arXiv subjects

Francesca De Santi

Publications and source records attributed to Francesca De Santi.

9 recordsLinked to original sources

arXiv2023

Diffusion of gravity waves by random space inhomogeneities in pancake-ice fields. Theory and validation with wave buoys and synthetic aperture radar

We study the diffusion of ocean waves by ice bodies much smaller than a wavelength, such as pancakes and small ice floes. We argue that inhomogeneities in the ice cover at scales comparable to that of the wavelength significantly increase diffusion, producing a contribution to wave attenuation comparable to what is observed in the field and usually explained by viscous effects. The resulting attenuation spectrum is characterized by a peak at the scale of the inhomogeneities in the ice cover, which could explain the rollover of the attenuation profile at small wavelengths observed in field experiments. The proposed attenuation mechanism leads to the same behaviors that would be produced by a viscous wave model with effective viscosity linearly dependent on the ice thickness. This may explain recent findings that viscous wave models require a thickness-dependent viscosity to fit experimental attenuation data. Experimental validation is carried out using wave buoy attenuation data and synthetic aperture radar image analysis.

physics.ao-ph↗

arXiv2020

SAR image wave spectra to retrieve the thickness of grease-pancake sea ice using viscous wave models

Young sea ice composed of grease and pancake ice (GPI), as well as thin floes, considered to be the most common form of sea ice fringing Antarctica, is now becoming the 'new normal' also in the Arctic. Investigations to determine how an increase in GPI is affecting the climate in the far north and globally, require specific tools to monitor the GPI's thickness distribution. Directional wave spectra from satellite SAR imagery are used to determine the change in wave dispersion as a wave train enters GPI fields. The ice cover thickness is then estimated by fitting the dispersion data with two models of wave propagation in ice cover ocean: the Keller's model and the close-packing (CP) model. For both models, an empirical constitutive equation for GPI viscosity as a function of the ice thickness is derived and discussed. Examples of GPI thickness retrievals are shown for a Sentinel-1 C band SAR image taken in the Beaufort Sea on 1 November 2015, and three CosmoSkyMed X band SAR images taken in the Weddell Sea on March 2019. The estimated GPI thicknesses are consistent with concurrent SMOS measurements and the available local samplings.

physics.ao-ph↗

arXiv2018

On the ocean wave attenuation rate in grease-pancake ice, a comparison of viscous layer propagation models with field data

The ability of viscous layer models to describe the attenuation of waves propagating in grease-pancake ice covered ocean is investigated. In particular, the Keller's model Keller [1998], the two-layer viscous model [De Carolis and Desiderio 2002] and the close-packing model [De Santi and Olla 2017] are extensively validated by using wave attenuation data collected during two different field campaigns (Weddell Sea, Antarctica, April 2000; western Arctic Ocean, autumn 2015). We use these data to validate the three models by minimizing the differences between the measured and model wave attenuation; the retrieved ice thickness is then compared with measured data. The three models allow to fit the observation data, but with important differences in the three cases. The close-packing model shows good agreement with the data for values of the ice viscosity comparable to those of grease ice in laboratory experiments. For thin ice, the Keller's model performance is similar to that of the close-packing model, while for thick ice much larger values of the ice viscosity are required, which reflects the different ability of the two models to take into account the effect of pancakes. The improvement of performance over the Keller's model achieved by the two-layer viscous model is minimal, which reflects the marginal role in the dynamics of a finite eddy viscosity in the ice-free layer. A good ice thickness retrieval can be obtained by considering the ice layer as the only source in the wave dynamics, so that the wind input can be disregarded.

physics.ao-ph↗

arXiv2017

Limit regimes of ice formation in turbulent supercooled water

A study of ice formation in stationary turbulent conditions is carried out in various limit regimes of crystal growth, supercooling and ice entrainment at the water surface. Analytical expressions for the temperature, salinity and ice concentration mean profiles are provided, and the role of fluctuations in ice production is numerically quantified. Lower bounds on the ratio of sensible heat flux to latent heat flux to the atmosphere are derived and their dependence on key parameters such as salt restitution in freezing and ice entrainment in the water column is elucidated.

physics.flu-dyn↗

arXiv2017

Effect of small floating disks on the propagation of gravity waves

A dispersion relation for gravity waves in water covered by disk-like impurities embedded in a viscous matrix is derived. The macroscopic equations are obtained by ensemble-averaging the fluid equations at the disk scale in the asymptotic limit of long waves and low disk surface fraction. Various regimes are identified depending on the disk radii and the thickness and viscosity of the top layer. Semi-quantitative analysis in the close-packing regime suggests dramatic modification of the dynamics, with orders of magnitude increase in wave damping and wave dispersion. A simplified model working in this regime is proposed. Possible applications to wave propagation in ice-covered ocean is discussed and comparison with field data is provided.

physics.flu-dyn↗

arXiv2016

Dispersive to non-dispersive transition and phase velocity transient for linear waves in plane wake and channel flows

