scholarly journals Unraveling the interplay of two counter-rotating helical vortices

2020 ◽  
Vol 5 (11) ◽  
Author(s):  
Alessandro Capone ◽  
Francisco Alves Pereira
Keyword(s):  
Helical Vortices

Microstructure-induced helical vortices allow single-stream and long-term inertial focusing

Lab on a Chip ◽  
2013 ◽  
Vol 13 (15) ◽  
pp. 2942 ◽  
Author(s):  
Aram J. Chung ◽  
Dianne Pulido ◽  
Justin C. Oka ◽  
Hamed Amini ◽  
Mahdokht Masaeli ◽  
...  
Keyword(s):  
Inertial Focusing ◽  
Single Stream ◽  
Helical Vortices

Numerical evaluation of the velocities induced by trailing helical vortices

AIAA Journal ◽  
1990 ◽  
Vol 28 (4) ◽  
pp. 754-756 ◽  
Author(s):  
D. H. Wood ◽  
G. Gordon
Keyword(s):  
Numerical Evaluation ◽  
Helical Vortices

Clear-Air Turbulence Near the Jet-Stream Maxima *

1955 ◽  
Vol 36 (2) ◽  
pp. 53-60 ◽  
Author(s):  
Leroy H. Clem
Keyword(s):  
Wind Speed ◽  
Physical Model ◽  
Wind Shear ◽  
Vertical Wind Shear ◽  
Vertical Wind ◽  
Stability Conditions ◽  
Jet Stream ◽  
Air Turbulence ◽  
Helical Vortices ◽  
High Level

The development of turbo-jet aircraft has made high-level clear air turbulence a major problem for aviation interests. This paper emphasizes the association of the majority of this turbulence with the pronounced vertical wind shear in and near the maximum wind speed centers that move along the jet stream. A physical model is proposed as a possible explanation of clear air turbulence, the associated cirrus bands and wind streaks in the jet maxima. This model is supported by an analogy drawn with similar low-level phenomena studied by Woodcock and others. The model can explain distribution of these features in the horizontal by means of helical vortices which are dependent upon proper vertical wind shear and stability conditions. The observed multiple layers in the vertical are also explained by this model. It is believed that the reason why most of the clear-air turbulence is found near the jet-stream maxima is simply because the necessary shear and stability conditions associated with this turbulence are most frequently fulfilled in that region.

Experimental and Computational Studies of Chaotic Stirring in Complex 3D Flows

2002 ◽  
Author(s):  
Fotis Sotiropoulos ◽  
Tahirih C. Lackey ◽  
S. Casey Jones
Keyword(s):  
Swirling Flow ◽  
Experimental Studies ◽  
Computational Studies ◽  
Poincaré Maps ◽  
Poincare Maps ◽  
Numerical Studies ◽  
Helical Vortices ◽  
3D Flows ◽  
Confined Flows ◽  
Particle Paths

Recent progress in experimental and computational studies of complex chaotically advected 3D flows is reviewed for the confined swirling flow in a cylindrical container with a rotating bottom and the open flow in a helical static mixer. The concept of Lagrangian averaging along particle paths, whose theoretical foundation stems from ergodic theory, is proposed as a powerful tool for constructing Poincare´ maps in numerical studies of confined flows. The same concept has also been employed to develop the first non-intrusive experimental technique for constructing Poincare´ maps in complex 3D flows. The potential of these ergodic concepts is demonstrated in computational and experimental studies for the confined swirling flow. Numerical computations for the helical mixer flow show that increasing the Reynolds number from Re = 100 to 500 leads to the appearance of unmixed islands in the flow. The mechanism that leads to the formation of such islands is shown to be linked to the growth of coherent helical vortices in the flow.

scholarly journals Local and global pairing instabilities of two interlaced helical vortices

2019 ◽  
Vol 863 ◽  
pp. 927-955 ◽  
Author(s):  
Hugo Umberto Quaranta ◽  
Mattias Brynjell-Rahkola ◽  
Thomas Leweke ◽  
Dan S. Henningson
Keyword(s):  
Growth Rates ◽  
Double Helix ◽  
Water Channel ◽  
Three Dimensional ◽  
Rotation Frequency ◽  
Quantitative Agreement ◽  
Driving Mechanism ◽  
Helical Vortices ◽  
Local Pairing ◽  
Real Flow

We investigate theoretically and experimentally the stability of two interlaced helical vortices with respect to displacement perturbations having wavelengths that are large compared to the size of the vortex cores. First, existing theoretical results are recalled and applied to the present configuration. Various modes of unstable perturbations, involving different phase relationships between the two vortices, are identified and their growth rates are calculated. They lead to a local pairing of neighbouring helix loops, or to a global pairing with one helix expanding and the other one contracting. A relation is established between this instability and the three-dimensional pairing of arrays of straight parallel vortices, and a striking quantitative agreement concerning the growth rates and frequencies is found. This shows that the local pairing of vortices is the driving mechanism behind the instability of the helix system. Second, an experimental study designed to observe these instabilities in a real flow is presented. Two helical vortices are generated by a two-bladed rotor in a water channel and characterised through dye visualisations and particle image velocimetry measurements. Unstable displacement modes are triggered individually, either by varying the rotation frequency of the rotor, or by imposing a small rotor eccentricity. The observed unstable mode structure, and the corresponding growth rates obtained from advanced processing of visualisation sequences, are in good agreement with theoretical predictions. The nonlinear late stages of the instability are also documented experimentally. Whereas local pairing leads to strong deformations and subsequent breakup of the vortices, global pairing results in a leapfrogging phenomenon, which temporarily restores the initial double-helix geometry, in agreement with recent observations from numerical simulations.

