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84 Chapter 5: Assessment of combustion noise in a premixed swirled combustor SPL (dB) − micro 5 180 160 140 120 100 80 Hybrid Computation Direct Computation: LES SPL (dB) − micro 6 180 160 140 120 100 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 6 Figure 5.13: Sound Pressure Levels from the direct and hybrid approaches SPL (dB) − micro 7 180 160 140 120 100 80 Hybrid Computation Direct Computation: LES 60 0 500 1000 1500 2000 2500 Frequency (Hz) Figure 5.14: Sound Pressure Levels from the direct and hybrid approaches waves. Figure 5.15 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 plotted. 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. 5.13 some zones of the spectrum still show differences between hybrid and direct computations. Let us focus on the low frequency zone of the spectra before the peak at 377 Hz. At this frequencies it is found that there is a significant hydrodynamic contribution in the direct computation that triggers higher pressure fluctuations. Figure 5.16 shows two interesting situations. The hybrid approach is based on the low Mach number assumption; in other words, the flow is considered stationary. Moreover, no strong gradients of the sound velocity ¯c are observed in the premixer region, which means that acoustic waves will propagate through this zone as if a quiescent medium was present. As a result, all acoustic modes of the chamber are excited when computing noise through the hybrid approach. Figure 5.16(a) shows that when considering frequencies close to the first eigen-frequency of the system (240 Hz), the quarter wave mode of the premixer is excited. Fluctuations of different nature are shown in Figure 5.16(b) which illustrates LES results. Although there is a strong coherence of the pressure fluctuations in the chamber region, these fluctuations do not come from acoustics. Moreover, the strong level of turbulence present in the premixer region prevents any important acoustic os-
5.3 Combustion noise Analysis 85 Pressure Fluctuation (Pa) M5 M6 M7 1000 Premixer Combustion chamber 500 0 −500 −1000 −0.2 −0.1 0 0.1 0.2 x (m) 0.3 0.4 0.5 (a) Quarter pressure Wave from Eq. (2.63) Hybrid Computation Pressure Fluctuation (Pa) M5 M6 M7 1000 Premixer Combustion chamber 500 0 −500 −1000 −0.2 −0.1 0 0.1 0.2 x (m) 0.3 0.4 0.5 (b) Quarter pressure Wave from LES Direct Computation Figure 5.15: Longitudinal pressure waves oscillating at 377 Hz cillation to be triggered in this region. Pressure Fluctuation (Pa) M5 M6 M7 600 Premixer Combustion chamber 400 200 0 −200 −400 −600 −0.2 −0.1 0 0.1 0.2 x (m) 0.3 0.4 0.5 (a) Pressure Wave from Eq. (2.63) Hybrid Computation Pressure Fluctuation (Pa) M5 M6 M7 600 Premixer Combustion chamber 400 200 0 −200 −400 −600 −0.2 −0.1 0 0.1 0.2 x (m) 0.3 0.4 0.5 (b) Pressure Wave from LES Direct Computation Figure 5.16: Longitudinal pressure waves oscillating at 251 Hz In Fig. 5.17 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. 5.17a). Obviously, when the pressure fluctuations not only contain acoustics but also hydrodynamic perturbations as in the direct computations (Fig. 5.17b), no pressure node can be observed and the resulting SPL is much higher than in the hybrid computation case. Similar conclusions can be drawn at frequencies around 1650 Hz where a pressure node is found close to Microphone 6 (Fig. 5.18). In order to not have any ambiguity measuring acoustic pressure fluctuations due to the presence of pressure nodes, a more appropriate quantity to evaluate at the microphone positions is
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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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134 BIBLIOGRAPHY [93] SENSIAU, C. S
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136 Chapter A: About the π ′ c =
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138 Chapter A: About the π ′ c =
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140 Chapter B: Publications
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142 Chapter B: Publications Computa
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144 Chapter B: Publications p ′ (
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148 Chapter B: Publications LES suc
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150 Chapter B: Publications SPL (dB
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152 Chapter B: Publications direct
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154 Chapter B: Publications [7] A.
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156 Chapter B: Publications Extract
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158 Chapter B: Publications As a co
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160 Chapter B: Publications SPL (dB
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