In this study we analyze the phase and group velocity of three-dimensional linear traveling waves in two sheared flows, the plane channel and the wake flows. This was carried out by varying the wave number over a large interval of values at a given Reynolds number inside the ranges 20-100, 1000-8000, for the wake and channel flow, respectively. Evidence is given about the possible presence of both dispersive and non-dispersive effects which are associated with the long and short ranges of wavelength. We solved the Orr-Sommerfeld and Squire eigenvalue problem and observed the least stable mode. It is evident that, at low wave numbers, the least stable eigenmodes in the left branch of the spectrum beave in a dispersive manner. By contrast, if the wavenumber is above a specific threshold, a sharp dispersive to non-dispersive transition can be observed. Beyond this transition, the dominant mode belongs to the right branch of the spectrum. The transient behavior of the phase velocity of small three-dimensional traveling waves was also considered. Having chosen the initial conditions, we then show that the shape of the transient highly depends on the transition wavelength threshold value. We show that the phase velocty can oscillate with a frequency which is equal to the frequency width of the eigenvalue spectrum. Furthermore, evidence of intermediate self-similarity is given for the perturbation field.

physics.flu-dyn↗

arXiv2015

Energy and water vapor transport across a simplified cloud-clear air interface

We consider a simplified physics of the could interface where condensation, evaporation and radiation are neglected and momentum, thermal energy and water vapor transport is represented in terms of the Boussinesq model coupled to a passive scalar transport equation for the vapor. The interface is modeled as a layer separating two isotropic turbulent regions with different kinetic energy and vapor concentration. In particular, we focus on the small scale part of the inertial range as well as on the dissipative range of scales which are important to the micro-physics of warm clouds. We have numerically investigated stably stratified interfaces by locally perturbing at an initial instant the standard temperature lapse rate at the cloud interface and then observing the temporal evolution of the system. When the buoyancy term becomes of the same order of the inertial one, we observe a spatial redistribution of the kinetic energy which produce a concomitant pit of kinetic energy within the mixing layer. In this situation, the mixing layer contains two interfacial regions with opposite kinetic energy gradient, which in turn produces two intermittent sublayers in the velocity fluctuations field. This changes the structure of the field with respect to the corresponding non-stratified shearless mixing: the communication between the two turbulent region is weak, and the growth of the mixing layer stops. These results are discussed with respect to experimental results with and without stratification.

physics.flu-dyn↗

arXiv2012

New observations in transient hydrodynamic perturbations. Frequency jumps, intermediate term and spot formation

Sheared incompressible flows are usually considered non-dispersive media. As a consequence, the frequency evolution in transients has received much less attention than the wave energy density or growth factor. By carrying out a large number of long term transient simulations for longitudinal, oblique and orthogonal perturbations in the plane channel and wake flows, we could observe a complex time evolution of the frequency. The complexity is mainly associated to jumps which appear quite far along within the transients. We interpret these jumps as the transition between the early transient and the beginning of an intermediate term that reveals itself for times large enough for the influence of the fine details of the initial condition to disappear. The intermediate term leads to the asymptotic exponential state and has a duration one order of magnitude longer than the early term, which indicates the existence of an intermediate asymptotics. Since after the intermediate term perturbations die out or blowup, the mid-term can be considered as the most probable state in the perturbation life. Long structured transients can be related to the spot patterns commonly observed in subcritical transitional wall flows. By considering a large group of 3D waves in a narrow range of wavenumbers, we superposed them in a finite temporal window with oblique and longitudinal waves randomly delayed with respect to an orthogonal wave which is supposed to sustain the spot formation with its intense transient growth. We show that in this way it is possible to recover the linear initial evolution of the pattern characterized by the presence of longitudinal streaks. We also determined the asymptotic frequency and phase speed wavenumber spectra for the channel and wake flows. The mild dispersion observed can in part explain the different propagation speeds of the backward and forward fronts of the spot.

physics.flu-dyn↗

arXiv2011

Kolmogorov scaling bridges linear hydrodynamic stability and turbulence

The way in which kinetic energy is distributed over the multiplicity of inertial (intermediate) scales is a fundamental feature of turbulence. According to Kolmogorov's 1941 theory, on the basis of a dimensional analysis, the form of the energy spectrum function in this range is the -5/3 spectrum. Experimental evidence has accumulated to support this law. Until now, this law has been considered a distinctive part of the nonlinear interaction specific to the turbulence dynamics. We show here that this picture is also present in the linear dynamics of three-dimensional stable perturbation waves in the intermediate wavenumber range. Through extensive computation of the transient life of these waves, in typical shear flows, we can observe that the residual energy they have when they leave the transient phase and enter into the final exponential decay shows a spectrum that is very close to the -5/3 spectrum. The observation times also show a similar scaling. The scaling depends on the wavenumber only, i.e. it is not sensitive to the inclination of the waves to the basic flow, the shape-symmetry of the initial condition and the Reynolds number.

physics.flu-dyn↗