scholarly journals Detecting vortical structures in time-resolved volumetric flow fields

2021 ◽  
Vol 1 (1) ◽  
Author(s):  
Karuna Agarwal ◽  
Omri Ram ◽  
Jin Wang ◽  
Yuhui Lu ◽  
Joseph Katz
Keyword(s):  
Near Field ◽  
Three Dimensional ◽  
Turbulent Shear ◽  
Time Resolved ◽  
Vortical Structures ◽  
Cavitation Inception ◽  
Turbulent Shear Layer ◽  
Velocity Gradients ◽  
Vortex Detection ◽  
Axial Stretching

The detection of three-dimensional coherent vortical structures that get advected as well as deformed with time is a challenge. However, it is critical for the statistical analysis of these vortices, for example, the quasi-streamwise vortices (QSVs) in the near field of a turbulent shear layer, where cavitation inception typically occurs. These structures exhibit underlying correlations among different properties that can be derived from the velocity gradients. Exploiting these correlations, a pseudo-Lagrangian vortex detection method is proposed that uses k-means clustering based on vorticity magnitude and direction, values of λ2, strain rate structure, axial stretching, and location. The method facilitates the finding that QSVs have pressure minima that are lower than those in the surrounding flow, including the primary spanwise vortices. These minima typically appear after a period of axial stretching and before contraction events.

Dissipative small-scale actuation of a turbulent shear layer

2010 ◽  
Vol 656 ◽  
pp. 51-81 ◽  
Author(s):  
B. VUKASINOVIC ◽  
Z. RUSAK ◽  
A. GLEZER
Keyword(s):  
Shear Layer ◽  
Large Scale ◽  
Near Field ◽  
Base Flow ◽  
Turbulent Shear ◽  
Small Scale ◽  
Vortical Structures ◽  
Turbulent Shear Layer ◽  
Vorticity Flux ◽  
Inviscid Instability

The effects of small-scale dissipative fluidic actuation on the evolution of large- and small-scale motions in a turbulent shear layer downstream of a backward-facing step are investigated experimentally. Actuation is applied by modulation of the vorticity flux into the shear layer at frequencies that are substantially higher than the frequencies that are typically amplified in the near field, and has a profound effect on the evolution of the vortical structures within the layer. Specifically, there is a strong broadband increase in the energy of the small-scale motions and a nearly uniform decrease in the energy of the large-scale motions which correspond to the most amplified unstable modes of the base flow. The near field of the forced shear layer has three distinct domains. The first domain (x/θ0 < 50) is dominated by significant concomitant increases in the production and dissipation of turbulent kinetic energy and in the shear layer cross-stream width. In the second domain (50 < x/θ0 < 300), the streamwise rates of change of these quantities become similar to the corresponding rates in the unforced flow although their magnitudes are substantially different. Finally, in the third domain (x/θ0 > 350) the inviscid instability of the shear layer re-emerges in what might be described as a ‘new’ baseline flow.

scholarly journals On the formation of the counter-rotating vortex pair in transverse jets

2001 ◽  
Vol 446 ◽  
pp. 347-373 ◽  
Author(s):  
L. CORTELEZZI ◽  
A. R. KARAGOZIAN
Keyword(s):  
Flow Field ◽  
Computational Study ◽  
Near Field ◽  
Three Dimensional ◽  
Vortex Pair ◽  
Transverse Jets ◽  
Vortical Structures ◽  
Jet In Crossflow ◽  
Dynamical Generation ◽  
Counter Rotating Vortex Pair

Among the important physical phenomena associated with the jet in crossflow is the formation and evolution of vortical structures in the flow field, in particular the counter-rotating vortex pair (CVP) associated with the jet cross-section. The present computational study focuses on the mechanisms for the dynamical generation and evolution of these vortical structures. Transient numerical simulations of the flow field are performed using three-dimensional vortex elements. Vortex ring rollup, interactions, tilting, and folding are observed in the near field, consistent with the ideas described in the experimental work of Kelso, Lim & Perry (1996), for example. The time-averaged effect of these jet shear layer vortices, even over a single period of their evolution, is seen to result in initiation of the CVP. Further insight into the topology of the flow field, the formation of wake vortices, the entrainment of crossflow, and the effect of upstream boundary layer thickness is also provided in this study.

