An experimental study on the aeroacoustics of wall-bounded flows: Sound emission from a wall-mounted cavity, coupling of time-resolved PIV and acoustic analogies

This thesis deals with the problem of noise. Sound is a constant presence in our lives. Most of the times it is something wanted and it serves a purpose, such as communication through speech or entertainment by listening to music. On the other hand, quite often sound is an annoying and unwanted by-product of some other activity necessary to us. This is what we usually refer to as noise. Noise does not simply create an unpleasant environment but can severely affect human health and have a serious impact on the natural environment as well. The noise pollution problem is addressed in many different ways: by clever urban planning, for instance, and by providing the heavily exposed buildings with acoustic barriers and insulation. Legislation also provides a limit on the maximum acceptable noise levels in many contexts such as working environments, discotheques and music headphones devices. An important effort in the fight against noise is obviously placed at the source of the problem itself, trying to make our tools and devices quieter. In order to do so it is essential to understand, and eventually be able to model, the processes through which sound is produced and propagated. The objective of this work is to develop and to employ experimental methods, based on the particle image velocimetry (PIV) measurement technique, for the estimate of the emitted sound from flow field measurements. In particular, the focus will be on the study of the noise that is generated by the unsteady loads on solid bodies immersed in the flow. The proposed technique, based on time-resolved PIV measurements, provides insight into the sound sources that is not possible to achieve with other established experimental methods. It also permits experimental investigation in those situations where other measurements would not be possible, for example, in the presence of a noisy environment or when a suitable anechoic tests facility is unavailable. In addition, it allows the estimation of the acoustic emission, coupled to the proper acoustic model, in the same fashion as done in hybrid computational approaches. This kind of experimental approach seems to be particularly suited for the study of aerodynamic sound from wall-bounded flows at low Mach number. The main limitation of a PIV-based method in general noise applications is in fact the poor temporal resolution currently available. However, for low enough velocities this is no longer a concern since the resolution is good enough to perform time resolved measurements. Moreover, an experimental technique might be advantageous with respect to computational studies for low Mach number and high Reynolds number flows with immersed solid bodies. The numerical simulation of the flow in the proximity of solid boundaries is in fact particularly complex. In this thesis, a rectangular wall-mounted shallow open cavity has been chosen as a test case, a series of experiments have been performed and different methods and solutions has been tested. In chapter 3 the main details of the experimental setup are given and Curle’s analogy is applied in its classical formulation. In this chapter we also give an estimate of the span-wise coherence of the flow, based on stereoscopic PIV measurements. Both the process of pressure calculation from PIV data and the application of Curle’s analogy are affected by several uncertainties, and simplifying hypotheses is necessary in the process. For this reason we also perform direct microphones measurements of the pressure fluctuations at the walls of the cavity and of the sound emitted. We can therefore compare those values with the estimated quantities from the PIV data in order to validate the results and to check the range of validity of the approach. We find that both the hydrodynamic pressure computed from the PIV data and the sound emission obtained applying Curle’s analogy have a frequency spectrum that is comparable to that of the direct microphones measurements. In section 3.7 we demonstrate that the larger flow structures, responsible for most of the sound, are rather two-dimensional and coherent in the span-wise direction. In chapter 4 we discuss the details of the implementation and solution of both Curle’s Analogy and the theory of vortex sound. In this chapter, in contrast with the implementation of the model performed in chapter 3, we take into account the presence of the non-compact wall in which the cavity is located by using the image principle in the derivation of the solution. The two methods are derived under the assumption of low Mach number and high Reynolds number and for a listener positioned in the far field. The two analogies perform quite well for the present test case and give very similar results, both in total intensity and in the spectral distribution of the emitted sound. In their application, however, they each have different strengths and weaknesses. The two solutions are in fact derived through quite different pathways, and the mathematical schemes used to solve the equations are sensitive to different factors. The choice for either of the two methods needs therefore to be carefully done in relation to the specific application. The use of the image principle seems to be crucial to properly estimate the sound emission for compact geometries placed in large non-compact surfaces. In chapter 5 we investigate the effect of the three-dimensional velocity fluctuations on the final result. We discuss the results obtained by time-resolved thin tomographic PIV measurements. These measurements provided enough velocity vectors in the span-wise direction to allow for the calculation of the differential quantities in that direction and therefore for the computation of the full source terms of both Curle’s analogy and Theory of Vortex Sound. Details about the new experimental setup and measurement technique were given as well. Results showed that the flow is indeed rather two-dimensional and that there is little difference between the analogies source terms computed two or three-dimensionally. Compared to two-dimensional measurements, volumetric PIV measurements are more expensive, require more complex setups and the obtained data requires a much longer processing time. Moreover, the quality of the data is often lower than that of planar PIV. At the same time thin tomographic measurements do not seem to add relevant information to the computation of the sound emission for our experimental case, that is representative of quasi two-dimensional wall-bounded flows where the main source of sound is determined by large span-wise coherent flow structures.Process and EnergyMechanical, Maritime and Materials Engineerin

Comparison of two acoustic analogies applied to experimental PIV data for cavity sound emission estimation

