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150 Chapter B: Publications SPL (dB) − micro 7 180 160 140 120 100 80 3 Millions Cells − LES 10 Millions Cells − LES Experimental Measurements . 0 500 1000 1500 2000 2500 Frequency (Hz) Figure 7: Sound Pressure Level assumption might be a misinterpretation of reality. Small pockets of propane-air presenting an equivalent ratio different for φ g may reach the flame influencing its dynamics and as a consequence the radiated sound. Secondly, perfect adiabatic walls were considered in the present LES. In [41] it was shown that accounting for heat transfer leads to a reduction in the SPL. Some computations, not shown here, were performed considering heat transfer by modeling approximate heat loss coefficients (as done in [41]). No significative changes on the radiated noise were observed. Nevertheless, heat transfer might still be modeled too aproximatively knowing that exact values of heat transfer coefficients are extremely difficult to be obtained experimentally. A proper simulation of conjugate heat transfer including convection, conduction and radiation might be important at some level for noise modeling in turbulent flames, but presently remains too cumbersome. Thirdly, resolution of the computation (mesh refinement and order of the numerical scheme) is clearly significant. High grid resolutions in the reactive region not only means a smaller influence of the combustion model but also to be able to account for the smallest turbulent structures present in the shear layer that might influence the coherence of the bigger scales and thus the global turbulent interaction with the flame. At last but not the least, another explanation lies in the experimental data. Measuring acoustics in a confined combustion chamber is only reliable when acoustic leakages are proved to be controlled. This is, anyway, a difficult task. It has been found by the concerned EC2 experimentalists that acoustic leakages are still not totally well managed. As a result, wrong evaluations of pressure fluctuations may arise. 5.2 Hybrid approach The acoustic outputs from both direct and hybrid approaches are compared for the 10 million cells mesh independently of experimental data. As sketched in Fig. 8, hydrodynamic pressure fluctuations are assumed to be small when considering results from the direct approach. Therefore, the acoustic field resulting from the hybrid approach is directly compared to the pressure fluctuation field coming from direct computations. The hybrid computation accounts for two steps. First, the source of combustion noise is computed by postprocessing the data obtained from the LES computation (instantaneous heat release rate in addition to mean flow parameters taken from the 10 million cells numerical results). Secondly, the simplified Phillips equation written in the zero Mach number limit given by Eq. 7 is solved in the frequency domain. 10
151 LES (AVBP) LES ASSUMPTION acoustic Direct Method Negligible hydrodynamic contribution COMPARISON ? Ac. Analogy acoustic Hybrid Method Figure 8: Exercise of comparison: Direct Approach Vs Hybrid Approach Overall good agreement is found between both direct and hybrid approaches in Fig. 9 which shows the sound pressure levels obtained by microphones 5 and 7 (see the location of M5, M7 in Fig. 1). SPL (dB) − micro 5 180 160 140 120 100 80 Hybrid Computation Direct Computation: LES . SPL (dB) − micro 7 180 160 140 120 100 80 Hybrid Computation Direct Computation: LES . 60 0 500 1000 1500 2000 2500 Frequency (Hz) 60 0 500 1000 1500 2000 2500 Frequency (Hz) (a) Microphone 5 (b) Microphone 7 Figure 9: Sound Pressure Levels from the direct and hybrid approaches It is interesting to notice that the hybrid computation manages to recover not only the magnitude of the acoustic pressure over almost all the spectrum, but also the shape of the acoustic waves. Figure 10 shows the strongest acoustic wave, the quarter wave mode, that resonates at 377 Hz. The pressure fluctuations along the axis of the combustor at different times within a cycle are observed. As both methods yield the same envelope of variations at this frequency, the pressure fluctuation recovered by the direct computation can be seen as almost completely caused by acoustics. In Fig. 9 some zones of the spectrum still show important gaps between hybrid and direct computations. For example in Fig. 11 two different types of pressure waves are observed at microphone 5 for the direct and hybrid computations in the region around 1000 Hz. Whereas a pure acoustic standing wave is obtained by the hybrid approach, a perturbed pressure wave is obtained in the direct computation results. A pure standing acoustic wave can naturally have an acoustic pressure node. If this pressure node is present close to the region of the measurement device a low value of pressure fluctuation will be obtained. This is what happens for microphone 5 in the zone close to 1000 Hz (fig. 11a). Obviously, when the pressure fluctuations not only contain acoustics but also hydrodynamic perturbations as in the 11
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UNIVERSITE MONTPELLIER II SCIENCES
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An expert is a man who has made all
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Y gracias familia mía!!, que así
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Abstract Today, much of the current
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4 CONTENTS 3.4.1 Preconditioning .
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List of Figures 1.1 Main sources of
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8 LIST OF FIGURES 5.19 Acoustic ene
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List of Tables 1.1 EU recommended r
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14 Chapter 1: General Introduction
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2 Computation of noise generated by
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22 Chapter 2: Computation of noise
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4 Validation of the acoustic code A
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60 Chapter 4: Validation of the aco
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5 Assessment of combustion noise in
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98 Chapter 6: Boundary conditions f
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