Boundary layer velocity measurements by a laser Doppler profile sensor with micrometre spatial resolution

Abstract

We have developed a differential laser Doppler profile sensor for distributed one-component velocity measurements with high spatial resolution. Two Doppler frequencies are measured simultaneously in order to determine the position as well as the velocity of individual tracer particles passing through the measurement volume. In the centre of the measurement volume the obtained uncertainty of the position is about 1.6 µm. The profile measurement has the advantage that no mechanical scanning is needed to obtain flow velocity profiles over a length of 5 mm. The profile sensor thus provides a tool for highly resolved instantaneous measurements of shear flows, which have strong velocity gradients.

Application of a Novel Laser-Doppler Velocimeter for Turbulence: Structural Measurements in Turbulent Boundary Layers

“…These include methods for estimating the particle acceleration (Lehmann et al 1990, Lehmann and Helbig, 2000, Lehmann, Nobach, and Tropea 2002and Nobach, Kinzel, and Tropea 2005, obtaining sub-measurement volume particle position resolution (Czarske et al , 2002(Czarske et al , and 2003, processing very closely-spaced-in-time Doppler bursts (Nobach 2002a), the ability to store vast amounts of burst data to be processed offline using high-speed PC-based digitizers, and robust, efficient estimation of signal time-delay correlation and spectra for non-evenly sampled data Tropea 2000, Nobach 2002b). …”

“…A technique for obtaining spatial resolution within LDV measurement volumes, already briefly discussed, was developed by Czarske et al (2002). This technique utilizes two measurements of the same component of velocity to achieve particle position resolution two orders of magnitude smaller than the measurement volume diameter, at absolute scales unobtainable for any other spatially-resolving flow velocity measurement technique with a dynamic velocity range approaching that of LDV.…”

“…The 20:1 absolute uncertainties in the y-and zparticle positions are each about 5.8/ 4 un for all velocity magnitudes. Although Czarske et al (2002) report an uncertainty smaller than this in their work with a similar LDV technique, the probe implemented therein has limited general applicability since it operates in a forward scatter mode which requires optical access on both sides of the flow facility. This difference results in stronger signals and reduces the frequency uncertainty considerably if noise sources are properly managed.…”

Optics and Spectroscopy for Fluid Characterization

“…Further details regarding the LDA measurement technique and experimental setups can be found in the review [92] and the book [26]. The laser Doppler velocity profile sensor (LDV-PS) invented by Czarske in 2001 consists of two pairs of intersecting laser beams that allow one to resolve the 1d position of the crossing scattering particle within the intersection volume [93][94][95]. Note that the photo detection is still accomplished with a point detector, which hinders high data rates, which means multiple seeding particles in the intersection volume.…”

Imaging Flow Velocimetry with Laser Mie Scattering

“…Further details regarding the LDA measurement technique and experimental setups can be found in the review [92] and the book [26]. The laser Doppler velocity profile sensor (LDV-PS) invented by Czarske in 2001 consists of two pairs of intersecting laser beams that allow one to resolve the 1d position of the crossing scattering particle within the intersection volume [93][94][95]. Note that the photo detection is still accomplished with a point detector, which hinders high data rates, which means multiple seeding particles in the intersection volume.…”