Alberto Francescutto

Enhancement of high-frequency acoustic cavitation effects by a low-frequency stimulation

The cavitation effects given by a high-frequency pulsed ultrasound field are studied with and without the stimulation of a low-frequency field. Sonoluminescence intensity and subharmonic one-half intensity of the high-frequency field are measured. The stimulation gives a sharp rise of both subharmonic and sonoluminescence intensities.

Bifurcations in ship rolling : experimental results and parameter identification technique

The steady rolling response in a regular beam sea of a 1 : 50 scale model of a destroyer in the bare hull condition has been studied. The restoring moment of the hull features strongly softening characteristics. This has led to the experimental evidence of bifurcations with jump in amplitude and phase during the tests at two different wave frequencies. The high quality measurements allowed the analysis of the gathered data through a single-degree-of-freedom roll motion equation. Exact numerical solutions have been used to obtain reliable values of the coefficients of the mathematical model to be used for the roll motion simulation. Approximate perturbative solutions have then been used to obtain a description of the domains of attraction of the system. An analysis of the stability of the steady state solutions in the range of frequencies where bifurcations and jumps were detected coupled with the analysis of the change of shape of the domains of attraction with the frequency allowed the differences found between the two physical situations to be explained.

On damping models in free and forced rolling motion

Abstract In this paper two different models for the damping moment to introduce in the rolling equation of the ship are proposed. They contain two terms, respectively linear-quadratic and linear-cubic in the angular velocity, and furthermore they foresee a non-linear term representing the dependence of the damping from the heeling angle. These models constitute a generalization of all the models up to now used in the naval literature. With the Bogoliubov-Krilov asymptotic method approximate relations, describing the decay curve of the free oscillations and the maximum roll amplitude in synchronism condition, are obtained. The analysis shows that the choice of the more realistic damping model cannot be based on the simple verification of a good fitting of the free oscillation decay curves. It is necessary to examine also the behaviour of the forced oscillations in synchronism. Finally, a plan of experiments which allows the determination of separate values for the different non-linear damping coefficients is proposed.

Cavitation activity stimulation by low frequency field pulses

The influence of a short-time action of a low-frequency ultrasound on the sonoluminescence generation by a high frequency pulsed field has been studied. This action remarkably lowers the cavitation thresholds and increases the sonoluminescence intensity. The stimulating effect of the low-frequency field action depends on its duration and on the intensities of both fields. Possible mechanisms of this effect are discussed.

Study into mechanisms of the enhancement of multibubble sonoluminescence emission in interacting fields of different frequencies.

The main factor of the enhancement of sonoluminescence (SL) emission by the interaction of two fields of highly different frequencies is the generation of new cavitation nuclei upon collapse of bubbles driven by the low-frequency (LF) field. The factors connected with the direct interaction of the two fields play a significant role in the enhancement of SL emission only in the case when intensities of the fields are less or not much higher than the corresponding thresholds of SL emission. The phenomena of afteraction of the LF field on cavitation generated by the high-frequency field is explained also by the generation of new nuclei upon collapse of bubbles driven by the LF fields.

Enhancement of sonoluminescence emission from a multibubble cavitation zone.

Investigations have been performed on various methods of increasing cavitation activity measured by the intensity of sonoluminescence. It is shown that the effect of the combined action of (a) pulsed modulation of an acoustic field, (b) liquid degassing and cooling and (c) increasing the static pressure considerably exceeds the sum of the effects achieved by each of these methods individually. A more than 250-fold increase of the sonoluminescence intensity has been attained compared with continuous irradiation under normal conditions (room temperature, atmospheric pressure, gas-saturated liquid). An interpretation of the results obtained is proposed.

On the effectiveness of constant coefficients roll motion equation

Abstract As known, the rolling motion characteristics, amplitudes and accelerations, greatly influence the ability of a ship to operate and survive in bad weather. On the other hand, traditional computer codes for seakeeping calculations fail the forecasting of large amplitude rolling. There is a great need of using semi-empirical damping models and coefficients. This stresses the importance of campaigns of measurements as described in the paper, to get a deeper insight into the physical-mathematical modelling of the different contributions to rolling equation. Experimental tests on nonlinear rolling in a regular beam sea of a Ro-Ro ship model have been conducted by varying both the wave steepness and the wave frequency. The use of a parameter estimation technique, based on the least squares fitting of the stationary numerical solution of the nonlinear rolling motion differential equation, allowed to obtain informations on the damping model and on the linear and nonlinear damping coefficients. These exhibit a quite strong dependence on frequency that reduces the efficiency of constant coefficients rolling equation to simulate large amplitude nonlinear rolling. The results indicate that a good quality prediction model of nonlinear rolling cannot be based on constant coefficients time domain simulations. These can infact lead to incorrect estimates of rolling amplitudes even when the parameters have been obtained through high level parameter estimation procedures based on experimental data. The analysis indicates also a marked dependence of the effective wave slope coefficient on wave amplitude. The introduction of both these dependences on the rolling equation allows to reproduce the experimental results with great accuracy even at large amplitudes.

Review of Available Methods for Application to Second Level Vulnerability Criteria

The International Maritime Organization (IMO) has begun work on the development of next generation intact stability criteria. These criteria are likely to consist of several levels: from simple to complex. The first levels are expected to contain vulnerability criteria and are generally intended to identify if a vessel is vulnerable to a particular mode of stability failure. These vulnerability criteria may consist of relatively simple formulations, which are expected to be quite conservative to compensate for their simplicity. This paper reviews methods which may be applicable to the second level of vulnerability assessment, when simple but physics-based approaches are used to assess the modes of stability failure, including pure-loss of stability, parametric roll, surf-riding, and dead-ship condition.

Steady‐state oscillations of gas bubbles in liquids: Explicit formulas for frequency response curves

The steady‐state nonlinear oscillations of a spherical gas bubble in an incompressible, viscous liquid subject to an acoustic pressure field are investigated by the multiscale perturbation method. As a result, simple formulas for the frequency response curves in the regions of the main resonance, first and second ultraharmonic and first and second subharmonic, are obtained to second order in the expansion.

Second Generation Intact Stability Criteria: on the validation of codes for direct stability assessment in the framework of an example application

ABSTRACT The Sub-Committee on Stability and Load Lines and on Fishing Vessels Safety (SLF) of the International Maritime Organization (IMO) has undertaken the development of so-called “Second Generation Intact Stability Criteria” (SGISC) with the intention of providing a new set of rules covering those phenomena which are not properly covered by present, mostly semi-empirical, requirements. The first two levels of the envisioned 3+1 tiers structure of SGISC are so-called “vulnerability assessment” levels: most of the discussion has so far been dedicated to these levels. At the highest level there is the so-called “Direct Stability Assessment”, which is also strictly linked with the development of ship-specific “Operational Guidance”. Recent discussion on the topic of “Direct Stability Assessment” (DSA) has touched the issue of “validation” of numerical codes to be employed at this level. Stimulated by, and in view of, the ongoing IMO discussion, this paper presents the results of a recent series of experiments in beam waves (mono-/bichromatic, irregular) and associated simulations based on a 6-DOF blended code. Nonlinear harmonic and sub-harmonic resonances are observed and simulated.