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Delegates are invited to meet and discuss with the poster presenters in this topic directly after the session 'Aerodynamics and rotor design' taking place on Wednesday, 12 March 2014 at 09:00-10:30. The meet-the-authors will take place in the poster area.

Volkmar Stenzel Fraunhofer IFAM, Germany
Co-authors:
Volkmar Stenzel (1) F P Andreas Momber (3) Yvonne Wilke (1)
(1) Fraunhofer IFAM, Bremen, Germany (2) Muehlhan AG, Hamburg, Germany (3) Fraunhofer IFAM, ,

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Abstract

Improvement of aerodynamic efficiency by functional coating

Introduction

Riblet- (shark-skin-) surfaces have proven to be efficient in the reduction of surface drag in different fields of application (aircrafts, ships, swimming, sailing-boats, etc.). The challenges of application of such surfaces on huge 3-dimensional surfaces like rotorblades are:
- ability to be produced on double-curved surfaces
- durability of the aerodynamic effect
- cost of application
The ability of such surfaces for the improvement of aero- or hydrodynamic performance of aircrafts and ships has been proven and validated. A new project has demonstrated how windplants could profit from this technology.


Approach

It has been known for some time that surfaces having a certain microstructure provide lower drag to liquids and gases under turbulent flow conditions.
A feature of such a microstructure is so-called “riblets” of well-defined shape and size oriented parallel to the flow direction. In popular scientific jargon, the term “artificial shark skin” is used for such surfaces. Figure shows an example of a low-drag microstructure produced at IFAM on a paint film.
Such structures could significantly reduce fuel consumption and improve performance (e.g. increase in speed) in a variety of application areas, including aircrafts, wind power turbines, rail vehicles, ships and pipelines.

Proof of the aerodynamic efficiency of such structures has been obtained from for example an Airbus A340 that was in scheduled service with Cathay Pacific Airways . A structured film was bonded to about 30% of the surface of this aircraft. Despite having the additional weight of the film and although not the whole surface was covered, it was demonstrated that the aircraft consumed about 1.5 % less kerosene.
There are however a number of reasons why bonding a film to the surface is not optimal:
- the bonding process, in particular to paraboloidal surfaces, is very difficult and sometimes impossible
- the film introduces additional weight that partly compensates the fuel saving
- the bonded film can become detached
- the film or adhesive can cause yellow discoloration when used externally
- the removal of the film increased the stripping-time for aircrafts significantly(the paint film on aircrafts has to be removed every 5 years)

In order to overcome these disadvantages, a process has been developed at IFAM that directly gives structure to a paint (see Figure 1). This involves an embossing step with simultaneous radiation curing.



Main body of abstract

Since the aerodynamic efficiency of riblet structures is proven the focus of current work lies on the proof of durability of such structured materials. The surfaces suffer from degradation by intensive UV-light, cleaning procedures (rotating brushes) and erosion. The goal of a current project is the improvement of the durability of riblet-structured paint surfaces and the measurement of the effect of wear on the drag-reducing properties.

The application process of a microstructured paint on large surfaces is based on a method that combines application, embossing and curing in one single process. A sketch of the application tool is shown in Figure 2 .

During application the device is moved over the surface to be coated. The process comprises the following steps:
- application of a paint film on the embossing foil (Item 1)
- the paint film moves under the soft roller and is pressed onto the surface and structured (because the foil carries the negative structure)
- the paint film is (partly) cured with the UV-lamp shining through the transparent foil
- the device leaves a track of micrstructured paint behind
The paint film has to have a portion of UV-curable resin that leads to a tack-free and non reflowing film. Several paint compositions are developed for usage on aircrafts (focus on UV-stability, flexibility and erosion resistance) and on ships (focus on water-stability, fouling-release or anti fouling properties). The paint compositions are usually dual-cure systems consisting of the UV-curable resin combined with other resins that give the required durability (e.g. aliphatic polyurethanes for aircraft usage). The paint film for aircraft application has to be reinforced by nanoparticles.
In order to investigate the durability of the coating and in particular the decrease of the aerodynamic effect due to wear a test program, consisting of lab-tests and in service investigations has been set up. The test procedure is illustrated in Figure 3 :

