Three-dimensional wake structure measurement using a modified PIV technique

Flapping wings continuously create and send vortices into their wake, while imparting downward momentum into the surrounding fluid. However, experimental studies concerning the details of the three-dimensional vorticity distribution and evolution in the far wake are limited. In this study, the three-dimensional vortex wake structure in both the near and far field of a dynamically scaled flapping wing was investigated experimentally, using volumetric three-component velocimetry. A single wing, with shape and kinematics similar to those of a fruitfly, was examined. The overall result of the wing action is to create an integrated vortex structure consisting of a tip vortex (TV), trailing-edge shear layer (TESL) and leading-edge vortex. The TESL rolls up into a root vortex (RV) as it is shed from the wing, and together with the TV, contracts radially and stretches tangentially in the downstream wake. The downwash is distributed in an arc-shaped region enclosed by the stretched tangential vorticity of the TVs and the RVs. A closed vortex ring structure is not observed in the current study owing to the lack of well-established starting and stopping vortex structures that smoothly connect the TV and RV. An evaluation of the vorticity transport equation shows that both the TV and the RV undergo vortex stretching while convecting downwards: a three-dimensional phenomenon in rotating flows. It also confirms that convection and secondary tilting and stretching effects dominate the evolution of vorticity.

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Three Dimensional Wake Structure of Free Planar Shear Flow Around Horizontal Cylinder

Direct and Large-Eddy Simulation VI ◽

10.1007/978-1-4020-5152-2_77 ◽

2006 ◽

pp. 669-676

Author(s):

Marcelo A. Vitola ◽

Edith Beatriz Camano Schettini ◽

Jorge Hugo Silvestrini

Keyword(s):

Shear Flow ◽

Three Dimensional ◽

Horizontal Cylinder ◽

Wake Structure ◽

Planar Shear

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Three-Dimensional Stereo Measurements in Bubbly Pipe Flows With Particle Tracking Velocimetry

10.1115/imece2000-2060 ◽

2000 ◽

Author(s):

Javier Ortiz-Villafuerte ◽

Yassin A. Hassan ◽

William D. Schmidl

Keyword(s):

Particle Tracking Velocimetry ◽

Bubbly Flow ◽

Three Dimensional ◽

Continuous Phase ◽

Analysis Method ◽

Two Phase ◽

Piv Measurements ◽

Piv Technique ◽

Piv Measurement ◽

Set Up

Abstract In a two-phase bubbly flow, the continuous phase velocity field can be captured using standard PIV techniques, and the gas phase configuration can be determined using, for example, a shadow technique. If the PIV system includes a three-dimensional shadow PIV measurement set up, the bubble locations in the standard PIV measurements can be determined. Then, the bubble images are removed, and a typical anlysis of the liquid phase can be performed. An experimental set up that combines the traditional PIV technique with a shadow technique will be described and sample PIV images will be analyzed to demonstrate an advanced velocity vector analysis method in a two-phase bubbly flow system.

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D112 Accuracy of the three dimensional velocity measurement by the novel light field PIV technique

The Proceedings of the Thermal Engineering Conference ◽

10.1299/jsmeted.2014._d112-1_ ◽

2014 ◽

Vol 2014 (0) ◽

pp. _D112-1_-_D112-2_

Author(s):

Syo OGAWA ◽

Tatsuya KAWAGUCHI ◽

Isao SATOH ◽

Takushi SAITO

Keyword(s):

Velocity Measurement ◽

Light Field ◽

Three Dimensional ◽

The Novel ◽

Piv Technique

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The effect of aspect ratio on the wake structure of finite wall-mounted square cylinders

Journal of Fluid Mechanics ◽

10.1017/jfm.2019.522 ◽

2019 ◽

Vol 875 ◽

pp. 929-960 ◽

Cited By ~ 3

Author(s):

Yendrew Yauwenas ◽

Ric Porteous ◽

Danielle J. Moreau ◽

Con J. Doolan

Keyword(s):

Boundary Layer ◽

Aspect Ratio ◽

Amplitude Modulation ◽

Dimensional Space ◽

Three Dimensional ◽

Cylinder Wake ◽

Near Wake ◽

Wake Structure ◽

Velocity Amplitude ◽

Aspect Ratios

This paper presents a combined experimental and large-eddy simulation study to characterise the effect of aspect ratio on the near-wake structure of a square finite wall-mounted cylinder (FWMC). The cylinder aspect ratios (span $L$ to width $W$) investigated in the experiments were $1.4\leqslant L/W\leqslant 21.4$ and the oncoming boundary-layer thicknesses were $1.3W$ and $0.9W$ at a Reynolds number based on cylinder width of $1.4\times 10^{4}$ and $1.1\times 10^{4}$, respectively. In complementary simulations, the cylinder aspect ratios investigated were 1.4, 4.3, 10 and 18.6. The cylinder wake structure was visualised in three-dimensional space using a vortex core detection method and decomposed to its oscillation modes using the spectral proper orthogonal decomposition (SPOD) technique. A parametric diagram is proposed to predict whether the time-averaged wake structure is a dipole or a quadrupole pattern, based on oncoming boundary-layer height and aspect ratio. Cellular shedding occurs when the aspect ratio is high with up to three shedding cells occurring across the span for aspect ratios $L/W>18$. Each of these cells sheds at a distinct frequency, as evidenced by the spectral content of the surface pressure measured on the side face and the near-wake velocity. Amplitude modulation is also observed in the vortex shedding, which explains the amplitude modulation of the acoustic pressure emitted by square FWMCs. SPOD is shown to be a viable method to identify the occurrence of cellular shedding in the wake.

