Synchronous Lagrangian and Eulerian Velocity Measurements IEEE SIXTH WORKING CONFERENCE ON CURRENT MEASUREMENT

A Phased Array Doppler Sonar (PADS), as developed at SIO, images both acoustic backscatter intensity and Doppler shift over areas up to 90 degrees in bearing by 500 m in range. Sequences of images are recorded continuously for up to two months, with individual “pings” 0.5 to 1.0 seconds apart. The images permit two independent estimates of the mean velocity to be formed over the imaged area: one from the Doppler shift (radial velocity), and another from tracking features from one frame to the next. For grazing incidence, the Doppler shifts provide an essentially Eulerian velocity estimate, an intensity-weighted average from the wave troughs downward. In contrast , the feature-tracking velocities are Lagrangian. A unique aspect of this is that both Eulerian and Lagrangian velocities are estimated from the same data. The difference between these two velocity estimates compares well with the Stokes’ drift calculated via linear theory from a resistance wire elevation array.