Evaluation of actuator disk theory for predicting indirect combustion noise Ashish Mishra, Daniel J. Bodony n Department of Aerospace Engineering, University of Illinois at Urbana-Champaign, Urbana, IL 61801, United States article info Article history: Received 22 November 2011 Received in revised form 20 September 2012 Accepted 22 September 2012 Handling Editor: P. Joseph Available online 27 October 2012 abstract Indirect combustion noise is believed to be a key component of turbofan engine core noise, but existing experimental data have not been able to definitively determine its importance. Instead, actuator disk theory (ADT) as developed by Cumpsty and Marble [The interaction of entropy fluctuations with turbine blade rows; a mechanism of turbojet noise, Proceedings of the Royal Society of London A 357 (1977) 323–344] is commonly used to estimate its contribution based on combustor exit conditions and changes in the mean flow across blade rows. The theory, which assumes planar propagation of acoustic, entropic, and vortical waves in the long wavelength limit, is assessed by comparing its predictions to those from two-dimensional compressible Euler calculations of idealized entropy disturbances interacting with a 1980s era NASA turbine stator. Both low-frequency planar waves of constant frequency and higher- frequency, localized entropy disturbances are considered, with the former being within ADT’s range of applicability and the latter outside of it. It is found that ADT performs well for the cut-on acoustic modes generated by the entropy-blade interaction but its accuracy suffers for the cut-off acoustic modes, which could impact indirect combustion noise predictions for turbines with closely spaced blade rows. & 2012 Elsevier Ltd. All rights reserved. 1. Introduction Modern aircraft turbo-fan engines generate significantly less noise relative to earlier models because of increases in engine bypass ratio, improved fan designs, and the introduction of nacelle chevrons. As a result the relative importance of the engine noise sources has changed such that jet noise, though still important, is not the sole dominant noise source. Noise reduction efforts now focus more heavily on broadband fan noise and on core noise generated by the compressor, the combustor, and the turbine. Future design changes of the turbine are expected to further increase the core noise contribution to the overall sound radiated by aircraft [1]. It is commonly assumed that the compressor is a secondary noise compared to the combustor and the turbine. However, there is little consensus on how the precise mechanisms by which the combustor and turbine contribute to the overall radiated sound power. One mechanism suggests that by combustion the combustor generates a very hot, unsteady gas which, through unsteady heat release, can generate a significant amount of noise. The noise generated by this process, termed direct combustion noise, exits via the turbine and engine nozzle system before reaching the acoustic field. Direct combustion noise due to isolated reacting jets has been investigated by several investigators [2–11] with theoretical results summarized by Dowling [12]. The large-eddy simulation of a non-premixed diffusion flame by Ihme et al. [8] showed Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/jsvi Journal of Sound and Vibration 0022-460X/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jsv.2012.09.025 n Corresponding author. Tel.: þ1 217 244 3844. E-mail address: bodony@illinois.edu (D.J. Bodony). Journal of Sound and Vibration 332 (2013) 821–838