A. Toffoli | Swinburne University of Technology, Hawthorn (original) (raw)

Papers by A. Toffoli

As the water depth decreases or the sea severity increases, nonlinearity becomes more relevant an... more As the water depth decreases or the sea severity increases, nonlinearity becomes more relevant and waves deviate from the Gaussian surface. Second-order wave theory is, in general, able to capture much of the nonlinearity. Measurements made in finite water depth, however, show that second-order wave profiles fail to restore second-order effects at high nonlinear levels. Influence of directional spreading and combined seas is also looked at.

An experimental model of reflection and transmission of ocean waves by an ice floe is presented.

ABSTRACT Wave group characteristics from water of intermediate depths ranging from relatively dee... more ABSTRACT Wave group characteristics from water of intermediate depths ranging from relatively deep to relatively shallow are discussed. The analysis is based on measurements of the surface elevation, which were collected with both floating and fixed sensors. Results indicate that, for similar spectral conditions, the general distributions of the run lengths are remarkably similar for the waves in the relatively deep and relatively shallow water depths. However, for less steep sea states, it has been observed that the group tendency is more pronounced for the deeper water location due to more relevant swell activities. The analysis of the fixed sensor data gives measured run lengths that are shorter than the ones obtained from wave buoys. For both locations, the empirical findings agree relatively well with probabilistic predictions using the approach given in [1] where a correlation parameter κ is estimated using spectral shape parameters.

ABSTRACT A wave crest distribution represents an important input to design of marine structures. ... more ABSTRACT A wave crest distribution represents an important input to design of marine structures. In current design practice the second-order theory-based crest distributions suggested by Forristall (2000) are commonly used. In the present study laboratory data of random directional wave fields have been used to investigate the combined effect of higher order nonlinearity and directional spreading on the wave crest distribution. Different seas states with a variety of combination of steepness and directional spreading have been considered, from long to short crested wave fields. The investigations have been supported by numerical simulations based on the potential Euler equations. A two-parameter Weibull distribution has been fitted to the experimental data. The Weibull parameters have been parameterized as a function of a general version of the Benjamin-Feir Index (BFI) for directional sea states recently presented by Mori et al. (2011). Long-term distributions of the one and two dimensional Benjamin-Feir Index as well as joint distributions of BFI and wave steepness and directional spreading have been proposed based on the hindcast data from the North Atlantic. Uncertainties related to the fits are discussed.

Geophysical Research Letters, 2011

1] We present an experimental and numerical investigation on the statistical properties of the su... more 1] We present an experimental and numerical investigation on the statistical properties of the surface elevation in crossing sea conditions. Experiments are performed in a very large wave basin (70 m × 50 m × 3 m) and numerical results are obtained using a higher order method for solving the Euler equations. Both experimental and numerical results indicate that the number of extreme events depends on the angle between the two interacting systems. This outcome is supported by recent theoretical investigations which have highlighted that the instability of wave packets may be triggered by the nonlinear interactions between coexisting, non-collinear wave systems. Citation: Toffoli, A., E. M. Bitner-Gregersen, A. R. Osborne, M. Serio, J. Monbaliu, and M. Onorato (2011), Extreme waves in random crossing seas: Laboratory experiments and numerical simulations, Geophys.

Extreme waves represent a serious threat for marine structures and operations. Numerical and theo... more Extreme waves represent a serious threat for marine structures and operations. Numerical and theoretical work has demonstrated that the modulational instability plays a relevant role in the formation of extreme waves. However, strong deviations from Gaussian statistics can only be expected if waves are rather long crested i.e. the spectral energy is concentrated on a narrow range of directions. For more realistic short crested seas (i.e. broad directional distributions), the effect of modulational instability becomes less prominent and, as a result, the occurrence of extreme waves does not exceed predictions from second-order theory. This transition between strongly to weakly non-Gaussian behavior is determined by a balance between nonlinearity (which promotes non-Gaussian behavior) and directionality (which suppresses non-Gaussian behavior). Thus, if there are circumstances when the nonlinearity is locally enhanced, we can expect that non-Gaussian behavior would persist also at broader directional spreads. In this respect, when waves propagates against an ambient current, wave steepness, and hence nonlinearity, increases as a consequence of the shortening of the wavelength, making nonlinear processes, such as the modulational instability mechanism, more likely. A number of laboratory experiments have been carried out to verify the behavior of regular and irregular waves when opposing a strong current. Most experimental results until now have been obtained in wave flumes, where only one-dimensional propagation can be addressed. For the present study, we have accessed the directional wave basin facility at Marintek in order to address the more general two dimensional problem, where a multi directional wave field propagates obliquely over a uniform current in partial opposition. The goal of the experiment was to investigate experimentally the role of increasing wave steepness due to wave-current interaction on the modulational instability mechanism and this for a wide range of wave directional spreadings. The results confirm that the current influences the nonlinear dynamics of the wave field and hence facilitates the formation of large amplitude waves. The effect of the wave-current interaction on the probability of occurrence of extreme wave has consequently been investigated.

