Joana Rocha

Flow-Induced Noise and Vibration in Aircraft Cylindrical Cabins: Closed-Form Analytical Model Validation

The turbulent boundary layer is a major source of interior noise in transport vehicles, mainly in aircraft during cruise flight. Furthermore, as new and quieter jet engines are being developed, the turbulent flow-induced noise will become an even more important topic for investigation. However, in order to design and develop systems to reduce the cabin interior noise, the understanding of the physical system dynamics is fundamental. In this context, the main objective of the current research is to develop closed-form analytical models for the prediction of turbulent boundary-layer-induced noise in the interior of aircraft cylindrical cabins. The mathematical model represents the structural-acoustic coupled system, consisted by the aircraft cabin section coupled with the fuselage structure. The aircraft cabin section is modeled as a cylindrical acoustic enclosure, filled with air. The fuselage structure, excited by external random excitation or by turbulent flow, is represented through two different models: (1) as a whole circular cylindrical shell with simply supported end caps and (2) as a set of individual simply supported open circular cylindrical shells. This paper presents the results obtained from the developed analytical framework and its validation through the successful comparison with several experimental studies. Analytical predictions are obtained for the shell structural vibration and sound pressure levels, for the frequency range up to 10,000 Hz.

Prediction of turbulent boundary layer induced noise in the cabin of a BWB aircraft

This paper discusses the development of analytical models for the prediction of aircraft cabin noise induced by the external turbulent boundary layer (TBL). While, in previous works, the contribution of an individual panel to the cabin interior noise was considered, here, the simultaneous contribution of multiple flow-excited panels is analyzed. Analytical predictions are presented for the interior sound pressure level (SPL) at different locations inside the cabin of a Blended Wing Body (BWB) aircraft, for the frequency range 0-1000 Hz. The results show that the number of vibrating panels significantly affects the interior noise levels. Itis shown that the average SPL,over the cabin volume, increases with the number of vibrating panels. Additionally, the model is able to predict local SPL values, at specific locations in the cabin, which are also affected with by number of vibrating panels, and are different from the average values.

Aeroelastic Control of a Wing with Active Skins Using Piezoelectric Patches.

In this paper, an experimental demonstration of aircraft flutter control using piezoelectric materials it is presented. Wind tunnel models were designed, fabricated, installed, and tested. Computational structural and aerodynamic wing models were created, in order to determine the wing natural frequencies and modal shapes, and to calculate its flutter speed. A digital control law was designed and implemented. Open and closed loop vibration and flutter tests were conducted in the wind tunnel, with excellent correlation achieved with computational predictions. A wing with active piezoelectric actuators distributed on the wing skin was designed and tested. The experimental results proved that the active wing exhibited significant aeroelastic control when compared with the corresponding passive wing.

Sound Radiation and Vibration of Composite Panels Excited by Turbulent Flow: Analytical Prediction and Analysis

The present study investigates the vibration and sound radiation by panels exited by turbulent flow and by random noise. Composite and aluminum panels are analyzed through a developed analytical framework. The main objective of this study is to identify the difference between the vibroacoustic behaviour of these two types of panels. This topic is of particular importance, given the growing interest in applying composite materials for the construction of aircraft structures, in parts where aluminum panels were traditionally being used. An original mathematical framework is presented for the prediction of noise and vibration for composite panels. Results show the effect of panel size, thickness of core, and thickness of face layers on the predictions. Smaller composite panels generally produced lower levels of sound and vibration than longer and wider composite panels. Compared with isotropic panels, the composite panels analyzed generated lower noise levels, although it was observed that noise level was amplified at certain frequencies.

An experimental study of the wall-pressure fluctuations beneath low Reynolds number turbulent boundary layers

A more complete understanding of the physical relationships, between wall-pressure and turbulence, is required for modeling flow-induced noise and developing noise reduction strategies. In this study, the wall-pressure fluctuations, induced by low Reynolds number turbulent boundary layers, are experimentally studied using a high-resolution microphone array. Statistical characteristics obtained using traditional cross-correlation and cross-spectra analyses are complimented with wall-pressure-velocity cross-spectra and wavelet cross-correlations. Wall-pressure-velocity correlations revealed that turbulent activity in the buffer layer contributes at least 40% of the energy to the wall-pressure spectrum at all measured frequencies. As Reynolds number increases, the low-frequency energy shifts from the buffer layer to the logarithmic layer, as expected for regions of uniform streamwise momentum formed by hairpin packets. Conditional cross-spectra suggests that the majority of broadband wall-pressure energy is concentrated within the packets, with the pressure signatures of individual hairpin vortices estimated to decay on average within traveling ten displacement thicknesses, and the packet signature is retained for up to seven boundary layer thicknesses on average.

