Targeted Manipulation: Airflow Across the Wing

Among the components whose modification could reduce noise, fuel consumption and exhaust emissions within the aviation industry is the engine. However, in conjunction with existing aircraft wings, the more efficient engines of tomorrow will also result in certain negative consequences. To find a solution, the University of Stuttgart’s Institute of Aerodynamics and Gasdynamics (IAG) is collaborating with an international research team in the INAFLOWT project, which is part of the EU's CleanSky 2 research program.

Aircraft manufacturers are predicting an increase in air traffic of over four per cent over the coming two decades. By itself, this increase will make it necessary for aircraft to consume less fuel and to make less noise. The Advisory Council for Aeronautics Research in Europe (ACARE), an advisory body to the EU, has issued a number of ambitious recommendations for the European aviation industry in connection with this. The goal is to achieve a reduction in CO2, nitrogen oxide and noise emissions by 2020 of 50, 80 and 50 percent respectively compared with the best technology available in 2000, and to achieve even further reductions of 75, 90 and 65 per cent respectively by 2050.

One starting point for improvements in these areas are the propulsion units, which give the aircraft the necessary thrust. To do so, they suck in huge volumes of air and blow it back out, which causes an enormous din, particular upon take off. Propulsion units with an extremely high bypass ratio are considered particularly promising in terms of reducing this noise. In engines of this type, a large part of the air entering the propulsion unit flows around the outside of the combustion chamber whilst only the smaller proportion flows through it, which increases the efficiency of the engine.

Nevertheless, these so-called Ultra-High- Bypass-Ratio (UHBR) propulsion units, which are still in the research stage, have much larger diameters. “This perturbs the aerodynamic profile of the wings under which they are attached”, explains Dr. Thorsten Lutz, Group Leader of Aircraft Dynamics at the University of Stuttgart's Institute of Aerodynamics and Gasdynamics. The UHBR impair the dynamic flow of air across the wing, which can result in flow separation, which, as Lutz explains,results in “a reduction in the potentially achievable lift”. So-called actuators, integrated within the wings, could remedy this situation. Actuators are engine components that convert electrical signals, such as commands from a control computer, into mechanical motion, or which convert other physical properties, such as pressure or temperature. How they would have to be dimensioned is what the Stuttgart-based institute is researching in collaboration with four other partners from Israel, The Czech Republic and Russia in the context of the EU's INAFLOWT project.

“Actuators, such as these, can blow out or suck up air via small openings on the leading edge of the wing”, Lutz explains. This modifies the flow filed and, therefore, as it were, counteract the flow separation caused by the UHBR propulsion units. “To work out the correct dimensions and control the timing of these actuators”, Lutz continues, “one has to understand the air flows, which become very complicated due to the blowing and sucking, and are also extremely difficult to calculate numerically”.

The task assignments within the project team are as follows: the two Israeli partners will develop the actuators and build a small model to be tested in their own wind tunnel; the Czech researchers will simulate the current flows inside the actuators with a view to enhancing their geometry, and Lutz and his team are responsible for simulating the reciprocal effects in the air flows around the wings and propulsion units in the absence of the actuators. “We have access to supercomputers at the University of Stuttgart's High-Performance Computing Center (HLRS), which will be absolutely necessary if we're to perform the calculations for this complicated model at all”, says Lutz. And, finally, the Russian partners will test the results on a larger model in one of the biggest wind tunnels in the world. The project results should be available by the end of 2020.

Michael Vogel