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Experimental investigations are presented on the characterization of flow turbulence and velocity fields around circular piers placed in isolated, tandem, and staggered arrangements in a rigid bed channel with identical flow conditions. Instantaneous velocity measurements are undertaken using a 16 MHz micro down-looking Acoustic Doppler Velocimeter (ADV) at different grid points along the flow depth. The streamline pattern obtained from the velocity fields is presented on vertical planes around the piers. Further, the resultant contours of vortices are plotted for all three types of arrangements, i.e., isolated, tandem, and staggered arrangements, to understand the strength of the vortices around the piers. Apart from the above investigations, the turbulence characteristics such as turbulence intensities, turbulent kinetic energy, velocity power spectra and Reynolds shear stresses at different planes are also presented for all three configurations of piers. The turbulence characteristics are used to identify the influence of one pier over others in the tandem and staggered arrangements compared to that of the isolated piers. The horseshoe vortex system, quantified by its vorticity and strength, is found to be predominant in the staggered configuration. A horseshoe vortex is formed at 0.36d, 0.9d and 0.35d (d is the diameter of the pier) upstream of the isolated pier, front piers of the tandem and front piers of the staggered case, respectively. A zone of recirculation is formed just upstream of the rear pier in the tandem case. The Reynolds shear stresses near the bed, turbulent kinetic energy and turbulence intensities are also found to be quite significant downstream of the front pier in the staggered arrangement compared to those in the isolated and tandem arrangements. The turbulent kinetic energy at the mid-flow depth is found to be 2.5 times higher than that near the bed. The velocity power spectra reveals that the strength of wake vortices is 2.5 times greater for the front piers of the staggered case than that of the tandem front piers with Strouhal numbers, S t = 0.26 and 0.112 for the staggered and tandem cases, respectively, near the bed. The present study enhances the understanding of the flow structure around isolated, tandem and staggered bridge piers. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]