Tip Vortices

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Low-Order Modeling of Wing-Tip Vortices Using an Augmented Vortex Lattice Method.

Low-Order Modeling of Wing-Tip Vortices Using an Augmented Vortex Lattice Method.

This approach is affirmed by Cosyn and Vierendeels [32], who performed CFD analyses of LAR wings at a Reynolds number of 100,000, using two different profiles. One wing was a flat plate and the other used an S5010 airfoil profile. The S5010 has a significantly rounded leading edge, which should support attached leading-edge flow, and therefore greater lift at a larger range of angles of attack than the flat plate. Yet, results showed very close agreement in lift and drag below angles of attack of 10 ◦ and 8 ◦ , respectively, regardless of the airfoil shape. This agreement continued up to increasing angles of attack with decreasing aspect ratios, demonstrating that for LAR wings operating at a Reynolds number near 10 5 , lift and drag are not greatly affected by the particular wing profile. These results are consistent with the findings and discussion of DeVoria and Mohseni, Taira and Colonius, and Cosyn and Vierendeels, all of whom reference the influence of tip vortices in re-attaching the separated leading-edge flow for LAR wings. For purposes of validating the LOM, these findings bolster the idea that the fully-attached leading-edge flow assumed by the VLM is valid for LAR wings at these lower ranges of angle of attack. These differences are recognized and discussed in Chapter 3.

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Effect of tip vortices on membrane vibration of flexible wings with different aspect ratios

Effect of tip vortices on membrane vibration of flexible wings with different aspect ratios

Viieru et al. [13] discussed recent progress in understanding the low Reynolds number unsteady fluid dynamics related to flapping wings, including pitching-up rotation, leading-edge vortices and wake-capturing mechanisms. They noticed that the structures of the leading-edge vortices changed with Reynolds number remarkably by investigating the flow structures at three Reynolds numbers, namely, 6000, 120 and 10. The leading edge vortex demonstrated an intense, conical structure, with a sustained spanwise flow at the vortex core, breaking down at nearly three-quarters of the span towards the tip at Reynolds number of 6000. The vortex break-down was lost, and it was found that the leading- edge vortices connected to the tip vortices at Reynolds number of 120. In addition to this, a vortex ring connecting the leading edge vortex, the tip vortex and the trailing vortex was observed at Re = 10.

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A Numerical Prediction of Tip Vortices from Tandem Propellers in the Counter Rotating Type Tidal Stream Power Unit

A Numerical Prediction of Tip Vortices from Tandem Propellers in the Counter Rotating Type Tidal Stream Power Unit

2) We can confirm the tip vortex stretching at downstream of the single and the counter-rotating propeller using a RANS model. But the tip vortices are dis- connected at interface surface according to the changing the numerical method at the region 4. So, it is necessary to study other methods (e.g. URANS, Dynamic mesh or Overset grid method etc.) to obtain more accurate results.

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First identification and quantification of detached-tip vortices behind a wind energy converter using fixed-wing unmanned aircraft system

First identification and quantification of detached-tip vortices behind a wind energy converter using fixed-wing unmanned aircraft system

trails, PIV (particle image velocimetry) measurements have increased the resolution and accuracy of wind tunnel exper- iments drastically (e.g. resolving Reynolds shear stress and turbulent kinetic energy; Zhang et al., 2012). But a common issue with wind tunnel measurements is that they usually suf- fer from scaling problems (Wang et al., 1996). Remote sens- ing techniques like lidar have also found their way into WEC wake evaluations. Various measurement strategies were de- veloped to visualise WEC wakes, e.g. in complex terrain (Barthelmie et al., 2018). Typical lidar scans provide a long- term measurement of a probed volume or plane. The spatial resolution (25–50 m), however, is comparably coarse. Lidars can provide a continuous monitoring of WEC wake struc- tures (e.g. wake centre, direction and wind velocity deficit) (Bodini et al., 2017) in homogeneous or even in complex ter- rain (Wildmann et al., 2018). Short-range continuous-wave lidars provide even higher spatial resolution for short focal distances and have been applied in WEC wake measurements (Menke et al., 2018), yet these measurements can still not resolve blade-tip vortices. UAS (unmanned aircraft system) measurements can provide in situ line measurements, cov- ering a small volume but with a high temporal and spatial resolution in (deca-) centimetre range. The coverage of these scales is important to measure detached-tip vortices in the near wake of a WEC.

