Acoustic Control of Flow Instabilities

  • J. E. Ffowcs Williams


The lecture reviews the common origin of sound and instabilities in weak perturbations of a smooth flow. The degree to which sound and instabilities interact, or indeed the degree to which they are separable, depends on their semantic definition. At one level they are merely secondary unsteady effects dependent on the main flow while at the other they display distinctive characteristics of propagation and exponential growth. Even if they exist as separable entities in a pure flow they remain coupled by the inevitable inhomogeneities of realistic geometrical arrangements so that in practice flow instability and sound are actually inseparable.

Sound, being essentially linear, admits to the attenuation brought by superposition of its phase-inverted replica. Anti-sound cancels sound, a principle now being implemented in rapidly growing noise control technology. The same principle implies change in stability whenever its coupled sound field is modified. Flows that are traditionally unstable can be stabilized by active systems. Whether all flows can be stabilized or not might depend on the fundamental constraint that it is impossible to forecast the future. More probably the limit is set by technical complexity and by the need to keep all perturbations within the linear regime where superposition is valid. The scope of the idea will only become clear as definite experiments are completed.

Experiments have progressed from the control of a small combustion system with a single instability mode to the acoustic suppression of violent unsteadiness at aero-engine reheat conditions. A wing section flexibly mounted in a wind tunnel has been brought out of flutter oscillation by switching on a wall-mounted loudspeaker. The fundamental Strouhal frequency in the Karman street wake of a circular cylinder has been suppressed by acoustic feedback and both rotating stall and surge have been avoided at conditions which made those instabilities inevitable without active control. An ONR programme to examine the applicability of these methods to a real gas turbine engine has led to a power increase of some 10% and to active methods of restoring the stable condition following the onset of surge. The lecture will conclude by reviewing these developments and issues that have arisen in developing the control methods. In particular it will raise the prospect of aerofoil sections controlled to operate with radically superior characteristics and speculate on that being an element of the performance improvement observed on the test engine.


Axial Compressor Flow Instability Centrifugal Compressor Wing Section Acoustic Feedback 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.


Unable to display preview. Download preview PDF.

Unable to display preview. Download preview PDF.


  1. 1.
    Berger, E. (1967). Suppression of vortex shedding and turbulence behind oscillating cylinders. Phys. Fluids 10, S191 - S193.ADSCrossRefGoogle Scholar
  2. 2.
    Crocco, L. and Cheng, S. I. (1956). Theory of combustion instability in liquid propellant rocket motors. Interscience.Google Scholar
  3. 3.
    Day, I. J. (1991). Compressor performance during surge. Proc. Tenth Int. Symp. on Airbreathing engines. Nottingham. Published by A.I.A.A.Google Scholar
  4. 4.
    Day, I. J. (1991a). Active Suppression of rotating stall and surge in axial compressors. A.S.M.E. paper 91-GT-87.Google Scholar
  5. 5.
    Dines, P. J. (1984). Active control of flame noise. Ph.D. thesis. Cambridge University, U.K.Google Scholar
  6. 6.
    Dowling, A. P., Bloxsidge, G. J., Hooper, N. and Langhorne, P. J. (1988). Active Control of Reheat Buzz. A.I.A.A. Journal, 26, pp. 783–790.ADSGoogle Scholar
  7. 7.
    Epstein, A. H., Ffowcs Williams, J. E. and Greitzer, E. M. (1986). Active suppression of aerodynamic instabilities in turbomachines. J. Prop. Power, Vol. 5, No. 2, pp. 204–211.CrossRefGoogle Scholar
  8. 8.
    Ffowcs-Williams, J. E. and Zhao, B. C. (1989). The active control of vortex shedding. J. Fluids and Structures, 3, 115–122.ADSCrossRefGoogle Scholar
  9. 9.
    Ffowcs Williams, J. E. and Huang, E. Y. (1989). Active stabilization of compressor surge. J. Fluid Mech. 204, pp. 245–262.ADSCrossRefGoogle Scholar
  10. 10.
    Ffowcs Williams, J. E. and Graham, W. R. (1990) An engine demonstration of active surge control. Proceedings of the gas turbine and aeroengine Congress and Exposition, June 11–14, Brussels, Belgium.Google Scholar
  11. 11.
    Heckl, M. (1985). Active control of Rijke tube. Ph.D. thesis, Cambridge University.Google Scholar
  12. 12.
    Huang, X. (1988). Active Control of Aerodynamic Instabilities. Ph.D. Thesis, Cambridge University, U.K.Google Scholar
  13. 13.
    Lueg, P. (1936). Process of silencing sound oscillations. U.S. Patent # 2043, 416.Google Scholar
  14. 14.
    Ludwig, G. R. and Nenni, J. P. (1980). Tests on an improved rotating stall control system on a J.85 turbojet engine. A.S.M.E. paper 80G.T.-17.Google Scholar
  15. 15.
    Nelson, P. A. and Elliot, S. J. (1991). Active noise control; a tutorial review. Proceedings of the Acoustical Society of Japan International Conference on the Active Control of Sound and Vibration. Tokyo.Google Scholar
  16. 16.
    Olson, H. F. and May, E. G.] (1953). Electronic sound absorber. J.A.S.A., 25, 1130–1136.Google Scholar
  17. 17.
    Paduano, J., Epstein, H., Longley, J. P., Valavani, L., Greitzer, E. M. and Guenette, G. R. (1991). Active control of a rotating stall in a low speed axial compressor. Proceedings of the A.S.M.E./I.G.T.I. Conference in Orlando.Google Scholar
  18. 18.
    Pinsley, J. E., Guenette, G. R., Epstein, A. H. and Greitzer, E.M. (1990). Active stabilization of centrifugal compressor surge. A.S.M.E. Gas Turbine Conference, Brussels.Google Scholar
  19. 19.
    Sunyach, M. and Ffowcs Williams, J. E. (1986). Contrôle actif des oscillations dans les cavités excitées par un écoulement. C. R. Acad. Sc. Paris, t. 303, Série 11, No. 12.Google Scholar

Copyright information

© Springer-Verlag New York, Inc. 1993

Authors and Affiliations

  • J. E. Ffowcs Williams
    • 1
  1. 1.Engineering Dept.Cambridge UniversityCambridgeUK

Personalised recommendations