Test section with the differential skin friction balance of the Berlin oil channel, the blue arrows are indicating the flow direction
Vortex structure over a riblet test surface, picture taken during PIV measurements
Numerical and Experimental Investigation to Reduce the Turbulent Drag by Means of Moveable Lamellas
The skin of fast swimming sharks is covered with a tiny grooved structure, which decreases the turbulent skin friction. This passive flow control method can be applied in the form of so called riblets as a technical application; a maximal skin friction reduction of 10% is possible.
In this project (sponsored by the German Research Foundation) the principle of the shark skin is combined with another flow control method. The very effective mechanism of drag reduction is an active flow control method: the introduction of lateral momentum due to a lateral oscillating wall. The combination of both mechanisms is studied by means of a DNS-simulation (at the ISTA of the TU Berlin) and by experimental investigation in oil channel of the DLR on a surface with a movable ribbed surface.
In a first approach the ribs are moved in a specific manner to gain insight into the influence of the different parameters like frequency and amplitude of the rib movement, rib height and distance and the Reynolds number dependency of the mechanism. The long term goal is to design a passively working flow control mechanism; in the next step a flexible design of the ribs is planned which should be driven by the fluid/structure interaction.
Riblets on Compressor Vanes
The manufacturing of hydraulic smooth surfaces for higher stages of multi-stage compressors is very complex due to the thin boundary layer. Especially for higher stages of a compressor this is not possible because of the very thin viscous sub layer of the boundary layer. Therefore it is interesting to design the manufacturing related roughness in a way that they do not cause additional drag; at best it should reduce the turbulent skin friction. This is the case when the manufacturing related roughness shows small tiny stream wise aligned grooves.
The project (sponsored by the German Research Foundation) clarifies the drag reducing properties of surfaces which are related to the manufacturing process. These surfaces are described with parameters common in surface measurement technique to be compared to optimised riblet geometries. The experimental investigation started with flat plate experiments and was carried on with experiments on compressor blades. The chosen manufacturing processes were laser ablation and grinding with dressed grinding disks. The thus produced riblet surfaces are examined in the Berlin oil channel in an enlarged scale.
The project is taking place in a multidisciplinary collaboration with the Institute for Measurement and Automatic Control (IMR), the Laser Zentrum Hannover e.V., the Institute of Production Engineering and Machine Tools (IFW) and the Institute for Turbomachinery (IFS) of the University of Hannover
''Production of low-drag surfaces on large structures''
Previous studies have shown that under turbulent flow conditions a surface with a fine ribbed structure generates a lower drag than a smooth, non-structured surface. Experimental tests involving a variety of applications (aircrafts, sailing yachts and laboratory experiments) have confirmed this.
Currently, the only means of creating these microstructures on large objects is the application of suitable films. However, this does have some disadvantages and is not possible in all instances.
Therefore the objective of the present project is to develop a process with a suitable coating material by means of which a structured coating can be applied to large-area surfaces and also to surfaces having complex curvatures. The application, structuring and drying of the coating shall be carried out in a single step. A small prototype of the application apparatus has been built and tested in a previous feasibility study, funded by the VolkswagenStiftung.
Finally, practical tests of the riblet paint structure will be carried out on a large object such as an aircraft or a rotor blade of a wind turbine. Amongst others the determination of the abrasion properties will be evaluated. Furthermore, the aerodynamic effectiveness of the worn surface will be determined by measurements in an oil channel on an enlarged structure, so that the fluid-mechanical parameters in air and oil are comparable.
Enlarged riblet surface model for experiments in the Berlin oil channel. The model equates to a riblet surface exposed to an abrasive test.