Recent Results in Laminar-Turbulent Transition

Methodic investigations of laminar-turbulent transition in wall-bounded she ar flows under controlled conditions are essential for untangling the various complex phenomena of the transition process occurring in flows at practical conditions. They allow understanding of the instability processes of the la minar flow, and thus enable the development of tools for flow control. On the one hand the laminar flow regime can be extended by delaying transition to reduce viscous drag, and on the other hand large-scale flow disturbances or transition can be forced in order to enhance momentum and mass ex change. Thus flow separation can be prevented, or mixing of fuel and air in combustion engines enhanced, for instance. The "DFG Verbund-Schwerpunktprogramm Transition" - a cooperative priority research program of universities, research establishments and indu stry in Germany - has been launched in April 1996 with the aim to explore transition by a coordinated use, development and validation of advanced experimental techniques and theoretical/numerical simulation methods, bin ding together all the appropriate resources available in Germany. At the very beginning of the six-year research period specifically selected test problems were to be investigated by various theoretical and experimental methods to identify and possibly rule out inadequate numerical or experimental methods. With respect to experiments it was planned to use multi-sensor-surface measuring techniques, the infrared measuring technique, and particle image velocimetry (PlV) in addition to hot-wire techniques to get instantaneous images of flows in sections, on surfaces, or within the complete flow field.

Recent results in laminar-turbulent transition Includes supplementary material: sn.pub/extras

Klappentext
This volume contains 24 papers presented at the international concluding colloquium of the German priority programme (DFG-Verbundschwerpunktprogramm) "Transition", held in April 2002 in Stuttgart. The unique and successful programme ran six years starting April 1996, and was sponsored mainly by the Deutsche Forschungsgemeinschaft, DFG, but also by the Deutsches Zentrum für Luft-und Raumfahrt, DLR, the Physikalisch-Technische Bundesanstalt Braunschweig, PTB, and Airbus Deutschland. The papers summarise the results of the programme and cover transition mechanisms, transition prediction, transition control, natural transition and measurement techniques, transition - turbulence separation, and visualisation issues. Three invited papers are devoted to mechanisms of turbulence production, to a general framework of stability, receptivity and control, and a forcing model for receptivity analysis. Almost every transition topic arising in subsonic and transonic flow is covered.

Zusammenfassung
Methodic investigations of laminar-turbulent transition in wall-bounded she­ ar flows under controlled conditions are essential for untangling the various complex phenomena of the transition process occurring in flows at practical conditions. They allow understanding of the instability processes of the la­ minar flow, and thus enable the development of tools for flow control. On the one hand the laminar flow regime can be extended by delaying transition to reduce viscous drag, and on the other hand large-scale flow disturbances or transition can be forced in order to enhance momentum and mass ex­ change. Thus flow separation can be prevented, or mixing of fuel and air in combustion engines enhanced, for instance. The "DFG Verbund-Schwerpunktprogramm Transition" - a cooperative priority research program of universities, research establishments and indu­ stry in Germany - has been launched in April 1996 with the aim to explore transition by a coordinated use, development and validation of advanced experimental techniques and theoretical/numerical simulation methods, bin­ ding together all the appropriate resources available in Germany. At the very beginning of the six-year research period specifically selected test problems were to be investigated by various theoretical and experimental methods to identify and possibly rule out inadequate numerical or experimental methods. With respect to experiments it was planned to use multi-sensor-surface measuring techniques, the infrared measuring technique, and particle image velocimetry (PlV) in addition to hot-wire techniques to get instantaneous images of flows in sections, on surfaces, or within the complete flow field.

Inhalt
Invited Lectures.- On a universal mechanism of turbulence production in wall shear flows.- A general framework for stability, receptivity and optimal control.- The equivalent forcing model for receptivity analysis with application to the construction of a high-performance skin perforation pattern for LFC.- 1 Transition Mechanisms.- DNS investigations on the laminar breakdown in a three-dimensional boundary-layer flow.- Absolute/convective instability investigation of primary and secondary crossflow vortices.- Direct numerical simulation of the development of asymmetric perturbations at very late stages of the transition process.- Systematic investigations of 3D acoustic receptivity with respect to steady and unsteady disturbances. Experiment and DNS.- Three-dimensional steady disturbance modes in the Blasius boundary layer a DNS study.- 2 Transition Prediction.- Industrial view on transition prediction.- Nonlinear nonlocal transition analysis code development and results .- Direct numerical simulation of instabilities in the compressible swept Hiemenz flow.- A combined numerical and experimental investigation of transition in a laminar separation bubble.- 3 Transition Control.- Generation and control of oblique Tollmien-Schlichting waves in a Blasius boundary layer.- DNS study of discrete suction in a 3-D boundary layer.- Active control of boundary-layer instabilities on an unswept wing.- Numerical investigations of active control of boundary-layer instabilities.- On the potential and the limitations of boundary-layer stabilization via active wave cancellation.- 4 Natural Transition and Measurement Techniques.- High resolution boundary layer profile sensors.- Propagation of Tollmien-Schlichting waves in a wing boundary layer.- Investigation of transitionalstructures in artificially excited boundary layer flows by means of stereo and multi-plane PIV.- Experimental investigations of natural and controlled transition on a laminar flow airfoil.- A hot-film measuring system for free flight conditions.- 5 Transition-Turbulence-Separation.- Comparison of two large-eddy simulations for a stalled airfoil flow using different finite-volume formulations.- Vernetzte Projektgruppen.- Visualization of unsteady flow structures in a high-performance computing environment.- Index of authors.

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