Jean-Paul Giovanangeli
Aix-Marseille University
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Featured researches published by Jean-Paul Giovanangeli.
Physics of Fluids | 1999
Nicolas Reul; Hubert Branger; Jean-Paul Giovanangeli
The evolution of the airflow instantaneous structure over an unsteady breaking wave propagating in a group is measured in detail using the digital particle image velocimetry technique. It is found that the boundary-layer over a breaking wave, the steepest in the group, separates at a point close to the sharp crest and reattaches in the front slope of the following wave. During breaking, the evolution of the turbulent vorticity is essentially unsteady and the recirculation zone of the separated flow takes the form of a large well-organized vortex. Links between the wave-crest geometry and geometrical features of the separation bubble have been established.
Journal of Fluid Mechanics | 1996
C. Mastenbroek; V. K. Makin; M. H. Garat; Jean-Paul Giovanangeli
Detailed observations of the air flow velocity, pressure and Reynolds stresses above water waves in a wave flume are presented. The static pressure fluctuations induced by the waves are observed following a new procedure that eliminates acoustical contamination by the wave maker. The measurements are analysed by comparing them with numerical simulations of the air flow over waves. In these numerical simulations the sensitivity to the choice of turbulence closure is studied. We considered both first-order turbulence closure schemes based on the eddy viscosity concept, and a second-order Reynolds stress model. The comparison shows that turbulence closure schemes based on the eddy viscosity concept overestimate the modulation of the Reynolds stress in a significant part of the vertical domain. When an eddy viscosity closure is used, the overestimated modulation of the Reynolds stress gives a significant contribution to the wave growth rate. Our results confirm the conclusions Belcher & Hunt reached on the basis of the rapid distortion theory. The ratio of the wind speed to the phase speed of the paddle wave in the experiment varies between 3 and 6. The observed amplitudes of the velocity and pressure perturbation are in excellent agreement with the simulations. Comparison of the observed phases of the pressure and velocity perturbations shows that the numerical model underpredicts the downwind phase shift of the undulating flow. The sheltering coefficients for the flow over hills and the growth rates of waves that are slow compared to the wind calculated with the Reynolds stress model are in excellent agreement with the analytical model of Belcher & Hunt. Extending the calculations to fast waves, we find that the energy flux to waves travelling almost as fast as the wind is increased on going from the mixing length turbulence closure to the Reynolds stress model.
Journal of Fluid Mechanics | 2008
Christian Kharif; Jean-Paul Giovanangeli; Julien Touboul; Laurent Grare; Efim Pelinovsky
The influence of wind on extreme wave events is investigated experimentally and numerically. A series of experiments conducted in the Large Air-Sea Interactions Facility (LASIF) shows that a wind blowing over a short wave group due to the dispersive focusing of a longer frequency modulated wave train (chirped wave packet) may increase the time duration of the extreme wave event by delaying the defocusing stage. A detailed analysis of the experimental results suggests that the air flow separation that occurs on the leeward side of the steep crests may sustain longer the maximum of modulation of the focusing-defocusing cycle. Furthermore it is found that the frequency downshifting observed during the formation of the extreme wave event is more important when the wind velocity is larger. The experiments have pointed out that the transfer of momentum and energy is strongly increased during extreme wave events. Two series of numerical simulations have been performed using a pressure distribution over the steep crests given by the Jeffreyssheltering theory. The first series corresponding to the dispersive focusing confirms the experimental results. The second series that corresponds to extreme wave events due to modulational instability shows that wind sustains steep waves which then evolve into breaking waves. Furthermore, it was shown numerically that during extreme wave events the wind-driven current may play a significant role in their persistence.
Journal of Physical Oceanography | 2007
V. K. Makin; Hubert Branger; William L. Peirson; Jean-Paul Giovanangeli
A model based on wind-over-waves coupling (WOWC) theory is used to simulate a laboratory experiment and to explain the observed peculiarities of the surface stress distribution above a combined wave field: wind-generated-plus-monochromatic-paddle waves. Observations show the systematic and significant decrease in the stress as the paddle wave is introduced into the pure wind-wave field. As the paddle-wave steepness is further increased, the stress level returns to the stress level characteristic of the pure wind waves. Further increase in the paddle-wave steepness augments the stress further. The WOWC model explains this peculiarity of the stress distribution by the fact that the paddle waves significantly damp the wind waves in the spectral peak. The stress supported by these dominant waves rapidly falls when the paddle wave is introduced, and this decrease is not compensated by the stress induced by the paddle wave. With further increase in the steepness of the paddle wave, the stress supported by dominant wind waves stays at a low level while the stress supported by the paddle waves continues to grow proportional to the square of the steepness, finally exceeding the stress level characteristic of the pure wind-wave field.
