The project aimed to develop a preliminary configuration of an optical sensor that is able to detect sub-mum-degradations of microstructured coatings (e. g. riblets) with highest accuracy. The availability of such sensor is mandatory for the establishment of drag-reduction riblet structures on aircraft wings and/or body components. Any type of degradations of the riblet structure are unfavourable for their functionality and negatively affect the aspect of fuel savings. This makes a sensor necessary, that allows to measure the state of riblet degradation with highest accuracy in the course of aircraft maintenance.
We proposed an optical sensor whose principle function is based on a simple physical phenomenon: the scattering of light. This sensor probes the riblet surface with a coherent laser beam and analyses the scattering pattern in direction of reflection.
Particularly the angular intensity distribution of the scattering pattern is unambiguous for the shape of the microstructure. Any type of riblet-degradation is causally related to an alteration of the scattering pattern. Taking into account a periodic riblet microstructure of triangular shape with degradation, the scattering intensity distribution becomes complex. Hence, a great effort is required for the clear determination of type and state of degradation from the scattering pattern.
We addressed this task by combined theoretical and experimental studies of the scattering pattern. Theory yields calculations of the scattering pattern in the far-field for a variety of riblet shapes, spatial frequencies and for both non-degraded and degraded riblets. These calculations were compared with experimental studies on differently coated riblet structures, i.e. on real structures. Parameters that are characteristic for the device function (accuracy and signal-to-noise ratio) were defined. The project ended with a preliminary configuration for an optical sensor with optimum specifications.
With the application of appropriate surface structuring on aircrafts, up to 8% fuel may be saved in regular air traffic. This not only decreases costs, but especially reduces exhaust of greenhouse gases significantly. Before these techniques can be introduced into productive environments, a controlling method for the quality of surface structuring had to be established to be used during fabrication and service, ensuring persistent quality of the structured coatings and a justified decision for surface renewal.
In this project, these important requirements for achieving the improvements defined above are fulfilled. We have shown that fast sampling is possible using noncontacting laser probing, and we have presented a working preliminary configuration for the sensor.
In the theoretical part, a model for the interaction between a probing laser beam and the surface was developed and the resulting wave front is derived. This was done using a combination of Huygens-Fresnel diffraction theory and geometrical optics. The model was then used to counsel the design of the experimental setup, to interpret the emerging data and to develop characteristic quantities for the sample, their derivation from the data and their signal-to-noise ratio.
In the experimental part, the interaction of laser light with the structured riblet surface was studied. For this purpose, an optical setup was installed to perform measurements of undegraded and degraded surfaces depending on a variety of experimental parameter like probe wavelength or angle of incidence.
The results of these measurements in the form of intensity distributions as a function of angles were constantly compared and checked with the theoretical calculations. A preliminary configuration of an optical setup with an optimized laser system was now available for further studies.
It allowed for very sensitive measurements of even slight degradations of the surfaces.
Here, it is regardless if the damage to the riblets is symmetrical or asymmetrical due to mechanical loss of material or if it is deriving from changes of reflectivity due to chemical processes of the riblet material itself.
A fast implementation in commercial products should be possible on the basis of this report.