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Prof. J.E. Ffowcs Williams
Prof. A.P. Dowling
Acoustic waves often play a major rôle in the self-excited oscillations of fluid flows. Earlier work on the thermoacoustic oscillations of a ducted flame has been extended to include realistic afterburner geometries(A7,A25). This linear theory is now being used by industry to avoid damaging pressure perturbations in the next generation of aircraft engines. For simpler geometries, the model has been extended to include nonlinear effects, thereby predicting both the amplitude and frequency of the oscillations at unstable operating conditions(A9).
Flow instability can also occur when an underexpanded jet impinges on a plane surface, a geometry that is relevant to vertical take-off aircraft. A theory has been developed, which highlights the physical mechanisms involved in these self-excited oscillations(A20), and compares well with experimental data for both moderately and highly(A21) underexpanded jets.
The noise of the turbulent boundary layer flow over a sonar system can mask an incoming sound signal. Work is in progress to identify the contributions to flow noise from the coherent vortical structures in the boundary layer(A8). This not only provides physical insight into the important noise producing events in a turbulent flow, but also indicates how the flow and boundary should be modified to reduce the noise.
All these phenomena are part of the aeroacoustics subject which is developing more and more to incorporate concepts of feedback control that, by eliminating unstable states, can transform the character of important high Reynolds number flows(A10).
Medical applications of aeroacoustics have led to good modelling of the biomechanics of snoring in humans and new ideas on the neuro-mechanics of respiratory instabilities(A18). Analysis of the snoring sounds is pointing to a technique(A27) for differentiating between the various instability mechanisms and allowing more confidence to be placed at the clinical diagnosis phase of treatment.
A significant amount of noise in aircraft cabins is due to the external flow. A simplified model for predicting this contribution has been developed(A13,A14). A new mathematical technique for evaluating summations of a form common in acoustics and electromagnetics has also arisen from this work(A16).
Prof. W.N. Dawes
An exciting development within the Department over the past year is the way we have been building on the intersections of the interests of a number of colleagues to produce a coherent, multi-disciplinary CFD group. A variety of research activities(A11,A12,A17) is now in place, imminent, or planned, combining expertise in acoustics, combustion and fluid mechanics bound together by a variety of CFD(A5,A6) codes. The strength of this collaboration is such that HEFCE rewarded us with a major grant under the Joint Research Equipment Initiative to acquire state of the art computer and visualisation systems.
Research (sponsored by the CEC, DRA, EPSRC, Rolls-Royce plc, and Shell Research Ltd) has continued with the aim of further improving the control and prediction of transition, turbulence, and combustion phenomena encountered in design and safety evaluations for complex engineering flows(A31). Work on aerofoil manipulator devices for turbulent flow control has concluded with a study of practical performance under flight-representative conditions(A35). The low-Re Reynolds Stress/Intermittency Transport model, developed to predict by-pass transition within the framework of the ERCOFTAC Transition Special Interest Group project coordinated from Cambridge(A33), has been refined by reference to Direct Simulation data as part of an EPSRC High Performance Computing Consortium(A29). It has been demonstrated that the model can now also handle non-local effects(A30), including the influence of variable free-stream turbulence length scale(A32), and effects of wake turbulence and flow curvature(A28). A method for extending the same approach to natural transition for external aerodynamic applications has been proposed(A34).
Work has now been completed on an experimental and theoretical investigation of non-planar wings in subsonic flow. It has been shown that a wing with downward sloping tips has a lower drag than a flat wing.
Dr D.J. Maull
Dr W.R. Graham
Research into the formation and evolution of trailing vortices behind aircraft has continued. A preliminary experimental study of prediction methods has been completed(A15), and work is proceeding under the "Eurowake" programme.
The theoretical analysis of three dimensional boundary layer flows has predicted the onset of a new and powerful instability mechanism that coincides with the observed location for the onset of laminar-turbulent transition. The experimental study has confirmed the presence of this new instability mechanism at the onset of transition. It is likely that these findings will have implications for other three-dimensional boundary layer flows, such as swept-wing flows.
Dr H. Babinsky
The control of shock/boundary-layer interactions has been studied extensively. Investigations into the use of passive control mechanisms are nearing completion and the results regarding three-dimensional interactions have been reported as part of the EUROSHOCK I programme(A39). The two-dimensional investigation is still under way. The findings indicate that a reduction of wave drag on transonic wings can be achieved, however an increase of viscous drag is also observed. It is hoped that active control incorporating suction can avoid the increase of viscous drag while retaining the beneficial effects on wave drag.
A new programme (EUROSHOCK II) has started which investigates the use of suction to actively control the shock/boundary-layer interaction. For this purpose one of the supersonic wind tunnels is being modified to incorporate an ejector based suction system. First tests with the new device are expected in the next year.
Papers on earlier work(A1,A2) on liquid crystal thermography, supersonic turbulent separation and roughness effects(A3) have now been published.
Dr W.R. Graham
The development of accurate low order models and effective control strategies is essential if unsteady fluid flows are to be controlled. In collaboration with MIT, the application of the Proper Orthogonal Decomposition and Optimal Control theory to the attenuation of vortex shedding behind a cylinder has been investigated(A36). Future research will concentrate on extending current formulations to more general geometries and types of control action.
