A correction method for turbulence measurements with a 3D acoustic Dopplervelocity profiler

A method is proposed to reduce the noise contribution to mean turbulence pa rameters obtained by 3D acoustic Doppler velocity profiler measurements. It is based on a noise spectrum reconstruction from cross-spectra evaluations of two independent and simultaneous measurements of the same vertical velo city component over the whole water depth. The noise spectra and the noise variances are calculated and removed for the three fluctuating velocity com ponents measured in turbulent, open-channel flow. The corrected turbulence spectra show a -5/3 slope over the whole inertial subrange delimited by the frequency band of the device, while the uncorrected turbulence spectra hav e flat high-frequency regions typical for noise effects. This method does n ot require any hypothesis on the flow characteristics nor does it depend on device-dependent parameters. The corrected profiles of turbulence intensit ies, turbulent kinetic energy, shear stress, and turbulent energy balance e quation terms, such as production, transport, and dissipation, are in bette r agreement with different semitheo-retical formulas and other measurements from the literature than those from the uncorrected data. Combined with th e use of a phase array emitter, the proposed correction method allows measu rements with a relative error under 10% in the outer flow region. The corre cted inner flow region measurements are still affected by errors that may o riginate from spatial averaging effects within the sample volume due to the high local velocity gradient or the lack of validity of the universal laws concerning turbulence quantities over a rough bed.