Detection and tracking of vortex phenomena using Lagrangian coherent structures

Huang, Yinjuan; Green, Melissa A.

doi:10.1007/s00348-015-2001-z

Cited by 53 publications

(25 citation statements)

References 49 publications

(50 reference statements)

Supporting

Mentioning

Contrasting

Order By: Relevance

“…Before that, the flow field is reconstructed using the POD method with 90 % of the total energy, so that the small-scale structures and noise can be filtered out. Now FTLE is a convenient method to identify the structure boundary with the ridges corresponding to the Lagrangian coherent structures (LCS) (Haller 2002; Huang & Green 2015; Krishna, Green & Mulleners 2018). According to previous studies (Shadden, Dabiri & Marsden 2006; Haller 2015; Huang & Green 2015; He et al.…”

Section: Resultsmentioning

confidence: 99%

Effect of excitation frequency on flow characteristics around a square cylinder with a synthetic jet positioned at front surface

Wang

Feng

et al. 2019

J. Fluid Mech.

View full text Add to dashboard Cite

The flow over a square cylinder controlled by a slot synthetic jet positioned at the front surface is investigated experimentally at different excitation frequencies. The Reynolds number based on the free-stream velocity and the side length of the square cylinder is 1000. The flow visualization was conducted using the laser-induced fluorescence technique. The velocity fields upstream and downstream of the square cylinder were measured synchronously with the two-dimensional time-resolved particle image velocimetry technique. Both the evolution of vortex structures and the characteristic frequencies of upstream and downstream flow fields are presented. The flow dynamics vary significantly with the excitation frequency at a fixed stroke length. During one excitation cycle, the synthetic jet vortex pair deflects to one side and later swings to the other side at a quite small excitation frequency of $f_{e}/f_{0}=0.6$, while it only deflects toward one side and does not turn to the other side at $f_{e}/f_{0}=1.0$. Compared with the natural case, the wake characteristics for the above two cases are not changed much by the synthetic jet adopted. At a moderate excitation frequency of $f_{e}/f_{0}=2.0$, the synthetic jet deflects upwards and downwards alternatively. The upstream flow field has a dominant frequency identical to half of the excitation frequency. Under the perturbations of the synthetic jet, two wake vortex pairs are formed per shedding cycle with a shedding frequency equal to that of the square cylinder without control. At a higher excitation frequency of $f_{e}/f_{0}=3.4$, the synthetic jet keeps deflecting to one side, and the upstream flow field is governed by the excitation frequency. The flow separation on the deflected side is suppressed effectively, and no periodic vortex shedding can be observed in the wake. Statistically, the velocity profiles also change with control. The recirculation bubble length in the wake is shortened, and the time-averaged velocity fluctuation is weakened remarkably. The control effects of the synthetic jet and the continuous jet are compared in this paper when placed at the front surface of a square cylinder.

show abstract

Section: Resultsmentioning

confidence: 99%

Effect of excitation frequency on flow characteristics around a square cylinder with a synthetic jet positioned at front surface

Wang

Feng

et al. 2019

J. Fluid Mech.

View full text Add to dashboard Cite

show abstract

“…Ridges of repelling and attracting fluid regions are obtained by thresholding forward-time and backward-time FTLE scalar fields. Topological changes of the flow field can be identified directly by tracking Lagrangian saddle points of the flow field, which are intersection points of forward-time and backward-time FTLE ridges, as shown by Huang and Green (2015). In this study the FTLE computation package developed by Peng and Dabiri (2009) is used to obtain FTLE fields.…”

Section: Data Processingmentioning

confidence: 99%

“…In continuation of earlier efforts by Kissing et al (2020), the occurrence of secondary structures is quantified with the approach introduced by Huang and Green (2015). This method identifies Lagrangian Coherent Structure (LCS) saddle points in the flow field by intersections of repelling and attracting ridges.…”

Section: Secondary Structure Occurrence (Sso)mentioning

confidence: 99%

Insights into leading edge vortex formation and detachment on a pitching and plunging flat plate

Kissing

Kriegseis

et al. 2020

Exp Fluids

View full text Add to dashboard Cite

The present study is a prelude to applying different flow control devices on pitching and plunging airfoils with the intention of controlling the growth of the leading edge vortex (LEV); hence, the lift under unsteady stall conditions. As a pre-requisite the parameters influencing the development of the LEV topology must be fully understood and this constitutes the main motivation of the present experimental investigation. The aims of this study are twofold. First, an approach is introduced to validate the comparability between flow fields and LEV characteristics of two different facilities using water and air as working media by making use of a common baseline case. The motivation behind this comparison is that with two facilities the overall parameter range can be significantly expanded. This comparison includes an overview of the respective parameter ranges, control of the airfoil kinematics and careful scrutiny of how post-processing procedures of velocity data from time-resolved particle image velocimetry (PIV) influence the integral properties and topological features used to characterise the LEV development. Second, and based on results coming from both facilities, the appearance of secondary structures and their effect on LEV detachment over an extended parameter range is studied. A Lagrangian flow field analysis based on finite-time Lyapunov Exponent (FTLE) ridges allows precise identification of secondary structures and reveals that their emergence is closely correlated to a vortex Reynolds number threshold computed from the LEV circulation. This threshold is used to model the temporal onset of secondary structures. Further analysis indicates that the emergence of secondary structures causes the LEV to stop accumulating circulation if the shear layer angle at the leading edge of the flat plate has ceased to increase. This information is of particular importance for advanced flow control applications, since efforts to strengthen and/or prolong LEV growth rely on precise knowledge about where and when to apply flow control measures. Graphical abstract

show abstract

“…Note for this test case, the plate is accelerating; see Fernando and Rival (2016) for details on the experiment in instantaneous load estimation, either using classical approaches or ones with drift-volume estimates, and further will also allow for complimentary understanding of topological features (Huang and Green 2015) by connecting sources of vorticity production on a body to its wake. Undoubtedly there remains much work in the development and validation of these Lagrangian-based analyses, and in turn, it is hoped that with access to new high-density Lagrangian tracking, we will further uncover the salient mechanisms of this broad class of unsteady flows.…”

Section: Figmentioning

confidence: 99%