The aim of the present study is to compare two different acoustic analogies applied to time-resolved particle image velocimetry (PIV) data for the prediction of the acoustic far-field generated by the flow over a rectangular cavity. Recent developments in laser and camera technology allow the possibility to extend PIV to the study of aeroacoustic phenomena in air flows at moderate speed (up to V=20m/s, Ma=0.08). We make use of these new possibilities to obtain estimates of the acoustic emission from time-resolved PIV data. We consider the model problem of the sound radiating from an open, two dimensional, shallow cavity with an aspect ratio between its length and depth of 2 at a Reynolds number of 3:0£104 based on the cavity length. The study is carried out combining high speed two dimensional PIV imaging and sound measurements. The emitted sound is then calculated using Curle’s analogy and vortex sound theory. The prediction of the acoustic fields obtained by applying the two methods are analyzed and compared with the measured sound. Results show that both the analogies estimate the overall sound pressure level quite well and within a few dB of each other. Vortex sound theory seems to provide a better estimate for the amplitude of the tonal component and its harmonics, but suffers from higher broadband noise compare to Curle’s analogy. This is due to the higher smoothing of the data involved in the computation of the source term in Curle’s analogy, which reduces noise but also smoothes out the signal. Results show that great care must be taken in the treatment of the experimental PIV data, especially in the techniques used for spatial and temporal differentiation. The data sets obtained by PIV measurements can be, in fact, quite noisy while the spatial and temporal resolution are still limited.Process and EnergyMechanical, Maritime and Materials Engineerin

A study on the application of two different acoustic analogies to experimental PIV data

The aim of the present study is to compare two different acoustic analogies applied to time-resolved particle image velocimetry (PIV) data for the prediction of the acoustic far-field generated by the flow over a rectangular cavity. We consider the model problem of sound radiating from an open, two-dimensional, shallow cavity with an aspect ratio of 2 at a Reynolds number of 3.0?×?104 (based on the cavity length). The study is carried out by simultaneous high-speed two-dimensional PIV and sound measurements. The instantaneous flow field is obtained from the PIV measurements. The emitted sound is then calculated using Curle’s analogy and Vortex Sound Theory. To our knowledge, Vortex Sound Theory is used here for the first time in combination with time-resolved PIV data. The acoustic analogies are derived through rather different pathways, and the mathematical schemes used to solve the equations are sensitive in a different way to factors such as data resolution, noise level, and complexity of the geometry. Both methods indicate that the trailing edge of the cavity is the main sound source. The predictions of the acoustic field obtained by applying the two methods are analyzed and compared with the measured sound. For the presented case, the results show that both analogies estimate the overall sound pressure level quite well and that they give very similar results, both in total intensity and in the spectral distribution of the emitted sound.Process and EnergyMechanical, Maritime and Materials Engineerin

High speed PIV applied to aerodynamic noise investigation

In this paper, we study the acoustic emissions of the flow over a rectangular cavity. Especially, we investigate the possibility of estimating the acoustic emission by analysis of PIV data. Such a possibility is appealing, since it would allow to directly relate the flow behavior to the aerodynamic noise production. This will help considerably in understanding the noise production mechanisms and to investigate the possible ways of reducing it. In this study, we consider an open cavity with an aspect ratio between its length and depth of 2 at a Reynolds number of 2.4 9 104 and 3.0 9 104 based on the cavity length. The study is carried out combining high speed two-dimensional PIV, wall pressure measurements and sound measurements. The pressure field is computed from the PIV data. Curle’s analogy is applied to obtain the acoustic pressure field. The pressure measurements on the wall of the cavity and the sound measurements are then used to validate the results obtained from PIV and check the range of validity of this approach. This study demonstrated that the technique is able to quantify the acoustic emissions from the cavity and is promising especially for capturing the tonal components on the sound emission.Aerodynamics & Wind EnergyAerospace Engineerin

On the suitability of direct application of acoustic theory to time-resolved tomographic PIV tested by DNS for low Mach number jet flows

In this paper, we investigate the minimum temporal requirements, spatial domain and spatial resolution requirements in order to make acoustic predictions by means of acoustic analogies, for low speed jet flows, by means of numerical simulations. This work is done to demonstrate the feasibility of indirect acoustic extrapolation from recent experiments undertaken by us and to develop robust techniques. For this paper we consider Direct Numerical Simulation of incompressible jets flows of Reynolds number 2,500 and Reynolds number 5,000 jets. Comparison of flow statistics and visualizations confirms that the simulations are representative of the jets obtained in our experiments. Lighthill’s and Powell’s aerocaoustic analogies are applied and demonstrated to yield indirect acoustic predictions of these jets, especially at 90º to the flow axis, where the coordinate origin dependency of Powell’s acoustic analogy is minimal. Finally we investigate the robustness of these indirect acoustic predictions when the DNS simulation data is reduced in temporal and spatial extent and resolution to those currently achievable by our Tomographic PIV setup. The results indicate that our current measurement capabilities are suitable for obtaining acoustic spectra up to a Strouhal number of 1.0. Moreover, in order to improve the spectral cut-off, it is preferable to increase the spatial resolution of the measurements at the expense of domain size and temporal resolution.Aerodynamics, Wind Energy & PropulsionAerospace Engineerin

Investigation of Cavity Flow Using Fast-Response Pressure Sensitive Paint

An experimental study was conducted to investigate the pressure fluctuations on the entire side wall of a rectangular cavity with an L/D of 5.67 using fast-response pressure sensitive paint. Additionally, the performance of four different passive flow-control devices was quantified. Experiments were conducted in the Trisonic Gasdynamics Facility at the Air Force Research Laboratory at Mach 0.7 and 1.5. The frequency spectrum (including Rossiter tones) and sound pressure levels (SPLs) obtained from the pressure sensitive paint (PSP) measurements are validated against data taken with conventional dynamic pressure sensors. The complex flow phenomena over the cavity wall were visualized, and full wall pressure spectra were calculated. The rod in cross flow showed the best peak suppression, followed closely by the flat spoiler. The large triangular step showed some peak suppression, while the ridges did not suppress the peaks at all. High resolution measurements of pressure fluctuations on the wall allowed for the visualization of SPL distribution over the entire cavity wall. This revealed a strong dependence between the Rossiter tone modes and the spatial distribution of SPL that was not possible to resolve with discrete pressure transducers