Experimental study of turbulent coherent structures using particle image velocimetry and hydrogen bubble visualisation t
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O R I G I NA L
Xuan Zhang
· Richard Simons · Jinhai Zheng
Experimental study of turbulent coherent structures using particle image velocimetry and hydrogen bubble visualisation techniques
Received: 23 April 2020 / Accepted: 1 October 2020 © Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract The characteristics of coherent structures in turbulent boundary layers were investigated experimentally using PIV and hydrogen bubble visualisation techniques. Turbulent boundary layers were generated and tested in two flumes with different scale, for two Reynolds numbers: Re = 32,000 and 71,200, both over a smooth bed. Measurements from the PIV technique and hydrogen bubble visualisation lead to the same research findings, suggesting a linear relationship between the instantaneous integral spatial scale and the instantaneous spacing between turbulent streaks. This new approach to determining the streak spacing provides some guidance for experimentalists in terms of experimental methodology. Results of hydrogen bubble visualisations provide additional evidence that streamwise vortices reside near low-speed streaks. Detailed statistical analysis further demonstrates the better performance of Burr distribution for the probability density function of streak spacing. Combining the measurements of flow field in the horizontal and vertical plane, streamwise vortices are observed to be tilted ± 8◦ in the (x, z) plane and shear layers are inclined to the wall at 30◦ in the (x, y) plane. Keywords Fluid dynamics · Turbulence · Bottom boundary layer · Coherent structures · Particle image velocimetry · Hydrogen bubble visualisation 1 Introduction Over the past few decades, noticeable progress has been made towards a more comprehensive understanding of turbulent boundary layers. Unidirectional turbulent currents have been shown to be influenced by hairpin vortices, which are often referred to as coherent structures. These vortices are believed to be the dominant feature in turbulent boundary layers [3,12–15,40]. Results from laboratory experiments and numerical models have made it clear that such coherent structures can be fundamentally classified into two types: the arch (alternatively known as hairpin or horseshoe) vortex and the ‘quasi-streamwise’ vortex [4,6,21,22,24,29,45,47]. Horseshoe vortices are typical features with an asymmetrical or partially rolled-up form (both apparent in spanwise and streamwise vorticity); ‘quasi-streamwise’ vortices imply that they are meandering in the streamwise direction. Low-speed streaks, which are observed to be persistent and relatively quiescent most of the time in viscous X. Zhang (B) · J. Zheng Key Laboratory of Ministry of Education for Coastal Disaster and Protection, Hohai University, Nanjing 210098, China E-mail: [email protected] X. Zhang · R. Simons Department of Civil, Environmental and Geomatic Engineering, University College London, Gower Street, London WC1E 6BT, UK X. Zhang · J. Zheng College of Harbor, Coastal and Offshore Engineering, Hohai University, Nanji
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