Laser Doppler velocimetry momentum defect measurements of cable drag at low to moderate Reynolds numbers: feasibility study

Abstract

The problem of measuring cable drag coefficients at low to moderate Reynolds numbers (100 < Re < 30,000) is addressed and alternative methods of measurement are analyzed. The very low forces and pressures involved at the lower Reynolds numbers render conventional measurement methods less suitable candidates. Laser Doppler velocity measurements, however, of the momentum defect in the wake appear capable of yielding sufficient accuracy « ± 5%) in the determination of the drag coefficient. This conclusion assumes that a test facility can be utilized with a sufficiently uniform flow field, low turbulence level and a free stream velocity which either remains stable during the wake survey measurement time interval or can be monitored independently. Water and air appear as almost equal in their merits as working fluids. with water slightly preferable as not requiring scattering particle seeding.