Physical modeling of unsteady turbulence in breaking tidal bores

A tidal bore is an unsteady flow motion generated by the rapid water level rise at the river mouth during the early flood tide under macrotidal and appropriate bathymetric conditions. This paper presents a study that physically investigates the turbulent properties of tidal bores. Results from some experimental measurements of free-surface fluctuations and turbulent velocities conducted on smooth and rough beds are reported. The free-surface measurements were conducted with Froude numbers of 1-1.7. Both undular and breaking bores were observed. Using an ensemble-averaging technique, the free-surface fluctuations of breaking tidal bores are characterized. Immediately before the roller, the free-surface curves gradually upwards. The passage of the bore roller is associated with some large water elevation fluctuations; the largest free-surface fluctuations are observed during the first half of the bore roller. The turbulent velocity measurements were performed at several vertical elevations during and shortly after the passage of breaking bores. Both the instantaneous and ensemble-averaged velocity data highlight a strong flow deceleration at all elevations during the bore passage. Close to the bed, the longitudinal velocity component becomes negative immediately after the roller passage, implying the existence of a transient recirculation. The height and duration of the transient are a function of the bed roughness, with a higher and longer recirculation region above the rough bed. The vertical velocity data presented some positive, upward motion beneath the front with increasing maximum vertical velocity with increasing distance from the bed. The transverse velocity data show some large fluctuations with nonzero ensemble average after the roller passage that highlight some intense secondary motion advected behind the bore front. DOI: 10.1061/(ASCE)HY.1943-7900.0000542. (C) 2012 American Society of Civil Engineers

Turbulence and aeration in hydraulic jumps: free-surface fluctuation and integral turbulent scale measurements

In an open channel, a change from a supercritical to subcritical flow is a strong dissipative process called a hydraulic jump. Herein some new measurements of free-surface fluctuations of the impingement perimeter and integral turbulent time and length scales in the roller are presented with a focus on turbulence in hydraulic jumps with a marked roller. The observations highlighted the fluctuating nature of the impingement perimeter in terms of both longitudinal and transverse locations. The results showed further the close link between the production and detachment of large eddies in jump shear layer, and the longitudinal fluctuations of the jump toe. They highlighted the importance of the impingement perimeter as the origin of the developing shear layer and a source of vorticity. The air–water flow measurements emphasised the intense flow aeration. The turbulent velocity distributions presented a shape similar to a wall jet solution with a marked shear layer downstream of the impingement point. The integral turbulent length scale distributions exhibited a monotonic increase with increasing vertical elevation within 0.2 < Lz/d1 < 0.8 in the shear layer, where Lz is the integral turbulent length scale and d1 the inflow depth, while the integral turbulent time scales were about two orders of magnitude smaller than the period of impingement position longitudinal oscillations

Air–water interface dynamic and free surface features in hydraulic jumps

Following previous experimental works (Mouaze et al., J. Fluids Engng. ASME 127(6) (2005) 1191–1193; Murzyn et al., Int. J. Multiphase Flow, 31(1) (2005) 141–154) on gas fraction characterisation in hydraulic jumps, the present paper aims to investigate free surface properties developing in these flows. Indeed, air–water interface exhibits a wide range of frequencies that free surface probes are not always enough accurate to follow. As a solution, two new free surface wire gauges (thin wires, ? = 0.05 mm, 1mm apart) have been built and calibrated. Accurate measurements of free surface fluctuations have been obtained with frequency resolution up to 12 Hz. Two sets of experiments have been made depending on Froude (Fr) and Weber (We) numbers. Experimental results showthat free surface mean and turbulent profiles exhibit discontinuities at the front of the toe characterized by a sudden high level of turbulence downstream followed by a zone of dissipation. Furthermore, free surface length scales have been estimated from correlation measurements. Similar features are found according to Fr andWe. Good agreement is shown with results deduced from a video analysis technique (Mouaze et al., J. Fulids Engng. ASME 127(6) (2005) 1191–1193). Lastly, comparison is made with Brocchini and Peregrine (Brocchini and Peregrine, J. Fluid Mech. 449 (2001a) 225–254; 449 (2001b) 255–290) depicting similar shapes for behaviour of vertical velocity fluctuations (??) as a function of longitudinal free surface length scal

Turbulence at free surface in hydraulic jumps

In the context of recent work by Brocchini &amp; Peregrine [1,2], this paper aims to document free surface profiles, and turbulence length scales in hydraulic jumps with Froude numbers between 1.98 and 4.82. Although information on bubble size, frequency and velocities in hydraulic jumps is available in the literature, there is not much data on the featuresof the free surface, or on mixing layer thickness. In the present case, measurements at the free surface have been realized with two miniature resistive wire gauges each comprising two parallel 50 micron diameter wires with a separation of 1mm. These instruments were calibrated dynamically over a range of frequencies up to 20 Hz. Furthermore optical probes were used to measure properties of the air phase within the jump, including void fractions (up to 98%). The present results extend the range of Froude numbers for which two-phase measurements in hydraulic jumps are available, and, in most respects, confirm earlier results obtained with different experimental techniques. Length scales at the free surface are deduced from cross-correlation analysis of wire gauge measurements, and are compared with similar data obtained from images of the surface

Flow visualisation and free surface length scales measurements in a horizontal jet beneath a free surface

