Jose M. Redondo

An Open Channel Flow Experimental and Theoretical Study of Resistance and Turbulent Characterization over Flexible Vegetated Linings

Hydraulic engineers and scientists working on river restoration recognize the needfor a deeper understanding of natural streams as a complex and dynamicsystem, which involves not only abiotic elements (flow, sediments) but alsobiotic or biological components. From this point of view, the role played byriverine vegetation in river dynamics and flow conditions becomesessential. Hydro-mechanic interaction between flow and flexible plantscovering a river bed is studied in this paper and some previous works arediscussed. Experimental tests and measurements of turbulence on the flow in anopen channelwere performed using plastic plants seeded in a gravel bed. Characterization of flowresistance (friction factors) due to vegetation flexible roughness fordifferent plant densities was attained; furthermore, measuring detailedturbulent velocityprofiles within and above submerged and flexed stems allowed us to distinguishdifferent turbulent regimes. Some interesting relationships wereobtained between the velocity field and the deflected height of the plants, suchas a linear fit between the non-dimensional flexural parameter and the relativedeflection of the plants. Turbulent stresses weremeasured showing two different regions: above and inside the vegetationdomain. The spectral interaction between the plant oscillations, their wakes and theturbulence at different heights, forces strongly anisotropic Reynolds tensors andin order to clarify turbulent processes and their complexstructure, theoretical concepts (Taylor, Kolmogorovs K41) and several dataanalysis (autocorrelation functions, integral scales) were applied.

Marine pollution in European coastal waters monitored by the ERS-2 SAR: a comprehensive statistical analysis

The oil pollution of the southern Baltic Sea, the North Sea, and the northwestern Mediterranean Sea has been studied within a two-year period from December 1996 until November 1998. In total, the authors have analyzed more than 700 synthetic aperture radar (SAR) images, which have been acquired over the three test areas by the Second European Remote Sensing Satellite (ERS-2). The authors present the results of their statistical analyses. Moreover, the authors introduce advanced image processing techniques for the classification of the observed radar signatures, namely the calculation of the fractal dimension, and discuss the possibility of identifying different types of oceanic phenomena.

Grid stirred turbulence: applications to the initiation of sediment motion and lift-off studies

Abstract A laboratory experimental set - up for studying the behaviour of sediment in presence of a turbulent field with zero mean flow is presented. Particular interest is shown on the initiation of sediment motion and in the sediment lift - off. Some examples of the results obtained with this set-up are shown. A turbulent velocity u′ lower than that u estimated by the Shield diagram is required to start sediment motion. The minimum u′ required to start sediment lift - off, is a function of sediment size, cohesivity and resting time. The lutocline height depends on u′, and the vorticity at the lutocline seems constant for a fixed sediment size. Combining grid stirring and image analysis, sediment vertical fluxes and settling speeds could be measured.

Structure Functions in Complex Flows

The turbulence in the ocean and atmosphere is most of the time non-homogeneous in nature. These spatial changes could affect the structure of the turbulence. In this work a classification is proposed to determine the intermittency and mixing ability. The variation of the structure functions and the scaling exponent in decaying non-homogeneous turbulence produced by a grid and by a jet is measured with a sonic velocimeter SONTEK3-D. We use Extended Self Similarity (ESS) to obtain better estimates of the scaling exponents of the structure functions of order up to the 6th. We study the variation of the absolute scaling exponents ζp and relative scaling exponents ¯ζp as a function of distance from the source of turbulence. In most cases, the absolute scaling exponent ζ3 is shown to vary as function of the separation distance l. On the other hand the relative scaling exponents ¯ζp depend on the location of the flow and in most cases the deviations from the Kolmogorov 1941 scaling are related to the intermittency.

Mixing in horizontally heterogeneous flows

An experimental study of mixing across density interfaces produced by laterally heterogenous turbulence is presented in this paper. The turbulence is generated by a flow or air bubles rising through a density interface produced by brine and fresh water. The mixing efficiency, η, of the process is measured comparing the increase in potential energy with the available kinetic energy. We find that there is a decrease in the global mixing efficiency of the process with the length of the tank, the shape of η(Ri) depends also on the air flow producing the turbulence, showing a geometrical limit to the ammount of kinetic energy which may be used for mixing.

