The accuracy of acoustic Doppler velocimetry (ADV) measurements in turbulent boundary layer flows over a smooth bed
Limnol. Oceanogr. Methods 5:23-33 (2007) | DOI: 10.4319/lom.2007.5.23
ABSTRACT: An evaluation of acoustic Doppler velocimetry (ADV) in the near-bed region of turbulent boundary layer flows is presented. Acoustic instruments have large sampling volumes compared with the smallest scales of motion in turbulent flows. This limits the accuracy of the technique, especially when making measurements close to the bed or in flows where large spatial gradients are present. These limitations are quantified by comparing ADV results to laser Doppler velocimetry (LDV) measurements from the same flow, and to direct numerical simulations of similar flows. A SonTek 16 MHz ADVField system was used in the evaluation. Measurements were made in a turbulent boundary layer over a smooth bed in a laboratory flume. The instrument was evaluated in both a fast and slow flow case with free stream velocities similar to those found in many natural environments. Additionally, results from an assessment of ADV sample volume size and position are reported. Mean velocities were within 5% accuracy down to 0.7 cm from the bed for the fast flow case and 1 cm for the slow flow case. ADV-derived Reynolds stresses matched well with those from LDV to within 1-2 cm of the bed, however turbulence intensities were found to deviate markedly up to 3-4 cm from the bed in some cases. In general, errors are largest close to the bed, but extend further away from the bed than reported in previous studies.