18th International Symposium on the Application of Laser and Imaging Techniques to Fluid Mechanics・LISBON | PORTUGAL ・JULY 4 – 7, 2016 Three-dimensional flow and load characteristics of flexible revolving wings at low Reynolds number R. van de Meerendonk 1* , M. Percin 1 , B. van Oudheusden 1 1: Dept. of Aerospace Engineering, Delft University of Technology, the Netherlands * Correspondent author: [email protected] Keywords: Flapping wing, Force/ Pressure from PIV data, Unsteady aerodynamics, Vortex flows HIGHLIGHTS • The goal of this experimental study is to investigate the effect of chordwise flexibility on the aerodynamics of revolving wings using tomographic particle image velocimetry (PIV) and direct force measurements. • Force measurements show that the lift-to-drag ratio is improved for decreasing flexural stiffness. • The PIV measurements and reconstructed pressure fields reveal how the vortical structures and the associated low pressure regions can be linked to the wing performance. ABSTRACT This study explores the flow field and fluid-dynamic loads generated by revolving low-aspect-ratio wings with different degree of chordwise flexibility, with an angle of attack of 45 deg in their undeformed condition, at a Reynolds number of 10,000. Phase-locked tomographic PIV measurements were performed in three volumes along the wing span for selected phases of the revolving motion. The evolution of major vortical structures including leading edge (LEV), starting & small-scale shed trailing edge (TEV), tip (TV) and root vortices (RV) are clearly revealed (see Fig 1). Pressure fields are reconstructed as well as the loads on the wings, and compared with direct force measurements. The lift generation of the rigid and moderately flexible wings is comparable while it is slightly reduced for the highly flexible wing. The drag shows a significant monotonic decrease with increasing flexibility resulting in an increased lift-to-drag ratio. It is also found that for the flexible wings the lift-to-drag ratio correlates well with the geometric angle of attack. The coherency of the vortex system increases with flexibility, with an increased outboard spanwise vorticity flux along the axis of the LEV. As the low pressure region accompanying the LEV becomes smaller with increasing flexibility, the total force acting on the wing is reduced, but it is also tilted more towards the lift direction due to the wing deformation. As a consequence, the lift component remains relatively high, also because the relatively high suction peak is located closer to the wing surface. Simultaneously, the drag is significantly suppressed for increasing flexibility, which is also reflected in the smaller size of the wake. Fig. 1 Vortical structures and reconstructed pressure fields for δ/c = 1.5, corresponding to a phase (rotation) angle of 36.7 deg. Left to right: rigid, moderate flexible and high flexible wing. Isosurfaces of Q-criterion in white (Q/(Vt/c) 2 = 3). Isosurfaces of reconstructed pressure field in blue p = -13 Pa and red p = 6 Pa (relative to ambient pressure).

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Page 1: Three-dimensional flow and load characteristics of ...ltces.dem.ist.utl.pt/lxlaser/lxlaser2016/final... · leading edge (LEV), starting & small-scale shed trailing edge (TEV), tip

18th International Symposium on the Application of Laser and Imaging Techniques to Fluid Mechanics・LISBON | PORTUGAL ・JULY 4 – 7, 2016

Three-dimensional flow and load characteristics of flexible revolving wings at low Reynolds number

R. van de Meerendonk1*, M. Percin1, B. van Oudheusden1

1: Dept. of Aerospace Engineering, Delft University of Technology, the Netherlands * Correspondent author: [email protected]

Keywords: Flapping wing, Force/ Pressure from PIV data, Unsteady aerodynamics, Vortex flows

HIGHLIGHTS

• The goal of this experimental study is to investigate the effect of chordwise flexibility on the aerodynamics of revolving wings using tomographic particle image velocimetry (PIV) and direct force measurements.

• Force measurements show that the lift-to-drag ratio is improved for decreasing flexural stiffness. • The PIV measurements and reconstructed pressure fields reveal how the vortical structures and the

associated low pressure regions can be linked to the wing performance.

ABSTRACT

This study explores the flow field and fluid-dynamic loads generated by revolving low-aspect-ratio wings with different degree of chordwise flexibility, with an angle of attack of 45 deg in their undeformed condition, at a Reynolds number of 10,000. Phase-locked tomographic PIV measurements were performed in three volumes along the wing span for selected phases of the revolving motion. The evolution of major vortical structures including leading edge (LEV), starting & small-scale shed trailing edge (TEV), tip (TV) and root vortices (RV) are clearly revealed (see Fig 1). Pressure fields are reconstructed as well as the loads on the wings, and compared with direct force measurements.

The lift generation of the rigid and moderately flexible wings is comparable while it is slightly reduced for the highly flexible wing. The drag shows a significant monotonic decrease with increasing flexibility resulting in an increased lift-to-drag ratio. It is also found that for the flexible wings the lift-to-drag ratio correlates well with the geometric angle of attack. The coherency of the vortex system increases with flexibility, with an increased outboard spanwise vorticity flux along the axis of the LEV. As the low pressure region accompanying the LEV becomes smaller with increasing flexibility, the total force acting on the wing is reduced, but it is also tilted more towards the lift direction due to the wing deformation. As a consequence, the lift component remains relatively high, also because the relatively high suction peak is located closer to the wing surface. Simultaneously, the drag is significantly suppressed for increasing flexibility, which is also reflected in the smaller size of the wake.

Fig. 1 Vortical structures and reconstructed pressure

fields for δ/c = 1.5, corresponding to a phase (rotation)

angle of 36.7 deg. Left to right: rigid, moderate flexible

and high flexible wing. Isosurfaces of Q-criterion in

white (Q/(Vt/c)2 = 3). Isosurfaces of reconstructed

pressure field in blue p = -13 Pa and red p = 6 Pa

(relative to ambient pressure).