Vertical cosine response of a faired ring acoustic current meter

Measurement of current in the horizontal plane requires that the azimuth of the fluid velocity vector projected onto the horizontal plane be accurately represented in Cartesian coordinates with a cosine response in the x and y direction. In the acoustic current meter that was studied, MAVS (Modular Acoustic Velocity Sensor, manufactured by Nobska Development, Inc.), the horizontal cosine response is excellent, varying from ideal by less than 5%. When a full vector current measurement is made, there is a further requirement that the projection of the fluid velocity vector onto the vertical or z axis have a cosine response to the elevation angle of the velocity vector. Structures supporting the acoustic transducers in an acoustic current meter necessarily create wakes and distort the flow. Minimizing this disturbance through faired supports improves the vertical cosine response; however, there remains a velocity defect of as much as 20% for flows exceeding a 45° elevation angle in MAVS. For flows elevated less than 30° from the horizontal plane, the deviation from ideal cosine response is less than 10% and over a 50° range in elevation angle the deviation from ideal cosine response is less than 5%.