Elliptic jets. Part 3. Dynamics of preferred mode coherent structure

1993 ◽  
Vol 248 ◽  
pp. 315-361 ◽  
Author(s):  
Hyder S. Husain ◽  
Fazle Hussain
Keyword(s):  
Coherent Structure ◽  
Near Field ◽  
Three Dimensional ◽  
Mixing Enhancement ◽  
Hot Wire ◽  
Vortical Structures ◽  
Mass Entrainment ◽  
Time Average ◽  
And Control ◽  
Made In

The dynamics of the preferred mode structure in the near field of an elliptic jet have been investigated using hot-wire measurements. A 2:1 aspect ratio jet with an initially turbulent boundary layer and a constant momentum thickness all around the nozzle exit perimeter was used for this study. Measurements were made in air at a Reynolds number ReDe (≡ UeDe/v) = 3.5 × 104. Controlled longitudinal excitation at the preferred mode frequency (StDe ≡ fDe/Ue = 0.4) induced periodic formation of structures, allowing phase-locked measurements with a local trigger hot wire. The dynamics of the organized structure are examined from educed fields of coherent vorticity and incoherent turbulence in the major and minor symmetry planes at five successive phases of evolution, and are also compared with corresponding data for a circular jet. Unlike in a circular jet, azimuthally fixed streamwise vortices (ribs) form without the aid of azimuthal forcing. The three-dimensional deformation of elliptic vortical structures and the rib formation mechanism have also been studied through direct numerical simulation. Differential self-induced motions due to non-uniform azimuthal curvature and the azimuthally fixed ribs produce greater mass entrainment in the elliptic jet than in a circular jet. The turbulence production mechanism, entrainment and mixing enhancement, and time-average measures and their modification by excitation are also discussed in terms of coherent structure dynamics and the rib-roll interaction. Various phase-dependent and time-average turbulence measures documented in this paper should serve as target data for validation of numerical simulations and turbulence modelling, and for design and control purposes in technological applications. Further details are given by Husain (1984).

Tip-Vortex Induced Cavitation on a Ducted Propulsor

2003 ◽  
Author(s):  
Christopher J. Chesnakas ◽  
Stuart D. Jessup
Keyword(s):  
Reynolds Number ◽  
High Speed ◽  
Three Dimensional ◽  
Tip Vortex ◽  
Number Range ◽  
Vortical Structures ◽  
Tip Vortices ◽  
Tip Vortex Cavitation ◽  
Cavitation Inception ◽  
Ducted Rotor

An extensive experimental investigation was carried out to examine tip-vortex induced cavitation on a ducted propulsor. The flowfield about a 3-bladed, ducted rotor operating in uniform inflow was measured in detail with three-dimensional LDV; cavitation inception was measured; and a correlated hydrophone/high-speed video system was used to identify and characterize the early, sub-visual cavitation events. Two geometrically-similar, ducted rotors were tested over a Reynolds number range from 1.4×106 to 9×106 in order to determine how the tip-vortex cavitation scales with Reynolds number. Analysis of the data shows that exponent for scaling tip-vortex cavitation with Reynolds number is smaller than for open rotors. It is shown that the parameters which are commonly accepted to control tip-vortex cavitation, vortex circulation and vortex core size, do not directly control cavitation inception on this ducted rotor. Rather it appears that cavitation is initiated by the stretching and deformation of secondary vortical structures resulting from the merger of the leakage and tip vortices.

