Vortex Design by Tip Shaping

• Applicant: Professor Dr.-Ing. Eike Stumpf
• Subject Area: Fluid Mechanics
• Term: from 2015 to 2017
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 252995383

It is proposed to revisit the effect of the variable axial velocity component in the core of vortices generated at the side-edge of lifting surfaces. Following results from other research groups as well as own preparatory work this velocity component has a) an effect on vortex instability phenomena in the immediate nearfield and b) an impact on maximum obtainable lift-to-drag ratio L/D. For both aspects open questions remain and in consequence the assumed potential of specific core axial velocities cannot yet be tapped by the design of aircraft wings or rotor blades. Own work has shown that for a given lift force axial velocity in the vortex core can be effectively manipulated by tip planform shaping and, thus, different tip planform shapes will be used here to alter the axial core velocity component. The proposed project combines complementary numerical and experimental work in order to allow a systematic analysis and interpretation of the flow phenomena. To increase physical understanding the focus of the test matrix, done both experimentally and numerically, is put upon three aspects: a) clarification of the role of viscid head loss and inviscid vortex topology on the axial velocity component, b) identification if conditions for onset of instability can be intentionally reached by tip shaping, and c) identification how optimum L/D coincides with the choice of tip and the related axial velocity. The test matrix includes as well sensitivity studies with respect to variation of angle of attack and Reynolds number. All gained knowledge is synthesized in engineering relationships relating identified tip design characteristics (e.g. tip loading) and flow features (e.g. multiple seperating vortices) to the axial velocity component in the vortex core respectively intentionally fulfilled instability criteria and increased L/D. Finally, the engineering relationships are verified using a classical Hörner and a modern raked tip. As a side product this study allows to qualify the Reynolds-Stress-Modeling approach for detailed nearfield investigations of side-edge vortices. The project aims at a future 'vortex design' that reduces aircraft or rotor drag and/or increases instability. Seeing the multitude of current and planned tip extensions it can provide a useful contribution to an eventually converged, physics-based tip design philosophy.

• DFG Programme: Research Grants