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1

Wake flow field of a wall-mounted pipe with spoiler on a rough channel bed

Devi Kalpana, Mishra Shivam, Hanmaiahgari Prashanth Reddy, Pu Jaan H.

Acta Geophysica

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2023

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Vol. 71, no. 6

2865--2881

EN

This research work focuses on the wake flow region of a cylinder with a spoiler on a rough bed under steady flow conditions. The acoustic Doppler velocimetry was used for the measurement of three-dimensional velocity data for two Reynolds numbers in a fully developed turbulent flow around the cylinder with a spoiler. The mean flow velocities, second-order turbulence structures, and conditional statistics were investigated in the wake region of the spoilered cylinder. The flow was separated from the spoiler with the formation of two shear layers between free surface flow and recirculating flow. It is observed that the flow is reattaching to the bed at 11D irrespective of the Reynolds number. Downstream of the cylinder, the mean velocity distributions are asymmetric due to the wall-wake effect, and the point of inflection is observed for each velocity profile at z = 0.40ẟ. The turbulence intensities, Reynolds stresses, and TKE are highly enhanced in the wake region of the cylinder as compared to their respective upstream values for both runs. The turbulence intensities, Reynolds normal stresses, Reynolds shear stresses, and turbulent kinetic energy are attaining peaks at z = 0.4ẟ for all the streamwise locations, and the peaks are found to be highest at x = 10D. The quadrant analysis results indicate that the sweeps are dominating bursting events in the inner and intermediate layers, while ejections are dominating in the outer layer of the wake region. As the hole size, H increases ejections stress fraction rises as compared to that of the sweeps in the wake region for z = 0.2-0.7 h.

2

Flow structure investigation over a pool-rife sequence in a variable width river

Tabarestani Ehsan Shahiri, Afzalimehr Hossein, Pham Quoc Bao

Acta Geophysica

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2022

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Vol. 70, no 2

713--727

EN

A comprehensive overview of flow characteristics in natural channels with bedforms is a vital issue in river management projects. Pool-rife sequences as common bedforms in the gravel-bed rivers significantly impact flow characteristics and turbulence intensity. The present study was taken by field investigation in the Babolroud River, Iran. A 95 m reach with variable width was chosen in this river and velocity components and shear stress were obtained in different sections. Quadrant analysis was also applied to determine the dominant bursting event in the pool section. The results revealed a phase shift for stream-wise velocity, near-bed velocities, and bed shear stress versus bed profile. In the pool, vertical velocity components were oriented downward near the bed and upward near the water surface, while in the rife section vectors were oriented towards the bed. The findings of quadrant analysis demonstrated the ejections and sweeps as a dominant event close to the bed and water surface, respectively.

3

Coherent structures around submerged circular and square cylinders due to change of orientation angle in steady current over plane bed

Bauri Krishna Pada

Acta Geophysica

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2022

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Vol. 70, no 5

2223--2250

EN

The investigation of the bursting process and coherent structures around circular and square cylindrical structures for various angular alignments over a rigid plane bed are introduced. The coherent structures and shear stresses perform a substantial characteristic in bed particle entrainment and transport from the bed around these structures. Hence, bed shear stress, power spectra are determined from the measured velocity data, in addition, third-order correlation, turbulence intensity, and kinetic energy were investigated using quadrant analysis to explore the correlation between coherent structures and sediment movement. Turbulent coherent structures due to the variations of submergence depth, velocity, orientation angle of the square cylinder, and cylinder size are also investigated. Further, the measured velocity data using Acoustic Doppler Velocimeter were validated with numerical simulations using COMSOL Multiphysics 5.0 with k-ω turbulence closure model. Outcomes indicate that the wake zone length rises with the rise of submergence ratio for the square cylinder with different angular alignments than the circular and square cylindrical structures. Bed shear stress is higher for the square cylinder with different angular alignments than the circular cylinder. The wake vortex strength increases with angular alignment signifying excessive movement of bed sediments from the bed around the cylindrical structures. The interaction events predominate near the bed and middle height of the cylinder, whereas ejection and sweep events dominate far away from the bottom toward the top of the cylinders. The stress fraction is maximum for square cylinders with higher alignment angles.

4

Flow Characteristics over a Gravel Bedform: Kaj River Case Study

Maddahi M. R., Afzalimehr H., Rowiński P. M.

Acta Geophysica

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2016

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Vol. 64, no. 5

1779--1796

EN

The present study deals with the turbulence structure in order to better understand the interaction of bedform and flow characteristics in a gravel-bed river. Data measured above a bedform is used to analyze the importance of coherent structures in turbulent transfer. The Reynolds stress and turbulence intensity in stream-wise direction illustrate significant difference along the bedform, showing a three-layer distribution at the crest and a convex one at the downstream of bedform. Quadrant analysis technique is used to picture momentum exchange above the considered bedform and to find the dominant event in bursting process of the gravel bedform. Quadrant analysis demonstrates that the mechanisms of bedforms generation in sand and gravel-bed rivers are similar and sweep is the dominant event in both rivers.