POD analysis on vortical structures in MVG wake by Liutex core line identification

Abstract Liutex is a vortex identification method that provides a vector interpretation of local fluid rotation. Liutex produces a vector quantity which can be used to determine the absolute and relative strength of a vortex, the local rotation axis of a vortex, the vortex core center, the size of the vortex core, and the vortex boundary. Vortex identification and visualization is essential in computational fluid turbulence analysis and fluid mechanics in general. Until Liutex, there has not been a way to identify the core of a vortex structure or even the center of rotation of a vortex structure. Since Liutex, tools have been created to assist in the identification and analysis of vortical structures. The Liutex Core Line has been developed to better understand turbulent fluid structures. A Liutex core is defined as a concentration of Liutex vectors and defined to be unique and the Liutex core line is the center of rotation of that Liutex core. Currently, iso-surfaces are the most popular way to visualize the structure of turbulent flow but there is no reason to believe that it is the best way to represent a vortex’s structure. Previous methods that use iso-surface are strongly threshold dependent and since the Liutex core line is unique, it is independent of threshold and can show the real vortex structure. In this paper we show the benefits and promises of the Liutex Core Line as a better way of representing vortex structures.

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A Time-Resolved PIV Study of Vortical Structures in the Near-Wall Region of an Impinging Round Jet

Volume 9: Heat Transfer, Fluid Flows, and Thermal Systems, Parts A, B and C ◽

10.1115/imece2009-13076 ◽

2009 ◽

Author(s):

Khaled J. Hammad ◽

Ivana M. Milanovic

Keyword(s):

Flow Field ◽

Turbulence Models ◽

Wall Region ◽

Vortex Identification ◽

Time Resolved ◽

Vortical Structures ◽

Reynolds Decomposition ◽

Gradient Based ◽

Pod Analysis ◽

Identification Techniques

Time-Resolved Particle Image Velocimetry (TR-PIV) was used to study the vortical structures resulting from a submerged water jet impinging normally on a smooth and flat surface. A fully developed turbulent jet, exiting a long pipe, and a semi-confined flow configuration ensured properly characterized boundary conditions, which allows for straightforward assessment of turbulence models and numerical schemes. The Reynolds number based on jet mean exit velocity was 23,000. The pipe-to-plate separation was varied between 2D and 7.6D. Turbulent velocity fields are presented using Reynolds decomposition into mean and fluctuating components. Proper Orthogonal Decomposition (POD) analysis was used to identify the most energetic coherent structures of the turbulent flow field. Three velocity gradient-based vortex identification techniques, 2nd invariant Q, λ2, and swirling strength, were found to perform equally well in identifying vortical structures along the impingement wall. The results clearly demonstrate the shortcomings of local vorticity as a vortex identifier in an impinging jet flow field.

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Vortex Identification Study of a Turbulent Cavity Flow

Volume 1: Flow Manipulation and Active Control; Bio-Inspired Fluid Mechanics; Boundary Layer and High-Speed Flows; Fluids Engineering Education; Transport Phenomena in Energy Conversion and Mixing; Turbulent Flows; Vortex Dynamics; DNS/LES and Hybrid RANS/LES Methods; Fluid Structure Interaction; Fluid Dynamics of Wind Energy; Bubble, Droplet, and Aerosol Dynamics ◽

10.1115/fedsm2018-83048 ◽

2018 ◽

Author(s):

Khaled J. Hammad

Keyword(s):

Particle Image ◽

Vortex Identification ◽

Cavity Depth ◽

Piv Measurements ◽

Vortical Structures ◽

Image Velocimetry ◽

Flow Features ◽

Shallow Cavity ◽

Pod Analysis ◽

Proper Orthogonal

A combined vortex identification and Proper Orthogonal Decomposition (POD) analysis is applied to high-resolution Particle Image Velocimetry (PIV) measurements of a turbulent flow past an open shallow cavity. The PIV measurements, at a cavity depth based Reynolds number of 42,000, capture the flow structure and turbulence, upstream, over, and downstream an open cavity having a length-to-depth ratio of four. Vorticity and second invariant Q of the velocity gradient tensor analysis are used to identify the vortical structures and the overall flow field features. POD analysis is applied to the vorticity and Q fields to identify the most energetic vortical structures and flow features. The results demonstrate the superiority of the combined Q-criterion and POD analysis in identifying distinct vortical structures and their evolution.

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Direct Numerical Simulation of Global Instability in a Hole-Tone Feedback System

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

10.1115/ajk2011-08034 ◽

2011 ◽

Author(s):

Kazuo Matsuura ◽

Masami Nakano

Keyword(s):

Pressure Fluctuation ◽

Feedback System ◽

Computational Results ◽

Helical Structures ◽

Tone Frequency ◽

Vortical Structures ◽

End Plate ◽

Fluctuation Field ◽

Air Jet ◽

Pod Analysis

Direct computations and experiments of a hole-tone feedback system are conducted. The mean velocities of an air-jet are 8 and 10 m/s in the computations, 6–13 m/s in the experiments. The diameters of a nozzle and an end plate hole are both 50 mm, and an impingement length between the nozzle and the end plate is 50 mm. The computational results agree well with the experimental data in terms of qualitative vortical structures and a relationship between the most dominant hole-tone frequency and a jet speed. Based on the computational results of the air-jet speed of 8 m/s, a Proper Orthogonal Decomposition (POD) analysis of the whole pressure fluctuation field is conducted. The 1st and 2nd POD modes are nearly in anti-phase, and alternatively appearing helical structures are observed upstream of the end plate hole in an isosurface plot of the eigenfunctions of the modes. Dominant behaviors of vortex shedding from the end plate hole are represented by the 3rd and 4th modes. As the result, dominant variation of the pressure fluctuation field is successfully extracted by the present POD analysis.

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