PhD from Barcelona University in Physics and from DAMTP, University of Cambridge (1987,1990). Professor of Applied Physics at UPC since 1991. Director of the Fluid Dynamics Laboratory. Chairman of the ERCOFTAC SIG14 on Stratified and Rotating Flows. Secretary of The Non Linear processes in Geophysics Division of the European Geosciences Union. Since 2009, Coordinator of the Pan European Laboratory on Non-Homogeneous Turbulence.
Abstract: The spontaneous formation of dendritic aggregates is observed in a two-dimensional confined layered system consisting of a film composed of liquid crystal, dye and solvent cast above a polymer substrate. The observed aggregates are promoted by phase separation processes induced by dye diffusion and solvent evaporation. The growth properties of the aggregates are studied through the temporal evolution of their topological properties (surface, perimeter, fractal dimension). The fractal dimension of the completely formed structures, when they are coexistent with different types of structures, is consistent with theoretical and experimental values obtained for Diffusion-Limited Aggregates. Under different experimental conditions (temperature and local dye concentration) the structure forms without interactions with other kinds of structures, and its equilibrium fractal dimension is smaller. The fractal dimension is thus not a universal property of the observed structures, but rather depends on the experimental conditions.
Abstract:
Departament de Fisica Aplicada, Universidad Politecnica de Catalunya C/ J G Salgado s/n, Campus Nord, Modul B-4, E-08034, Barcelona, Spain
E-mail: redondo@fa.upc.es
Abstract. Marine pollution has been highlighted thanks to the advances in detection techniques as well as increasing coverage of catastrophes (e.g. the oil tankers Amoco Cadiz, Exxon Valdez, Erika, and Prestige) and of smaller oil spills from ships. The new satellite based sensors SAR and ASAR and new methods of oil spill detection and analysis coupled with self-similar statistical techniques allow surveys of environmental pollution monitoring large areas of the ocean. We present a statistical analysis of more than 700 SAR images obtained during 1996–2000, also comparing the detected small pollution events with the historical databases of great marine accidents during 1966–2004 in European coastal waters. We show that the statistical distribution of the number of oil spills as a function of their size corresponds to Zipf's law, and that the common small spills are comparable to the large accidents due to the high frequency of the smaller pollution events. Marine pollution from tankers and ships, which has been detected as oil spills between 0.01 and 100 km2, follows the marine transit routes. Multi-fractal methods are used to distinguish between natural slicks and spills, in order to estimate the oil spill index in European coastal waters, and in particular, the north-western Mediterranean Sea, which, due to the influence of local winds, shows optimal conditions for oil spill detection.
Abstract: The difficulty in developing models for waves in turbulent flows is a
key problem in the analysis of the complexity of turbulence. We present a method
to find and filter perturbations that are generated by the flow of deterministic waves
from the power spectrum in the atmospheric boundary layer (ABL). The perturba-
tion model proposed assumes that the amplitude and frequency of such waves decay
with time exponentially. For illustrative purposes, we apply the technique to three
time series of wind velocities obtained with a sonic anemometer. This analytical
procedure allows us to filter waves of the proposed structure with a 99% significance
level in the power spectrum. We have applied the same method to 540 such wind
series, all painting similar results. We then compare the fractal dimension of the
original series to those from which the waves have been removed. We find that
the fractal dimension of the filtered waves is slightly less than that of the original
series. Finally, we consider the fractal dimension of the studied series as a function
of the length-scales and dissipation rate of kinetic energy per unit mass. Our results
suggest an increase of fractal dimension with both length-scale and dissipation rate
of kinetic energy.
Abstract: Spatial and temporal variations in the distribution of the marine picoplanktic cyanobacteria population and mixing conditions were found in
the Ebro River estuary outflow to the Mediterranean Sea in Spain. Six sampling surveys were undertaken between July 1999 and February 2000
for distances up to 15 km from the river mouth. Measurements were taken of flow velocity, salinity, temperature, depth and picocyanobacteria
(PCB) abundances. Gradient Richardson (Rig) and Reynolds (Re) numbers were determined to evaluate hydrodynamics. In summer, large values
of Rig arise from the small flow rates, and small values of velocity shear between the surface fresh water layer and the bottom saline layer;
conversely, in winter the large flow rates and attendant large velocity shears between the layers give rise to small values of Rig. Flow conditions
in the fall are an intermediate case between the summer and winter cases. Vertical abundance distributions were resolved through the river water,
interfacial region, and the bottom salt wedge; longitudinal gradients of PCB abundances were also resolved. Seasonal differences in the PCB
abundance values were observed. Analysis of cell numbers (C) showed that the variable dC/dS, the dependence of cell number upon salinity
gradient (dS ) was critical. A non-dimensional number; the cellular gradient number (Cg) is introduced. Cg has useful biological interpretations
that can potentially be included in ecological modeling. For example, Cg ¼ 1 pertains to perfect adaptability of the organism to adjust to changing
environmental conditions, whereas Cg ¼ 0 describes total mortality. For a system with strong advection there is insufficient time for cells to
adapt to the changing environment, and so those cell counts are unchanged. This is the case for the Ebro estuary in winter as advection of salinity
(and hence PCB abundance) dominates the other loss processes for large flow rates.