Assessment of Surface-Based Aeroacoustic Noise From Blades of a Vertical-Axis Wind Turbine

Surface-based sources of aerodynamically-generated noise for the 17-m troposkien shape vertical-axis wind turbine are predicted using Farassat’s Formulation 1A of the Ffowcs Williams-Hawkings equation. By discretizing the three-dimensional turbine blades over the height of the turbine into constant-radius sections, the blades were aerodynamically modeled in two-dimensions in the horizontal plane by an unsteady panel method to obtain results for surface pressures and velocities. The acoustic pressure generated by the blades throughout their rotations was determined by the combination of loading and thickness noise sources on the surface of the blade sections in the time-domain. The simulation results were compared to experimental results for the acoustic pressure power spectral density. The sound pressure level around the turbine was found to have a slight dipole radiation pattern, caused primarily by the loading acoustic pressure on the blades.Copyright

The Influence of Boundary Layer Parameters on Interior Noise

Predictions of the wall pressure in the turbulent boundary of an aerospace vehicle can differ substantially from measurement due to phenomena that are not well understood. Characterizing the phenomena will require additional testing at considerable cost. Before expending scarce resources, it is desired to quantify the effect of the uncertainty in wall pressure predictions and measurements on structural response and acoustic radiation. A sensitivity analysis is performed on four parameters of the Corcos cross spectrum model: power spectrum, streamwise and cross stream coherence lengths and Mach number. It is found that at lower frequencies where high power levels and long coherence lengths exist, the radiated sound power prediction has up to 7 dB of uncertainty in power spectrum levels with streamwise and cross stream coherence lengths contributing equally to the total.

Design and evaluation of a high-speed aeroacoustic wind tunnel

During cruise flight, the main source of acoustic noise radiation within the aircraft cabin occurs as a result of structural vibrations of the flexible fuselage panels due to the random pressure fluctuation field imposed by the Turbulent Boundary Layer (TBL) [1]. Conventional methods of describing these interactions have relied on the use of various statistical semiempirical models which predict the behavior of the TBL pressure fluctuation spectrum. Each model however makes fairly different predictions with regards to the spectrum in the various frequency regions. There is therefore a need to not only assess these inconsistencies with experimental methods, but also to clarify the fundamental physical relationship between the turbulent structures and the resulting fluctuation signature. The present phase of this research initiative at Carleton University involves the design and fabrication of the new High- Speed Aeroacoustic Wind Tunnel (HSAWT), which is capable of studying wall pressure fluctuation behavior at subsonic flow speeds up to typical cruise flight conditions.

Modal analysis of a parameterized model of pathological vocal fold vibration

Computational models of vocal fold vibration are used to investigate the physics that governs speech. The effect of a polyps mass, stiffness, position, and aspect ratios on the natural frequencies and modes of vibration of a vocal fold are studied using an in-house finite element code. Polyp mass has the most significant effect, damping vibration. The damping effect of the polyps excess mass is mitigated by increases in stiffness and influence from the regions of the vocal folds which are fixed to cartilage. Distribution of mass into the glottis is found to maximize the damping effect of the polyps excess mass.

A study on minimizing the noise from a trailing edge by the use of optimized serrations

An optimization study is presented in which the noise produced by flow over the trailing edge of a flat plate is minimized via serrations. The theoretical optimum configuration is found by changing the geometry of the serrations with a non-linear gradient based algorithm. The theoretical noise produced by the trailing edge is determined using the semi-empirical model developed by Howe. As expected, the configuration of the trailing edge serrations that produced the least noise over all frequencies (from 20 Hz to 20 kHz) was found to be those with the smallest width (approaching zero) and largest root-to-tip distance (up to the assigned upper-bound). The optimum serration geometry for individual frequencies was found to be highly dependent on the upper bound given to the height. In addition to the optimization study, an analysis is presented of a method for modifying the noise model to capture the high-frequency noise increase (relative to a straight trailing edge) that is regularly observed in experiments...