Frédérique Larrarte

Turbulent velocity profile in fully-developed open channel flows

The determination of velocity profile in turbulent narrow open channels is a difficult task due to the significant effects of the anisotropic turbulence that involve the Prandtl’s second type of secondary flow occurring in the cross section. With these currents the maximum velocity appears below the free surface that is called dip phenomenon. The well-known logarithmic law describes the velocity distribution in the inner region of the turbulent boundary layer but it is not adapted to define the velocity profile in the outer region of narrow channels. This paper relies on an analysis of the Navier–Stokes equations and yields a new formulation of the vertical velocity profile in the center region of steady, fully developed turbulent flows in open channels. This formulation is able to predict time averaged primary velocity in the outer region of the turbulent boundary layer for both narrow and wide open channels. The proposed law is based on the knowledge of the aspect ratio and involves a parameter CAr depending on the position of the maximum velocity (ξdip). ξdip may be derived, either from measurements or from an empirical equation given in this paper. A wide range of longitudinal velocity profile data for narrow open channels has been used for validating the model. The agreement between the measured and the computed velocities is rather good, despite the simplification used.

Study of the shear stress in narrow channels : application to sewers

The management of deposits in combined sewers is of vital importance from a technical and management point of view. In combined sewers, there are a wide range of shape of cross-sections that have not been characterised from a hydraulic aspect and where it is difficult to measure accurately the near wall velocity and shear stress. A research programme, therefore, is in progress to study how flows behave in such channels and the numerical simulations can be used systematically to investigate some parameters viz. velocity field, shear stress. In the present study, a numerical method was employed to compute distribution of shear stress in whole of the cross sections. The effect of turbulence modelling in simple and combined cross section was investigated. A comparison between one-dimensional and three-dimensional flow equations was carried out to estimate distribution of the shear stress. The three-dimensional model results were in close agreement with experimental data, whereas one-dimensional equations could not fit with experimental values.

Velocity Distribution in Open Channel Flows: Analytical Approach for the Outer Region

AbstractThis paper presents an integration procedure for the Reynolds-averaged Navier-Stokes equations for the determination of the distribution of the streamwise velocity using the vertical component. This procedure is dedicated to the outer region and central part of channels. The proposed model is applicable to both rough and smooth flow regimes, provided the velocity at the inner-outer boundary has been properly defined. To generate a simplified expansion, a number of hypotheses are proposed, focusing in particular on the analytical modeling of the vertical component by adopting a negligible viscosity. The proposed hypotheses are validated by the experimental data existing in the literature. The proposed simplified expansion is studied through a sensitivity analysis and proved consistent in regards to model experimental data. The proposed model seems capable of demonstrating different kinds of flows, including dip phenomenon flow patterns.

Coupling acoustic devices for monitoring combined sewer network sediment deposits.

Combined sewer networks display some unique features that are not easily reproducible in the laboratory and have yet to be well understood. The transport of organic and mobile sediments, present in large quantities despite the use of optimal design practices, dramatically changes sewer flow patterns and the interaction between flow and sediment. To understand both the flow patterns and transport features of these complex environments, it is therefore necessary to install instrumentation in situ. For this paper, two distinct acoustic techniques were coupled in order to measure both the sediment interface and velocity. For this purpose, a 2 MHz rotating head acoustic profiler and a 10 MHz acoustic Doppler velocimeter (ADV) were jointly installed to survey the position of sediment deposits and measure velocity patterns. Results were compared with measurements recorded in different environments, where both coarser and finer soft deposits were present. These results typically showed good agreement between the interfaces detected using velocity measurements on coarse deposits, while the presence of soft deposits demonstrated the need for coupling measurements techniques that can correctly assess the sediment interface.

Velocity and suspended solids distributions in an oval-shaped channel with a side bank

This study focuses on field measurements of both the velocity and total suspended solids within a combined sewer through use of a two-dimensional sampler called Hydre. The paper begins by presenting this instrument and its experimental applications. The discussion centers on the recorded velocities and concentrations; moreover, analyses of results demonstrate the ability of this Hydre device to provide robust suspended solids and velocity profiles in sewers for a range of flow conditions.

Acoustic measurement and morphological features of organic sediment deposits in combined sewer networks

The performance of sewer networks has important consequences from an environmental and social point of view. Poor functioning can result in flood risk and pollution at a large scale. Sediment deposits forming in sewer trunks might severely compromise the sewer line by affecting the flow field, reducing cross-sectional areas, and increasing roughness coefficients. In spite of numerous efforts, the morphological features of these depositional environments remain poorly understood. The interface between water and sediment remains inefficiently identified and the estimation of the stock of deposit is frequently inaccurate. In part, this is due to technical issues connected to difficulties in collecting accurate field measurements without disrupting existing morphologies. In this paper, results from an extensive field campaign are presented; during the campaign a new survey methodology based on acoustic techniques has been tested. Furthermore, a new algorithm for the detection of the soil-water interface, and therefore for the correct esteem of sediment stocks is proposed. Finally, results in regard to bed topography, and morphological features at two different field sites are presented and reveal that a large variability in bed forms is present along sewer networks.

Additional elements regarding the muddy layer in combined sewers

Abstract For 25 years, major efforts have been undertaken to improve the basic knowledge of solid dynamics in sewers. However, various aspects still require further research. The French national project ‘SER’ was conducted from 2009 to 2012, in providing the opportunity for new experimental set-ups to enhance knowledge of solid dynamics. The present work has originated from this ‘SER’ project and benefited from the various research tasks completed. The experimental site and set-up are briefly introduced before displaying the daily dry weather pattern of the muddy layer observed at the top of the deposit. A discussion is provided on the conditions most favourable to the constitution and origin of such a layer. A mass balance is also presented, showing that the muddy layer is moving into the sewer network on a daily basis.

Coude et champs de vitesses en réseaux d'assainissement

ABSTRACT More and more measurement devices are implemented in sewer networks in order to achieves various kind of purposes. Selecting sections for conducting these measurements may prove difficult, as the efficiency of available devices strongly rely on hydraulic conditions. The current research project uses numerical and experimental investigations in order to improve the representativeness of the measurements made in sewer networks by taking account of each site specificities. Our purpose is to create an aid for designing the instrumentation of sewer sections. This paper presents the application to the velocity fields downstream of a bend.