Abstract: Two sets of turbulence decaying experiments have been performed with the aim of focusing in the middle of a strungly stratified density interface. The experiments have been done under two different external conditions: a) stirring (non-rotating) decaying 2D turbulence experiments and b) rotating decaying 2D turbulence experiments.
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 each other for urban canopies associated with H/wb
of 1 or 3. Values of shear dU/dz increase toward ground level for an urban canopy associated with H/wb
1. For an urban canopy associated with H/wb 3, localized peaks of shear dU/dz exist at the building height
and at ground level, with values of shear decreasing to zero at building midheight and far above the building
height. A consequence of the different forms of the shear layers of the two urban canopies is that the
ground-level dispersion coefficient is likely to be greater for urban canopies associated with H/wb 1 than
for those associated with H/wb 3 because of an increased ventilation and exchange mechanism for cities
such as Los Angeles relative to cities such as New York City that possess urban canyons.
Abstract: Turbulent mixing is a very important issue in the study of geophysical phenomena because most fluxes
arising in geophysics fluids are turbulent. The diffusion of physical quantities in nature is due to turbulence.
This is a key factor in the study of atmospheric phenomenae, specially at the planetary boundary layer.
We study turbulent mixing due to convection using a laboratory experimental model with two miscible
fluids of different density with an initial top heavy density distribution. Mixing is produced by the
evolution of a two-dimensional array of forced turbulent plumes. Measurements include density data
and flow visualization. The conclusions of this experimental model relate the mixing efficiency and the
volume of the final mixed layer to the Atwood number, ranging from 0.010 to 0.134. We find that the
number of plumes diminishes, the mixing efficiency reduces. We performed additional experiments
releasing a fixed volume with negative buoyancy in a lighter fluid as a line of plumes (from one to
nine) with a fixed Atwood number of 0.03, measuring the height of the final mixed layer as functions
of the number of plumes and the available mixing volume.
Abstract: Hydraulic engineers and scientists working on river restoration recognize the need for
a deeper understanding of natural streams as a complex and dynamic system, which involves not
only abiotic elements (flow, sediments) but also biotic or biological components. From this point of
view, the role played by riverine vegetation in river dynamics and flow conditions becomes essential.
Hydro-mechanic interaction between flow and flexible plants covering a river bed is studied in this
paper and some previous works are discussed. Experimental tests and measurements of turbulence
on the flow in an open channel were performed using plastic plants seeded in a gravel bed. Characterization
of flow resistance (friction factors) due to vegetation flexible roughness for different plant
densities was attained; furthermore, measuring detailed turbulent velocity profiles within and above
submerged and flexed stems allowed us to distinguish different turbulent regimes. Some interesting
relationships were obtained between the velocity field and the deflected height of the plants, such as
a linear fit between the non-dimensional flexural parameter and the relative deflection of the plants.
Turbulent stresses were measured showing two different regions: above and inside the vegetation
domain. The spectral interaction between the plant oscillations, their wakes and the turbulence
at different heights, forces strongly anisotropic Reynolds tensors and in order to clarify turbulent
processes and their complex structure, theoretical concepts (Taylor, Kolmogorov’s K41) and several
data analysis (autocorrelation functions, integral scales) were applied.
Abstract:
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.
Abstract: A series of novel image analysis techniques have been used to study surf-zone hydrodynamics taking advantage of recent advances in digital processing of images taken from video recordings of the sea surface near the coast. The use of image analysis allows the estimation of both spatial and temporal characteristics of wave fields, surface circulation and mixing in the surf zone. The dispersion of blobs of dye released at different distances from the coastline under very different sea conditions is used to measure surface eddy diffusivities. A preliminary set of field measurements were done in the Ebro Delta where the methodology was tested. Further experiments have been performed at Vilanova, Spain and Recife, Brazil.
There is an increase of diffusivity with wave height but only if the wave Reynolds number, Rw, is greater than 103. No such trend is observed for Rw greater than 106. The other important factors are wind speed and tidal and longshore currents.
