school of mechanical engineering Eyal Ginsburg
School of Mechanical Engineering Seminar
Wednesday, November 8, 2017 at 14:00
Wolfson Building of Mechanical Engineering, Room 206
Current and Wave velocities measurements with X-band Doppler radar in a laboratory environment
Eyal Ginzburg
MSc. student of Prof. Lev Shemer and Dr. Yaron Toledo
The history of radio oceanography can be traced back to the middle of the 20th century. Generally, Doppler Effect is used to measure the relative speed of a moving target with respect to the radar. Using Doppler-shift-based radars gives the ability to measure the wave celerity and the current velocities, using the Bragg-resonance backscattering mechanism. However, recent studies have shown that different parameters, like the wind speed and the frequency of non-Bragg water-waves, may have an influence on the radar backscattered signal. While numerous studies have been made on Bragg and non-Bragg scattering radars, there are no studies at-present to verify field measurement thus experiments in controlled laboratory environment are needed.
The main objective of the study is to perform laboratory measurements using the same methodology as in field measurements, with the ability to change wave, current and wind parameters. The radar which was used in this study was not originally constructed for water surface measurements. Hence, a set of preliminary experiments dealing with the direct reflection from moving solid reflective bodies was carried out. A second set of experiments was focused on radar measurements within a wind-wave flume in which mechanical waves can be excited, along with winds and ambient currents that can be controlled. The spectra analysis of the radar backscatter corresponding to different wind forcing shows a good agreement with the theoretical interpretation of the signal as a sum of the Bragg wave Doppler frequency and one of the water surface drift velocities due to the wind shear stress. For high wind speeds the main wave component due to the specular reflection is also manifested.
As part of the current study, some preliminary tests were performed. Firstly, a flow system, which enables generation of current with different mean velocities, along- and opposite to the wind direction in the flume, was designed and constructed. Additionally, measurements of wind driven shearing current profiles were carried out for different wave conditions with a Pitot tube, which gives rather accurate information about the surface current. This is extremely important for the radar measurements interpretation.