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Experimental investigation of wing tip vortices in the near-field

Experimental investigation of wing tip vortices in the near-field

Abstract. Results of an experimental investigation related to near-field wing tip vortices are presented. The measurements were carried out using a PIV-system in T-1K wind tunnel of KNRTU-KAI. Q-criterion and cross- sectional lines method were used to determine vortex core locations, which showed a good agreement. It is shown that the circulation of tip vortices remains constant at low to moderate angles of attack, and decreases in the stream-wise direction for higher angles of attack. It is also shown that the vortex core radius increases in the stream-wise direction, taking larger values at higher angles of attack.

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Vortex formation and drag on low aspect ratio, normal flat plates

Vortex formation and drag on low aspect ratio, normal flat plates

Coutanceau (1992), who performed similar experiments with low AR circular cylinders with one end free. However, the mechanism responsible for keeping the LEVs attached to the plate near the tip was not that hypothesized by Ellington et al. (1996), who studied the LEV of a flapping model of a hawkmoth. Ellington et al. attributed the prolonged attachment of the hawkmoth LEV to a strong root-to-tip spanwise flow within its core. They hypothesized that this spanwise flow convects vorticity out of the LEV and toward the tip, which slows the accumulation of circulation within the LEV and thus keeps it attached longer. The spanwise flow within the LEVs of the present study (from tip to root) was the opposite of that found by Ellington et al. Therefore, another mechanism must prevent the LEVs from pinching-off. Based on the results of the current work, this author proposes that it is instead the low pressure created at the tip by the induced 3-D flow from the tip vortices that keeps the LEVs attached there and limits their strength. This low pressure is also responsible for the high drag generated. The spanwise flow within the LEVs is a result of the flow induced by the tip, and not the mechanism actually responsible for the attachment of the LEVs near the tip. Since Ellington et al. also observed a significant tip vortex, it may be the cause for the behavior of the hawkmoth LEV. The discrepancy in spanwise flows between that investigation and the present work may simply be due to differences in kinematics. The hawkmoth wing has a sweeping shoulder motion that could create a pressure gradient from root to tip strong enough to generate a spanwise flow, within the LEV core, in that direction.

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The vortex wake of blackcaps (Sylvia atricapilla L ) measured
using high speed digital particle image velocimetry (DPIV)

The vortex wake of blackcaps (Sylvia atricapilla L ) measured using high speed digital particle image velocimetry (DPIV)

We found no support for the hypothesis that the tail should reduce the cost of flight by utilizing the induced flow between wing root vortices. From this hypothesis it would, for example, be predicted that wing root vortices, of the opposite sense of rotation to the wing tip vortices, should be present in the tailless bird throughout the downstroke [as is the case in bats (Hedenström et al., 2007; Johansson et al., 2008; Hedenström et al., 2009)]. In the bird with the tail these wing root vortices would, optimally, either be absent or have reduced circulation. Not only did we find no differences in the wake morphology between the two birds but also none of the normalized quantitative measures of circulation differed. Furthermore, the wake pattern found did not match any of the two expected wake morphologies outlined above. Also, in a thrush nightingale, where we studied the wake of the same individual, with and without a tail (using a 10 Hz, two- dimensional DPIV setup), we found no qualitative or quantitative differences in the wake (L.C.J., personal observation). We thus conclude that this hypothesis is not supported by the data, at least in these two species. During steady level flight the tail is often furled in birds, which is a potential explanation for why we find no differences between the bird with and the bird without a tail. During manoeuvres the tail is spread, which will result in the tail being better positioned to utilize the wing root vortices, and the tail presumably has a more aerodynamically active function than during steady flight.

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Wake Development behind Paired Wings with Tip and Root Trailing Vortices: Consequences for Animal Flight Force Estimates

Wake Development behind Paired Wings with Tip and Root Trailing Vortices: Consequences for Animal Flight Force Estimates

techniques. They indicate reduced strength of the bound circulation between the wings that results in a lower span efficiency but potentially also a higher degree of manoeuvrability [23]. That even swifts, which nearly spent their entire life on the wing [30] and presumably would benefit from high energy efficiency, exhibit root vortices gives reason to believe that this may be an inevitable feature–at least to some degree–in flying animals. We have demonstrated that paired vortex wakes are time- varying structures and that the location of PIV measurement planes will have an effect on animal flight aerodynamic analyses that should be considered at the stage of experimental design. Because the estimation of lift forces relies on unaltered wakes over the time interval between generation and measurement, consid- eration is required with respect to positioning of PIV planes and to the theoretical models with which the empirical data are analysed. Of course, the dynamic wakes generated by birds, bats and insects are not identical to the simplified wake topology we produced in this study using fixed wings. Furthermore, root vortices found in the wakes of animals are usually weaker than tip vortices because of the presence of the body and mostly persist only throughout the downstroke, or parts thereof. However, their existence will have an

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Aerodynamics of bird flight

Aerodynamics of bird flight

The wing-tip-slots do not eliminate completely the vortices at the wing tips (induced vortices), especially in lower aspect-ratio-wings, as, e.g., in passerines. They are generated during the downstroke phase and strengten by gradually engulfing the trailing edge vortex sheet. The strong downward-induced velocity thus effectively enhances the lift. Traditionally, tip vortices are considered as a nuisance increasing the drag, however, in low-aspect-ratio-wings they can favourably enhance the lift. The downward velocity behind the flyer is accompanied by an upward velocity field outside the wake, and in V-flight formation of migrating birds this can be used by the follower´s wing as an energy saving contribution to the generated lift (Fig.7) [1].

