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Experimental Analysis of Relations Between Coherent Turbulent Structures and Formation of Bedforms

100%

Termini D. , Sammartano V.

tom Vol. 55, nr 3-4

125-143

The present paper describes an experimental investigation on the interactions between flow turbulence and sediment motion. During the experiments, detailed measurements of flow velocity components were carried out using an Acoustic Doppler Velocimeter (ADV). The occurrence of turbulent events (inward interaction, ejection, sweep and burst) was verified through conditioned quadrant analysis. The quantitative information on the spatial and temporal evolution of turbulent events was obtained through space-time correlations of the conditioned data. As the primary objective was to analyse how turbulent structures influence formation of bedforms, the spatial scale of turbulent event evolution has been compared with the wavelength of bed-forms (alternate bars) observed on the bed. The analysis has essentially highlighted that such spatial scale compares well with the wavelength of the bars.

Reinvestigation on Mixing Length in an Open Channel Turbulent Flow

63%

Kundu S. , Kumbhakar M. , Ghoshal K.

tom Vol. 66, no. 1

93--107

The present study proposes a model on vertical distribution of streamwise velocity in an open channel turbulent flow through a newly proposed mixing length, which is derived for both clear water and sedimentladen turbulent flows. The analysis is based on a theoretical consideration which explores the effect of density stratification on the streamwise velocity profile. The derivation of mixing length makes use of the diffusion equation where both the sediment diffusivity and momentum diffusivity are taken as a function of height from the channel bed. The damping factor present in the mixing length of sediment-fluid mixture contains velocity and concentration gradients. This factor is capable of describing the dip-phenomenon of velocity distribution. From the existing experimental data of velocity, the mixing length data are calculated. The pattern shows that mixing length increases from bed to the dip-position, having a larger value at dip-position and then decreases up to the water surface with a zero value thereat. The present model agrees well with these data sets and this behavior cannot be described by any other existing model. Finally, the proposed mixing length model is applied to find the velocity distribution in wide and narrow open channels.The derived velocity distribution is compared with laboratory channel data of velocity, and the comparison shows good agreement.

Time-averaged Turbulent Flow Characteristics over a Highly Spatially Heterogeneous Gravel-Bed

63%

Sarkar S.

tom Vol. 64, no. 5

1797--1824

The present study focuses on the time-averaged turbulence characteristics over a highly spatially-heterogeneous gravel-bed. The timeaveraged streamwise velocity, Reynolds shear and normal stresses, turbulent kinetic energy, higher-order moments of velocity fluctuations, length scales, and the turbulent bursting were measured over a gravel-bed with an array of larger gravels. It was observed that the turbulence characteristics do not vary significantly above the crest level of the array as compared to those below the array. The nondimensional streamwise velocity decreases considerably with a decrease in depth below the array. Below the array, the Reynolds shear stress (RSS) deviates from the gravity-law of RSS distributions. Turbulence intensities reduce below the crest level of the gravel-bed. The third-order moments of velocity fluctuations increase below the crest level of the gravel-bed and give a clear indication of sweeps as the predominating event which were further verified with the quadrant analysis plots. The turbulent length scales values change significantly below the crest level of the gravel-bed.