Respond of Bedforms to Velocity Power Spectra of Acoustic-Doppler Velocimetry Data in Rough Mobile Beds
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Respond of Bedforms to Velocity Power Spectra of Acoustic-Doppler Velocimetry Data in Rough Mobile Beds Ratul Das* Department of Civil Engineering, National Institute of Technology, Agartala, Tripura, 799055 India *e-mail: [email protected] Received March 15, 2019; revised August 6, 2019; accepted August 6, 2019
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. Keywords: bedforms, velocity power spectra, immobile-bed, turbulent production, energy dissipation, clearwater flows DOI: 10.1134/S0097807820050176
INTRODUCTION The state-of-the-art of flows over bedforms has been in the focus of research for a long time due to the interactions of transported particles with the overlying flows. In particular, significant attempts are made in many literatures to address different spectral subregimes during bedform development: yet, the influence of sediment mobility on spectral characteristics are not well understood and really challenging [2, 6, 9, 10, 13, 40]. In general, a velocity power spectrum is used to address different spectral sub-regimes and to assess turbulent kinetic energy (TKE) dissipation rate due to sediment mobility on bed. It is very common to observe spikes in the velocity signals measured by an acoustic doppler velocimeter (ADV) owing to the fact of Doppler noise, high-frequency fluctuations and sometimes on account of limitations of instrument used for recording the velocity signals which produce unrealistic spectral scaling regime [4, 10, 22, 29, 42, 43]. Therefore, many researchers adopted despiking of datasets in order to obtain an acceptable inertial subrange of velocity power spectra for analysis. Acceleratio
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