Stability of helical tip vortices in a rotor far wake

2007 ◽  
Vol 576 ◽  
pp. 1-25 ◽  
Author(s):  
V. L. OKULOV ◽  
J. N. SØRENSEN
Keyword(s):  
Rotor Blades ◽  
Tip Vortex ◽  
Tip Vortices ◽  
Radial Extent ◽  
Helical Vortices ◽  
Constant Pitch ◽  
Bladed Rotor ◽  
The Stability ◽  
Further Development ◽  
Far Wake

As a means of analysing the stability of the wake behind a multi-bladed rotor the stability of a multiplicity of helical vortices embedded in an assigned flow field is addressed. In the model the tip vortices in the far wake are approximated by infinitely long helical vortices with constant pitch and radius. The work is a further development of a model developed in Okulov (J. Fluid Mech., vol. 521, p. 319) in which the linear stability of N equally azimuthally spaced helical vortices was considered. In the present work the analysis is extended to include an assigned vorticity field due to root vortices and the hub of the rotor. Thus the tip vortices are assumed to be embedded in an axisymmetric helical vortex field formed from the circulation of the inner part of the rotor blades and the hub. As examples of inner vortex fields we consider three generic axial columnar helical vortices, corresponding to Rankine, Gaussian and Scully vortices, at radial extents ranging from the core radius of a tip vortex to several rotor radii.The analysis shows that the stability of tip vortices largely depends on the radial extent of the hub vorticity as well as on the type of vorticity distribution. As part of the analysis it is shown that a model in which the vortex system is replaced by N tip vortices of strength Γ and a root vortex of strength − N/Γ is unconditionally unstable.

Vortex Breakdown in Swirling Fuel Injector Flows

2007 ◽  
Author(s):  
Kris Midgley ◽  
Adrian Spencer ◽  
James J. McGuirk
Keyword(s):  
Vortex Breakdown ◽  
Near Field ◽  
Aerodynamic Characteristics ◽  
Relative Strength ◽  
Turbulence Energy ◽  
Fuel Injector ◽  
Swirl Number ◽  
Helical Vortices ◽  
Helical Vortex ◽  
Air Mixing

It is well known that the process of vortex breakdown plays an important role in establishing the near-field aerodynamic characteristics of fuel injectors, influencing fuel/air mixing and flame stability. The precise nature of the vortex breakdown can take on several forms, which have been shown in previous papers to include both a precessing vortex core (PVC) and the appearance of multiple helical vortices formed in the swirl stream shear layer. The unsteady dynamics of these particular features can play an important role in combustion induced oscillations. The present paper reports an experimental investigation, using PIV and hot-wire-anemometry, to document variations in the relative strength of PVC and helical vortex patterns as the configuration of a generic fuel injector is altered. Examples of geometric changes which have been investigated include: • The combination of an annular swirl stream with and without a central jet; • variation in geometric details of the swirler passage, e.g. alteration in the swirler entry slots to change swirl number, and variations in the area ratio of the swirler passage. The results show that these geometric variations can influence: • the axial location of the origin of the helical vortices (from inside to outside the fuel injector); • the strength of the PVC. For example, in a configuration with no central jet (swirl number S = 0.72) the helical vortex pattern was much less coherent, but the PVC was much stronger than when a central jet was present. These changes modify the magnitude of the turbulence energy in the fuel injector near field dramatically, and hence have an important influence on fuel air mixing patterns.

On the Stability of Taylor - Couette Flow With Axial Flow

2012 ◽  
Author(s):  
Emna Berrich ◽  
Fethi Aloui ◽  
Jack Legrand
Keyword(s):  
Flow Direction ◽  
Axial Flow ◽  
Diffusion Method ◽  
Diffusion Limit ◽  
Taylor Flow ◽  
Concentric Cylinders ◽  
Reflected Light ◽  
Helical Vortices ◽  
Taylor Couette ◽  
The Stability

An experimental investigation of Taylor-Couette flows with axial flow is presented. Two techniques are used: Visualization using the Kalliroscope and Electro-diffusion method using electrochemical probes. The fluid is confined between concentric cylinders. It is constituted by an electrochemical solution seeding with 2% of a rheoscopic liquid AQ-1000 (Kalliroscope Corp., U.S.A.). The rheoscopic liquid contains small particles reflecting light in dependence on their orientation imposed by the flow direction. The reflected light intensity of Kalliroscope flakes allows a qualitative study of the flow. While the polarography technique allows the measurement of diffusion limit current intensities delivered by the electrochemical probes. The frequency responses of the probe to the flow allow the determination of the instantaneous and local mass transfer and the instantaneous wall shear rate. Two protocols were adopted to study the effect of an axial flow superposed to Couette-Taylor flows and the history flow effect. The first one consists to impose an azimuthal flow to the inner cylinder. When the regime was established, we superposed the axial flow. This protocol was named “the direct protocol”. While the second protocol consists to impose firstly the axial flow on the gap of the system then the azimuthal flow. We named it “the inverse protocol”. We demonstrated that the Couette-Taylor flow with axial flow is strongly dependent on the flow history (the protocol). For the same Taylor number and axial Reynolds number, the resulting flow is completely different. An axial flow superposed to Couette-Taylor flow can delay the instabilities apparition; generate the displacement of the Taylor vortices in the same direction as the axial flow or in the opposite direction; and modify the instability character of the flow by developing helical vortices or wavy helical vortices.

Dynamics of helical vortices in a superconducting wire

1996 ◽  
Vol 264 (3-4) ◽  
pp. 204-212 ◽  
Author(s):  
M. Shvartser ◽  
M. Gitterman ◽  
B.Ya. Shapiro
Keyword(s):  
Superconducting Wire ◽  
Helical Vortices
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