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148 Chapter B: Publications LES succeed in predicting the central recirculation zone satisfactorily. The LES on the ‘refined’ mesh is however more accurate for the outer region, particularly for the radial velocity. The fluctuating velocity field is characterized by rms profiles. Figure 5 shows that on the ‘coarse’ mesh a high overprediction of velocity fluctuations is obtained in both axial and radial components. On the fine grid however the LES clearly recovers the experimental velocity fluctuating field. 50 x = 7 mm x = 17 mm 50 x = 27 mm 50 x = 7 mm 50 x = 17 mm 50 x = 27 mm 50 40 40 40 40 40 40 30 30 30 30 30 30 20 20 20 20 20 20 z (mm) 10 0 −10 10 0 −10 10 0 −10 10 0 −10 10 0 −10 10 0 −10 −20 −20 −20 −20 −20 −20 −30 −30 −30 −30 −30 −30 −40 −40 −40 −40 −40 −40 −50 −20 0 20 40 −50 −50 −20 0 20 40 −20 0 20 40 (a) Mean Axial Velocity (m/s) −50 −50 −50 −20 0 20 −20 0 20 −20 0 20 (b) Mean Radial Velocity (m/s) Figure 4: Velocity Profiles: ◦ Experimental PIV measurements – – – LES 3 million cells, —— LES 10 million cells Acoustics and flame dynamics of the system represented by the heat release are, on the contrary, more difficult to evaluate than the mean and fluctuating velocity field. The mean value of heat release is similar in both LES and is close to the 40kW experimental thermal power, as can be observed in Fig. 6(a). Different values in the variations of heat release are however obtained for each LES. Strong and more regular fluctuations of heat release are obtained with the ‘coarse’ mesh while smaller and less periodic fluctuations are given by the ‘refined’ mesh computation. It is likely that the coarser mesh does not capture enough small turbulent scales and trigger too large turbulent eddies. These large coherent structures might clearly have an influence on the flame dynamics and thus in the large fluctuations of heat release. The value of the rate of change of heat release integrated over the whole volume of the combustor has also been computed for the two different meshes and is shown in Fig. 6(b). On the finer mesh a quieter flame is modeled, considering the smaller values of rate of change of heat release compared to those obtained from the coarse mesh. As a consequence, smaller rms pressure values should be expected on the finer mesh. Acoustics in the chamber is rather characterized by the Sound Pressure Level (SPL) at a given point than rms values of the pressure. Figure 7 compares the SPL values at the microphone 7 (see the location of M7 in Fig. 2) of the computations on the refined and coarse meshes to the experimental measurements. Both LES clearly overestimate the sound levels with a significant improve on the finer. It is then found that in order to correctly evaluate the dynamics of a flame and the acoustics generated by this one it is not enough to satisfactorily model the fluctuating velocity field as shown in Figs. 4 and 5. As stated before, computing acoustic pressure fluctuations is very challenging since these values are very small compared to the aerodynamic fields. Several additional phenomena can play an important role. First, the performed LES assume a perfect premixed mixture of air and fuel in the reactive region. This 8
149 50 x = 7 mm x = 17 mm 50 x = 27 mm 50 x = 7 mm 50 x = 17 mm 50 x = 27 mm 50 40 40 40 40 40 40 30 30 30 30 30 30 20 20 20 20 20 20 z (mm) 10 0 −10 10 0 −10 10 0 −10 10 0 −10 10 0 −10 10 0 −10 −20 −20 −20 −20 −20 −20 −30 −30 −30 −30 −30 −30 −40 −40 −40 −40 −40 −40 −50 0 10 20 −50 0 10 20 −50 0 10 20 −50 −50 −50 0 10 20 0 10 20 0 10 20 (a) RMS Axial Velocity (m/s) (b) RMS Radial Velocity (m/s) Figure 5: Velocity Profiles: ◦ Experimental PIV measurements – – – LES 3 million cells, —— LES 10 million cells 7 x 104 Time (s) 6 3 million Cells 10 million Cells ∫ ˙qdV ˙Q = 5 4 3 2 1 0 0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04 0.045 0.05 a) Heat release integrated over the entire volume (W ) 1 x 108 Time (s) ∫ (d ˙q/dt) dV V 0.5 0 −0.5 3 million Cells 10 million Cells −1 0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04 0.045 0.05 b) Rate of change of heat release integrated over the entire volume (W/s) Figure 6: Heat Release and rate of change of heat release 9
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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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42 Chapter 3: Development of a nume
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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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74 Chapter 5: Assessment of combust
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96 Chapter 6: Boundary conditions f
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Page 138 and 139: Bibliography [1] AGARWAL, A., MORRI
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Page 142 and 143: 132 BIBLIOGRAPHY [62] NICOUD, F., B
Page 144 and 145: 134 BIBLIOGRAPHY [93] SENSIAU, C. S
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