58 research outputs found
Llum i ones electromagnètiques
Els coneixements que l’home ha adquirit durant els dos darrers segles, en els camps de la
ciència i de la tècnica li han augmentat la capacitat de modificar les pròpies condicions de vida en
un grau molt més alt del que s’havia aconseguit en tota la resta de la història de la humanitat.
Són moltes les ciències i les tecnologies que neixen durant els segles XIX i XX; i són molt
grans, i qualitativament importants, els canvis que experimenten les disciplines que ja tenien història.
L’Òptica FÃsica, l’estudi de la naturalesa de la llum i la seva interacció amb la matèria, és una
bona mostra d’aquestes progressions espectaculars. El coneixement de la naturalesa electromagnètica
de la llum ha conduït, en molt pocs anys, a la rà dio, a la televisió, als models actuals de l’estructura
de la matèria, a la cosmologia moderna, a l’energia nuclear i a les noves concepcions relativistes
de l’espai i del temps
A subcritical instability of wave-driven alongshore currents
The development of shear instabilities of a wave-driven alongshore current is
investigated. In particular, we use weakly nonlinear theory to investigate the possibility
that such instabilities, which have been observed at various sites on the U.S. coast and in
the laboratory, can grow in linearly stable flows as a subcritical bifurcation by resonant
triad interaction, as first suggested by Shrira eta/. [1997]. We examine a realistic
longshore current profile and include the effects of eddy viscosity and bottom friction. We
show that according to the weakly nonlinear theory, resonance is possible and that these
linearly stable flows may exhibit explosive instabilities. We show that this phenomenon
may occur also when there is only approximate resonance, which is more likely in
nature. Furthermore, the size of the perturbation that is required to trigger the instability is
shown in some circumstances to be consistent with the size of naturally occurring
perturbations. Finally, we consider the differences between the present case examined and
the more idealized case of Shrira et a/. [ 1997]. It is shown that there is a possibility of
coupling between triads, due to the richer modal structure in more realistic flows,
which may act to stabilize the flow and act against the development of subcritical
bifurcations. Extensive numerical tests are called for
Generation and nonlinear evolution of shore-oblique/transverse sand bars
The coupling between topography, waves and currents in the surf zone may selforganize
to produce the formation of shore-transverse or shore-oblique sand bars on
an otherwise alongshore uniform beach. In the absence of shore-parallel bars, this has
been shown by previous studies of linear stability analysis, but is now extended to the
finite-amplitude regime. To this end, a nonlinear model coupling wave transformation
and breaking, a shallow-water equations solver, sediment transport and bed updating
is developed. The sediment flux consists of a stirring factor multiplied by the depthaveraged
current plus a downslope correction. It is found that the cross-shore profile
of the ratio of stirring factor to water depth together with the wave incidence angle
primarily determine the shape and the type of bars, either transverse or oblique to
the shore. In the latter case, they can open an acute angle against the current (upcurrent
oriented) or with the current (down-current oriented). At the initial stages of
development, both the intensity of the instability which is responsible for the formation
of the bars and the damping due to downslope transport grow at a similar rate with
bar amplitude, the former being somewhat stronger. As bars keep on growing, their
finite-amplitude shape either enhances downslope transport or weakens the instability
mechanism so that an equilibrium between both opposing tendencies occurs, leading
to a final saturated amplitude. The overall shape of the saturated bars in plan view
is similar to that of the small-amplitude ones. However, the final spacings may be
up to a factor of 2 larger and final celerities can also be about a factor of 2 smaller
or larger. In the case of alongshore migrating bars, the asymmetry of the longshore
sections, the lee being steeper than the stoss, is well reproduced. Complex dynamics
with merging and splitting of individual bars sometimes occur. Finally, in the case of
shore-normal incidence the rip currents in the troughs between the bars are jet-like
while the onshore return flow is wider and weaker as is observed in nature
Self-organization mechanisms for the formation on nearshore crescentic and transverse sand bars
The formation and development of transverse and crescentic sand bars in the coastal
marine environment has been investigated by means of a nonlinear numerical model
based on the shallow-water equations and on a simpli ed sediment transport parameterization.
