Prof. Dr.-Ing. Dr. h. c. Uwe Hampel

Experimental Thermal Fluid Dynamics
Phone: +49 351 260 2772

Laser Doppler Anemometer (LDA) and Phase-Doppler Particle Anemometer (PDPA)

Measuring principle

The department possesses two modern laserinterferometric measurement devices for particle velocimetry measurement (LDA, Dantec Corp.) and a combined particle velocity and size measurement (PDPA, Aerometrics/TSI Inc.). Both systems enable the determination of velocity (LDA, PDPA) and size of particles or small gas bubbles within a small volume in a flow and with high accuracy. The principle is based on the measurement of the Doppler shift of light that is scattered from the moving particles in a small volume of two crossing laser beams. Thereby the Doppler shift is measured from the temporal spacing of the interference fringes. By proper arrangement of multiple detectors the PDPA can additionally determine the size of the particles from the phase shift of the Doppler signals between the given spatial locations. Both techniques are inherently self-calibrating and are thus essentially independent from optical measurement conditions. The measurement is without intrusion into the flow and thus especially suitable in problems and places, where conventional probes are difficult to apply: reverible flows, chemically aggressive substances, high temperatures and pressure. Further, it is possible to perform measurements in moving systems, such as in reciprocating engines and turbo machines.
For more details see: DANTEC, TSI.

System description

The LDA allows a one-dimensional and two-dimensional measurement of local velocity. A completion of the system for three-dimensional measurements is planned for the future. The LDA system comprises a SpectraPhysics argon laser, an optical Bragg modulator, two measurement probes, and a PC-controlled traversing unit for the probe(s). The Aerometrics PDPA has one probe for one-dimensional velocity and particle size measurements.

LDA - Bragg cell + laser
LDA probe
PDPA system (photo from manufacturer)

System parameters


laser type argon
max. optical power 8 W
laser class according to DIN EN 60825-1 4
probe 1 medium measurement in air
measurement type 1D
wavelength 514 nm
ray separation 30 mm
probe 2 medium measurement in water
measurement type 2D
wavelength 514 nm and 488 nm
ray separation 15 mm
system velocity measurement range up to 500 m/s

Aerometrics (TSI) PDPA

laser type argon
max. optical power 300 mW
wavelength 514 nm
  laser class according to DIN EN 60825-1 3b
system system number of probes 1
velocity measurement range up to 500 m/s
particle size range 10 µm ... 1 mm
dynamic range of size measurement approx 50:1


U. Hampel