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Journal of Flow Visualization and Image Processing
SJR: 0.161 SNIP: 0.312 CiteScore™: 0.5

ISSN Imprimir: 1065-3090
ISSN On-line: 1940-4336

Journal of Flow Visualization and Image Processing

DOI: 10.1615/JFlowVisImageProc.2012003945
pages 253-274

AERODYNAMIC PERFORMANCE AND FLOW STRUCTURE OVER A THIN AIRFOIL UNDER SMOOTH AND TURBULENT CONDITIONS AT LOW REYNOLDS NUMBERS

Sridhar Ravi
Department of Environmental Physics, University of Tubingen, Paul-Ehrlich-Str. 17, Tubingen, 72076 T, Germany
P. Petersen
Department of Environmental Physics, University of Tubingen, Paul-Ehrlich-Str. 17, Tubingen, 72076 T, Germany
S. Watkins
Department of Environmental Physics, University of Tubingen, Paul-Ehrlich-Str. 17, Tubingen, 72076 T, Germany
M. Marino
Department of Environmental Physics, University of Tubingen, Paul-Ehrlich-Str. 17, Tubingen, 72076 T, Germany
J. Watmuff
Department of Environmental Physics, University of Tubingen, Paul-Ehrlich-Str. 17, Tubingen, 72076 T, Germany

RESUMO

Micro Air Vehicles (MAVs) are small remotely controlled or autonomous aircrafts that fly relatively slowly (50,000 < Reynolds number < 200,000) and very close to the Earth surface. MAVs are very difficult to fly in the outdoor environment as they are exposed to both low Reynolds number effects and high levels of turbulence. To shed light on the influence of turbulence on the flow structure over an airfoil and on aerodynamic performance, smoke-flow visualization and surface pressures measurements were taken over a thin flat-plate airfoil under nominally smooth and turbulent conditions. A high level of detail on various flow structures that developed over the airfoil in nominally smooth flow including the laminar separation bubble (LSB), formation of instabilities in the separated shear layer, and vortices shed on reattachment of the shear layer, is presented. At elevated levels of free-stream turbulence, the flow field over the airfoil was noted to be considerably different whereby enhanced roll-up of the leading edge separated shear layer occurred. This resulted in the formation of large leading edge vortices which had a significant influence on the aerodynamic loads experienced by the airfoil. The surface pressures indicated that airfoil performance was higher under smooth flow conditions at lower angles of attack (<10 deg). However an increase in turbulence resulted in a decrease of the lift-curve-slope, an increase in maximum lift, and significant delay in stall rendering airfoil performance to be greater at higher angles of attack. Turbulence had a relatively lower influence on the pressure drag experienced by the airfoil and the pitching moment coefficient was found to be sensitive to the ambient turbulence level only from angles of attack >6 deg.


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