Aeroacoustics and Compressible Flows

Introduction

The aeroacoustic research within the center relates with both low-Mach number flow applications (e.g. ducted flows) and high-Mach number flows (e.g. free and confined transonic and supersonic flows) relevant to aeronautical applications.

For low-Mach number aeroacoustics, the focus is naturally on lower frequencies, for which the coupling between the acoustic source and the surrounding geometry is strong. This implies that more details of the geometry need to be included and that phenomena such as whistling are frequent. In the low frequency regime the effect of fluid-structure interaction is more prominent. Here, sound generated by vibrating structures or the influence of a vibrating structure on the scattering of waves are studied. As examples of such fluid-structure interactions one can think of the process of voice production in human larynges as well as snoring (often associated with obstructive airway disorders).

Figure 1. Simulation of the propagation of plane waves in a duct with an orifice

Over the years, system identification techniques were developed to extract linear acoustic source data from experimental tests. For in-duct sources so called multi-port models, dependent on the geometrical configuration and frequency content of the source, are used. Typical application areas are sound generation from IC-engines, ducted fans and flow constrictions in ducts.

The coupling between sound and flow at a boundary often allows for the existence of unstable modes. Such growing modes can dissipate, generate or amplify sound and can also affect the mean flow. One important application of this type of research is liners, i.e. the perforated wall elements used to attenuate sound in for instance aero-engines. Of interest is also the related flow – acoustic interaction at sharp edges or other points of flow separation.

Recent investigations are related to sound propagation in ducts at low Mach-number flows. For example, flow-acoustic interactions that can occur in regions with flow separation are studied. For flows at a sudden area variation in ducts we have shown that for low Helmholtz numbers, the flow effects on the acoustic scattering can be described via the use of the Kutta condition, by only modelling the region in the vicinity of the sharp edge where the flow separates. Within the research area, the experimental techniques to determine transmission and reflection properties, the passive part, of complex acoustic systems have been developed. These techniques have been applied to investigate the scattering of sound at, e.g., open pipe terminations and in-duct orifice plate constrictions.

For compressible, high-Mach number flow problems related with aeronautical applications (e.g. high subsonic and supersonic jets, jet pumps) research is carried out on evaluating various noise suppression technologies. Thus, the knowledge built on understanding the problem of noise generation in hot compressible jets using high-fidelity Large Eddy Simulation (LES) calculations is employed to develop efficient flow control technologies, e.g. in supersonic jets for controlling shock-wave location and strength, reduce the acoustic noise radiation and improve performance of the propulsion device. The compressible LES flow solution allows a direct calculation of the near-field acoustics. Moreover, far-field acoustic predictions are carried based on acoustic analogies with acoustic sources calculated from the LES solution.

Figure 2. Mach number distribution and near-field acoustic pressure fluctuation corresponding to a supersonic jet exhausting a convergent-divergent nozzle geometry.

Research Overview

The overall research goals associated with the Aeroacoustics and Compressible Flows research within FLOW are:

  • To obtain detailed understanding of the generation and scattering of sound in internal flows for efficient acoustic characterization of in-duct low-Mach number flows.
  • To combine acoustic and flow control in internal flows with application to adaptive liners and new concepts for noise control.
  • To provide physics-based guidance for development of more efficient noise suppression technologies towards environmental friendly propulsion systems.

Research environment

The research within the area of Aeroacoustics and Compressible Flows is carried out at the Marcus Wallenberg Laboratory KTH (MWL) and within the Applied CFD group at KTH-Mechanics. The research at MWL has concentrated on low Mach number internal flows, such as flows in ducts and pipes. In particular the work has focused on the development of linear aeroacoustic models and experimental techniques. The aeroacoustics research at KTH-Mechanics looks into understanding sound/noise generation mechanisms in flow scenarios related with turbomachinery, turbulent jets, or biological flows (e.g. voice production).

Research groups

Facilities

  • The Marcus Wallenberg Laboratory for Sound and Vibration Research ( MWL ),
  • FLOW centre has access to the different super-computer resources managed by the Swedish Centre for Infrastructures ( SNIC ).

Collaborating organizations

GKN Aerospace, Volvo Cars, Scania, Volvo GTT, BorgWarner, University of Cincinnati, Chalmers, TU Berlin, ECL-Lyon, ONERA, COMOTI, Poli Valencia, Poli Milano, von Karmann Institute

Key publications

Gojon R., Bogey C., Mihaescu M., "Oscillation Modes in Screeching Jets.", AIAA Journal, 56(7): 2918-2924, 2018.

Ceci A., Gojon R., Mihaescu M., "Large Eddy Simulations for Indirect Combustion Noise Assessment in a Nozzle Guide Vane Passage.", Flow,
Turbulence and Combustion, https://doi.org/10.1007/s10494-018-9964-9, 2018.

Sundström E., Semlitsch B., Mihaescu M., "Acoustic signature of flow instabilities in radial compressors.", Journal of Sound and Vibration,
434: 221-236, 2018.

Lim S.M., Dahlkild A., Mihaescu M., "Aerothermodynamics and exergy analysis in radial turbine with heat transfer.", Journal of Turbomachinery, 140(9), 2018.

Gojon R., Baier F., Gutmark E., and Mihaescu M., Temperature effects on the aerodynamic and acoustic fields of a rectangular supersonic jet, AIAA paper, AIAA 2017-0002, 2017.

Semlitsch B. and Mihaescu M., Flow phenomena leading to surge in a centrifugal compressor, Energy - The International Journal, vol. 103, pp. 572-587, 2016.

Du L., Holmberg A., Karlsson M. and Åbom M., Sound amplification at a rectangular T-junction with merging mean flows. Journal of Sound and Vibration, 367, 69-83, 2016.

Sack S., Åbom M., and Efraimsson G., On Acoustic Multi-Port Characterisation Including Higher Order Modes. Acta Acoustica united with Acustica, 192(5), 834-850, 2016.

Holmberg A., Karlsson M., and Åbom M., Aeroacoustics of rectangular T-junctions subject to combined grazing and bias flows - An experimental investigation. Journal of Sound and Vibration, 340, 152-166, 2015.

Kårekull O., Efraimsson G., and Åbom M., Revisiting the Nelson-Morfey scaling law for flow noise from duct constrictions. Journal of Sound and Vibration, 357, 233-244, 2015.

Semlitsch B., Wang Y., and Mihaescu M., Flow effects due to valve and piston motion in an internal combustion engine exhaust port, Energy Conversion and Management, vol. 96, pp. 18-30, 2015.

Sundström E., Semlitsch B., and Mihaescu M., "Centrifugal Compressor: The Sound of Surge," AIAA Paper, AIAA 2015-2674,2015.

Karlsson M. and Åbom M.,Aeroacoustics of T-junctions- An experimental investigation, J. Sound and Vibration. 329(10), 1793-1808, 2010.

Kierkegaard A., Boij S. and Efraimsson G., A frequency domain linearized Navier-Stokes equations approach to acoustic propagation in flow ducts with sharp edges, J. Acous. Soc. Am. 127(2), 710-719, 2010.