Mario Felli

Statistical properties of wall pressure fluctuations over a forward-facing step

This work describes an experimental study of the flow field and wall pressure fluctuations induced by quasi-two-dimensional incompressible turbulent boundary layers overflowing a forward-facing step (FFS). Pressure fluctuations are measured upstream and downstream of an instrumented FFS step model installed inside a large scale recirculation water tunnel, while two-dimensional (2D) velocity fields are measured close to the step via 2D particle image velocimetry (PIV). The overall flow physics is studied in terms of averaged velocity and vorticity fields for different Reynolds numbers based on the step’s height. The wall pressure statistics are analyzed in terms of several indicators, including the root mean squares and probability density functions of the pressure fluctuations, demonstrating that the most relevant flow structure is the unsteady recirculation bubble formed at the reattachment region downstream of the step. Pressure spectra and cross correlations are computed as well, and the convection vel...

Analysis of the Propeller-hull Interaction by LDV Phase Sampling Techniques

In the present paper the results of the wake survey in two stern sections of a twin screw ship model, located just upstream the propeller disk and behind the directional rudder, are presented. Experiments have been performed in a large towing tank by using a Laser Doppler Velocimeter and phase sampling techniques, in order to analyse wake evolution with the propeller angular position. The main features of the propeller installation are highlighted as well as the strong and complex interaction of the propeller with the hull wake, especially in the brackets region and down stream the directional rudder.

Wall Pressure Fluctuations Over a Forward-Facing Step

This work describes part of the results obtained from an extended experimental campaign aimed at the characterization of the wall pressure fluctuations induced by twodimensional incompressible turbulent boundary layers overflowing a Forward-Facing Step. Pressure fluctuation measurements were carried out using an array of piezoresistive microphones flush mounted at the wall upstream and downstream of a FFS step model installed within a large scale water tunnel. Particle Image Velocimetry measurements have been also performed in order to clarify the flow physics. The acquired pressure data were post-processed in order to obtain spectral and physical quantities including the Sound Pressure Level and the convection velocity along the whole model.

A VERSATILE FULLY SUBMERSIBLE STEREO-PIV PROBE FOR TOW TANK APPLICATIONS

A unique, highly modular and flexible underwater system for stereoscopic particle image velocimetry (PIV) measurements has been designed, manufactured and tested. The instrument is intended for planar three-dimensional velocity measurements in large facilities such as water tow tanks and tunnels. The performance of the system is assessed in four major stereoscopic configurations. Errors under 2% for the inplane components and 4% for the out-of-plane components are found. The system is tested in the INSEAN large circulating water channel where the measurement of the flow around a model ship oriented at a moderate yaw angle is performed and puts into evidence the main features of the flow.

Far field pressure fluctuations and coherent structures in a low Mach number turbulent jet flow

Simultaneous far field pressure/in-flow velocity fluctuations were measured in a turbulent jet flow at low Ma number. Measurements were conducted using 2 component Laser Doppler Velocimetry technique and standard microphones. The velocity signals, acquired in several positions within the flow field, were conditioned on the far field pressure peaks and signatures of ensemble averaged noise emitting events were retrieved. The results reported here definitely confirm previous outcomes obtained on the same apparatus showing that intermittent structures generated within the turbulent region of the jet, i.e. around 10 diameters downstream the jet exit, give a relevant contribution to the overall far field noise.

Velocity and Wall Pressure Correlations Over a Forward Facing Step

This work describes an experimental study of the flow field and wall pressure fluctuations induced by quasi-twodimensional incompressible turbulent boundary layers overflowing a forward-facing step (FFS). Pressure fluctuations are measured upstream and downstream an instrumented FFS step model installed inside a large-scale recirculation water tunnel while two dimensional velocity fields are measured close to the step via time-resolved PIV. The overall flow physics is studied in terms of averaged velocity and vorticity fields for different Reynolds number based on the step’s height. Pressure spectra and cross-correlations are measured as well, and the convection velocity characterizing the propagation of acoustic and hydrodynamic perturbations is computed as a function of the distance from the vertical side of the step. The evolution of the overall Sound Pressure Level measured at the wall shows that the most critical flow structure is the reattachment region downstream the step where an unsteady recirculation bubble is formed.

Pressure-density gradient correlations in the near-field of a transonic jet

The relation between pressure fluctuations and density gradients in the near-field of a jet at M=0.9 is studied in this paper. For this purpose, an experiment was performed where the density gradients were acquired with the Background Oriented Schlieren technique. This optical technique measures virtual displacements of a background pattern when seen through the compressible flow. Simultaneously, pressure fluctuations were measured with two microphones at 2D away from the shear layer. These signals were decomposed in sound and pseudo-sound using a wavelet based separation technique. The objective is to study the degree of correlation between these three quantities: density gradient, sound and pseudo-sound. Moreover, a cross-conditional analysis is done in order to relate intermittency events in the pressure signal with variations in the density gradient field. The results of the cross-correlation indicate that only the hydrodynamic component is correlated with the density gradients. Furthermore, the correlation is located after the potential core. In addition, the results of the cross-condtional analysis show that fluctuations of the density gradient related with intermittent events in the pressure signal are mainly after the potential core.

Hydrodynamic and Hydroacoustic Phenomena in the Propeller Wake-Rudder Interaction

The present paper describes the major mechanisms underlying the hydroacoustic and hydrodynamic perturbations in a rudder operating in the wake of a free running marine propeller. The study was based on a holistic approach which concerned time resolved visualizations and detailed flow measurements around the rudder as well as wall-pressure fluctuation measurements over the rudder surface, at different deflection angles.Copyright