Dario E Calderon

Volumetric measurements and simulations of the vortex structures generated by low aspect ratio plunging wings

Volumetric three-component velocimetry measurements have been performed on low aspect ratio wings undergoing a small amplitude pure plunging motion. This study focuses on the vortex flows generated by rectangular and elliptical wings set to a fixed geometric angle of attack of α = 20°. An investigation into the effect of Strouhal number illustrates the highly three-dimensional nature of the leading edge vortex as well as its inherent ability to improve lift performance. Computational simulations show good agreement with experimental results, both demonstrating the complex interaction between leading, trailing, and tip vortices generated in each cycle. The leading edge vortex, in particular, may deform significantly throughout the cycle, in some cases developing strong spanwise undulations. These are at least both Strouhal number and planform dependent. One or two arch-type vortical structures may develop, depending on the aspect ratio and Strouhal number. At sufficiently high Strouhal numbers, a tip vortex ring may also develop, propelling itself away from the wing in the spanwise direction due to self-induced velocity.

Lift enhancement of a rectangular wing undergoing a small amplitude plunging motion

We present an experimental study on a plunging rectangular wing with semi aspect ratio of 2, at low Reynolds numbers of 1×104- 3×104. Time-averaged force measurements have been presented as a function of non-diniensional frequency, alongside particle image velocimetry (PIV) measurements at the midspan plane. We study the effect of oscillations at low amplitudes and various angles of attack. The presence of multiple peaks in lift have been identified for this three-dimensional wing, thought to be related to the natural shedding frequency of the stationary wing. Wing/vortex and vortex/vortex interactions have been identified which may also contribute to the selection of optimal frequencies. Lift enhancement is observed to become more notable with increasing amplitude, coupled with a slight shift in the peak frequencies, to lower Strouhal numbers, with increasing angle of attack. Despite the highly three-dimensional nature of the flow, lift enhancements up to 180% are possible

Lift-Enhancing Vortex Flows Generated by Plunging Rectangular Wings with Small Amplitude

Experiments in a water tunnel have been carried out on low-aspect-ratio rectangular wings undergoing a small-amplitude harmonic plunge motion at Reynolds numbers 10,000 and 20,000. A series of measurement techniques have been used, including force measurements, hot film, particle image velocimetry, and volumetric velocimetry measurements, to study the lift enhancement as a function of forcing frequency. Multiple peaks in the time-averaged lift have been observed, occurring at frequencies in the order of natural vortex-shedding frequencies of the stationary wings. It is postulated that interaction between the leading-edge and trailing-edge vortices contributes to the selection of the optimal frequencies for the time-averaged lift. At a specific Strouhal number, an adverse interaction between the vortices results in a vortex dipole that directs flow upstream. A comparison between a NACA 0012 and flat-plate profile provides further insight into the advantages and disadvantages of using a thinner profile in l...

On the absence of asymmetric wakes for periodically plunging finite wings

It has previously been shown that, at high Strouhal numbers, oscillating airfoils can produce deflected jets that can create very high lift-coefficients for otherwise symmetric scenarios. These deflected jets form through pairing of the trailing-edge vortices to create asymmetric vortex couples that self-propel at an angle to the freestream, resulting in an asymmetric flow field and non-zero lift. In this paper results are presented that indicate these high-lift deflected jets cannot form for finite wings. Instead of the straight vortex tubes that pair and convect at an angle to the freestream observed for effectively infinite wings, finite wings exhibit vortex tubes that break into two branches near the tip forming double helix structures. One branch connects with the last vortex; one branch connects with the next vortex. This creates a long “daisy chain” of interconnected trailing edge vortices forming a long series of vortex loops. These symmetric flow fields are shown to persist for finite wings even to Strouhal numbers more than twice those required to produce asymmetric wakes on plunging airfoils. Two contributing reasons are discussed for why deflected jets are not observed. First the tip vortex creates three-dimensionality that discourages vortex coupling. Second, the symmetry of the circulation of the interconnected vortex loops, which has been confirmed by the experiments, is a natural consequence of the vortex topology. Therefore, the asymmetry in trailing edge vortex strength previously observed as characteristic of deflected jets cannot be supported for finite wings.

Wake structure of plunging finite wings

The vortex shedding characteristics of finite aspect ratio plunging wings, set to a zero geometric angle of attack, have been examined using force, particle image velocimetry and volumetric velocimetry measurements. Dual mode deflected jets have so far only been observed for two-dimensional airfoils. In this study, we investigate whether one can observe this phenomenon with finite aspect ratio wings, to significantly improve the time-averaged lift by exploiting the proper deflection mode. A gradual transition between the existence of these deflection modes in the 2D case and its absence on a finite aspect ratio wing is observed by gradually increasing the spacing between the tip of the wing and endplate. We believe that tip vortices may be preventing the formation of these deflected jets. Volumetric measurements illustrate that intertwined vortex loops are formed downstream of the wing, consisting of trailing edge and tip vortices. A double helix structure is observed within the trailing edge vortex, allowing the two consecutive loops to remain interconnected.

Low aspect ratio oscillating flexible wings at low Reynolds numbers

To identify the role of flexibility in lift generation, the force, deformation and flow fields of rigid and flexible low aspect ratio (sAR = 1.5) wings plunging with a fixed post-stall angle of attack of 15° and amplitude of 15% of chord were experimentally measured. It was shown that spanwise flexibility can significantly enhance lift. The peak increase in lift coefficient over the stationary case is more than three times larger for the flexible wing compared to the rigid wing. This increase is associated with significant deformation of the wing. The root is sinusoidally plunged with small amplitude but this motion is amplified along the span resulting in a larger tip motion. The amount it is amplified strongly depends on Strouhal number. A Strouhal number of Src = 1.5 was selected for detailed flow field measurements due to it being associated with high lift, near the natural frequency, and comparable with a previous study for high aspect ratio wings. For this Strouhal number the rigid wing exhibits a LEV dipole. This is where the clockwise upper-surface LEV pairs with the counterclockwise lower-surface LEV to form a vortex ring that self-advects upstream and away from the wings upper surface. Conversely the flexible wing experiences deformation resulting in tip amplitude 1.53 times greater than the root amplitude and tip phase lag of 98°. This deformation inhibits the LEV dipole. Instead a strong upper-surface LEV forms during the downward motion and convects close to the airfoil upper surface thus explaining the significantly higher lift.