C. A. Ozen

Flow structure on a rotating wing: Effect of steady incident flow

The flow structure along a rotating wing in steady incident flow is compared to the structure on a rotating wing in quiescent fluid, in order to clarify the effect of advance ratio J (ratio of free-stream velocity to tip velocity of wing). Stereoscopic particle image velocimetry leads to patterns of vorticity, velocity, and Q-criterion (constant values of the second invariant of the velocity gradient tensor), as well as streamlines, which allow identification of critical points of the flow. The effective angle of attack is held constant over the range of J, and the wing rotates from rest to a large angle that corresponds to attainment of the asymptotic state of the flow structure.Priortotheonsetofmotion,thewingisathighangleofattackandthesteady incident flow yields a fully stalled state along the wing. After the onset of rotation, the stalled region quickly gives rise to a stable leading edge vortex. Throughout the rotation maneuver, the development of the flow structure in the leading edge region is relatively insensitive to the value of J. In the trailing-edge region, however, the structure of the shed vorticity layer is strongly dependent on the value of J. Further insight into the effects of J is provided by three-dimensional patterns of spanwiseoriented vorticity, spanwise velocity, and Q-criterion. C 2013 AIP Publishing LLC.

Control of vortical structures on a flapping wing via a sinusoidal leading-edge

The flow structure generated by a flapping wing in the form of a plate is fundamentally altered if the leading-edge has a sinusoidal shape. It is possible to attenuate both the positive and negative spanwise flows along the plate surface, as well as the onset and development of large-scale concentrations of positive and negative streamwise vorticities at inboard locations. These alterations of the inboard flow structure have an insignificant influence on the structure of the tip vortex.

Shallow flow past a cavity: Coupling with a standing gravity wave

Shallow, fully turbulent inflow past a cavity can give rise to highly organized oscillations, due to coupling between: (i) the inherent instability of the separated turbulent layer along the opening of the cavity, and (ii) a gravity standing wave within the cavity. Techniques of particle image velocimetry and pressure measurements are employed to characterize the occurrence of the fully coupled state of the oscillation, relative to its uncoupled state. At a threshold value of inflow velocity, the frequency of the inherent instability of the turbulent separated shear layer locks-on to the frequency of the gravity standing wave. Moreover, the amplitude of the spectral peak of the pressure fluctuation, both at the impingement corner of the cavity and within the cavity, attains maximum values when lock-on occurs. Occurrence of a fully coupled or locked-on state substantially alters the time-averaged flow structure. Enhanced magnitudes of patterns of Reynolds stress and transverse velocity fluctuation occur al...

Observations of Flow Structure Changes with Aspect Ratio for Rotating Insect Wing Planforms

The effect of wing aspect ratio for flapping and rotating wings in insect-like flight regimes is not well understood. In this study the effect of aspect ratio on the vortex structures formed over altered fly wings has been investigated. A numerical model was employed to simulate the flow over these wings which were rotating at a constant angular velocity. It was found that increasing the wing aspect ratio resulted in the formation of a dual co-rotating vortex structure near the leading-edge of the wing and the strengthening of the spanwise velocity within the core of the downstream vortex. Increasing the wing aspect ratio also resulted in an unsteady region of flow being developed out near the wing tip in which vortex structures are shed from the wing. The formation of this region was found to be due to the interaction of the leading-edge vortex with the vorticity at the trailing edge and not due to leading-edge vortex shedding as the dual leading-edge vortex structure was found to remain attached to the wing for all aspect ratios. This interaction did result in the reduction of the lift and drag coefficients for high aspect ratios.

Flow Structure on a Rotating Wing: Effect of Wing Aspect Ratio and Shape

ow structure at a large angle of rotation, corresponding to the steady{state lift plateau. When the aspect ratio of the rectangular wing is suciently large, and at large radial distance from the center of rotation, degradation of the organized swirl of the leading{edge vortex occurs; the structure of the separated layers from the leading{ and trailing{ edges of the wing indicate that the eects of rotation are severely diminished. Simultaneously, there is loss of an identiable tip vortex; it is coherent only for the smallest aspect ratio. Despite these changes of patterns of the leading{edge and tip vortices, the structure of the root vortex remains the same with changes of aspect ratio. These major features of the vortex system also occur along the model wing of a fruit y; their form is remarkably similar to the structure along the rectangular wing of the same aspect ratio.