Geophysical Research Letters, 2010

As the water depth decreases or the sea severity increases, nonlinearity becomes more relevant an... more As the water depth decreases or the sea severity increases, nonlinearity becomes more relevant and waves deviate from the Gaussian surface. Second-order wave theory is, in general, able to capture much of the nonlinearity. Measurements made in finite water depth, however, show that second-order wave profiles fail to restore second-order effects at high nonlinear levels. Influence of directional spreading and combined seas is also looked at.

An experimental model of reflection and transmission of ocean waves by an ice floe is presented.

ABSTRACT Wave group characteristics from water of intermediate depths ranging from relatively dee... more ABSTRACT Wave group characteristics from water of intermediate depths ranging from relatively deep to relatively shallow are discussed. The analysis is based on measurements of the surface elevation, which were collected with both floating and fixed sensors. Results indicate that, for similar spectral conditions, the general distributions of the run lengths are remarkably similar for the waves in the relatively deep and relatively shallow water depths. However, for less steep sea states, it has been observed that the group tendency is more pronounced for the deeper water location due to more relevant swell activities. The analysis of the fixed sensor data gives measured run lengths that are shorter than the ones obtained from wave buoys. For both locations, the empirical findings agree relatively well with probabilistic predictions using the approach given in [1] where a correlation parameter κ is estimated using spectral shape parameters.

ABSTRACT A wave crest distribution represents an important input to design of marine structures. ... more ABSTRACT A wave crest distribution represents an important input to design of marine structures. In current design practice the second-order theory-based crest distributions suggested by Forristall (2000) are commonly used. In the present study laboratory data of random directional wave fields have been used to investigate the combined effect of higher order nonlinearity and directional spreading on the wave crest distribution. Different seas states with a variety of combination of steepness and directional spreading have been considered, from long to short crested wave fields. The investigations have been supported by numerical simulations based on the potential Euler equations. A two-parameter Weibull distribution has been fitted to the experimental data. The Weibull parameters have been parameterized as a function of a general version of the Benjamin-Feir Index (BFI) for directional sea states recently presented by Mori et al. (2011). Long-term distributions of the one and two dimensional Benjamin-Feir Index as well as joint distributions of BFI and wave steepness and directional spreading have been proposed based on the hindcast data from the North Atlantic. Uncertainties related to the fits are discussed.

Geophysical Research Letters, 2011

1] We present an experimental and numerical investigation on the statistical properties of the su... more 1] We present an experimental and numerical investigation on the statistical properties of the surface elevation in crossing sea conditions. Experiments are performed in a very large wave basin (70 m × 50 m × 3 m) and numerical results are obtained using a higher order method for solving the Euler equations. Both experimental and numerical results indicate that the number of extreme events depends on the angle between the two interacting systems. This outcome is supported by recent theoretical investigations which have highlighted that the instability of wave packets may be triggered by the nonlinear interactions between coexisting, non-collinear wave systems. Citation: Toffoli, A., E. M. Bitner-Gregersen, A. R. Osborne, M. Serio, J. Monbaliu, and M. Onorato (2011), Extreme waves in random crossing seas: Laboratory experiments and numerical simulations, Geophys.

Extreme waves represent a serious threat for marine structures and operations. Numerical and theo... more Extreme waves represent a serious threat for marine structures and operations. Numerical and theoretical work has demonstrated that the modulational instability plays a relevant role in the formation of extreme waves. However, strong deviations from Gaussian statistics can only be expected if waves are rather long crested i.e. the spectral energy is concentrated on a narrow range of directions. For more realistic short crested seas (i.e. broad directional distributions), the effect of modulational instability becomes less prominent and, as a result, the occurrence of extreme waves does not exceed predictions from second-order theory. This transition between strongly to weakly non-Gaussian behavior is determined by a balance between nonlinearity (which promotes non-Gaussian behavior) and directionality (which suppresses non-Gaussian behavior). Thus, if there are circumstances when the nonlinearity is locally enhanced, we can expect that non-Gaussian behavior would persist also at broader directional spreads. In this respect, when waves propagates against an ambient current, wave steepness, and hence nonlinearity, increases as a consequence of the shortening of the wavelength, making nonlinear processes, such as the modulational instability mechanism, more likely. A number of laboratory experiments have been carried out to verify the behavior of regular and irregular waves when opposing a strong current. Most experimental results until now have been obtained in wave flumes, where only one-dimensional propagation can be addressed. For the present study, we have accessed the directional wave basin facility at Marintek in order to address the more general two dimensional problem, where a multi directional wave field propagates obliquely over a uniform current in partial opposition. The goal of the experiment was to investigate experimentally the role of increasing wave steepness due to wave-current interaction on the modulational instability mechanism and this for a wide range of wave directional spreadings. The results confirm that the current influences the nonlinear dynamics of the wave field and hence facilitates the formation of large amplitude waves. The effect of the wave-current interaction on the probability of occurrence of extreme wave has consequently been investigated.

Geophysical Research Letters, 2010