Unsteady Force Generation and Vortex Dynamics of Pitching and PlungingAirfoils at Low Reynolds Number.

Unsteady Aerodynamics;Vortex Dynamics;Flapping Wing Aerodynamics;Low Reynolds Number Aerodynamics;Pitching and Plunging Airfoils;Aerospace Engineering;Engineering;Aerospace Engineering

Baik, Yeon SikUkeiley, Lawrence S. ;

University of Michigan

关键词: Unsteady Aerodynamics;    Vortex Dynamics;    Flapping Wing Aerodynamics;    Low Reynolds Number Aerodynamics;    Pitching and Plunging Airfoils;    Aerospace Engineering;    Engineering;    Aerospace Engineering;   

Others  :  https://deepblue.lib.umich.edu/bitstream/handle/2027.42/84556/yeonb_1.pdf?sequence=1&isAllowed=y

瑞士|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

An experimental study of two-dimensional pitching and plunging airfoils at low Reynolds number (Re) is presented. The main goals of the study are to provide a better understanding of flapping wing aerodynamics for application to the design of micro air vehicles (MAV) and to obtain detailed experimental data for validation of computational models. The effects of Re (range O(10^3) to (10^4)) and airfoil shape (SD7003 and flat plate) are investigated for two kinematics consisting of sinusoidal effective angle of attack profiles: a shallow stall kinematics (A_eff(t) range 2.4° to 13.6°), and a deep stall kinematics (A_eff(t) range -6° to 22°). Re effects are observed only for the SD7003 airfoil and shallow stall kinematics at low Re, where laminar-to-turbulent transition was observed in the separated flow region during the down stroke motion that produced 20% more lift coefficient compared to high Re cases. At high Re, the flow remains attached and the measured lift coefficient is in good agreement with unsteady linear theory. For the flat plate airfoil, a sharp leading edge produces boundary layer separation which forms a closed separation region for shallow stall kinematics or a leading edge vortex (LEV) for the deep stall kinematics. These flow features result in ~50% higher lift coefficient compared to unsteady linear theory predictions, and clearly document the significance of LEV in lift generation.To elucidate the role of reduced frequency, k, and Strouhal number, St, on force generation and LEV dynamics, parametric studies were conducted for the deep stall kinematics at fixed Re. It is found that the flow topology is similar at all conditions with formation of a LEV. An increase in k delayed the formation of a LEV which in turn affected the vortex evolution and unsteady force generation. The LEV core trajectory in the frame of reference moving with the airfoil is found to be independent of k and St, and solely a function of the effective angle of attack profile. Increasing St increases peak values of the lift coefficient by altering pitching and plunging rates. Peak lift coefficient values exceeding 4 are measured for St = 0.32.