Liutex (vortex) core definition and automatic identification for turbulence vortex structures

Xu, Hanlin; Cai, Xiaoshu; Liu, Chaoqun

doi:10.1007/s42241-019-0066-5

Cited by 50 publications

(10 citation statements)

References 26 publications

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“…where λ ci is the imaginary part of the complex conjugate eigenvalues and r is the real eigenvector of velocity gradient tensor. Thereafter, multiple vortex identification methodologies have been developed including the Liutex iso-surfaces, Liutex-Ω method [36,37], Liutex core line method [38,39], objective Liutex method [40], etc. In addition, a particular feature of the Liutex vector is that it is free from shear contamination.…”

Section: Introductionmentioning

confidence: 99%

Flow Field Analysis of a Turbulent Channel Controlled by Scalloped Riblets

Yu,

Huang,

Wang

et al. 2023

Aerosp. Res. Commun.

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Riblets are small protruding surfaces along the direction of the flow, and are one of the most well-known passive turbulent drag reduction methods. We investigated a scalloped riblet, the shape of which was constructed by smoothly connecting two third-order polynomials and was not as sharp in the tip as corresponding triangular riblets with the same height-width ratio. Numerical simulations were performed for turbulent channel flow with and without riblet control at an estimated optimum width of W+=20 and a height-width ratio of 0.5. A drag reduction rate of 5.77% was obtained, which is generally larger than the reported drag reduction rates of corresponding triangular riblets from the literature. Mean flow fields and second-order statistics of velocity, vorticity, and Liutex, a quantity introduced to represent vortices, were reported. It was found that streamwise vortices just above the riblet tips, which have a length scale of 200−300 in wall units, have an important influence on those statistics and thus the turbulence generation cycle and the drag reduction mechanism. Pre-multiplied energy spectra of streamwise velocity and the Liutex component were reported to reveal the length scales in the flow field. Instantaneous vortical flow fields visualized by the Liutex method were provided with emphases on the streamwise vortices just above riblet tips. It should be noted that the class of scalloped riblets is suitable for investigations on the influences of curvatures at the tip and the valley of the riblet in future.

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Section: Introductionmentioning

confidence: 99%

Flow Field Analysis of a Turbulent Channel Controlled by Scalloped Riblets

Yu,

Huang,

Wang

et al. 2023

Aerosp. Res. Commun.

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“…These will directly influence one's understanding of the hydraulic loss caused by vortex rope evolution. Recently, Liu et al introduced the Ω method and Liutex/Rotex method [4] to determine the vortex. The Liutex/Rotex method has provided better performance and more reliability in revealing vortex structures [5,6].…”

Section: Introductionmentioning

confidence: 99%

Application of Liutex and Entropy Production to Analyze the Influence of Vortex Rope in the Francis-99 Turbine Draft Tube

2022

Teh. vjesn.

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The vortex structures in a draft tube of a Francis turbine are confused, primarily when cavitation occurs, which may induce severe vibration and noise. Understanding this vortex's periodical precession and investigating the vortex rope influence in the draft tube cone is required for stable operation at off-design conditions. This study captured the cavitating flow with high accuracy by utilizing the SST turbulence and ZGB cavitation models. The Omega and Liutex methods are employed and compared with the traditional Q -criterion to determine the vortex rope behavior in the Francis-99 turbine draft tube. The result showed that the Liutex/Rortex method is advantageous over the Q -criterion for representing the vortex rope structure. The Liutex method can effectively remove the boundary layer's influence on visual vortex structures from the draft tube's wall. Moreover, the local hydraulic loss distribution was clarified by using entropy production. By comparing Liutex magnitude distribution and entropy production rate, the vortex rope behavior effects on local hydraulic loss were significantly illuminated.

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“…By changing the precone angle of the upstream wind turbine in the range of -15°~15°, the wake characteristics of the tandem turbine rotors are explored. Finally, the post-processing program of the latest third-generation W vortex identification method Xu et al, 2019) is compiled, to visualize the wind turbine wake vortex structure under the precone variation. By comparing the differences of the first, second, and third vortex identification methods, the influence mechanism of the upstream wind turbine wake vortex structure on the downstream wind turbine is further understood, thus proving valuable to the conceptualization of wind farms.…”

Section: Introductionmentioning

confidence: 99%

Research on complex wake interference of aligned rotors considering the precone variation of the upstream wind turbine

Zhang

Sun

et al. 2022

Front. Mar. Sci.

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To understand the interaction wake effects of aligned horizontal-axis wind turbines, a refined and low-cost wake simulation framework is developed by combining the actuator line (AL) with a local dynamic Smagorinsky (LDS) model. Firstly, the published performance of the NREL-5MW wind turbine is used to corroborate the accuracy of the AL-LDS coupling model. Secondly, the research is extended to predict the interference between two tandem rotor wakes. The influence of the precone characteristics on the overall wake and power output of the tandem wind turbines is explored by modifying the upstream wind turbine and introducing the latest third-generation Ω vortex identification method in the post-processing procedure. According to results, the tandem wind turbines have complex wake interference, with the precone angle variation always reducing the global output power, and leading to an unstable downstream turbine wake. This is detrimental to system lifespan due to high performance fluctuation and stress and as well may have ecological implications due to sediment alteration. From results, an upwind turbine with precone angle (negative) can lead to higher downstream but lower upstream turbine performance, compared with a non-tilted turbine. Despite the optimal precone angle of the upstream wind turbine is 0°, a -2.5° proves more advantageous owing to the large flexible deformation of the turbine wingtips. The findings can serve as a continuous incentive for improving wind farm numerical simulation.

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Liutex (vortex) core definition and automatic identification for turbulence vortex structures

Cited by 50 publications

References 26 publications

Flow Field Analysis of a Turbulent Channel Controlled by Scalloped Riblets

Flow Field Analysis of a Turbulent Channel Controlled by Scalloped Riblets

Application of Liutex and Entropy Production to Analyze the Influence of Vortex Rope in the Francis-99 Turbine Draft Tube

Research on complex wake interference of aligned rotors considering the precone variation of the upstream wind turbine

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