Engineering systems employing turbulent combustion usually have high levels of noise production, due both to direct and indirect combustion-noise sources. Direct sources, due to unsteady gas expansion in flames, have been widely studied [1–3]. Indirect sources include entropy noise, caused by entropy patches and vorticity noise, caused by vortices. Entropy patches and vortices produce sound waves as they exit the combustion chamber through a nozzle or turbine. Some sound waves are radiated into the environment, and some are reflected back into the combustion chamber. The latter can produce new entropy patches and vortices, which in turn produce sound waves as they exit the combustion chamber. Under unfavorable circumstances this results in a feedback loop which promotes combustion instability. Thermoacoustic combustion-chamber instabilities are a potential issue driven by indirect-combustion noise in aeroengines, electrical-power generation turbines [2, 3], and are a well-known problem in large solid rocket motors [4–9].

In order to cultivate fundamental understanding of complex phenomena such as indirect-combustion noise, it is standard practice to design experiments in which only one effect is dominant [10–15]. A good example of this is Bake et al.’s [10] canonical entropy-noise experiment. Moreover, this practice of studying indirect-noise sources in isolation has also successfully been used for the development of analytical and numerical indirect-combustion noise models [16–18].

Of the two indirect-combustion noise sources, entropy noise has been the most widely studied, as evidenced by the high number of citations of two seminal articles by Marble & Candel [16] and Ffowcs Williams & Howe [17]. Vorticity noise, in contrast, has received far less attention.

When it comes to vorticity noise, one should distinguish between sound produced by vorticity oriented normal to the main flow, and that produced by vorticity oriented parallel to the main flow (axial vorticity) [12, 19]. The former is relevant for large solid rocket motor applications and is an essentially-unsteady sound production mechanism [8, 9, 20, 20]. The latter is expected to be an issue in gas turbines and aeroengines, in which combustion is normally swirl stabilised. In these systems a significant permanent axial vorticity component is present the perturbations of which are a potential source of sound when interacting with the combustion chamber exit.

During this seminar, I will present contributions to the fundamental understanding of parallel component vorticity noise. The results I will discuss where obtained when I was a postdoc at the DLR’s engine acoustics lab in Berlin-Charlottenburg using a purpose built experimental setup. I will show that sound production in this setup is due to a quasi-steady mechanism. To illustrate essentially-unsteady vorticity noise, I will present work – performed when I was a PhD student at ArianeGroup/CentraleSupélec/von Kármán Institute – on normal component vorticity noise with relevance for large solid rocket motor applications.

[1] Strahle, W. C., “On combustion generated noise,” Journal of Fluid Mechanics, Vol. 49, No. 2, 1971, pp. 399–414. doi:10.1017/S0022112071002167.

[2] Dowling, A. P., and Mahmoudi, Y., “Combustion Noise,” Proceedings of the Combustion Institute, Vol. 35, No. 1, 2015, pp. 65–100. doi:10.1016/j.proci.2014.08.016.

[3] Morgans, A. S., and Duran, I., “Entropy Noise: A Review of Theory, Progress and Challenges,” International Journal of Spray and Combustion Dynamics, Vol. 8, No. 4, 2016, pp. 285–298. doi:10.1177/1756827716651791.

[4] Dotson, K. W., Koshigoe, S., and Pace, K. K., “Vortex Shedding in a Large Solid Rocket Motor Without Inhibitors at the Segmented Interfaces,” Journal of Propulsion and Power, Vol. 13, No. 2, 1997, pp. 197–206. doi:10.2514/2.5170.

[5] Hulshoff, S. J., Hirschberg, A., and Hofmans, G. C. J., “Sound production of vortex nozzle interactions,” Journal of Fluid Mechanics, Vol. 439, 2001, pp. 335–352. doi:10.1017/S0022112001004554.

[6] Anthoine, J., Buchlin, J.-M., and Hirschberg, A., “Effect of Nozzle Cavity on Resonance in Large SRM: Theoretical Modeling,” Journal of Propulsion and Power, Vol. 18, No. 2, 2002, pp. 304–311. doi:10.2514/2.5935.

[7] Hirschberg, L., Hulshoff, S. J., Collinet, J., Schram, C., and Schuller, T., “Vortex nozzle interaction in solid rocket motors: A scaling law for upstream acoustic response,” Journal of the Acoustical Society of America, Vol. 144, No. 1, 2018, pp. EL46–EL51. Doi:10.1121/1.5046441.

