Cross-Spectral Phase Method for Distinguishing Waves from Turbulence in Single-Point Boundary Layer Flow Measurements

Cross-spectral analysis was applied to fixed single-point current measurements obtained using a Modular Acoustic Velocity Sensor (MAVS) from a shallow deployment in a tidal channel in Vineyard Sound south of Woods Hole, Massachusetts. Distinct phase signatures at different frequency bands were found in the cross power spectra between the vector components of velocity and between velocity and pressure. Velocity and pressure data had been obtained in burst samples both during periods when the flow was dominated by tidal currents and during periods when the flow was dominated by surface waves. Our analyses show that the phase signature in the cross power spectra can be used to effectively separate waves from turbulence over a broad range of scales that we sampled. In addition, the phase signature of the Cartesian velocity components has been derived in terms of the acoustic axes that comprise the raw measurements of velocity from MAVS. This association is used to distinguish between turbulence contributions and electronic noise at the intermediate to high frequency bands. By separating flow fluctuations due to turbulence from those due to waves a more accurate and representative observation of boundary layer dynamics can be obtained from a single-point vector flow measurement in the bottom boundary layer. Where these bands can be separated in frequency, Reynolds stress as well as structural eddies can be distinguished from waves