Currently, however, DGV is considered to be more suitable for the measurement of high speed flows (estimated to be above 30ms-1) because of the limit on velocity resolution which is approximately equal to 1ms-1, the absolute error of a very good DGV system. It is one of the very few Mie-scattering whole-field velocimetry techniques that could be truly regarded as instantaneous with the use of a short duration (~10 ns) pulsed laser.
High power CW or single-pulsed laser system (single longitudinal mode with good laser beam quality) coupled with high performance frequency stablisation system (error signal detection feedback control), mechanically stable optical workbench, specially commissioned absorption line filters (filled with pure molecular iodine vapour or other atomic vapour, one for imaging and the other for on-line absorption calibration) with thermal stablisation control oven system (PID controller), optical spectrum analyser, fast oscilloscope, power meters with CW or pulsed head adaptors, non-polarizing beam splitter plates or cubes, interference filters, neutral density filters, 1/4 wave plates, high energy mirrors, lenses and optics mounts, fibre optics, laser sheet generation optics, camera calibration target (eg. USAF resolution target), low noise (cooled) and high quality CCD camera (two CCDs are needed for dual-camera DGV or PDV), a high performance computer (large memory and high capacity hard disk) with a high performance frame grabber card (a dual frame grabber card is needed for dual-camera DGV or PDV, plus additional electronics for synchronisation and control) and image processing software, CD burner for data storage (optional), D/A & A/D converter board, seeding particles generator and rotating disc for velocity calibration.
Alternative Methods using Iodine Absorption for Velcoimetry:
Komine, H., 1990. System for measuring velocity field of fluid flow utilizing a laser-Doppler spectral image converter. U.S. Patent 4,919,536.
J.F., 1995. Development of Doppler global velocimetry as a flow
diagnostics tool. Meas. Sci. Technol. 6, pp.769-783.
Roehle I and Willert C E 2001, Extension of Doppler global velocimetry to periodic flows
Meas. Sci. Technol. 12 (April 2001) 420-431
Nobes, D.S., Ford, H.D. and Tatam, R.P. 2004 "Instantaneous, three-component planar Doppler velocimetry using imaging fibre bundles" Experiments in fluids 36 (1), pages 3-10.
A state-of-the-art but technically complicated and challenging (truly instantaneous, near real-time) planar flow measurement technology. Several universities and aerospace research institutes in the US and Europe (e.g. Oxford University, Cranfield University, DLR, ONERA) have been actively involved with this technique. Main driving thrust behind the technology is directed by NASA (Langley Research Center, Ames Research Center) and with the intention of future full-scale aircraft real-time in-flight measurements in NASA's Dryden Flight Research Center. NASA's Glenn Research Center is also currently establishing a DGV programme. DLR (German Aerospace) has been one of the many contributors in advancing the DGV technique. DLR is currently also developing a micro-DGV system. A major advancement in the technique has been made by the research team at Cranfield.