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Anisotropy and vortices in Bose Einstein condensates

Anisotropy and vortices in Bose Einstein condensates

Abstract. The fast rotation of a Bose Einstein condensate in a harmonic trap leads to a hexagonal lattice of vortices, called the Abrikosov lattice. Vortices are described as the zeros of the condensate wave function, which is a solution to a nonlinear Schrdinger equation type. We study the solutions of this equation on a specific eigenspace of the limiting problem, the Lowest Landau Level. Upper and lower bounds for the energy allow to determine the presence of a vortex lattice or the lack of visible vortices, as a function of the anisotropy and the rotation. The study of the critical regime is open and would need some numerical simulations. This constitutes a project in collaboration with Xavier Blanc and Nicolas Lerner.

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Direct numerical simulation of the flow around an aerofoil in ramp-up motion

Direct numerical simulation of the flow around an aerofoil in ramp-up motion

angle of incidence has reached the final value, the lift experiences a first over- shoot and then suddenly decreases towards the static stall asymptotic value. The transient instantaneous flow is dominated by the generation and detachment of the dynamic stall vortex, a large scale structure formed by the merging of smaller scales vortices generated by an instability originating at the trailing edge. New insights on the vorticity dynamics leading to the lift overshoot, lift crisis, and the damped oscillatory cycle that gradually matches the steady condition are discussed using a number of post-processing techniques. These include a detailed analysis of the flow ensemble average statistics and coherent structures identification carried out using the Q-criterion and the finite-time Lyapunov exponent technique. The results are compared with the one obtained in a companion simulation considering a static stall condition at the final angle of incidence α = 20 ◦ . C 2016 AIP Publishing LLC.

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Vortex merger in surface quasi geostrophy

Vortex merger in surface quasi geostrophy

Note that 2D, SQG and 3D QG dynamics share common features for the merging process, such as a critical merger distance for localized vortices, the formation of larger or smaller eddies and of filaments, or the possibility of partial merger. But only 3D QG dynamics allows the vertical tilting of vortices, which can lead to their merger at certain depths, while a part of the initial vortices does not merge at other depths (see for instance Reinaud and Dritschel, 2005). This supplementary degree of freedom is important for the dynamics of deep oceanic vortices, such as meddies for instance, considering their complex potential vor- ticity distribution.

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Motion of two point vortices in a steady, linear, and elliptical flow

Motion of two point vortices in a steady, linear, and elliptical flow

Thus a consequence of Theorem 3.4 is that, if two vortices are close to each other, then their period of rotation around each other is what it would be if there were no background flow, while, if the vortices are far apart, then that period is approximately what it would be if the vortices did not affect each others’ motion.

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The matrix model of vortex-beam
quadrefringence in a uniaxial crystal

The matrix model of vortex-beam quadrefringence in a uniaxial crystal

turn, entails destruction of the high-order optical vortices in the beam components. The individual vortices inside the beam get jumbled. It is difficult to speak what of the beams the optical vortex belongs to. Detailed information about the vortex behaviour in the singular jumble is provided by vortex trajectories plotted in the coordinates ‘ ( ) x y , versus α o ′ : Re { Ψ ( ) ± m l , ( x y z , , , α o ) } = 0, Im { Ψ ± ( ) m l , ( x y z , , , α o ) } = 0 . Fig. 4 illustrates the vortex trajectories for the simplest case of singly charged initial vortex-beam with the right-hand circular polarisation. The trajectories shown in Fig. 4 are traced by the vortices in the left-hand polarised component of the beam inside the crystal. They are computed for both the theoretical and model solutions. The qualitative portrait of the trajectories in those cases is much the same [6, 7]. There are three traits in the trajectory behaviour:

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Analysis of Low Speed Aerodynamics of Double Delta Wing

Analysis of Low Speed Aerodynamics of Double Delta Wing

Recent research work has been in the area of high- angle of attack aerodynamics on delta-shaped wings. As discussed earlier, lift “increase as the angle of attack increases. Unfortunately, there are limits to the advantages provided by the delta wing vortices [1]. As the angle of attack increases, there is a abrupt breakdown in the vortex structure. This process is known as vortex bursting. As a result, there is stagnation in the core axial flow and an expansion in radial size [2]. Once this is reached, there is no increase in the lift aft of the burst point.