Dr J.F.A. Sleath
Work has continued on coastal bottom boundary layers and sediment transport due to wave action and preliminary results have been submitted for publication. Work on wave/current interactions over rippled beds continues(A26). Particular attention is being paid to the way in which the drift currents are modified by bed roughness.
Dr P.A. Davidson
Fluid mechanics plays a critical role in the production and casting of metals. For example, aluminium is produced by electrolysis in large reduction cells. This process is highly energy intensive, requiring around 30 Mwatt hrs to produce one tonne of semi-finished product (about seven times the energy required to produce steel). Yet the bulk of the energy expended in reduction cells is not required for electrolysis, but rather is lost in resistive heating of the electrolyte. Attempts to increase the efficiency of these cells, by reducing the volume of electrolyte, have been fundamentally limited by the appearance of a magnetohydrodynamic(A4) instability which manifests itself as a sloshing of the electrolyte. The fundamental nature of these instabilities is still poorly understood.
Research Council awards in this area are supporting our work on:
A1. BABINSKY, H., EDWARDS, J.A. Automatic liquid crystal thermography for transient heat transfer measurements in hypersonic flow. Experiments in Fluids, 21, 227-236 (1996).
A2. BABINSKY, H., EDWARDS, J.A. On the incipient separation of a turbulent hypersonic boundary layer. Aeronautical Journal, 100, 209-214 (1996).
A3. BABINSKY, H., TAKAYAMA, K. CFD validation strategies for compressible flow using interferometry. 34th Aerospace Sciences Meeting and Exhibition, Reno, NV, paper AIAA 96-0438 (January 1996).
A4. DAVIDSON, P.A. Analytical models of rotary electromagnetic stirring. IMA Journal of Mathematics Applied in Business and Industry, 7, 89-108 (1996).
A5. DAWES, W.N. Construction of CAD based unstructured mesh models for 3D Navier-Stokes simulations. 15th International Conference on Numerical Methods in Fluid Dynamics, Monterey, CA (June 1996).
A6. DAWES, W.N. The generation of 3D, stretched, viscous unstructured meshes for arbitrary domains. 41st ASME International Gas Turbine and Aeroengine Congress, Birmingham, ASME paper 96-GT-55 (June 1996).
A7. DHANAK, M.R., CHAO, S.I., DOWLING, A.P. On a model for surface pressure fluctuations induced by the wall region of a turbulent boundary layer. Flow Noise Modelling, Measurement and Control, 1995 ASME International Mechanical Engineering Congress and Exposition, San Francisco, CA (November 1995); Edited by R.M. Lueptow, W.L. Keith, T.M. Farabee, 87-94, NCA vol-19/FED vol-230, (ASME, 1995).
A8. DHANAK, M.R., DOWLING, A.P. On the pressure fluctuations induced by coherent vortex motion near a surface. 26th American Institute of Aeronautics and Astronautics Fluid Dynamics Conference, San Diego, CA, paper AIAA 95-2240 (June 1995).
A9. DOWLING, A.P. Nonlinear acoustically coupled combustion oscillations. Proceedings, 2nd AIAA/CEAS Aeroacoustics Conference, State College, Pennsylvania, paper AIAA 96-1749 (May 1996).
A10. FFOWCS WILLIAMS, J.E. Aeroacoustics. Journal of Sound and Vibration, 190, (3), 387-398 (1996).
A11. FREIDRICHS, S., HODSON, H.P., DAWES, W.N. Aerodynamic aspects of endwall film cooling. 41st ASME International Gas Turbine and Aeroengine Congress, Birmingham, ASME paper 96-GT-208 (June 1996).
A12. FRIEDRICHS, S., HODSON, H.P., DAWES, W.N. Distribution of film cooling effectiveness on a turbine endwall measured using the ammonia and diazo technique. Proceedings, ASME International Gas Turbine and Aeroengine Congress and Exposition, Houston, TX ASME Paper 95-GT-1 (June 1995).
A13. GRAHAM, W.R. Boundary layer induced noise in aircraft. Part 1: the flat plate model. Journal of Sound and Vibration, 192, (1), 101-120 (1996).
A14. GRAHAM, W.R. Boundary layer induced noise in aircraft. Part 2: the trimmed flat plate model. Journal of Sound and Vibration, 192, (1), 121-138 (1996).
A15. GRAHAM, W.R. Experimental assessment of the extended Betz method for wake vortex prediction. Proceedings, 78th AGARD Fluid Dynamics Panel Symposium, Trondheim, Norway, 9-1-9-12, AGRAD-CP-584 (May 1996).
A16. GRAHAM, W.R. Stationary phase for summations. (Letters to the Editor) Journal of the Acoustical Society of America, 99, (5), 3233-3235 (1996).
A17. HODSON, H.P., DAWES, W.N. On the interpretation of measured profile losses in unsteady wake turbine blade interaction studies. 41st ASME International Gas Turbine and Aeroengine Congress, Birmingham, ASME paper 96-GT-494 (June 1996).
