Laser Doppler Anemometry (LDA) / Laser Doppler Velocimetry (LDV)

 


Laser Doppler Anemometry (LDA) or Laser Doppler Velocimetry (LDV) is a single point optical measuring technique which enables the velocity of the seeded particles (~0.5 - 5 microns (in air) or 1~20 microns (in water)) conveyed by a fluid flow to be measured in a non-intrusive manner. Generally speaking, for all Mie-scattering velocimetry techniques, the seeded particles must be big enough to scatter sufficient light for signal detection (good signal to noise ratio) but small enough to follow the flow (laminar or turbulent) faithfully (in-synchronisation with the localised fluid motion). By analysing the Doppler-equivalent frequency of the laser light scattered (intensity modulations within the crossed-beam probe volume)
by the seeded particles within the flow, the local velocity of the fluid can be determined.  The area of interest within the flow field is sampled by a crossed-beam in a point by point manner. Recently, Strunck et al 2004 have demonstrated in experiments that higher spatial resolution can be yielded with commercial 3D LDA systems.  They recommended to manufactures that they should provide the users of modern LDA systems with the arrival times of the burst signals of each LDA channel when coincidence mode is employed.

This technique (reference-beam LDA) was first reported by Yeh and Cummins in 1964.  Forty years on, LDA has become a well established optical velocimetry technique which is used extensively in fluid flow studies.  It could be used to serve as a bench-mark validation tool for planar velocimetry techniques (eg. DPIV, DPTV, DGV).

 

Basic One-component LDA Equipments:  

Laser system (Continuous-Wave - CW, single colour for single channel), transmission optics (e.g. Bragg cell, lenses, beam expanders, beam splitter, mirrors, prisms, fibre cable link with laser beam manipulator), receiving optics (e.g. lenses, pinhole, interference filter, photomultiplier), signal processor units (e.g.. fringe-counting, spectral analysis, photon-correlation), traversing mechanism (manual or automated) for transmitting and receiving optics, oscilloscope, seeding generation (solid or liquid vapour) and computer (large capacity hard disk) with a data acquisition board and data handling software.  The more compact and easy to handle type of LDA system has fibre transmission and receiving optics.

A three-component LDA system

 

Picture of a three-dimensional LDA system

 

 

Schematic of the three-dimensional LDA system

 

Three-dimensional in-cylinder LDA measurement

 


Original Paper:


Yeh, Y. & Cummins, H., 1964. Localised fluid flows measurements with a He-Ne laser spectrometer. Appl. Phys. Lett., 4, pp. 176-178.

 

 General Reference:


Drain, L.E., 1980. The Laser Doppler Technique, John Wiley Press, USA. 
Durst, F., Melling, A., & Whitelaw, J.H., 1976. Principles and practice of laser Doppler anemometry. Academic Press, London.
Durrani, T.S., and Greated, C.A., 1977. Laser Systems for Flow Measurement, Plenum, New York.
Strunck V, Sodomann T, Muller H and Dopheide D., 2004 How to get spatial resolution inside probe volumes of commercial 3D LDA systems. Experiments in Fluids 36 141-145.

Albrecht H. E. ,  Borys M.,  Damaschke N., and Tropea C.,  2003 Laser Doppler and Phase Doppler Measurement Techniques. Springer, Berlin Heidelberg New York.

Verdict and Comment:


A popular and mature point-wise flow measurement technology with a proven track record. LDA is to become the standard in fluid research laboratories all over the world.