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Experiments by Invited Researchers

 

Investigation into the physical relationships between water-worked gravel be armours and turbulent in-channel flow patterns

Project acronym: HyIII-Hull-01
Name of Group Leader: Dr Katinka Koll, TU Braunschweig, Leichtweiß-Insti
User-Project Title: Investigation into the physical relationships between water-worked gravel be armours and turbulent in-channel flow patterns
Facility: TES
Proceedings TA Project: Investigation into the physical relationship between water-worked gravel bed armours and turbulent in-channel flow patterns
Data Management Report: There is no Data Management Report available for this project

User-Project Objectives

Summary:

Traditional methods for describing turbulent flow account for the effects of a rough sediment boundary using a representative grain size. However, recent studies of gravel bed structure have shown that grain orientation differs for static and dynamic armour layers. Both bed types may be characterised by the same roughness height and surface grain size distribution, but the different arrangement of surface grains results in different longitudinal and transverse roughness scales. The first objective of this study is to investigate how the systematic difference in surface structure affects the spatial and temporal organisation of near bed turbulent flow. The second objective is to compare techniques used to measure velocity and turbulence over rough sediment beds using both Particle Image Velocimetry (PIV) and Laser Doppler Anemometry (LDA). The third objective is to use the data collected to better understand the influence of bed structure on the transfer of fluid momentum both within the roughness elements of the bed and throughout the flow.

The spatially-averaged streamwise velocity profiles and distributions of form induced stresses for the same flow conditions show that grain arrangement has a significant influence on flow roughness. The results indicate that representative surface grain size or bed surface roughness height based on simple statistics of bed elevations cannot accurately account for the resistance imposed by a water-worked gravel bed on the flow. The shape of the vertical profiles as well as the absolute values obtained by PIV measurements depend only marginally on the number of verticals (168 compared to 20). Compared to LDA data, only the longitudinal velocity component shows good agreement. This result is partly associated with the chosen experimental set-up, which aimed at the description of the complete flow field rather than a small section.

Figure 1. View looking downstream showing one of the sediment trays being lifted  before being rotated.

Figure 2. Experimental instrumentation showing LDA on central gantry support, and PIV light tube on the left with external submersible cameras in black tube to the right.

 

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