Turbulence anisotropy in a compound meandering channel with different submergence conditions

Abstract

(100-150 words) 8The understanding of the physical processes related to flows on compound meandering channels is a 9 challenge given their highly 3D and complex characteristics. Three-dimensional ADV measurements 10 were made in three cross-sections on a 1:20 physical Froude model of a real reach in River Mero (A 11 Coruña) for bankfull flow and flood conditions. General characteristics and processes within the flow 12 are herein described and characterized, such as momentum and mass exchange between the main 13 channel and the floodplains. Time-averaged velocities and Reynolds stresses are presented and 14 discussed. The spatial distribution of turbulence in several positions along a meander bend is 15 analyzed in this paper. The characterization of the turbulent field in these highly 3D complex flows 16 highly depends on the used reference system, and the intense local variation of turbulence makes a 17 global and fixed coordinate system of petty use. An independent technique, regardless the 18 coordinate system of the measurements, is thus the best way to analyse these flows. The anisotropy 19invariants technique was used to analyze the evolution of the magnitude and nature of anisotropy 20 along the meander. The degree and nature of anisotropy was identified, and their relation to flow 21 structures, such as vortices in the contact between the main channel and the floodplains, was 22 analyzed using the quadrant analysis technique. 23Keywords: meanders, compound channels, turbulence anisotropy, physical model, ADV 24 Introduction 25River management represents a challenge from an engineering, environmental and social point of 26 view. In particular, floodplains are among the most productive and diverse ecosystems in the world 27 due to the regular deposition of nutrient rich sediments (Viers et al. 2005). The environmental value 28 of these ecosystems is unquestionable given their high biodiversity and their role on water 29 purification and on the fixation of soil and nutrients. Furthermore, floodplains can be seen as natural 30 systems providing food availability and flood protection. The capacity to act as a natural protection 31 against floods is conditioned and intrinsically related to the local hydrodynamics, erosion and 32 sedimentation processes and to the global resistance of the river reach. Flow properties, more 33 specifically related to turbulent phenomena, need a good physical understanding so engineers can 34 address such important issues such as flood management, morphology evolution and spreading of 35 pollutants in river flows (van Balen et al., 2010). The physical processes underlying the formation of 36 meanders have been the subject of intensive and detailed research (i.e. da Silva, 2006; de Marchis 37 and Napoli, 2008;Stoesser et al., 2010), although the theoretical developments taking into account 38 compound meandering channels are still scarce. 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 ...

Fluvial eco-hydraulics and morphodynamics: New insights and challenges

“…Particular attention is devoted to flow hydrodynamics in meandering bends [15,17]. The links between cross-sectional flow pattern, bed deformation and bank erosion in high-curvature bends are explored and peculiar features of turbulent flow structure in compound meandering channels are presented.…”

“…Flow turbulence characteristics along a real compound meander reach in River Mero (Coruña) are explored by Mera et al [15] in the paper Turbulence anisotropy in a compound meandering channel with different submergence conditions, with the aid of data collected in a 1:20 laboratory channel.…”

Spiral Vortex Motion Fluid Analysis and Possibilities of Its Use in the Hydraulic System

“…Одним з основних припущень при розрахунку трубопроводів вважається, що круглий переріз буде оптимальною формою для протікання рідини, так як при заданій площі периметр тертя буде мінімальним, а пропускна спроможністьмаксимальною. Але в таких випадках не розглядається вплив вихрового та спірального характеру руху рідин, на що звертали увагу деякі вчені та натуралісти [9][10][11].…”

Turbulence in Rivers

“…In the transition between these limiting states of turbulence, two characteristic types of turbulent structures are found: pancake-shaped turbulence, which corresponds to a situation where two of the fluctuation components are equal and considerably larger than the third, and cigar-shaped structures where two of the turbulence components, and hence the normal stresses values, are equal and the third has a substantially higher value. The analysis based on the Lumley plots, combined with other observational techniques, such as the quadrant analysis (Lu and Willmarth 1973;Nakagawa and Nezu 1977), permits the establishment of a proper framework for the analysis of turbulence and forms the basis of useful simplifications for the analysis of complex 3D flows (Mera et al 2014). The knowledge of the nature of turbulent eddies in river flows allows the establishment of restricted spatial frameworks for the analysis of these flows, as will be seen in Sect.…”

“…Experimental studies that focus on compound meandering morphologies are scarce (Shiono and Muto 1998;Shiono et al 2008). Mera et al (2014) characterized the hydrodynamics and turbulence patterns in a real compound meandering channel using the anisotropy invariants combined with quadrant analysis techniques.…”