Reynolds stress resolution from a modular acoustic velocity sensor

Correlation of fluctuations in velocity components in a boundary layer flow represents turbulent exchange of momentum across streamlines. Averaged over many burst and sweep events, the correlation of velocity fluctuations normal to the boundary with streamwise velocity fluctuations is Reynolds stress. Exchange of fluid is the dominant stress carrying process from the viscous sublayer to the outer boundary layer. In this region, millimeters to meters, the stress is constant and a single measurement of Reynolds stress represents the shear stress at the boundary. The Modular Acoustic Velocity Sensor, MAVS, measures the 3D vector velocity in a 10-cm diameter volume, a size suitable for resolving Reynolds stress. Such a measurement is valuable for studies of sediment transport in which the shear stress on the bottom is the forcing function for erosion and a barrier to deposition. Noise level in still fluid, correlations of velocity while towed in unsheared fluid, and comparison with BASS (Benthic Acoustic Stress Sensor) in an inertial sublayer test whether MAVS can measure Reynolds stress. Flow distortion by the transducer supports and correlation of fluctuations in velocity components in the wake are the potential limits on MAVS for Reynolds stress sensing. Electronic noise and drift are not a problem. If one assumes 1/3 of the fluctuations in the wake correlate (downstream with vertical) then the limit for rms flow noise in the wake is about 3% of the flow velocity. Experimental verification shows the Modular Acoustic Velocity Sensor, MAVS, does meet the noise criteria for measuring Reynolds stress. The noise limits should permit MAVS to resolve Reynolds stress. Towing in a tank provides noise figures for unsheared flow (no Reynolds stress). Deployment with BASS on a tripod in a boundary layer flow in a tidal channel gives a comparison of Reynolds stress between a known sensor and MAVS. The results from the towing tests and the tidal channel can be compared to give a lower limit of Reynolds stress that MAVS can detect and the expected error in stress as a function of speed