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Institute of Turbomachinery and Fluid Dynamics
Logo Leibniz Universität Hannover
Institute of Turbomachinery and Fluid Dynamics
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Laser Doppler Anemometer (LDA)

Field of Application

  • Non-contact measurement of velocities and turbulent flow quantities
  • High temporal resolution allows the measurement of highly turbulent flow

Measured Quantities

  • Two-dimensional velocity vector
  • Turbulent quantities of fluctuating flow

Description

The Laser Doppler technique is based on the contact-free measurement of the time it takes for a fluid particle to travel through an interference pattern of two laser beams. The disturbance in the interference pattern by the light, which is reflected off the moving particles is detected and from this the flow rate is deduced.

The LDA measurement technique is a non-invasive, i.e. it is not affecting the flow with the probes, and calibration free method which measures absolute values. LDA is used when measureing unsteady flow with high temporal and spatial resolution.

When two coherent laser beams cross each other a stripe pattern, the so-called interference fringe pattern is produced by interference. The finite thickness of the beams leads to a 3-dimensional interference ellipsoid in which the flow is measured. The particles added to the flow pass through the ellipsoid volume and reflect the laser light. A photo detector captures this reflection and converts the so-called Doppler burst into an electrical signal. From this electrical signal, the time t which it takes for a particle to pass through a light or dark area can be determined. With the time t and the fringe spacing δ, the speed of the particle can be calculated. To resolve the direction in which the particle passes through the volume, the interference pattern is shifted in frequency by 40 MHz using a Bragg cell. This creates a moving interference pattern which allows the determination of both the speed and the direction of the flow.

One laser beam can measure one directional component. Hence, 3-dimensional measurements require three laser beams of different wavelengths. At TFD a two-dimensional LDA is used to measure the flow field close to the housing of the diffuser test rig.

Contact

Dominik Frieling, M.Sc.