Laser Doppler Velocimetry
Figure 1: LDV laser measuring the bubbly wake
of a NACA 0015 foil
Laser Doppler Velocimetry is a non-invasive technique used to accurately measure fluid velocity at a point. The point measurement volume is defined by the intersection of two laser beams. The flow is seeded with particles that accurately follow the flow. As a particle passes through the measurement volume created by the intersecting laser beams, light is scattered onto a detector. The frequency of the resultant Doppler burst signal is directly proportional to the velocity of the particles. For each component of velocity, a pair of laser beams is required. Different wavelength (color) beams are used for each component to ensure different wavelengths are scattered by the particles ensure accurate determination of each component.
Due to its non-intrusive nature, LDV can be performed in small-scall, recirculating, turbine flows as well as in hostile enviroments. Its very fast response allows for measurement of turbuelence and frequency spectra. The determination of the velocity by analysis of the Doppler frequencies elminates the need for calibration when performing LDV measurements.
The water tunnel at SAFL is equipped with Laser Doppler Velocimetry (LDV) for velocitry measurement in the flow and Phase Doppler Anemometry (PDA) for the measurement of bubble size distribution. Both LDV and PDA capabilities are incorporated into a single system manufactured by TSI Inc. The PDA system consists of three primary components. The optical system utilizes a 5W Argon-ion laser and fiber optics. Two focal lengths have been selected which provide the capability to measure particles in the size range of 10µm < d < 3000µm. The optical system is expandable for measurement of particles in other size ranges. The signal processor utilizes digital electronics for extremely high data acquisition rate and simultaneous measurement of particle size and velocity. LDV/PDA software provides menu driven control for data acquisition, analysis and storage. On-line histograms, distributions and correlations of size and velocity can be generated.
Figure 2: Schematic of the TSI LDV System used at SAFL