Luca Marino

Shock wave formation in the collapse of a vapor nanobubble.

In this Letter, the dynamics of a collapsing vapor bubble is addressed by means of a diffuse-interface formulation. The model cleanly captures, through a unified approach, all the critical features of the process, such as phase change, transition to supercritical conditions, thermal conduction, compressibility effects, and shock wave formation and propagation. Rather unexpectedly for pure vapor bubbles, the numerical experiments show that the process consists in the oscillation of the bubble associated with the emission of shock waves in the liquid, and with the periodic disappearance and reappearance of the liquid-vapor interface due to transition to super- or subcritical conditions. The results identify the mechanism of shock wave formation as strongly related to the transition of the vapor to the supercritical state, with a progressive steepening of a focused compression wave evolving into a shock which is eventually reflected as an outward propagating wave in the liquid.

Characterization of the three-dimensional instability in a lid-driven cavity by an adjoint based analysis

In this paper we investigate the three-dimensional stability of the 2D flow generated in a cavity by the motion of two facing walls. An adjoint-based analysis of the most unstable global mode will be performed in order to localize the core of the instability.

Stability and Sensitivity Analysis of Non-Newtonian Flow through an Axisymmetric Expansion

This paper deals with a linear stability analysis of the flow in a circular pipe with a sudden expansion. We consider both Newtonian and non-Newtonian fluid models and a thorough comparison is presented. The stability analysis is completed by an adjoint-based investigation on the sensitivity characteristics of perturbations. The results are discussed and compared, when it is possible, to those already published in the pertinent literature.

Aeroacoustics and aerodynamics of impinging supersonic jets: Analysis of the screech tones

The interaction between acoustics and aerodynamics of a supersonic jet is an actual fundamental topic which has been a matter of discussion in the last decades. The present paper is devoted to the experimental analysis of free and impinging jets with particular attention on the effect of an impinging surface on screech tones. The acoustics is studied using free-field microphones, while Particle Image Velocimetry is used to investigate the velocity field. The analysis of acquired data allowed to verify and explain the coupling between acoustic discrete tones and mean and fluctuating flow velocities.

Experimental analysis of UAV-propellers noise

A series of experiments was carried out to investigate the acoustical behaviour of different propellers for values of the advance ratio spanning from zero to about one. At zero advance ratio the propellers were tested in a small anechoic chamber and the spatial distribution of the acoustic signature and of the sound pressure level has been evaluated. The preliminary data at positive advance ratios were obtained in the wind tunnel of the Aeronautical Laboratory of the Department of Mechanics and Aeronautics in Rome and are also presented here. In addition to the experimental investigation, a numerical analysis, based on a simple aerodynamic model, was performed and the results are compared with the test data and discussed in this paper.

Diffuse interface modeling of a radial vapor bubble collapse

A diffuse interface model is exploited to study in details the dynamics of a cavitation vapor bubble, by including phase change, transition to supercritical conditions, shock wave propagation and thermal conduction. The numerical experiments show that the actual dynamic is a sequence of collapses and rebounds demonstrating the importance of nonequilibrium phase changes. In particular the transition to supercritical conditions avoids the full condensation and leads to shockwave emission after the collapse and to successive bubble rebound.

The velocity distribution function in direct simulation Monte Carlo method with an application to extended thermodynamics

A numerical experiment is carried out to prove the difference between the value of the kinetic temperature and that of the thermodynamic temperature in a gas in the presence of molecular transport. As a reference situation the velocity distribution function is evaluated between two concentric cylinders at different wall temperatures. A direct simulation Monte Carlo method (DSMC) is adopted for various Knudsen numbers Kn and comparisons are made with existing data. The results prove quantitatively that the difference between the two differently defined temperatures increases with Kn and with the temperature gradient, as predicted by the theory for systems which are almost in thermodynamical equilibrium. The present article does not aim to validate DSMC, but rather to illustrate how the method can be used to address fundamental issues in gas dynamics.

A note about three-dimensional exact dynamical solutions for neo-Hookean materials

Two classes of exact three-dimensional dynamical solutions for a neo-Hookean elastic slab are given.

Analysis of the characteristic acoustic tones of an impinging jet

The acoustic emission of a supersonic jet is an interesting and fundamental issue due to the several technological applications and the diverse physical aspects to be still clarified. An experiment ...

A T-junction device allowing for two simultaneous orthogonal views: application to bubble formation and break-up

A novel design for the classical microfluidic device known as T-junction is proposed with the purpose of obtaining a simultaneous measurement of the in-plane velocity components in two orthogonal planes. A crucial feature of the proposed configuration is that all three velocity components are available along the intersection of the two planes. A dedicated optical set-up is developed to convey the two simultaneous views from the orthogonal planes into the sensor of a single camera, where a compound image is formed showing on either half one of the two views. A commercial micro-particle image velocimetry system is used to measure the velocity in the two planes. Feeding the T-junction with a liquid continuous phase and a dispersed gas phase, the velocity is measured by phase averaging along the bubble formation and break-up process showing the potentialities of the new design. The accuracy analysis shows that the error is dominated by a systematic component due to the thickness of the measurement slice. The error can be reduced by applying confocal microscopy to the present system with no further modifications so as to reduce the thickness of the measurement slab thereby reducing the error. Moreover, by sweeping the planes across the region of interest, a full three-dimensional reconstruction of the velocity field can be readily obtained. Finally, the simultaneous views offer the possibility to extract the principal curvatures of the bubble meniscus thereby providing access to the Laplace pressure.