Most fluid flows of practical importance are fully three-dimensional, so the non-linear instability properties of three-dimensional flows are of particular interest. In some cases the three-dimensionality may have been caused by a finite amplitude disturbance whilst, more usually, the unperturbed state is three-dimensional. Practical applications where transition is thought to be associated with non-linearity in a three- dimensional flow arise, for example, in aerodynamics (swept wings, engine nacelles, etc.), turbines and aortic blood flow. Here inviscid `cross-flow' disturbances as well as Tollmien-Schlichting and Görtler vortices can all occur simultaneously and their mutual non-linear behaviour must be understood if transition is to be predicted. The non-linear interactions are so complex that usually fully numerical or combined asymptotic/numerical methods must be used.
Moreover, in view of the complexity of the instability processes, there is also a growing need for detailed and accurate experimental information. Carefully conducted tests allow us to identify those elements of a particular problem which are dominant. This assists in both the formulation of a relevant theoretical problem and the subsequent physical validation of predictions. It should be noted that the demands made upon the skills of the experimentalist are high and that the tests can be extremely sophisticated - often making use of the latest developments in flow diagnostic techniques, automated high speed data gathering, data analysis, fast processing and presentation.

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Preface. 1. Behaviour of Oblique Waves. The Effect of Nonlinear Critical Layers on Boundary Layer Transition; M.E. Goldstein. Experimental Investigation of Laminar-Turbulent Transition Process in Supersonic Boundary Layer Using Controlled Disturbances; Yu.G. Ermolaev, et al. An Active Resonant Triad of Mixed Modes in a Symmetric Shear Flow; X. Wu. Wave Packets Described by the Uniformly Valid Model of Three-Dimensional Boundary Layer; I.V. Savenkov, E.D. Terent'ev. Experiments on a 2:1 Resonance in the Blasius Boundary Layer; J.J. Healey. Linear and Nonlinear Unstable 3D Waves for Boundary Layers in Differentially Heated Enclosures; R.A.W.M. Henkes, P. Le Quéré. Non-Linear Three-Dimensional Wave Packets in Axisymmetric Flows; I.V. Savenkov. 2. Görtler Instability. The Nonlinear Evolution of Inviscid Görtler Vortices in 3-D Boundary Layers: The Effects of Non-Dominant Viscosity in the Critical Layer; A. Dando. On the Secondary Instability of Görtler Vortices in Three-Dimensional Boundary Layers; S.R. Otto. On the Nonlinear Evolution of Görtler Vortices in Curved Mixing Layers; S.O. Seddougui, S.R. Otto. The Nonlinear Evolution of Inviscid Secondary Instabilities of a Fully Nonlinear Görtler Vortex Flow; N.D. Blackaby. 3. Instability and Transition in Swept-Wing Flow. Generation Development and Interaction of Instability Modes in Swept-Wing Boundary Layers; Y.S. Kachanov. Non-Linear Instability and Transition in Flow Near a Swept Leading Edge; M. Danks, D.I.A. Poll. Design and Control of Crossflow Instability Field; Y. Kohama, et al. Influence of Spanwise Pressure Gradient on Transition Prediction in Transonic Flow; C. Masson, et al. Nonlinear Waves in Three-Dimensional Boundary Layer;O.S. Ryzhov, E.D. Terent'ev. Spiral Instability of Fanning Swirling Jets; V. Shtern, U. Dallmann. 4. Non-Linear Processes. Non-Linear Evolution of Point-Source Disturbances in an Adverse Pressure Gradient Laminar Boundary Layer; A. Seifert. The Non Linear Behaviour of Modulated Tollmien&endash;Schlichting Waves; A.F. Medeiros, M. Gaster. Non-Linear Instability and 3-D Transition in the Flow Past a Circular Cylinder at Low Reynolds Number; H. Persillon, et al. Nonlinear Development of Travelling Waves in a Three-Dimensional Boundary Layer; W. Müller, et al. Generation of Secondary Instability Modes by Localized Surface Suction-Blowing; M.V. Ustinov. 5. DNS and PSE. Nonlinear Analysis with PSE Approach; G. Casalis, et al. On the Birth and Evolution of Disturbances in Three-Dimensional Boundary Layers; F.P. Bertolotti. Nonlinear Crossflow Disturbances and Secondary Instabilities in Swept-Wing Boundary Layers; M.R. Malik, et al. 6. Cross-flow Instability. Cross-Flow Instability with Periodic Distributed Roughness; T.C. Korke, F.K. Knasiak. Non-Linear Transition Paths in Boundary Layers with Cross-Flow; F.T. Smith. Nonlinear Crossflow Saturation in Three-Dimensional Boundary Layers; W. Koch. Influence of Cross-Flow Influence on Nonlinear Tollmien&endash;Schlichting/Vortex Interaction; D. Davis, F.T. Smith. On the Nonlinear Evolution of a Stationary Cross-Flow Vortex in a Fully Three-Dimensional Boundary Layer Flow; J.S.B. Gajjar. 7. Wall Effects and Stability Modifiers. Effect of Wall Compliance on Rotating-Disc Boundary-Layer Stability; P.W. Carpenter, A.J. Cooper. Nonintegrable, Multiple Scale Formulations; N. Daher. 8. Transition