Abstract:
We describe the presentations at the international meeting/workshop on Mixing in Geophysical Flows that took place at Vilanova i la Geltru, near Barcelona during the 20, 21 and 22nd of March 1997. There were more than 100 participants from 20 countries with 66 oral and poster presentations covering experimental and theoretical aspects of rotating and stratified fluids as well as field observations. The main topics discussed at the workshop were stratified flows, rotating stratified flows, gravity waves, instabilities and mixing, convection, experiments and numerical simulations of Geophysical flows and turbulent mixing. The papers are summarised in this report giving a state-of-the-art overview of present research in geophysical turbulent mixing.
Abstract: Probability distribution of basic instabilities appearing in stratified flows and point
density fluctuations have been studied. Various parameters of the mixing process have been
changed in the experiments, to investigate mixing. Detailed flow visualization as well as
density measurements have been used in zero-mean-flow laboratory experiments involving
grid-stirred turbulent mixing across a density interface and bubble-induced mixing. The
overall mixing efficiency of the processes depends on the local Richardson number as well
as on the local vorticity. Parameter distributions of low and high mixedness corresponding
to different instabilities are presented, showing that dipolar vortices penetrating the
interface are the most efficient mixing instabilities.
Notes: Britter, R.E., 1984. Diffusion and decay in stably-stratified turbulent flows. In: J.C.R. Hunt (Editor),
Turbulence and Diffusion in Stable Environments. Clarendon Press, Oxford, pp. 13-24.
E, X. and Hopfinger, E.J., 1986. On mixing across an interface in stably stratified fluid. J. Fluid Mech.,
166: 227-240.
Fernando, H.J.S., 1991. Turbulent mixing in stratified fluids. Annu. Rev. Fluid Mech., 23: 455-493.
Hopfinger, E.J., 1987. Turbulence in stratified fluids, a review. J. Geophys. Res., 92: 5287-5302.
Itsweire, E.C., Helland, K.N. and van Atta, C.W., 1986. The evolution of grid-generated turbulence in a
stably stratified fluid. J. Fluid Mech., 162: 299-338.
Koop, C.G. and Browand, F.K., 1979. Instability and turbulence in a stratified fluid with shear. J. Fluid
Mech., 135: 135-147.
Linden, P.F., 1975. The deepening of a mixed layer in a stratified fluid. J. Fluid Mech., 71: 385-405.
Linden, P.F., 1980. Mixing across a density interface produced by grid turbulence. J. Fluid Mech., 100:
3-29.
McEwan, A.D., 1983a. Internal mixing in stratified fluids. J. Fluid Mech., 128: 59-80.
McEwan, A.D., 1983b. The kinematics of stratified mixing through internal wave breaking. J. Fluid
Mech., 128: 47-58.
Redondo, J.M., 1987. Difusion turbulenta en fluidos estratificados. Ph.D. Thesis, Univ. Barcelona.
Redondo, J.M. and Cantalapiedra, I.R., 1993. Mixing in horizontally heterogeneous flows. Appl. Sci.
Res., 51: 217-222.
Rohr, J.J., Itsweire, E.C. and van Atta, C.W., 1984. Mixing efficiency in stably stratified decaying
turbulence. Geophys. Astrophys. Fluid Dyn., 29: 221-226.
Turner, S.T., 1973. Buoyancy Effects in Fluids. Cambridge University Press.
Abstract: The oil pollution of Gulf of Lion in the NW Mediterranean has been studied with SAR
images during the period 1999 -2005. We have analyzed these SAR images with respect
to other surface features such as wind, river plumes, eddies and convergence areas.
Some results of our statistical analysis are presented showing that the NW
Mediterranean is most polluted along the main ship traffic routes, but comparatively
less that near other routes in the Indic and the Pacific. The oils spill index is higher than
one. The sizes of the detected oil spills vary over a large range, and if the statistics of
the largest accidents are also considered on a longer timescale, we show that Zipf's Law,
relating the frequency and the size of the spill in a hyperbolic fashion is applicable.
Advanced image analysis techniques, such as the calculation of the multi-fractal
dimensions of the observed SAR signatures, have been applied to distinguish between
natural slicks and antropogenic spills. Fractal dimensions can also be used to predict
the time of release of the spill, non-dimensionalised with local turbulent dissipation.
The multi-scale appearance and the topological structure of the slicks and spills may
also be used as a useful measure of the diffusivity, yielding additional information
which in turn may improve automated detection algorithms and be used in numerical
models.
Abstract: 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). They present the results of their statistical analyses. Moreover, they 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
Abstract:
Vertical microstructure and mixing in zero mean flows are investigated. A microconductivity probe described in Redondo (1987) with a DANTEC micropositioner is used to measure the evolution of the density profiles. The evolution of the microlayer formation as the turbulence decays is found to be different for the different solutes (heat, salt, and sugar), showing vertical scales between the rod diameters and the Ozmidov scale.
