Alessandro Di Marco

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.

Wall Pressure Fluctuations Induced by Supersonic Turbulent Boundary Layer

Wall pressure fluctuations induced on a rigid wall by a turbulent boundary layer in supersonic regime are the subject of the present review. The sound and vibration of the structure subjected to the correlated fluctuating forces are modeled and predicted presuming the knowledge of a forcing function related to the wavenumber–frequency spectrum of the boundary layer pressures. In this framework the main results obtained in measurements of equilibrium supersonic turbulent boundary layers and in recent numerical simulations are reviewed evidencing the actual limitations of both methodologies. More emphasis is devoted to the experimental and numerical analysis made by the authors respectively on the pressure field generated on the external surface of a launcher model and on the modelization of the pressure coherence function generated by a supersonic flow in different conditions over a flat plate.

Multi-variate statistics of the wall pressure field beneath supersonic turbulent boundary layers

This paper describes a statistical analysis of the wall pressure fluctuations generated at the wall by a supersonic turbulent boundary layer. The investigation is aimed at the evaluation of cross-spectral features and cross-correlations, these quantities being the main factors to be considered for the wall pressure statistical modeling. Wall pressure data are collected from an existing data-base obtained by a direct numerical simulation (DNS). The flow conditions considered include three Mach numbers, 2, 3 and 4, which span a relatively large range of Reynolds numbers. The influence of Mach and Reynolds number on the crossstatistics is analyzed and an accurate description of the pressure field is achieved with the use of the Corcos model.

Investigation of the flow and the acoustics generated by a cylindrical cavity

The flow and the acoustic response of a cylindrical cavity with an aspect ratio (Depth / Diameter) of 1.375 are studied for different Mach numbers (M∞ =0 .02 to 0.22). The shear layer and the wake generated by the cavity are investigated using Hot Wire Anemometry while the flow inside the cavity is investigated using Particle Image Velocimetry and wall pressure measurements. The experimental results are combined with the results of a DES numerical simulation: new insights into the internal and external flows of a cylindrical cavity are described. Two pairs of counter-rotating longitudinal vortices are found in the cavity wake. A simple spectral decomposition of the wall pressure signals into frequency bands gives information of the topology of the flow inside the cavity. The self-sustained oscillations generated by the shear layer instabilities are also studied. The three first hydrodynamic modes of the shear layer are found to generate lock-on at the acoustic modes of the test-section/cavity system.

Pressure Fluctuations in the Transonic Boundary Layer of a VEGA Launcher Model

Wall pressure fluctuations generated in a transonic boundary layer are measured along the surface of a VEGA launcher model for different Mach numbers at transonic conditions and angles of incidence. Pressure signals are measured simultaneously in several positions by means of pressure transducers located along the scaled model. Spectral analyses are applied to characterize the pressure fluctuations in the Fourier domain and results are interpreted within the framework of literature spectral models and in account of the flow physics. Specific attention is addressed towards the cross-correlation functions and the convection velocities measured in the region where the payload would be located. The time domain analysis reveals that the propagation of pressure perturbations within the turbulent boundary layer is driven by two different mechanisms, one associated to hydrodynamic pseudo-sound effects and the other to pure acoustic effects. Possible sources of the acoustic perturbations are identified and the effects of the Mach number and the angle of incidence are also discussed.

Aeroacoustic Wind Tunnel Testing of a 1:6.5 Model Scale Innovative Regional Turboprop

The aeroacoustics of an innovative regional turboprop aircraft is experimentally investigated on a 6.5 scaled model. The measurement campaign was performed at the RUAG Large Subsonic Wind Tunnel in Emmen. The model was mounted on a ceiling strut in the 7-by-5 meters test section, not acoustically treated. The aeroacoustic noise generated by the aircraft model was evaluated analyzing the pressure fluctuations acquired through a phased array of 144 microphones installed on a flat-plate on the test section floor. Pressure fluctuations were acquired in different configurations the most important ones being: the baseline solution (no innovative device applied) with engine on and the model equipped with lined Flap. For the engine on configuration, take-off and approach settings were tested, whereas only the landing configurations was investigated for the baseline lined flap comparison. The main parameters varied during the tests were: the propeller thrust, the propeller revolutions per minute, the speed of the air-flow, the incidence angle of the aircraft and the position of the microphone array. Data were rocessed and then analyzed in the frequency domain and using a conventional beamforming algorithm to retrieve the sound

A Tomographic Directivity Approach to Frequency Domain Beamforming

Aeroacoustic measurements in wind tunnels are a common tool in the determination of sound sources on scaled models. Most of the algorithms used are based on the assumption that the unknown aeroacoustic sources radiate sound waves omnidirectionally, thus monopole sources are utilized. For the prediction of the noise footprint of aircrafts however, it is essential to have information on the directivity of the airframe sources. An approach to estimating this directivity is to use different array positions for measurement relative to the model which leads to different observation angles. The evaluation of measurements at several observation angles holds several issues, for example the difficult source localization due to the three-dimensionality of the point spread function and partial shadowing of sources at large observation angles. This paper therefore presents an approach to estimate the source positions taking into account information from all four observation angles at once. The positions estimated in this fashion are subsequently used for an improved prediction of source power and eventually lead to an estimation of directivity of the observed aeroacoustic sources.

Boundary layer noise Part 1: generation mechanisms

Boundary layer noise concerns the generation of acoustic waves as an effect of the interaction of a fluid with a moving surface. Several issues are related to the noise generation mechanisms in such a configuration. In the present description we focalize mainly onto the case of an infinite flat plate and two main distinct situations are considered. The first one deals with the prediction of the far field noise as accomplished from the classical integral theories, and the main formulations, including Curle’s approach, are briefly reviewed. A novel approach based on the computation of the surface transpiration velocity is also presented. The second aspect concerns the interior noise problem and it is treated from the view point of the fluid dynamic effects rather than from that of the structural dynamics. Attention is focused on the statistical properties of the wall pressure fluctuations and a review of the most effective theoretical models predicting statistical quantities is given. The discussion is completed by a short review of the pressure behavior in realistic situations, including the separated boundary layers in incompressible and compressible conditions and the effect of shock waves at transonic Mach numbers.