Respond of Bedforms to Velocity Power Spectra of Acoustic-Doppler Velocimetry Data in Rough Mobile Beds

Abstract

Present work aims to insight the influence of bedform transport on velocity power spectra of acoustic Doppler Velocimeter (ADV) in order to address different spectral sub-regimes. Laboratory experiments were conducted over two types of rough bed (d50 = 4 mm): (a) immobile rough bed and (b) mobile bedform. The near-bed spectral characteristics during bedform transport are compared with those in immobile rough bed. An ADV probe, named Vectrino+, manufactured by Nortek with acoustic frequency of 10 MHz was used to measure the velocity fluctuations in flows. Instrument noise and high-frequency fluctuation results furious spikes in the velocity signals and therefore, filtration of contaminated data was very much essential to obtain clear velocity power spectra. In this study the acceleration threshold method was applied successfully for filtering the data sets and the velocity power spectra of filtered data set are found to be well fitted the Kolmogorov “–5/3 scaling-law” in the inertial sub-range. Importantly, the spectral sub-regime at low frequencies with a spectral slope of about −1.0 occurred owing to the fact of bedforms development. The results of Taylor microscale and Kolmogorov scale reveal an amplification of eddy sizes in the near-bed flow region attributed to bedform transport and the drastic reduction in pressure energy diffusion in budget analysis implied gain in near-bed turbulence production.