Topological Characteristics of Coherent Structures in the Turbulent Boundary Layer Measured by Tomo-PIV

2013 ◽  
Vol 718-720 ◽  
pp. 801-806
Author(s):  
Hai Ping Tian ◽  
Shao Qiong Yang ◽  
Nan Jiang
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Coherent Structures ◽  
Velocity Structure ◽  
Three Dimensional ◽  
Time Resolved ◽  
Multi Scale ◽  
Velocity Gradients ◽  
Velocity Structure Function ◽  
Topological Characteristics

Database of time series of the instantaneous three-dimensional three-component (3D-3C) velocity vector field, measured by tomographic time-resolved PIV(Tomo-PIV) in a water tunnel, was analyzed to investigate spatial topologies of coherent structures in the turbulent boundary layer (TBL). A new concept of spatial locally averaged velocity structure function of turbulence is put forward to describe the spatial dilation or compression of the multi-scale coherent structures in the TBL. According to the physical mechanism of dilation or compression of multi-scale coherent vortex structures in the turbulent flow, a new conditional sampling method was proposed as well to extract the spatial topological characteristics of physical quantities of coherent structures, such as fluctuating velocities, velocity gradients, velocity strain rates and vorticity during the bursting process in the Tomo-PIV database. Furthermore, the anti-symmetric structures are the typical spatial topologies characteristics for the velocity gradients and vorticity during coherent structures burst.

A numerical study of the evolution and structure of a turbulent shear layer under a free surface

1998 ◽  
Vol 354 ◽  
pp. 239-276 ◽  
Author(s):  
WU-TING TSAI
Keyword(s):  
Free Surface ◽  
Shear Layer ◽  
Numerical Study ◽  
Horseshoe Vortex ◽  
Turbulent Shear ◽  
Turbulent Shear Flow ◽  
Vortical Structures ◽  
Turbulent Shear Layer ◽  
Initial Vorticity ◽  
Vorticity Transport

Results from direct numerical simulations of an unsteady turbulent shear layer with a free surface are presented. The emphasis is on the interaction dynamics of the free surface with the coherent vortices in the underlying turbulent shear flow as well as the resulting free-surface signatures. Instantaneous vortex lines and isosurfaces of enstrophy indicate that coherent horseshoe vortical structures emerge from the random initial vorticity field. These horseshoe vortices impinge, break and reconnect onto the free surface, and then appear as two vortex connections with opposite signs on the surface. The two identified vortical structures correspond to ‘splatting’ and ‘swirling’ events, which have been observed in other experiments and simulations of free-surface/turbulence flows. Though free-surface depressions form near the vertical-vorticity centres in the connection processes, only a low correlation (≈50% to 60%) is found between the free-surface roughness (vertical deformation) and the connected normal vorticity. On the other hand, the free-surface curvatures and the tangential free-surface vorticities are better correlated (≈80% to 90%). The balance of enstrophy and the vorticity transport show that stretching and viscous dissipation along the direction of the vorticity vector dominate the vortex dynamics near the free surface. These two transport mechanisms are found to be responsible for the cancellation of the spanwise vorticity of the horseshoe-vortex heads and the annihilation of the surface-connected normal vorticities.

Flow Structure of an Inclined Jet in Crossflow

2005 ◽  
Author(s):  
Bertrand P. E. Dano ◽  
James A. Liburdy
Keyword(s):  
Flow Structure ◽  
Near Field ◽  
Detection Algorithm ◽  
Three Dimensions ◽  
Two Dimensional ◽  
Vortical Structures ◽  
Jet In Crossflow ◽  
Turbulent Characteristics ◽  
Component Velocity ◽  
Vortex Detection

The velocity and turbulence fields associated with a streamwise 45° inclined jet in a crossflow are investigated in three dimensions. Using Stereo-PIV data, full 3D rendering of the 3-component velocity field and turbulent characteristics are achieved and used to study the near field flow structure of the jet flow. A two dimensional vortex detection algorithm is also used to further assess the vortical structures in the flow.

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