Optical measurement techniques

 

3D Laser Doppler Anemometry (LDA)

Schematics for LDA measurement system Copyright: © RWTH Aachen | IST

Laser Doppler Anemometry (LDA) is a non-invasive measurement method which allows the punctual measurement of steady and unsteady flow velocities in up to three spatial directions. In the two-beam method, two laser beams of the same wavelength are focused for each velocity direction and superimposed at their focal point. The overlapping area thus formed defines the measuring volume and is characterized by an interference fringe pattern that forms parallel to the beam cross axis. If particles are added to the flow and their scattered light is detected as it passes through the interference fringe pattern, the registered scattered light signal is called a Doppler burst. For signal processing, this Doppler burst is split into a direct current and an alternating current component, whereby the Doppler frequency is determined over the period of the alternating current component. The Doppler frequency and the interference fringe distance can thus be used to determine a particle velocity perpendicular to the beam crossing axis.

Calibration is not necessary with this measurement method, since the interference fringe distance is only a function of the optical setup. It is also advantageous that only one optical access is required for measurements in closed measurement sections, since the laser light transmitter and receiver can both be positioned orthogonally to the same optical access.

Application

At IST, the 3D LDA system is used, for instance, on the jet engine centrifugal compressor and on the free jet wind tunnel. For this purpose, a traversable 3D probe is available, which allows velocity and angle measurements in three spatial directions due to the spatial arrangement of three pairs of beams. The measuring volume is traversed by a 6-axis industrial robot.

Measured variables

  • 3-dimensional Velocity Vector
  • Unsteady Velocity Field
 

Particle Image Velocimetry (PIV)

Schematics for PIV measurement system Copyright: © RWTH Aachen | IST

The PIV measurement technique is a laser-optical velocity measurement method that allows the instantaneous acquisition of two- or three-dimensional velocity fields in fluids. It can be seen as an extension of flow visualization, since it allows not only qualitative but also quantitative measurement of the flow field. The measuring principle is based on the determination of the displacement of the particles introduced into the fluid. These particles are illuminated in a pulsed laser light section. A camera records the scattered light of the illuminated particles in the flow. This is done by a double exposure or two time-shifted single exposures of a CCD chip.

By statistical evaluation of the recorded images, the velocity vectors of the particles can be determined from the displacement and the time of the exposure interval.

Operation at IST

The non-contact measuring method PIV does not lead to any local influence on the flow, which is why it can also be used in very small dimensions with possibly high flow velocities. At IST the method is used in centrifugal compressor diffusers, for instance. PIV allows the wide-range measurement of the quantitative velocity field. Thus, it is possible to quickly detect flow phenomena such as detachments. The results obtained provide excellent data for the validation of flow solvers and thus contribute to the further development and improvement of turbomachinery.

Measured variables

  • 2-dimensional velocity field