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
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,
Basic One-component LDA Equipments:
Laser system (Continuous-Wave -
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
(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
A three-component LDA
Picture of a three-dimensional LDA
Schematic of the
three-dimensional LDA system
Three-dimensional in-cylinder LDA
Yeh, Y. & Cummins, H., 1964. Localised fluid flows measurements with a He-Ne laser spectrometer. Appl. Phys. Lett., 4, pp. 176-178.
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.
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.
is to become the standard in fluid research laboratories all over the world.