On streamwise vortical structures in the near-field of axisymmetric shear layers

Meccanica ◽  
1994 ◽  
Vol 29 (4) ◽  
pp. 403-410 ◽  
Author(s):  
R. Suprayan ◽  
H. E. Fiedler
Keyword(s):  
Near Field ◽  
Shear Layers ◽  
Vortical Structures ◽  
Axisymmetric Shear

Linear and nonlinear processes in two-dimensional mixing layer dynamics and sound radiation

2009 ◽  
Vol 625 ◽  
pp. 321-351 ◽  
Author(s):  
LAWRENCE C. CHEUNG ◽  
SANJIVA K. LELE
Keyword(s):  
Near Field ◽  
Shear Layers ◽  
Wave Radiation ◽  
Instability Wave ◽  
Sound Generation ◽  
Instability Waves ◽  
Vortical Structures ◽  
Vortex Pairing ◽  
Linear And Nonlinear ◽  
Field Sound

In this study, we consider the effects of linear and nonlinear instability waves on the near-field dynamics and aeroacoustics of two-dimensional laminar compressible mixing layers. Through a combination of direct computations, linear and nonlinear stability calculations, we demonstrate the significant role of nonlinear mechanisms in accurately describing the behaviour of instability waves. In turn, these processes have a major impact on sound generation mechanisms such as Mach wave radiation and vortex pairing sound. Our simulations show that the mean flow correction, which is required in order to accurately describe the dynamics of large-scale vortical structures, is intrinsically tied to the nonlinear modal interactions and accurate prediction of saturation amplitudes of instability waves. In addition, nonlinear interactions are largely responsible for the excitation and development of higher harmonics in the flow which contribute to the acoustic radiation. Two flow regimes are considered: In supersonic shear layers, where the far-field sound is determined by the instability wave solution at sufficiently high Mach numbers, it is shown that these nonlinear effects directly impact the Mach wave radiation. In subsonic shear layers, correctly capturing the near-field vortical structures and the interactions of the subharmonic and fundamental modes become critical due to the vortex pairing sound generation process. In this regime, a method is proposed to combine the instability wave solution with the Lilley–Goldstein acoustic analogy in order to predict far-field sound.

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.

Transition to global instability in transverse-jet shear layers

2010 ◽  
Vol 661 ◽  
pp. 294-315 ◽  
Author(s):  
J. DAVITIAN ◽  
D. GETSINGER ◽  
C. HENDRICKSON ◽  
A. R. KARAGOZIAN
Keyword(s):  
Shear Layer ◽  
Near Field ◽  
Single Mode ◽  
Shear Layers ◽  
Global Instability ◽  
Frequency Tracking ◽  
Transverse Jet ◽  
Critical Range ◽  
Crossflow Velocity ◽  
Lock In

In a recent paper (Megerianet al.,J. Fluid Mech., vol. 593, 2007, pp. 93–129), experimental exploration of the behaviour of transverse-jet near-field shear-layer instabilities suggests a significant change in the character of the instability as jet-to-crossflow velocity ratiosRare reduced below a critical range. The present study provides a detailed exploration of and additional insights into this transition, with quantification of the growth of disturbances at various locations along and about the jet shear layer, frequency tracking and response of the transverse jet to very strong single-mode forcing, creating a ‘lock-in’ response in the shear layer. In all instances, there is clear evidence that the flush transverse jet's near-field shear layer becomes globally unstable whenRlies at or below a critical range near 3. These findings have important implications for and provide the underlying strategy by which active control of the transverse jet may be developed.

Effects of Surface Tension in the Near Field Wave Breaking of Ships

2008 ◽  
Author(s):  
Fabrizio Pistani ◽  
Angelo Olivieri ◽  
Emilio Campana
Keyword(s):  
Surface Tension ◽  
Free Surface ◽  
Flow Velocity ◽  
Wave Breaking ◽  
Near Field ◽  
Surface Elevation ◽  
Vortical Structures ◽  
High Speeds ◽  
Resistance Curves ◽  
Surface Tension Forces

