An adaptive flow separation control system is designed and implemented as an essential part of a novel high-lift device
for future aircraft. The system consists of MEMS pressure sensors to determine the flow conditions and adaptive lips to
regulate the mass flow and the velocity of a wall near stream over the internally blown Coanda flap. By the oscillating lip
the mass flow in the blowing slot changes dynamically, consequently the momentum exchange of the boundary layer
over a high lift flap required mass flow can be reduced. These new compact and highly integrated systems provide a real-time
monitoring and manipulation of the flow conditions. In this context the integration of pressure sensors into flow
sensing airfoils of composite material is investigated. Mechanical and electrical properties of the integrated sensors are
investigated under mechanical loads during tensile tests. The sensors contain a reference pressure chamber isolated to the
ambient by a deformable membrane with integrated piezoresistors connected as a Wheatstone bridge, which outputs
voltage signals depending on the ambient pressure. The composite material in which the sensors are embedded consists
of 22 individual layers of unidirectional glass fiber reinforced plastic (GFRP) prepreg. The results of the experiments are
used for adapting the design of the sensors and the layout of the laminate to ensure an optimized flux of force in highly
loaded structures primarily for future aeronautical applications. It can be shown that the pressure sensor withstands the
embedding process into fiber composites with full functional capability and predictable behavior under stress.
Using more and more controlled systems in future aircraft the need of flexible sensors to be applied on curved aircraft structures increases. An appropriate substrate material for such flexible sensors is polyimide, which is available both as ready-made foil and as liquid polyimide to be spun-on. Latest results in producing and processing of polyimide layers with a thickness of down to 1 μm including designs for thin foil sensors are presented respectively. The successful processing of liquid polyimide is outlined first including the spin-on procedure, soft bake and curing for polymerization. Parameters for spin-on volume and rotation speed on glass substrates along with a comparison with ordinary polyimide foil are presented. High-precision structuring of the polyimide layer is performed either by etching (wet-etching as well as dry etching in a barrel etcher) or ablative removal using a femtosecond laser. In combination with a layer of silicon nitride as an inorganic diffusion barrier a reliable protection for water tunnel experiments can be realized. The fabrication of a protection layer and test results in water with protected sensors are presented. The design of a hot-film anemometric sensor array made on spin-on polyimide is demonstrated. With a thickness of down to 7 μm the sensors can be applied on the surface of wind tunnel models and water tunnel models without impacting the flow substantially. Additionally both the concept and recent results of a silicon sensor integrated in a polyimide foil substrate that can measure pressure as a complementary measurand for aeronautics are illustrated.
In this paper, a set of flexible aeroMEMS sensor arrays for flow measurements in boundary layers is presented. The
sensor principle of these anemometers is based on convective heat transfer from a hot-film into the fluid. All sensors
consist of a nickel sensing element and copper tracks. The functional layers are attached either on a ready-made
polyimide foil or on a spin-on polyimide layer. These variants are necessary to meet the varying requirements of
measurements in different environments. Spin-on technology enables the use of very thin PI layers, being ideal for
measurements in transient flows. It is a unique characteristic of the presented arrays that their total thickness can be
scaled from 5 to 52 μm. This is essential, because the maximum sensor thickness has to be adapted to the various
thicknesses of the boundary layers in different flow experiments. With these sensors we meet the special requirements of
a wide range of fluid mechanics. For less critical flow conditions with much thicker boundary layers, thicker sensors
might be sufficient and cheaper, so that ready-made foils are perfect for these applications. Since the presented sensors
are flexible, they can be attached on curved aerodynamic structures without any geometric mismatches. The entire
development, starting from theoretical investigations is described. Further, the micro-fabrication is explained, including
all typical processes e.g. photolithography, sputtering and wet-etching. The wet-etching of the sensing element is
described precisely, because the resulting final dimensions are critical for the functional characteristics.
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