Flow field diagnostics - Topological flow changes and spatio-temporal flow structure

Dallmann, U.; Herberg, T.; Gebing, H.; Su, Wei; Zhang, H.-Q.

doi:10.2514/6.1995-791

Cited by 21 publications

(22 citation statements)

References 7 publications

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“…The sectional streamlines fail to show the primary vortex, because the local vortex axis considerably deviated from the spanwise direction (Dallmann et al, 1995). Similar phenomena for possibly the same reason occurred when ␣ m =40° (Fig.·8D).…”

Section: Leading-edge Vortices On Flapping Wingsmentioning

confidence: 49%

“…In this sense, the vortex pairs in Fig.·7B, which can be identified as dual LEV, are those within the range 0-0.58r. The main reason for the absence of foci in the primary vortex region at 0.43r and 0.58r is the relatively large deviation of the local vortex axis from the spanwise direction (or the laser-sheet normal), which changed the sectional topological structure (Dallmann et al, 1995). Although the primary vortex increased in size along the spanwise direction (showing a conical shape) and showed severe vorticity diffusion at the outer wing, it remained attached to the wing surface.…”

Section: The Journal Of Experimental Biology Dual Leading-edge Vorticmentioning

confidence: 99%

“…(2) The spatial resolution of CFD and PIV in prior studies may have been inadequate to reveal the fine flow structures (Liu et al, 1998;Maybury and Lehmann, 2004). (3) ␣ m in prior studies was in the range of 40-45°, and the PIV laser-sheets were always vertical to the spanwise direction; at this relatively low ␣ m the primary vortex could not be illustrated by sectional streamlines as the local vortex axis deviated from the spanwise direction (Birch et al, 2004;Dallmann et al, 1995), which will be shown below.…”

Section: The Journal Of Experimental Biology Dual Leading-edge Vorticmentioning

confidence: 99%

“…Surprisingly, the dual LEV structure was also evident on these very narrow wings, as with the lower AR cases. The failure to illustrate the primary vortex in the case of AR=5.8 and 7.5 via sectional streamlines could be also due to the large deviation of local vortex axis from the spanwise direction (Dallmann et al, 1995).…”

Section: The Effect Of Armentioning

confidence: 99%

“…the distribution of the critical points. (2) If the plane of interest is not vertical to the local vortex axis, the planar flow field topological structure may change (Dallmann et al, 1995). Thus, the absence of a focus in a vorticity-condensed region alone may be not sufficient to rule …”

Section: Dye Flow Visualizationmentioning

confidence: 99%

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Dual leading-edge vortices on flapping wings

Yuan

Shen

Lai

2006

Journal of Experimental Biology

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). This type of vortical structure, at certain high Reynolds numbers, is analogous to the spiral conical vortex generated on each leading edge of a delta wing (Birch et al., 2004). However, whereas considerable importance is attached to the force performance and power efficiency of flapping flight (Birch and Dickinson, 2003;Sun and Tang, 2002a;Sun and Tang, 2002b), few studies have addressed the detailed structure of the LEV.In a recent flow visualization of a robotic wing moving in the motion of dragonfly hovering (Y.L., G.X.S. and W. H. Su, manuscript submitted), we observed that the LEV with intensive spanwise flow did not develop along the leading edge but moved inboard, leaving a space for the formation of a minor vortex outside the primary vortex (see Fig.·1). In fact, a similar LEV pair has been reported in butterfly free flight using smokewire visualization (Srygley and Thomas, 2002). This vortical system reminds us of the dual LEV structure on non-slender delta wings, which is defined as the large and small same-sense vortices (the primary vortex and minor vortex, respectively) located close to the leading edge on both sides of the secondary separation in-and outboard, respectively (Gordnier and Visbal, 2003;Taylor and Gursul, 2004). This interesting flow behavior implies that the LEV generated in flapping motions may involve sub-structures, and merit in-depth exploration.With both visualizations the two vortices were reported to rotate in the same direction, but it is difficult to distinguish them directly from the flow images. In this paper, dye flow visualization and high-resolution DPIV measurements were conducted to reveal the detailed features of the LEV region. Based on the validation of the dual LEV, systematic investigations were performed by altering the kinematic (Re, from 160 to 3200, and ␣ m , from 10 to 80°) and geometric (AR, from 1.3 to 10) parameters. The well-known case of the fruit fly was re-examined to see whether the dual LEV had been missed in the previous studies. All measurements were obtained with the robotic wings flapping in the hovering condition. Materials and methodsModel wing planform The planform of the simplified model dragonfly wing (AR=5.8, AR=R/c; where R is model wing length, measured from the translational axis to the wingtip; c is mean wing chord length) An experimental investigation was performed with two aims: (1) to clarify the existence of the dual leading-edge vortices (i.e. two vortices with the same sense of rotation located close to the leading edge above the leeward wing surface) observed on flapping wings in previous studies; (2) to study systematically the influences of kinematic and geometric parameters on such a vortical structure. Based on a scaled-up electromechanical model flapping in a water tank, the leading-edge vortex (LEV) cores were visualized via dye flow visualization, and the detailed substructures of LEV were revealed through digital particle image velocimetry (DPIV) with high spatial resolution. Five wing aspect ratios (AR) (1.3, 3.5, 5....

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Section: Leading-edge Vortices On Flapping Wingsmentioning

confidence: 49%

Section: The Journal Of Experimental Biology Dual Leading-edge Vorticmentioning

confidence: 99%

Section: The Journal Of Experimental Biology Dual Leading-edge Vorticmentioning

confidence: 99%

Section: The Effect Of Armentioning

confidence: 99%

Section: Dye Flow Visualizationmentioning

confidence: 99%

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Dual leading-edge vortices on flapping wings

Yuan

Shen

Lai

2006

Journal of Experimental Biology

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Dynamics of flow near a separation point

Uruba

Knob

2008

Proc Appl Math and Mech

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Dynamical behavior of a boundary layer separation is studied using the experimental approach. The dynamical nature of the phenomenon is demonstrated on a simple case of a flat–plate boundary layer in adverse pressure gradient. The Time–Resolved PIV technique was utilized for monitoring instantaneous structure of the separation region and its time development. Distinctive coherent structures and their dynamical behavior were identified using the Proper Orthogonal Decomposition method. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

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Strong Interaction Phenomena

Basics of Aerothermodynamics

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Flow field diagnostics - Topological flow changes and spatio-temporal flow structure

Cited by 21 publications

References 7 publications

Dual leading-edge vortices on flapping wings

Dual leading-edge vortices on flapping wings

Dynamics of flow near a separation point

Strong Interaction Phenomena

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