international geoscience and remote sensing symposium | 1989
Larry F. Bliven; Jean-Paul Giovanangeli; G. Norcross
Rainfall modification of directional scatterometer response from the sea surface was simulated in wind-wave tank experiments. Data show that for the range of conditions in laboratory experiments, rain enhances radar cross section for all azimuthal angles relative to wind direction. This result broadens previous measurements, which showed that scatterometer response increases with increasing rainfall for radars pointing upwind. But more to the point, the data also show that the directional dynamic-range of scatterometry diminishes rapidly as rainfall rate increases. Thus, while it may be possible to determine wind speed and direction during rain, it will require adequate system sensitivity.
Journal of Fluid Mechanics | 1985
Jean-Paul Giovanangeli; A. Memponteil
This paper describes an experimental study, conducted in the I.M.S.T. air–sea interaction tunnel, of waves excited on a water surface by a periodic train of vortices in the air flow above. The water surface, under some conditions, shows a rapidly developing resonant response, while in the non-resonant case waves propagate both upstream and downstream at speeds different from, but dependent upon, the vortex-convection speed.
IEEE Transactions on Geoscience and Remote Sensing | 1999
Nicolas Reul; Hubert Branger; Larry F. Bliven; Jean-Paul Giovanangeli
The authors conducted experiments in the large wind-wave tank at IRPHE to examine the influence of longer waves propagating at oblique angles to the wind direction on microwave backscattering. Measurements were made of wind, wave directional spectra, and cross sections from a 13.5-GHz scatterometer at 30/spl deg/ from nadir incidence angle. the authors characterize the scatterometric azimuthal scans with respect to the magnitude and the direction of the maximum backscattered power. Scans of solely wind waves are used to assess the effects of oblique waves, for which the scans yield trends that are related to the oblique-wave direction relative to the wind and the oblique-wave steepness. In particular, cross-section maxima are enhanced by oblique waves and the direction of maxima rotates from the wind axis toward the oblique-wave direction. These scatterometer data, in addition to other oblique-wave studies, led the authors to conclude that wind vectors produced from standard retrieval algorithms for scatterometers should be used with prudence in regions with significant veering winds, such as near meteorological fronts.
International Journal of Remote Sensing | 1995
Larry F. Bliven; Valérie Billat; Piotr Sobieski; Albert Guissard; Hubert Branger; Jean-Paul Giovanangeli
To characterize scatterometer returns from the sea surface near meteorological fronts, we investigated microwave scattering from seas in which long waves are at oblique angles to short waves. We simulate the effects of veering winds on C- and K-u-band scatterometers by using models in which the short waves align with the wind friction velocity u*, but the long waves are at oblique angles to the u* direction. The analysis reveals two main effects due to the rotation of the long wave slope probability density distribution. Azimuthally averaged normalized radar cross-section a(0) decreases as the oblique angle increases. Additionally, two regimes exist. In the small angle regime, azimuthal scans of normalized radar cross-section sigma degrees exhibit features similar to the classic double-maxima pattern for non-veering wind conditions, but the axis of sigma degrees maxima is rotated toward the long-wave axis. In the large angle regime, more than two maxima are apparent in azimuthal scans. Therefore it may be inappropriate to use standard three term Fourier cosine models for some veering wind conditions.
international geoscience and remote sensing symposium | 1989
C. Kharif; Jean-Paul Giovanangeli; L. Bliven
Modulation of a rain wave pattern by longer waves has been studied. An analytical model taking into account capillarity effects and obliquity of short waves has been developed. Modulation rates in wave number and amplitude have been computed. Experiments were carried out in a wave tank. First results agree with theoretical models, but higher values of modulation rates are measured. These results could be taken into account for understanding the radar response from the sea surface during rain.
international geoscience and remote sensing symposium | 1994
Larry F. Bliven; Valérie Billat; Piotr Sobieski; Albert Guissard; Hubert Branger; Jean-Paul Giovanangeli
To characterize scatterometer returns from the sea-surface near meteorological fronts, the authors investigated microwave scattering from seas in which long-waves are at oblique angles to short-waves. They simulate the effects of veering winds on a C-band scatterometer by using models in which the short-waves align with the wind-friction velocity u*, but the long-waves are at oblique angles to the u* direction. As the oblique angle increases, tilt effects due to the long-waves led to unusual scan patterns with reduced azimuthally-averaged radar cross-sections a/sub 0/.<<ETX>>