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In order to comprehend potential changes in the hydrodynamics of flow under downward seepage condition when water is extracted from a threshold alluvial channel, investigations in a laboratory flume were carried out. Experiments in the present study, were organized in two categories: in absence of downward seepage and in presence of downward seepage. Increased in transport of bed-material in the channel was observed on introduction of downward seepage. Instantaneous velocity samples were collected employing an acoustic Doppler velocimeter (ADV) to calculate various length scales of turbulent motions such as: integral length scale, Taylor microscale, and Kolmogorov's microscale. An increase in the values of these length scales has been observed under the influence of downward seepage. Three sub-ranges in the velocity power spectra both for seepage and no-seepage situations are confirmed by the results of the spectral analysis in the channel near-bed area. Toward the channel bed, spectral densities were found increasing for both types of experiments. At every measurement location, velocity power spectra of the seepage experiment shifted toward higher spectral densities and higher frequencies than the power spectra of the no-seepage experiment. Moreover, the power spectra of seepage experiment were found flatter than that of the no-seepage experiment. This flatness in the velocity power spectra indicates an increased bed roughness when a threshold alluvial channel is subjected to downward seepage.

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]

Bora is a strong, usually dry temporally and spatially transient wind that is common at die eastern Adriatic Coast and many other dynamically similar regions around the world. One of the Bora main characteristics is its gustiness, when wind velocities can reach up to five times the mean velocity. Bora often creates significant problems to traffic, structures and human life in general. In this study, Bora velocity and near-ground turbulence are studied using the results of three-level high-frequency Bora field measurements carried out on a meteorological tower near the city of Split, Croatia. These measurements are analyzed for a period from April 2010 until June 2011. This rather long period allows for making quite robust and reliable conclusions. The focus is on mean Bora velocity, turbulence intensity, Reynolds shear stress and turbulence length scale profiles, as well as on Bora velocity power spectra and thermal stratification. Tire results are compared with commonly used empirical laws and recommendations provided in the ESDU 85020 wind engineering standard to question its applicability to Bora. The obtained results report some interesting findings. In particular, the empirical power- and logarithmic laws proved to fit mean Bora velocity profiles well. With decreasing Bora velocity there is an increase in the power-law exponent and aerodynamic surface roughness length, and simultaneously a decrease in friction velocity. This indicates an urban-like velocity profile for smaller wind velocities and a rural-like velocity profile for larger wind velocities. Bora proved to be near-neutral thermally stratified. Turbulence intensity and lateral component of turbulence length scales agree well with ESDU 85020 for this particular terrain type. Longitudinal and vertical turbulence length scales, Reynolds shear stress and velocity power spectra differ considerably from ESDU 85020. This may have significant implications on calculations of Bora wind loads on structures. [ABSTRACT FROM AUTHOR]