The application of backscatter laser Doppler techniques for velocity measurements at vessels and ship models

Various arrangements are discussed for backscatter laser velocimeters to be used for velocity measurements in the wake field of ship models. Laboratory experiments show that a fringe mode laser Doppler arrangement that employs the photon correlation technique gives the best results when operating with low power lasers. An experimental (coaxial) optical system using a 5 mW He-Ne laser is described. Results are reported for velocity measurements in the wake field of a ship model as carried out at the Netherlands Ship Model Basin at Wageningen. The experiments show that the photon correlation technique is suited for ship model work when using a coaxial optical system that transmits the laser beams and collects the backscattered light. Several suggestions for improvement are made to reduce the time of measurement. The use of a 50 mW argon laser when employing photon correlation is proposed, and it is expected that a laser power of at least 1 W is required for operation with a conventional laser Doppler signal processor, such as a frequency tracker. Finally, the use of the laser Doppler technique is discussed for full scale measurements on in-voyage vessels at sea. A laser Doppler system capable of measuring velocities at a range of 10 m is proposed.