Laminar-Turbulent Transition Research at DLR

Summary

DLR institutes and divisions in Braunschweig, Göttingen and Cologne are presently involved within several European projects on transition research and Hybrid Laminar Flow Control. DLR also contributes to transition research within FESTIP (Future European Space Transportation Investigation Programme) and ESA/ESTEC-TRP hypersonic flow programmes The following review, however, concentrates on recent results on the physics of laminar-turbulent transition and on non-empirical transition prediction for subsonic and transonic flows. Such investigations were performed at DLR Göttingen.

The main goal of this laminar-turbulent transition research is the construction of a reliable, non-empirical transition prediction method for transonic flows over wings of commercial airplanes. This transition-prediction method should aid industry in handling cases that are intractable using any simpler, but empirical prediction techniques. Experimental identification and theoretical / numerical modeling of both the receptivity mechanisms that generate disturbances inside the boundary layer and the subsequent mechanisms that drive transitional flows into turbulence are indispensable to reach the goal of a non-empirical transition prediction. The transition-research plan at DLR Göttingen rests on a physical insight into these processes not only as a necessary prerequisite of new and more reliable transition predictions for industrial applications but also for testing efficient laminar-flow control concepts. The present article is a shortened and updated version of the paper [1] “Status of DLR’s non-empirical transition prediction project” by St. Hein et al.. The reader is kindly asked to check that paper [1] for references on any detailed work. In addition to the results presented below we refer to the book of abstracts of the Second European Forum on Laminar Flow Technology [2], where DLR’s contributions to the wind tunnel and flight experiments utilizing so called Natural and Hybrid Laminar Flow Control technologies have been presented.

Each of DLR’s instability and transition experiments has been designed to concentrate on certain aspects of laminar-turbulent transition. This reduces the number of phenomena present in the individual experiment and thereby allows a much deeper insight into the physical mechanisms. New theories and new numerical codes had to be developed to deal with such nonlinear and nonlocal flow instabilities as well as disturbance amplifications and interactions. Moreover, some of the experiments have been supplied with disturbance generators that allow controlled excitation of disturbances within the boundary layer. Together with some advanced measurement techniques this further improves the possibilities of analysing the phenomena observed and helps to compare with theory.

Investigations of receptivity issues have considered the disturbances generated by small, steady and time-varying, localized surface roughness. Downstream, detailed measurements of the nonlinear stages of transition were made. Data on the effect of free-stream turbulence on transition were also collected [6]. Such data were used to verify and improve the theoretical and numerical instability on transition models. Due to the relatively high turbulence levels in wind-tunnels, however, the experimental results are not directly applicable to the free-flight transonic flows of interest. Thus, once the theoretical and numerical tools have been tested and proven reliable, the tools can be applied to the “quiet” flows found in actual flight.