Visualization of vortical flows around a rapidly pitching wing and propeller

This study was conducted to visually investigate flows related to fixed-wing vertical-takeoff-and-landing micro air vehicles, using the smoke-wire technique. In particular, the study examines transition between forward flight and near-hover. The experimental model consists of a rigid Zimmerman wing and a propulsion system with contra-rotating propellers arranged in a tractor configuration. The model was pitched about the wing’s aerodynamic center at approximately constant rates using a five-axis robotic arm. Constant-rate pitching angles spanned 20° to 70°. No-pitching and four pitching-rates were used, along with three propulsive settings. Several observations were made during no-pitching tests. Turbulent wakes behind blades and laminar flow between them produces pulsations in the boundary layer. These pulsations alter the boundary layer from a laminar to turbulent state and back. An increase in lift and drag in the presence of a slipstream is a result of competing effects of the propulsive slipstream: (a) suppression of flow separation and increased velocity over the wing and (b) decrease of the effective angle of attack. Higher nose-up pitching-rates generally lead to greater trailing-edge vortex-shedding frequency. Nose-up pitching without a slipstream can lead to the development of a traditional dynamic-stall leading-edge vortex, delaying stall and increasing wing lift. During nose-up pitching, a slipstream can drive periodically shed leading-edge vortices into a larger vortical-structure that circulates over the upper-surface of a wing in a fashion similar to that of a traditional dynamic-stall leading-edge vortex. At lower nose-up pitching-rates, leading-edge vortices form at lower angles of attacks. As a slipstream strengthens, a few things occur: separation wakes diminish, separation occurs at a higher angle of attacks, and downward flow-deflection increases. Similar effects are observed for nose-up pitching, while nose-down pitching produces the opposite effects.

[1]  Richard E Kuhn,et al.  Investigation of the aerodynamic characteristics of a model wing-propeller combination and of the wing and propeller separately at angles of attack up to 90 degrees , 1956 .

[2]  G. Batchelor,et al.  An Introduction to Fluid Dynamics , 1968 .

[3]  S. M. Batill,et al.  Visualization of the laminar-turbulent transition in the flow over an airfoil using the smoke-wire technique , 1980 .

[4]  T. Mueller.  On the historical development of apparatus and techniques for smoke visualization of subsonic and supersonic flows , 1980 .

[5]  M. V. Dyke,et al.  An Album of Fluid Motion , 1982 .

[6]  Joseph Shang,et al.  Numerical investigation of the flow structure around a rapidly pitching airfoil , 1987 .

[7]  L. B. Schiff,et al.  Visualization and wake surveys of vortical flow over a delta wing , 1988 .

[8]  Bruce J. Holmes,et al.  Wing laminar boundary layer in the presence of a propeller slipstream , 1988 .

[9]  M. Visbal,et al.  Investigation of the flow structure around a rapidly pitching airfoil , 1989 .

[10]  Miguel R. Visbal,et al.  Unsteady vortex structure over a delta wing , 1994 .

[11]  Ismet Gursul,et al.  Origin of Vortex Wandering over Delta Wings , 2000 .

[12]  Mo Samimy,et al.  A Gallery of Fluid Motion , 2004 .

[13]  Ismet Gursul,et al.  Review of Unsteady Vortex Flows over Slender Delta Wings , 2005 .

[14]  Tyson L Hedrick,et al.  Software techniques for two- and three-dimensional kinematic measurements of biological and biomimetic systems , 2008, Bioinspiration & biomimetics.

[15]  Peter Ifju,et al.  Flow characteristics of a three-dimensional fixed micro air vehicle wing , 2008 .

[16]  Sergey V Shkarayev,et al.  Aerodynamic Design of Micro Air Vehicles for Vertical Flight , 2008 .

[17]  Sergey V Shkarayev,et al.  Simulator Development for Transition Flight Dynamics of a VTOL MAV , 2010 .

[18]  P. Durbin,et al.  A Stereoscopic PIV Study of a Near-field Wingtip Vortex , 2010 .

[19]  Jon Ahn,et al.  A Computational Study on the Aerodynamic Influence of a Pusher Propeller on a MAV , 2010 .

[20]  Roshan Antony,et al.  Experimental Studies on a Propelled Micro Air Vehicle , 2011 .

[21]  Sergey V Shkarayev,et al.  Longitudinal Aerodynamics of a Vertical Takeoff and Landing Micro Air Vehicle , 2011 .

[22]  G. Abate,et al.  Longitudinal aerodynamics of rapidly pitching fixed-wing micro air vehicles , 2012 .