Turbulent skin friction reduction with riblets

Model of a reproduced shark skin

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Force balance of the oil channel

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Drag reducing riblet surface milled in aluminium
Skin friction reducing riblet surfaces consist of very small grooves with sharp ridges which have to be aligned with the mean flow. The grooves have to be designed in a way that they appear to the flow as a hydrodynamically smooth surface, i.e. they are almost completely submerged in the viscous sublayer. The skin friction reduction is obtained by the reduction of the turbulent spanwise motion near the wall. The riblet surfaces developed by DLR reduce the skin friction of turbulent flow up to 10%.
The Berlin oil channel was designed to investigate and optimize drag reducing surfaces (riblets). The very small structures and skin friction forces require a very accurate direct measurement technique.
The Berlin oil channel allows by a specially designed force balances accurate measurements of the skin friction of smooth and structured surfaces.
By the use of technical white oil as working fluid, the structures under investigation (e.g. riblet surfaces) can be enlarged by a factor of about one hundred compared to experiments in air or water.
By the use of the differential force balance a high measurement accuracy is obtained. The channel is driven by two ship propellers, flow velocities between 0.3 to 1.3 m/s can be reached. In the test section of the channel a fully developed turbulent channel flow is obtained. The Reynolds number of this channel calculated with the channel width 0.25 m and the mean velocity and the cinematic viscosity ν = 11.5·10-6m²/s ranges from Rech = 8,000 to Rech = 33,000.
The test facility offers the possibility to investigate the drag properties of technical roughness. This is of interest in order to reduce the production costs, for example of blades for the use in turbo machines as well as for the production of wind tunnel test models, where the quality of the model surface is of great importance.

Test section with the differential skin friction balance of the Berlin oil channel, the blue arrows are indicating the flow direction

Riblets data

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.