Doppler Global Velocimetry (DGV) / Planar Doppler Velocimetry (PDV)

Doppler Global Velocimetry / DGV  or Planar Doppler Velocimetry / PDV, was first described in a US Patent by Komine (US) in 1990.

 
In particular, since DGV (or PDV) relies on light scattering (single exposure) by clouds of particles (high concentration) carried by the flow, rather than the imaging of each individual particle, it has the ability to generate velocity vector field information more quickly than LDA, and over a plane of larger area (~3 x 3 m²) than that for DPIV. Each pixel within the sensor array could yield a potential velocity vector, therefore, a mega pixels imaging camera would potentially output a million velocity vectors at video rate.  
In DGV, the velocity information is obtained by means of an optical & spectroscopic frequency converter (a pre-selected linear spectral line optical transfer function), known as an absorption line filter (ALF), that transforms the Doppler shifted frequency of light scattered by the particles (~0.5 to 5 microns (in air)) in the flow to real intensity variations in the imaging plane. Once this transformation is completed, the converted Doppler signal intensity map can then be processed by light intensity detectors (CCD camera) and computers to obtain a velocity map of the flow of interest. To eliminate the problem of both scattering signal and illumination intensity variations spatially in the measurement window, the Doppler signal intensity map is normalised by a reference intensity map from the same view of the flow. 
The measurement area within the flow field is defined by the position and physical dimension of a fan of laser light and the measured velocity component is dependent on the direction of laser sheet propagation and the CCD camera viewing angle. 

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.

Typical one-component DGV / PDV Equipments:

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.

Related Techniques:

Filtered Rayleigh Scattering

Point Doppler Velocimetry

Alternative Methods using Iodine Absorption for Velcoimetry:

Planar Laser-Induced Fluorescence

Original Paper:

Komine, H., 1990. System for measuring velocity field of fluid flow utilizing a laser-Doppler spectral image converter. U.S. Patent 4,919,536.

General Reference:

Meyers J.F., 1995. Development of Doppler global velocimetry as a flow diagnostics tool. Meas. Sci. Technol. 6, pp.769-783.
McKenzie, R.L., 1996. Measurement capabilities of planar Doppler velocimetry using pulsed lasers. Appl. Optics 35, No.6, pp. 948-964.
Smith, M.W., 1998. Application of a planar Doppler velocimetry system to a high Reynolds number compressible Jet , 36th AIAA Aerospace Sciences Meeting and Exhibit , Reno, Nevada , AIAA 98-0428 .
Special feature: Molecular Filter Based Diagnostics. Meas. Sci. Technol. Vol.12, No. 4. April 2001.

Elliott, G.S. & Beutner, T.J. 1999 Molecular filter based planar Doppler velocimetry. Progress in Aerospace Sciences Volume 35, Issue 8 Pages 799-845

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.

Verdict and Comment:

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.

Proposed DGV measurements with a higher power laser delivery system

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A simple DGV iodine absorption filter prediction in C