Reliability of ADV measurements for the analysis of the universal laws of turbulence

Open-channel flows are one of the most attractive, fascinating, and complex phenomena in nature. Complex thalweg paths and cross-section geometries, coarse sediment beds, aquatic vegetation, and human-built structures (e.g., bridge piers or abutments) make the prediction of the flow evolution very hard. Owing to the high complexity of the aforementioned features, the flow characteristics are often investigated under controlled conditions in a laboratory. The present work aims at providing the reliability of a laboratory measurement instrument that can be easily used in situ. It is important to underline the importance of rough beds, which are typical of southern Europe rivers, in engineering practice; from environmental engineering interventions (e.g., safeguarding of aquatic life) to sediment transport phenomena, countermeasures against erosion and local scour (e.g., at bridge piers/abutments), many phenomena can be addressed by using coarse sediments. Researchers and engineers need mathematical tools in order to investigate problems like turbulence. Many techniques were developed in the past, like spectral analysis, which is the most accredited technique to analyze turbulent phenomena (e.g., characteristic scales and energy cascade). Anyway, most of the physical discussions are treated within the real space, which is an important and meaningful tool of analysis in turbulence. The wide range of turbulent eddies present in a river are large coherent structures, which are commonly called energy-containing scales. These coherent structures arise in the bottom boundary layer, where a prominent shear is present, and their generation is due to the interaction between regions with different momentum. Different scales are employed in the mechanisms of sediment entrainment in river flows. For instance, the knowledge of the energy amount through different scales (from larger to smaller) can help to understand how they act on the Reynolds stress. As can be easily imagined, all these quantities and phenomena can be investigated by exploring the entire range of turbulent scales with highly sophisticated laboratory instruments. The present work is based on a laboratory campaign aimed at investigating the effect of highly rough beds (with low relative submergence) on the characteristics of wall turbulence. The measurements are performed with an Acoustic Doppler Velocimetry (ADV), that is able to explore part of the isotropic range of flow velocity frequencies, in which the universal laws of turbulence are valid.