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Mitglied derHelmholtz-Gemeinschaft For the development of robust methods to significantly reduce friction drag caused by the turbulent boundary layer the DFG research group FOR1779 analyses the effect of transversal waves on an extended aluminum surface in wind tunnel experiments. The transversal surface waves running in the span direction (orange arrow) cause an interaction with the near-surface flow field (blue arrow).An actuation system developed by ZEA-2 generating the transversal waves by phase-shifted driven, parallel orientated and uniformly distributed electromagnetic actuators. These actuators are glued to the aluminum surface and hence are mechanically coupled. Sketch of the electromagnetic actuator system.For the generation of the running transversal waves, the actuators are driven by the FPGA RIO board PXI-7831 R from National Instruments via an analogue amplifier front-end. The Virtex-II FPGA Chip included in this board provides automated calculated smooth signal transition when adapting the signal characteristics on the external change of the wave parameters amplitude, frequency and wavelength. This prevents discontinuous signals slope with high frequency interference on the aluminum surface. The high frequency interference by rapid parameter changes without smooth signal transition is shown on the left versus a parameter change with calculated smooth signal transitions on the right.We implemented the smooth wavelength transition based on direct digital synthesis (DDS). This method ensures a smooth wavelength transition by means of an intelligent manipulation of the readout steps of a stored waveform look-up Smooth Wavelength Transition MethodAs mentioned in the previous chapter, the look-up table is sampled with different step sizes as a function of the wave frequency. Each amplitude value of the signal has its own sample in the memory which is known as the memory point P 0 combining an amplitude value to the sample at S 0 ,Phase shift of 60°by moving the zero crossing point at 2π to the point S3.The method of changing the phase, without high-frequency interferences caused by rapid changes in the voltage and its effects on the aluminum surface, is a negative or positive time shift of the signal by moving the zero crossing point by increasing or decreasing the readout steps while the memory controller is at the sample of the amplitude value P n of the changing phase.This assumption could be made by the characteristic of a periodic and zerocentered signal that the phase value always appears positive or negative every quartered period. By the time the calculated phase value reaches the memory point S 0 , the zero crossing following after a half-period is shifted to the sample position of S 3 . This results in a faster sampling CS New of the memory so that the zero crossing is reached earlier.The developed electromagnetic actuator system is used for experiments to develop robust methods for turbulent friction resistance reduction by transversal surface waves. In case of changing inflow conditions wav...

A novel fatigue testing device for bending loading was developed that resembles loading conditions in aerodynamic drag-reduction systems, creating transversal surface waves to modify the surfaces' turbulent boundary layer. The setup uses Lorentz force actuators and is employed in fully reversed loading (R = −1) at a testing frequency of around 100 Hz. Tests were run on riblet-structured Alclad 2024 T351 material. The damage mechanisms for this material are found to be similar for axial and flexural loading. In the high cycle fatigue regime up to a few 106 load cycles, the S/N-curves are also comparable for both loading modes. One distinct difference between the loading modes is a less pronounced threshold behavior in the S/N-curve for bending. Furthermore, a higher stress level has to be applied for fatigue failure in this case.

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