of velocimetry experience severe difficulties.

Utilisation

The essential product of this research proposal is a non-perturbing technique for velocity measurements in air flows. The anticipated beneficiaries comprise that part of the fluid mechanics and aerodynamics community that is involved in flow velocimetry. Particle-based velocimetry techniques, notably PIV and LDA, are at present commercially available, and standard equipment in aerodynamics laboratories. Their drawbacks, for want of anything better, are either taken for granted or corrected for in post-processing. A technique that could be applied also in those cases where such particle-based techniques fail, is expected to find similar general acceptance. The company OFS (Optical Flow Systems) has expressed its interest in possible commercial prospects of the research proposed here.
Indeed, many of the larger international (mainly US-based) aerodynamics research facilities (like NASA, US Air Force) currently maintain research projects focussed at alternative velocimetry techniques.
In consult with two potential users in The Netherlands, the National Aerospace Laboratory (NLR) and the laboratory for Aero- and Hydrodynamics of the TU Delft, we have identified two specific problem areas in velocimetry that we intend to tackle. These are small scale turbulence and base flows. In the case of small scale turbulence the problem lies in the required seed particle density: If also the smallest scales in a flow are to be resolved, the seed particle density must become so high that it starts to effect the flow properties or may even render it opaque. The solution to this problem, that is, in situ creation of molecular rather than particulate tracers at only those locations where you need them, is the fundamental property of MTV. In the case of base flows, the inverse problem arises. The inertia of seed particles prohibits sufficient seeding of the recirculation zone downstream of flow bodies with a blunt base (like rockets, or re-entry vehicles). Here again, MTV circumvents the problem by creating molecular tracers where they are needed. In both cases that we want to pursue in this proposal, therefore, the problems created by global particulate seeding (PIV, LDA) are solved by local molecular seeding (MTV).

Meer informatie kunt u vinden op http://www.sci.kun.nl/mlf.

NMi, NLR, ILA en Philips zijn bij dit project betrokken.