Influence of Discrete Suction Orifices on Effectiveness of HLFC Surfaces

Kurzfassung

To effectively apply Hybrid Laminar Flow Control (HLFC) for drag reduction of transport aircraft, it is desirable to achieve the largest possible laminar flow region with given suction power. Therefore, experimental as well as numerical studies are conducted at DLR. In past wind tunnel tests with a flat plate with suction in the DNW-NWB, differences in the effectiveness of different suction orifice geometries have been found. These are investigated in more detail in recent wind tunnel measurements. Two slotted geometries are compared with nearly homogeneous suction through a four-layer wire mesh. Transition locations are determined by infrared thermography, hot-wire and hot-film sensors are used to determine amplitudes of Tollmien-Schlichting waves. Accompanying the experiments, Direct Numerical Simulations (DNS) of Tollmien-Schlichting waves propagating over discrete suction orifices are carried out. They resolve the three-dimensional disturbances introduced by the suction orifices. It can be shown in agreement between experiment and simulation, that certain slot patterns lead to additional stabilisation by longitudinal vortices. This leads to a further down-stream shift of transition additionally to the known stabilising effect of suction. The results match the findings from Fransson et al., where a stabilising effect from longitudinal vortices, introduced by discrete roughness elements, is described. The results suggest that by targeted design of suction surfaces, their effectiveness can be improved significantly, which can enhance the benefit of an HLFC system.