The Shear Layer above and in Urban Canopies

Abstract The nature and role of the shear layer, which occurs at the level of the average building height in urban canopies, are poorly understood. Velocity data are analyzed to determine the characteristics of the shear layer of the urban canopy, defined as the broad, linear segment of the mean velocity profile in a region of high shear. Particle image velocimetry measurements in a water tunnel were undertaken to resolve velocity profiles for urban canopies of two geometries typical of Los Angeles, California, and New York City, New York, for which the aspect ratios (average building height-to-width ratio) H/wb are 1 and 3, respectively. The shear layers evolve with distance differently: For H/wb = 1 the urban canopy shear layer extends quickly from above the building height to ground level, whereas for H/wb = 3 the urban canopy shear layer remains elevated at the vicinity of the building height, only reaching to a depth of z/H ∼ 0.5 far downstream. Profiles of the mean velocity gradient also differ from...

Coastal and interfacial mixing. Laboratory experiments and satellite observations

Abstract Studies of mixing in coastal waters have been made using different parameters like the Rossby Deformation radius, the Rossby number and the flux and gradient Richardson numbers. In order to describe the structure of density interfaces and fronts and their hydrodynamic behavior, we have analyzed in laboratory conditions different mixing processes near density interfaces to investigate experimentally the structure of these interfaces. The behaviour of both horizontal and vertical mixing fronts is studied and compared. We use a video analysis technique where mixing processes are recorded and analyzed by means of the DigImage software. We present in our analysis the evolution of the fronts and the local velocity fields, in term of the local gradient Richardson number in both vertical and horizontal mixing events. The horizontal structures are also compared with satellite observations of coastal induced eddies (Platonov and Redondo, 1999).

Observations From Grid Stirred Turbulence. Grid Characterization and Application to Sediment Lift-Off Experiments

The characterization of sediment behavior is very important in coastal dynamics, deep sea topography, global models of circulation, etc. This paper presents some experimental techniques used to study sediments extracted from the sea. A stirring grid is used to obtain a well-known turbulent velocity field which interacts with the sediment bed. The velocity field is modeled from measurements with Acoustic Doppler Velocimetry, and compared with classical results.

Montaje experimental para el estudio de la mezcla generada por penachos turbulentos

A simple laboratory model of turbulent mixing between two miscible fluids under an initial situation of top heavy stratification in a gravitational field has been performed. The mixing processes are generated by the evolution of a discrete set of unstable forced turbulent plumes. We describe the experimental setup, the laboratory instruments and the way the mixing processes are. We also describe the characteristic and construction of the instruments that are classified in 3 groups: fluid flow instruments, electric instruments and video-digitalisation instruments. This laboratory model lets the corresponding turbulent mixing processes measuring the density profiles and the heights of the fluid layers by means of shadowgraph and direct flow visualization. words: Turbulent mixing; turbulent plumes; experimental setup; conductivity probes; shadowgraph; venturimeter; digitalisation.

CONVECTIVE AND RAYLEIGH-TAYLOR INSTABILITIES IN STRATIFIED FLUIDS

Lateral variations of Buoyancy occur frecuently in geophysical situations and local accelerations of diferent density fluid regions produce Rayleigh-Taylor instability at the interfaces. Detailled flow visualization and point density and temperature measurements are used in a laboratory experiment where convective instability is produced by setting a heat flux distribution using thermoelectric elements by means of Peltier effect cooling and heating. The interaction of the local convective instabilities with a density interface is studied in both stable and unstable situations. Reynolds and Prandtl number variations are studied using different fluids. In the high Re regime, turbulent mixing occurs across the density interface. In these cases the flow patterns are also studied using a numerical model using a two dimensional Boussinesq approximation with sub-grid turbulent parametrization based in a kinematic simulation.