Author(s): Manousos Valyrakis; Da Liu
Linked Author(s): Manousos Valyrakis
Keywords: No Keywords
Abstract: Acoustic Doppler velocimetry profilers (ADVPs) are widely used to assess flow conditions, both at laboratory flumes and at natural streams during fieldwork, because of their robustness and ease of use. Acquired velocity profile measurements, can be used to directly assess bulk flow characteristics, as well as the mean and turbulent flow field. They can also be further processed to extract near bed shear stresses, with a wide range of eco hydraulics, engineering and geomorphology applications. However, the ease of use when customarily the default probe settings are chosen and the lack of sufficient detailed documentation to guide the user on appropriately selecting settings for the flow conditions at hand, render their use risky and the results obta ined in this manner questionable. Given the significance of acquiring accurate flow diagnostics measurements, with which ecological and hydraulic studies can be conducted with confidence, this study aims to evaluate the performance of ADVPs in hydrodynamic measurements, under various probe configurations. Specifically, the impact of different probe settings on obtained results (from raw data of velocity time series, to post-processed mean velocity profiles and near bed turbulent kinetic energy computed by instantaneous velocities), are assessed, for a wide range of combinations of the probe’s settings. Signal to noise ratio, correlations and number of spikes are used as indicators of the quality of velocity time series. Goodness of fit measures of the acquired velocity profiles to the predicted function for wall bounded flows (log Law of the Wall), is used to assess the performance of the ADVP under certain combinations of the ADVP probe’s configurations. Estimated boundary shear stresses and Nikuradse’s equivalent sand roughness, , are also assessed. The impact of probe settings on shear stress estimation is assessed. All these results demonstrate that there is a significant dependence on the exact settings of the probe configuration – implying that if the default settings are used, it is quite probable that the velocimetry results and derived measurements may involve a good level of error. The exact configuration that may yield optimal results may depend on the actual flow and boundary charac teristics. Thus, this study aims to demonstrate the importance of checking for these optimal (rather than using the default) settings and offers a guideline based on performance metrics, to facilitate assessing these.