EXPERIMENTS ON DRAG-REDUCING SURFACES AND THEIR OPTIMIZATION WITH AN ADJUSTABLE GEOMETRY

Previous research has established that surfaces with tiny ribs (riblet s) aligned in the streamwise direction can reduce the turbulent wall-s hear stress below that of a smooth surface. Typical skin-friction redu ctions have been found to be about 5%. The results of the present inve stigation, however, demonstrate a considerable improvement over this v alue. This improvement is achieved by a systematic experimental optimi zation which has been guided by theoretical concepts. A key feature of our experiments is the utilization of an oil channel. Previous experi ments in wind tunnels had to contend with very small riblet dimensions which typically had a lateral rib spacing of about 0.5 mm or less. By contrast, in our oil channel, the ribs can have a lateral spacing of between about 2 and 10 mm. This increased size of the surface structur es enables test surfaces to be manufactured with conventional mechanic al methods, and it also enables us to build test surfaces with adjusta ble geometry. In addition, the Berlin oil channel has a novel shear st ress balance with an unprecedented accuracy of +/-0.3 %. This latter f eature is a prerequisite for a systematic experimental optimization. I n the present investigation, surfaces with longitudinal ribs and addit ional slits are studied. The experiments cover a fairly large range of parameters so that the drag reduction potential of a surface with rib s and/or slits is worked out conclusively. A large parameter range is made possible because of the adjustability of the surfaces as well as the automatic operation of the oil channel. In particular, the followi ng tests were run. (i) Shear stress measurements with conventional rib let con figurations, i.e. with triangular and semi-circular grooves, h ave been carried out. These measurements were necessary in order to es tablish the connection between our oil channel data and previous data from wind tunnels. As was previously established, we found a drag redu ction of about 5%. (ii) An adjustable surface with longitudinal blade ribs and with slits was built and tested. Both groove depth and slit w idth could be varied separately and continuously during the experiment . It turned out, that slits in the surface did not contribute to the d rag reduction. Nevertheless, these investigations show how perforated surfaces (e.g. for boundary-layer control) can be designed for minimal parasitic drag. On the other hand, with closed slits, an optimal groo ve depth for the rib surface could be determined, i.e. half of the lat eral rib spacing. For this configuration, we found an 8.7% skin-fricti on reduction. By carefully eliminating deleterious effects (caused by little gaps, etc.), the skin-friction reduction could be improved to a record value of 9.9%. (iii) A quantitative comparison between theory and experiment was carried out. The theory is based on the assumption that riblets impede the fluctuating turbulent crossflow near the wall. In this way, momentum transfer and shear stress are reduced. The simp lified theoretical model proposed by Luchini (1992) is supported by th e present experiments. (iv) For technological applications of riblets, e.g. on long-range commercial aircraft, the above thin-blade ribs are not practical. Therefore, we have devised a surface that combines a s ignificantly improved performance (8.2 %) with a geometry which exhibi ts better durability and enables previously developed manufacturing me thods for plastic riblet film production to be used. Our riblet geomet ry exhibits trapezoidal grooves with wedge-like ribs. The flat floor o f the trapezoidal grooves permits an undistorted visibility through th e transparent riblet film which is essential for crack inspection on a ircraft.