In the context of the recent studies on turbulence/free surface interactions of Brocchini and Peregrine [M. Brocchini, D.H. Peregrine, The dynamics of strong turbulence at free surfaces. Part 1. Description, J. Fluid Mech. 449 (2001) 225–254; M. Brocchini, D.H. Peregrine, The dynamics of strong turbulence at free surfaces. Part 2. The boundary conditions, J. Fluid Mech. 449 (2001) 255–290] and Murzyn et al. [F. Murzyn, D. Mouaze, J.R. Chaplin, Optical fibre probe measurements of bubbly flow in hydraulic jumps, Int. J. Multiphase Flow 31(1) (2005) 141–154], this paper presents flow visualizations and free surface measurements of the flow generated at the free surface by a horizontal jet beneath this air/water interface. The main goal is focused on the free surface motion. The shadowgraph technique is used for the first part of this study and two wire gauges have been built to describe the free surface oscillations. Description of typical free surface length scales and frequencies are also obtained and shown through a correlation technique for different positions of the jet beneath the air/water interface. Comparisons of the flow visualizations with free surface length scales confirm that this shadowgraph technique is well defined for this kind of flows and very promising. Front velocity of the jet is deduced from the pictures taken with a digital camera and a comparison is made with theoretical work of Brocchini and Peregrine [M. Brocchini, D.H. Peregrine, The dynamics of strong turbulence at free surfaces. Part 1. Description, J. Fluid Mech. 449 (2001) 225–254; M. Brocchini, D.H. Peregrine, The dynamics of strong turbulence at free surfaces. Part 2. The boundary conditions, J. Fluid Mech. 449 (2001) 255–290] to define the nature of the flow

Optical fibre probe measurements of bubbly flow in hydraulic jumps

This paper describes measurements of void fractions, bubble frequencies and bubble sizes in hydraulic jumps with Froude numbers 2.0, 2.4, 3.7 and 4.8. In each case data were obtained with a dual-tip optical fibre probe at a large number of points throughout the jump. Across the lower part of the flow, dominated by air entrainment into a region of turbulent shear, void fractions follow a Gaussian distribution. In the upper region, dominated by interactions with the free surface, the void fraction follows the form of an error function. The intersection between these two profiles provides a well-defined boundary between the two regions. Comparisons are made with measurements at higher Froude numbers by Chanson &amp; Brattberg (2000) revealing a very large measure of compatibility between the two sets of data

Turbulence at the free surface

International audienc

Free surface length scale estimation in hydraulic jumps

In this paper we aim to document free surface elevations and free surface length scales in hydraulic jumps with Froude numbers between 1.98 and 4.82. Although information on bubble size, frequency, and velocities in hydraulic jumps is available in the literature, there is not much data on the features of the free surface, or on mixing layer thickness. In the present case, measurements at the free surface have been realized with two "homemade" miniature resistive wire gauges made of two parallel 50 mu m diameter wires 1 mm apart. These instruments were calibrated dynamically over a range of frequencies up to 20 Hz. The present results extend the range of Froude numbers for which two-phase measurements in hydraulic jumps are available. In most respects, it confirms earlier results obtained with different experimental techniques. Length scales at the free surface are deduced front a cross-correlation analysis of wire gauge measurements, and are compared with similar data obtained from images of the surface

Globex: wave dynamics on a gently sloping laboraty beach

As waves approach the shore, non-linearity in their dynamics becomes increasingly important. Most of our understanding of wave non-linearity has resulted from theoretical work, laboratory experiments and field studies on beaches slopes steeper than about 1:40. Here, very strong non-linear processes happen locally and on a short time scale, as demonstrated by narrow surf zones with plunging or collapsing breakers. The non-linearity on lower sloping beaches, typical of high-energy dissipative environments, has a different character, as it can now build up over a long period of time in a cross-shore extensive area. This second case of strong non-linearity is not well understood. This contribution serves to introduce the GLOBEX project, during which a high-resolution (in space and time) data set of the cross-shore evolution of short and infragravity waves was collected on a low-sloping (1:80) non-mobile laboratory beach for a range of wave conditions. Various other presentations at the conference will build on this introductory contribution

Near-Infrared Spectroscopy Technology for Soil Nutrients Detection Based on LS-SVM

Part 1: Decision Support Systems, Intelligent Systems and Artificial Intelligence ApplicationsInternational audienceThe detection method of the soil nutrients (organic matter and available N, P, K) were analyzed based on the near infrared spectroscopy technology in order to decision-making for precision fertilization. 54 samples with 7m×7m was collected using DGPS receiver positioning in a soybean field. The soil organic matter, available nitrogen (N), available phosphorus (P), available potassium (K) content was determined, the near-infrared diffuse reflectance spectrum of the soil samples were obtained by FieldSpec3 spectrometer. 54 samples were randomly divided into 40 prediction sets and 14 validation sets. After smoothing, the eight principal components of original spectra were extracted by principal component analysis (PCA). Prediction model of soil organic matter, available nitrogen (N), available phosphorus (P), potassium (K) were respectively established with the eight principal component as input and soil nutrients by measured as the output, and the 14 validation samples were predicted. The results showed that the soil organic matter, available nitrogen (N), available phosphorus (P), potassium (K) prediction model were set up with principal component analysis and LS-SVM, which the correlation coefficients between the prediction value and measurement value were 0.8708, 0.7206, 0.8421 and 0.6858, the relative errors of the LS-SVM prediction was smaller and those mean values were 1.09%, 1.06%, 4.08% and 0.69%. The method of soil organic matter content prediction is feasible