[8] Hirschberg, L., Hulshoff, S. J., Collinet, J., Schram, C., and Schuller, T., “Influence of Nozzle Cavity on Indirect Vortex- and Entropy-Sound Production,” AIAA Journal, Vol. 57, No. 7, 2019, pp. 3100–3103. doi:10.2514/1.J058138.

[9] Hirschberg, L., and Hulshoff, S. J., “Lumped-Element Model for Vortex-Nozzle Interaction in Solid Rocket Motors,” AIAA Journal, Vol. 58, No. 7, 2020, pp. 3241–3244. doi:10.2514/1.J058673.

[10] Bake, F., Richter, C., Muhlbauer, B., Kings, N., Rohle, I., Thiele, F., and Noll, B., “The Entropy-Wave Generator (EWG): A reference case on entropy noise,” Journal of Sound and Vibration, 2009, pp. 574–598. doi:10.1016/j.jsv.2009.05.018.

[11] Kings, N., and Bake, F., “Indirect combustion noise: noise generation by accelerated vorticity in a nozzle flow,” International Journal of Spray and Combustion Dynamics, Vol. 2, No. 3, 2010, pp. 253–266. doi:10.1260/1756-8277.2.3.253.

[12] Hirschberg, L., Bake, F., Knobloch, K., and Hulshoff, S. J., “Swirl-Nozzle Interaction Experiments: Influence of Injection- Reservoir Pressure and Injection Time,” AIAA Journal, Vol. 59, No. 7, 2021, pp. 2806–2810. doi:10.2514/1.J060291.

[13] De Domenico, F., Rolland, E., Rodrigues, J., Magri, L., and Hochgreb, S., “Compositional and entropy indirect noise generated in subsonic non-isentropic nozzles,” Journal of Fluid Mechanics, Vol. 910, 2021, pp. A5 1–31. doi:10.1017/jfm.2020.916.

[14] Wellemann, M., and Noiray, N., “Experiments on sound reflection and production by choked nozzle flows subject to acoustic and entropy waves,” Journal of Sound and Vibration, Vol. 492, 2021, p. 115799. doi:10.1016/j.jsv.2020.115799.

[15] Hirschberg, L., Bake, F., Knobloch, K., Rudolphi, A., Kruck, S., Klose, O., and Hulshoff, S. J., “Swirl-Nozzle Interaction Experiment: Quasi-Steady Model Based Analysis,” Experiments in Fluids, Vol. 62, No. 175, 2021, pp. 1–16. doi:10.1007/s00348- 021-03271-y.

[16] Marble, F. E., and Candel, S. M., “coustic disturbance from gas non-uniformities convected through a nozzle,” Journal of Sound and Vibration, Vol. 55, 1977, pp. 225–243. doi:10.1016/0022-460X(77)90596-X.

[17] Ffowcs Williams, J. E., and Howe, M. S., “The generation of sound by density inhomogeneities in low Mach number nozzle flows,” Journal of Fluid Mechanics, Vol. 70, No. 3, 1975, pp. 605–622. doi:10.1017/S0022112075002224.

[18] Leyko, M., Moreau, S., Nicoud, F., and Poinsot, T., “Numerical and analytical modelling of entropy noise in a supersonic nozzle with a shock,” Journal of Sound and Vibration, Vol. 330, No. 16, 2011, pp. 3944–3958. doi:10.1016/j.jsv.2011.01.025, cOMPUTATIONAL AERO-ACOUSTICS (CAA) FOR AIRCRAFT NOISE PREDICTION - PART A.

[19] Hirschberg, L., Hulshoff, S. J., and Bake, F., “Sound Production due to Swirl–Nozzle Interaction: Model-Based Analysis of Experiments,” AIAA Journal, Vol. 59, No. 4, 2021, pp. 1269–1276. doi:10.2514/1.J059669.

[20] Hirschberg, L., Schuller, T., Collinet, J., Schram, C., and Hirschberg, A., “Analytical model for the prediction of pulsations in a cold-gas scale-model of a Solid Rocket Motor,” Journal of Sound and Vibration, Vol. 19, 2018, pp. 445–368. doi:10.1016/j.jsv.2018.01.025.