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Structure and Migration of 2D Vortices

Structure and Migration of 2D Vortices

The evolution of the vortices was followed numerically for ∼ 200 rotations (up to 1000 rotations in some cases) and for a wide range of the possible parameters. From our sample two families of vortices can be distinguished as shown in figure 2. The family of incompressible vortices corresponds to big vortices with large χ θ and R 0 ' −0.1; they have closed streamlines and excite only very weak

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Dynamics of near shore vortices

Dynamics of near shore vortices

In chapter 2 we discussed how a sector of a vortex ring can be used to model a vortex in a wedge-like domain and we observed that this construct is mathematically exact. We also observed that this model could be seen as an extension to three dimensions of the two-dimensional model that uses parallel straight-line vortices, or, equivalents, point vortices. If point vortices are taken, the model is valid as it stands only if applied to unbounded fluids or if the vortices are in between two parallel planes (see

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Spherical vortices in rotating fluids

Spherical vortices in rotating fluids

A popular model for a generic fat-cored vortex ring or eddy is Hill’s spherical vortex (Phil. Trans. Roy. Soc. A vol. 185, 1894, p. 213). This well-known solution of the Eu- ler equations may be considered a special case of the doubly-infinite family of swirling spherical vortices identified by Moffatt (J. Fluid Mech. vol. 35(1), 1969, p. 117). Here we find exact solutions for such spherical vortices propagating steadily along the axis of a rotating ideal fluid. The boundary of the spherical vortex swirls in such a way as to exactly cancel out the background rotation of the system. The flow external to the spherical vortex exhibits fully nonlinear inertial wave motion. We show that above a crit- ical rotation rate, closed streamlines may form in this outer fluid region and hence carry fluid along with the spherical vortex. As the rotation rate is further increased, further concentric ‘sibling’ vortex rings are formed.

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Downstream and soaring interfaces and vortices in 2-D stratified wakes and their impact on transport of contaminants

Downstream and soaring interfaces and vortices in 2-D stratified wakes and their impact on transport of contaminants

flow with a self-induced fine structure of an initially smooth stratification. To better understand the flow past obstacles and to compare observations with analytical and numerical solutions, the typical approach has been to investigate first of all the flow at similar ranges of parameters past obsta- cles of simple or perfect shapes, such as strips, right circu- lar cylinders and spheres. In this spirit, the structure of a flow around an obstacle including upstream disturbances, an internal wave field and a downstream wake have been stud- ied both theoretically and experimentally. It is well known that vortices and vortex arrays play an important role in the transport of substances, heat and momentum. Much less is known about the effect of high gradient interfaces and rea- sons of their formations. Common theoretical and experi- mental methods are generally directed to study dynamics of regular macroscopic elements of a flow. It is difficult to in- vestigate small-scale irregularities both in a laboratory and in the environmental conditions due to the spatial or tem- poral smoothing and interactions of sensors or visualization elements (seed particles, drifting balloons in the atmosphere and drifters in the ocean) with the flow. This study reports on some analytical and laboratory experiments which em- ploy high-resolution optical techniques to provide a pattern of density gradient and velocity fields in a stratified flow around obstacles. It is important that the experimental tech- niques are directed to observe disturbances of the real fluid being tested and not additional substances (dye, solid parti- cles) that change the physical properties of the medium.

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Interaction of electron vortices and optical vortices with matter and processes of orbital angular momentum exchange

Interaction of electron vortices and optical vortices with matter and processes of orbital angular momentum exchange

In view of the similarities of EVs and OVs it is clearly natural to contemplate whether an experiment similar to that by Araoka et al. [16] on the handedness of processes involving the possibility of OAM exchange with internal dynamics should be carried out for the EV case. As far as the authors are aware, the only experiment to date involving the transfer of OAM of electron vortices is that by Verbeeck et al. [8], who specifically dealt with the case l = ±1 to investigate the electron-energy- loss spectroscopy signals from magnetized Fe films using EV beams. The transitions involved are those by core electrons participating in electric dipole allowed transitions between discrete atomic energy levels within a rigid condensed-matter background of essentially fixed Fe atoms. The expectation was that the two EELS signals, one from l = 1 and the other from l = −1, would be different, suggesting that the EELS revealed an intrinsic chirality of the medium. This was indeed the case in the experiment by Verbeeck et al. The question, however, arises as to the theoretical basis for the observed dichroism.

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