The test procedure consists of the following steps:
- Application of test fields on several areas of the aircraft
- Regular molding of the test fields after each 4-6 weeks in service
- Measuring the topography of the worn microstructure and creation of a CAD-file
- Production of an upscaled model via rapid prototyping
- Measurement of the model in an oil channel and generation of drag reduction data
The in-service test will be continued.
In parallel measurements of the drag-reducing effect of riblets applied on airfoils and on torpedo-like specimen have been performed in a wind-tunnel and in a water-channel respectively.

Since producibility and durablty could be proven the next question was: Can wind energy benefit from this type of coating in the same way?
In order to answer this question a test campaign was carried out, using a wind-channel model of a DU-W-300 profile (figure 4). This model was tested with the new riblet-coating and with a smooth coating for comparison. The test was performed with 50 m/s and 75 m/s wind speed.
The measurements of lift and drag have shown that the riblet-coated model gave an approx. 30% increase of lift/drag ratio (at relevant angles of attack).


Conclusion

Surface-drag-reduction values for the application of riblet-surfaces of water –vehicles by 5.2 % have been measured. A reduction of 6.0 % total drag of a wing-profile could be measured in a wind-channel experiment on an aircraft profile. An incease of lift/drag ration by 30% was measured on a windenergy profile in an appriate wind-channel experiment.
These values clearly indicate the potential of the riblet-technology for fuel-saving oin the transportation sector or for increase of energy production of windplants.
Of course for a given riblet spacing the drag reduction is optimal for a particular range of Reynolds numbers. That means that for e.g. an aircraft with a typical cruising speed or for a container freighter that is designed for specific cruising conditions an optimal riblet-spacing can be applied and a high benefit can be expected.
If an application is intended on objects that operate with very different speeds in air or water, the benefit that can be achieved from riblets should be calculated very carefully before the effort to apply such surfaces is made.

The production of a drag-reducing paint surface on large parts of ships and aircrafts is feasible. A nanoparticle-reinforced paint based on aliphatic polyurethane resins and UV-curable urethane-acrylates possesses a durability that shows a high probability for a lifetime of a microstructured surface of several years under aviation in-service conditions. Clearly the potential to fulfill the requirements of lifetime for wind-energy applicatons is given.

The project led to following conclusions:
- The production of a drag-reducing paint surface for ships, aircrafts and wind-rotorblades is feasible
- The durability under service conditions (aircraft) turned out to be promising
- The measured drag-reducing effects for aircratfs and ships range from 6.25% to approx. 3.5%
- The effect on aerodynamic profiles is higher, because additionally to the reduction of drag there is a delay of separation
- The efficiency of wind-rotorblades could be improved significantly by application of riblet-structured coatings.





Learning objectives
- Aerodynamic effect of riblet-surfaces
- Application-technology for riblet-structured paints
- Properties (durability etc.) of riblet-structured paints
- Benefit for wind-rotorblades
- Next stes to be carried out in order to utilize this technology in practice



References
1.. D.W. Bechert, M. Bruse, W. Hage, Experiments with three-dimensional riblets as an idealized model of shark skin, Experiments in Fluids 28(2000) 403-412
2. Jane’s: All the World’s Aircraft 1997-1998, Jane’s Information Group, Coulsdon, Surrey, England (1997)
3. Patents DE 10346124 B4 and DE 102006004644 B4
4. V. Stenzel, Y. Wilke, W. Hage, Drag-reducing paints for the reduction of fuel consumption in aviation and shipping, Progress in Organic Coatings, Volume 70, Issue 4, April 2011, Pages 224–229