Silvano Grizzi

Using Wavelet transforms and Linear Stochastic Estimation to study nearfield pressure and turbulent velocity signatures in free jets

A data post-processing technique (denoted as WLSE) based on the combination of wavelet transforms with the Linear Stochastic Estimation (LSE) method is applied to pressure and velocity signals obtained in an isothermal subsonic co-axial jet. The aim of the method is to identify and clarify the role of intermittent turbulent events in the generation of nearfield pressure fluctuations. To this purpose, a wavelet-based filtering procedure is performed to extract intermittent events from the pressure signals and the LSE technique is applied to identify velocity fluctuations which are linearly associated with the selected pressure fluctuations. The methodology comprises two main steps: (i) Pressure signal filtering using the wavelet transform and decomposition of the given signal into intermittent and non-intermittent components; (ii) Reconstruction using LSE of velocity fluctuations linearly associated with the wavelet-filtered part of the pressure signals. Preliminary results, showing encouraging outcomes, are presented and discussed.

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.

Statistical Analysis of the Pressure Field in the near Region of a M = 0.5 Circular Jet

In a recent paper [1] a novel technique providing the separation between hydrodynamic and acoustic components of pressure signals has been presented and applied to near field pressure signals measured in a turbulent jet at M = 0.5. The database is further analyzed herein with the scope of clarifying the physical mechanisms underlying the generation of acoustic pressure fluctuations in the near field. The analysis of cross-correlations between pressure and acceleration and the statistics of the potential core fluctuations, confirm that the region at the end of the potential core plays a fundamental role in this sense. A conditional sampling technique based on the use of wavelet transform identifies ensemble averaged velocity traces of the noise sources. The velocity signatures are interpreted as induced by wavepackets, in the region close to the jet exit, and ring vortices in the far region.

Experimental investigation of the Near-Field Noise generated by a Compressible Round Jet

An experimental study of the pressure field generated by a subsonic, single stream, round jet is presented. The investigation is conducted in the near-field region at subsonic Mach numbers (up to 0.95). The main task of the present work is the analysis of the near-field acoustic pressure and the characterization of its spectral properties. To this aim, a novel postprocessing technique based on the application of wavelet transforms is presented. The method accomplishes the separation of nearly Gaussian background fluctuations, interpreted as acoustic pressure, from intermittent pressure peaks induced by the hydrodynamic components. With respect to more standard approaches based on Fourier filtering, the new technique permits one to recover the whole frequency content of both the acoustic and the hydrodynamic contributions and to reconstruct them as independent signals in the time domain. The validity of the method is verified through several tests on measured signals and comparisons against literature data. The near field acoustic pressure is analyzed in terms of spectral content, Sound Pressure Level and directivity and the results are compared to published far-field observations. Simultaneous velocity/pressure measurements have been also performed using a hot wire probe located in the region close to the nozzle exit. The analysis of the velocity/pressure correlations clearly shows that the main noise sources are located close to the end of the potential core while the largest hydrodynamic pressure peaks are generated further downstream.

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