A18. HUANG, L., GUZ, A., FFOWCS WILLIAMS, J.E. Neuro-mechanics of respiration. Proceedings, 19th IUTAM International Congress of Theoretical and Applied Mechanics, Kyoto, Japan, 774 (August 1996).
A19. JIANG, Z., BABINSKY, H., MEGURO, T., TAKAYAMA, K. Numerical study on transient shock wave flows in a tube with a sudden change in its cross section. 34th Aerospace Sciences Meeting and Exhibit, Reno, NV, paper AIAA 96-0291 (January 1996).
A20. KUO, C.Y., DOWLING, A.P. Oscillations of a moderately underexpanded choked jet impinging upon a flat plate. Journal of Fluid Mechanics, 315, 267-291 (1996).
A21. KUO, C.Y., DOWLING, A.P. Shock oscillations of underexpanded jets impinging upon flat plates. Proceedings, 2nd AIAA/CEAS Aeroacoustics Conference, State College, PA, paper AIAA 96-1747 (May 1996).
A22. LINGWOOD, R.J. Absolute instability of the boundary layer on a rotating disk. Journal of Fluid Mechanics, 299, 17-33 (1995).
A23. LINGWOOD, R.J. Absolute instability of the rotating-disk boundary layer - theory and experiment. Advances in Turbulence VI, Proceedings, 6th European Turbulence Conference, Lausanne, Switzerland (July 1996); Edited by S. Gavrilakis et al, pp 365-368 (Kluwer, 1996).
A24. LINGWOOD, R.J. An experimental study of absolute instability of the rotating-disk boundary-layer flow. Journal of Fluid Mechanics, 314, 373-405 (1996).
A25. MacQUISTEN, M.A., DOWLING, A.P., Combustion oscillations in a twin stream afterburner. Journal of Sound and Vibration, 188, (4), 545-560 (1995).
A26. MARIN, F., SLEATH, J.F.A. Interaction houle-courant au-dessus d'un fond de rides. Journees Nationales: Genie Cotier - Genie Civil, Dinard, France, 79-86 (April 1996).
A27. QUINN, S.J., HUANG, L., ELLIS, P.D.M., FFOWCS WILLIAMS, J.E. The differentiation of snoring mechanisms using sound analysis. Clinical Otolaryngology, 21, 119-123 (1996).
A28. SAVILL, A.M. COST-ERCOFTAC SIG 10: Transition, BRITE-EURAM AERO-CT92-0050 project workshop, Thessalonki, Greece, 21-23 September 1995: meeting report, ERCOFTAC Bulletin, 29, 7-9 (1996).
A29. SAVILL, A.M. Exploiting DNS data for improved turbulence modelling. Proceedings, High Performance Computational Engineering in the UK Conference, Daresbury Laboratory, DRAL (March 1996).
A30. SAVILL, A.M. Non-local and inhomogeneity corrections to stress transport modelling of transition. Proceedings, 7th UMIST Biennial Colloquium on Computational Fluid Dynamics, Manchester, paper 2, 5 (May 1996).
A31. SAVILL, A.M. One point closures applied to transition. In: Turbulence and Transition Modelling, ERCOFTAC/IUTAM Summer School, Stockholm, Sweden (June 1995); Edited by D. Hennington, et al, Chapter 6, 233-268 (Kluwer Academic, 1996).
A32. SAVILL, A.M. Predicting the effect of variable free-stream turbulence length-scale on transition with turbulence models. Proceedings, 6th European Turbulence Conference, Lausanne, Switzerland (July 1996).
A33. SAVILL, A.M. Progress in the prediction of by-pass transition; a report on the collaborative testing exercise. ERCOFTAC Report-Back on Turbulence Modelling Workshops , Lausanne, Switzerland (June 1996).
A34. SAVILL, A.M. Transition prediction with turbulence models. In: Transitional Boundary Layers in Aeronautics, Proceedings of the Royal Netherlands Academy of Arts and Sciences; Edited by R.A.W.M. Henkes, J.L. van Ingen, 311-319 (Elsevier, 1996).
A35. SQUIRE, L.C., SAVILL, A.M. Experimental results on aerofoil manipulators at high subsonic speeds. Experiments in Fluids, 21, (4), 275-285 (1996).
A36. TANG, K.Y., GRAHAM, W.R., PERAIRE, J. Active flow control using a reduced order model and optimum control. 27th American Institute of Aeronautics and Astronautics Fluid Dynamics Conference, New Orleans, LA, paper AIAA 96-1946 (June 1996).
A37. WOODBURN, P.J., BRITTER, R.E. CFD simulations of a tunnel fire - Part I. Fire Safety Journal, 26, 35-62 (1996).
A38. WOODBURN, P.J., BRITTER, R.E. CFD simulations of a tunnel fire - Part II. Fire Safety Journal, 26, 63-90 (1996).
A39. YEUNG, A.F.K., SQUIRE, L.C., FAUCHER, X. The passive control of the interaction between swept shocks and boundary layers. EUROSHOCK Report TR AER29249/1.2, European Community, Brussels, project number 2064 (1996).