Episode 3. A journey to PIV flow measurements at ship scale.

Sea Trials Other

Another option to measure the flow around a ship could be to use LDV (Laser Doppler Velocimetry). LDV is a technique that uses the Doppler shift of laser light to measure the velocity of fluid flows. This works by crossing two beams of coherent laser light. Transmitting optics focus the beams to intersect at their waists, where they interfere and generate a set of interference fringes. As particles (either naturally occurring or artificial) contained in the fluid pass through the fringes, they reflect light that is then collected by receiving optics and focused on a photodetector. The particle and thereby the fluid velocity can be determined from the Doppler shift of the light hitting the photodetector.
The LDV technique is widely used in model tests to measure the wakefield; there were comparatively few attempts to use LDV at the ship scale. For example, MARIN attempted to measure the flow on MV Valovine in the GRIP EU research project in 2014. Unfortunately, those measurements were not successful. One of the challenges was related to setting up the unit on a vessel; vibrations in the aft end of the ship (due to machinery and propeller cavitation) make intersecting the two beams very challenging. Furthermore, the data rate of LDV that measures in only one point might be limited, requiring long measurement times. Finally, intersecting one set of beams is challenging, resulting in one component of velocity; for the second component, another set of beams would need to be intersected at the same point where the first pair of beams is, increasing the complexity and likelihood of measurement failure even further.  
To be continued...

LDV unit used at GRIP project in 2014

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