ENGINEERING AND CFD

Passenger transportation by aircrafts is responsible for a considerable share of the global CO₂ emission budget. Although aircraft engines have become more efficient during the past, there is an increasing pressure on airline companies and manufacturers to accomplish the growth of air traffic without increasing the emission of greenhouse gases. At present, alternative technologies such as electric propulsion or hydrogen fuelled engines are investigated. These, however, do not reduce the overall energy consumption of the aircraft.

In the past, various ways were explored to lower the amount of fuel necessary per flight by e.g. using passive drag reduction concepts in form of riblets. Riblets were developed after analysing the effect of microstructures in the skin of sharks. They reduce friction drag, which is one component of the overall drag of an aircraft and makes up to 50 percent of the total drag for standard narrow- and wide-body aircraft. Friction drag occurs due to the viscosity of a fluid, i.e., air for aircrafts and water for sharks. For slender bodies moving in a viscous fluid the transition of the fluid velocity from the aircraft surface, where it has the speed of the aircraft, to the speed of the outer flow, e.g., zero velocity for still air, occurs within a very thin layer close to the aircraft skin. This thin layer is known as the boundary layer.

For passenger aircrafts in cruise flight the boundary layers are most often in turbulent state. That is, the fluid in the boundary layer exhibits a seemingly chaotic and unpredictable behaviour. The turbulence in the boundary layers increases the friction drag on the skin of the wings and fuselage, which is why there is a large interest to manipulate the turbulence in the boundary layers with the goal of drag reduction.

In this project, a recent approach of active surface movement was applied for the first time to a wing profile section in turbulent flow. In general, the flat surface is deformed to the shape of a sine-wave of low amplitude and this wavy surface is then continuously transformed such that the crests and valleys of the surface are constantly moving in the spanwise direction, i.e., a direction perpendicular to the main flow direction. The generation of these spanwise travelling waves requires the surface to execute only a movement in wall normal direction, in the same way as the stadium waves during a soccer game. The target of the project was to show that the total drag, which the wing section experiences, can be lowered by this actuation of the wing surface. Previous investigations using generic test case scenarios showed that the technique works in principle. The goal in this project was to confirm that the drag reduction effect can be maintained in a realistic application.

The principle mode of operation can be observed in video 1, where turbulent structures above the actuated wall are visualized. The interaction of the oscillating wall with the boundary layer modifies the turbulent flow structures. Dependent on the wave parameters, the skin-friction drag reduces by up to 30 percent. Figure 1 shows the turbulent flow around the NACA4412 wing section, to which the drag reduction technique of traveling transversal waves is applied.

Video 2 shows the time-dependent flow around the wing section. In the lower right corner the instantaneous friction drag coefficient is depicted by a red line. As soon as the actuation is started, which happens after about 15 seconds from the start of the video, the drag immediately begins to decrease below the level of the non-actuated wing section (black line). On average, the friction drag is reduced by about 13 percent and the total drag including the pressure drag is reduced by 9 percent.