By assuming normally approaching waves and a saturated surf zone,
rhythmic patterns develop from a planar slope where random perturbations of
small amplitude have been superimposed. Two types of bedforms appear: one is a
crescentic bar pattern centred around the breakpoint and the other, herein modelled
for the rst time, is a transverse bar pattern. The feedback mechanism related to the
formation and development of the patterns can be explained by coupling the water
and sediment conservation equations. Basically, the waves stir up the sediment and
keep it in suspension with a certain cross-shore distribution of depth-averaged concentration.
Then, a current flowing with (against) the gradient of sediment concentration
produces erosion (deposition). It is shown that inside the surf zone, these currents
may occur due to the wave refraction and to the redistribution of wave breaking
produced by the growing bedforms. Numerical simulations have been performed in
order to understand the sensitivity of the pattern formation to the parameterization
and to relate the hydro-morphodynamic input conditions to which of the patterns
develops. It is suggested that crescentic bar growth would be favoured by high-energy
conditions and ne sediment while transverse bars would grow for milder waves and
coarser sediment. In intermediate conditions mixed patterns may occur
What determines the wavelength of self-organized shoreline sand waves?
Shoreline undulations extending into the bathymetric contours with a length scale larger than that of the rhythmic surf zone bars are referred to as shoreline sand waves. Many observed undulations along sandy coasts display a wavelength in the order 1-7 km. Several models that are based on the hypothesis that sand waves emerge from a morphodynamic instability in case of very oblique wave incidence predict this range of wavelengths. Here we investigate the physical reasons for the wavelength selection and the main parametric trends of the wavelength in case of sand waves arising from such instability. It is shown that the existence of a minimum wavelength depends on an interplay between three factors affecting littoral drift: (A) the angle of wave fronts relative to local shoreline, which tends to cause maximum transport at the downdrift flank of the sand wave, (B) the refractive energy spreading which tends to cause maximum transport at the updrift flank and (C) wave focusing (de-focusing) by the capes (bays), which tends to cause maximum transport at the crest or slightly downdrift of it. Processes A and C cause decay of the sand waves while process B causes their growth. For low incidence angles, B is very weak so that a rectilinear shoreline is stable. For large angles and long sand waves, B is dominant and causes the growth of sand waves. For large angles and short sand waves C is dominant and the sand waves decay. Thus, wavelength selection depends on process C, which essentially depends on shoreline curvature. The growth rate of very long sand waves is weak because the alongshore gradients in sediment transport decrease with the wavelength. This is why there is an optimum or dominant wavelength. It is found that sand wave wavelength scales with λ0/β where λ0 is the water wave wavelength in deep water and β is the mean bed slope from shore to the wave base
Frontshear and backshear instabilities of the mean longshore current
An analytical model based on Bowen and Holman [1989] is used to prove the
existence of instabilities due to the presence of a second extremum of the background
vorticity at the front side of the longshore current. The growth rate of the so-called
frontshear waves depends primarily upon the frontshear but also upon the backshear and
the maximum and the width of the current. Depending on the values of these parameters,
either the frontshear or the backshear instabilities may dominate. Both types of waves
have a cross-shore extension of the order of the width of the current, but the frontshear
modes are localized closer to the coast than are the backshear modes. Moreover, under
certain conditions both unstable waves have similar growth rates with close wave numbers
and angular frequencies, leading to the possibility of having modulated shear waves in the
alongshore direction. Numerical analysis performed on realistic current profiles confirm
the behavior anticipated by the analytical model. The theory has been applied to a current
profile fitted to data measured during the 1980 Nearshore Sediment Transport Studies
experiment at Leadbetter Beach that has an extremum of background vorticity at the front
side of the current. In this case and in agreement with field observations, the model
predicts instability, whereas the theory based only on backshear instability fai led to do so
Shoreline sand waves along the catalan coast
The beach of Calella, north of Barcelona, in the Catalan coast, features a series of shoreline sand waves with wavelengths ranging from 700 to 1400 m that match with similar undulations in the -5 m bathymetric line. Historical satellite images from 2002 till 2010 show that these undulations slightly change in time. The wave climate on that stretch of the Catalan coast has a large proportion of waves from the E-NE and from the SW, i.e., with high angles with respect to shore normal rending the shoreline potentially unstable. Here we show that those sand waves might be due to that instability. Model results, both Linear Stability Analysis and nonlinear time evolution, show that the shoreline is nearly at the threshold for instability and that the emergent wavelengths are roughly consistent with the observed ones.Postprint (published version
- …