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Development of an international survey attitude scale: measurement equivalence, reliability, and predictive validity

Measurement Instruments for the Social Sciences ◽

10.1186/s42409-019-0012-x ◽

2019 ◽

Vol 1 (1) ◽

Author(s):

Edith de Leeuw ◽

Joop Hox ◽

Henning Silber ◽

Bella Struminskaya ◽

Corrie Vis

Keyword(s):

Factor Structure ◽

Predictive Validity ◽

Measurement Equivalence ◽

Three Dimensional ◽

Attitude Scale ◽

Three Dimensions ◽

Willingness To Participate ◽

Effective Measure ◽

Development And Validation ◽

Structure Measurement

AbstractDeclining response rates worldwide have stimulated interest in understanding what may be influencing this decline and how it varies across countries and survey populations. In this paper, we describe the development and validation of a short 9-item survey attitude scale that measures three important constructs, thought by many scholars to be related to decisions to participate in surveys, that is, survey enjoyment, survey value, and survey burden. The survey attitude scale is based on a literature review of earlier work by multiple authors. Our overarching goal with this study is to develop and validate a concise and effective measure of how individuals feel about responding to surveys that can be implemented in surveys and panels to understand the willingness to participate in surveys and improve survey effectiveness. The research questions relate to factor structure, measurement equivalence, reliability, and predictive validity of the survey attitude scale.The data came from three probability-based panels: the German GESIS and PPSM panels and the Dutch LISS panel. The survey attitude scale proved to have a replicable three-dimensional factor structure (survey enjoyment, survey value, and survey burden). Partial scalar measurement equivalence was established across three panels that employed two languages (German and Dutch) and three measurement modes (web, telephone, and paper mail). For all three dimensions of the survey attitude scale, the reliability of the corresponding subscales (enjoyment, value, and burden) was satisfactory. Furthermore, the scales correlated with survey response in the expected directions, indicating predictive validity.

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The wake structure and thrust performance of a rigid low-aspect-ratio pitching panel

Journal of Fluid Mechanics ◽

10.1017/s0022112008000906 ◽

2008 ◽

Vol 603 ◽

pp. 331-365 ◽

Cited By ~ 127

Author(s):

JAMES H. J. BUCHHOLZ ◽

ALEXANDER J. SMITS

Keyword(s):

Aspect Ratio ◽

Strouhal Number ◽

Three Dimensional ◽

Leading Edge ◽

Horseshoe Vortex ◽

Dimensional Structure ◽

Peak Efficiency ◽

Thrust Coefficient ◽

Wake Structure ◽

Thrust Performance

Thrust performance and wake structure were investigated for a rigid rectangular panel pitching about its leading edge in a free stream. For ReC = O(104), thrust coefficient was found to depend primarily on Strouhal number St and the aspect ratio of the panel AR. Propulsive efficiency was sensitive to aspect ratio only for AR less than 0.83; however, the magnitude of the peak efficiency of a given panel with variation in Strouhal number varied inversely with the amplitude to span ratio A/S, while the Strouhal number of optimum efficiency increased with increasing A/S. Peak efficiencies between 9% and 21% were measured. Wake structures corresponding to a subset of the thrust measurements were investigated using dye visualization and digital particle image velocimetry. In general, the wakes divided into two oblique jets; however, when operating at or near peak efficiency, the near wake in many cases represented a Kármán vortex street with the signs of the vortices reversed. The three-dimensional structure of the wakes was investigated in detail for AR = 0.54, A/S = 0.31 and ReC = 640. Three distinct wake structures were observed with variation in Strouhal number. For approximately 0.20 < St < 0.25, the main constituent of the wake was a horseshoe vortex shed by the tips and trailing edge of the panel. Streamwise variation in the circulation of the streamwise horseshoe legs was consistent with a spanwise shear layer bridging them. For St > 0.25, a reorganization of some of the spanwise vorticity yielded a bifurcating wake formed by trains of vortex rings connected to the tips of the horseshoes. For St > 0.5, an additional structure formed from a perturbation of the streamwise leg which caused a spanwise expansion. The wake model paradigm established here is robust with variation in Reynolds number and is consistent with structures observed for a wide variety of unsteady flows. Movies are available with the online version of the paper.

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Wake Structure Behind Circular Cylinder by Plasma Actuators

ASME-JSME-KSME 2011 Joint Fluids Engineering Conference: Volume 1, Symposia – Parts A, B, C, and D ◽

10.1115/ajk2011-13015 ◽

2011 ◽

Cited By ~ 1

Author(s):

Shunsuke Yamada ◽

Koui Shibata ◽

Hikaru Yanagihara ◽

Takahiro Doi ◽

Hitoshi Ishikawa ◽

...

Keyword(s):

Circular Cylinder ◽

Bluff Body ◽

Three Dimensional ◽

Plasma Actuators ◽

Dimensional Structure ◽

Separation Flow ◽

Driving Frequency ◽

Dbd Plasma ◽

Wake Structure ◽

Body Boundary

The separation flow causes the decrease of the driving efficiency of the fluid machines. It is important to control the separation on the bluff body, boundary layer and so on. The purpose of the present study is to control the separation on a circular cylinder and investigate the wake structure using the induced by dielectric barrier discharge (DBD) plasma. The electrode of the DBD plasma was mounted. In previous study, it is reported that the three dimensional wake structures are effective for the drag reduction. We investigate the three dimensional structure of the wake due to three dimensional jets of plasma actuators. The plasma actuators have pulse driving frequency of 0, 0.22, 1.0 and 2.0. A voltage of 4 kVpp and a frequency of 10 kHz are applied to the electrode. The velocity profiles behind the circular cylinder were measured by X type hot-wire anemometer at a Reynolds number of 1.0 × 103. The wake structure changes because the roll up of vortex is suppressed using pulse driving of the plasma actuators. The effect of plasma actuators is evaluated based on the half width and Reynolds stress in the wake.

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