When model experiments are performed the viscous and surface tension forces are not scaled accordingly. Thus not all of the features of the flow can be satisfactorily reproduced at model scale. A comparative set of experiments for measuring the model resistance, the free surface elevation and the flow velocity in the near field, have been carried out for models of different scales for evaluating the influence of the dimensions in reproducing the complete wave breaking dynamics. The resistance curves of the models show that the scale effect is present both for low and high speeds. Comparison of the averaged surface elevation reveals that the largest model possess already some of the full scale features. The comparison of the flow velocity fields highlights substantial differences among the models in the formation of the vortical structures. The influence of these vortices on the free surface is discussed and a correlation with surface scars is proposed.

scholarly journals Contributions of the wall boundary layer to the formation of the counter-rotating vortex pair in transverse jets

2011 ◽  
Vol 676 ◽  
pp. 461-490 ◽  
Author(s):  
FABRICE SCHLEGEL ◽  
DAEHYUN WEE ◽  
YOUSSEF M. MARZOUK ◽  
AHMED F. GHONIEM
Keyword(s):  
Boundary Layer ◽  
Boundary Condition ◽  
Near Field ◽  
Vortex Pair ◽  
Transverse Jets ◽  
Vortical Structures ◽  
Wall Boundary Layer ◽  
Wall Boundary ◽  
Jet Trajectory ◽  
Counter Rotating Vortex Pair

Using high-resolution 3-D vortex simulations, this study seeks a mechanistic understanding of vorticity dynamics in transverse jets at a finite Reynolds number. A full no-slip boundary condition, rigorously formulated in terms of vorticity generation along the channel wall, captures unsteady interactions between the wall boundary layer and the jet – in particular, the separation of the wall boundary layer and its transport into the interior. For comparison, we also implement a reduced boundary condition that suppresses the separation of the wall boundary layer away from the jet nozzle. By contrasting results obtained with these two boundary conditions, we characterize near-field vortical structures formed as the wall boundary layer separates on the backside of the jet. Using various Eulerian and Lagrangian diagnostics, it is demonstrated that several near-wall vortical structures are formed as the wall boundary layer separates. The counter-rotating vortex pair, manifested by the presence of vortices aligned with the jet trajectory, is initiated closer to the jet exit. Moreover tornado-like wall-normal vortices originate from the separation of spanwise vorticity in the wall boundary layer at the side of the jet and from the entrainment of streamwise wall vortices in the recirculation zone on the lee side. These tornado-like vortices are absent in the case where separation is suppressed. Tornado-like vortices merge with counter-rotating vorticity originating in the jet shear layer, significantly increasing wall-normal circulation and causing deeper jet penetration into the crossflow stream.

Periodic Flow Between Low Aspect Ratio Parallel Jets

2003 ◽  
Vol 125 (2) ◽  
pp. 389-392 ◽  
Author(s):  
Elgin A. Anderson ◽  
Deryl O. Snyder ◽  
Jonathan Christensen
Keyword(s):  
Bluff Body ◽  
Near Field ◽  
Shear Layers ◽  
Periodic Flow ◽  
Field Region ◽  
Hot Wire ◽  
Low Aspect Ratio ◽  
Confining Effect ◽  
Two Component ◽  
Nozzle Plate

The behavior of symmetric parallel jets was investigated experimentally. Two-component hot-wire surveys of the velocity field were performed over a jet region extending from the nozzle plate to a distance seven times the spacing between the nozzles. The objective of this study was to investigate an observed periodic behavior in the near-field region between parallel jets that increases in frequency as the nozzle widths decrease. This behavior was found to occur in parallel jets where nozzle widths are greater than 0.5 times the jet spacer width. The phenomena are attributed to bluff body shedding in the near field and a confining effect of the outer shear layers.

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).

The elliptic whistler jet

1999 ◽  
Vol 397 ◽  
pp. 23-44 ◽  
Author(s):  
HYDER S. HUSAIN ◽  
FAZLE HUSSAIN
Keyword(s):  
Flow Visualization ◽  
Near Field ◽  
Major Axis ◽  
Field Mass ◽  
Vortical Structures ◽  
Air Jet ◽  
Mass Entrainment ◽  
Circular Jets ◽  
Secondary Vortices ◽  
Axis Switching

Elliptic jets have decided advantages for technological applications over circular jets; this paper explores further advantages achieved by jet forcing due to self-excitation. Using hot-wire measurements and flow visualization, we have studied an elliptic whistler (i.e. self-excited) air jet of 2:1 aspect ratio which, in contrast to an elliptic jet issuing from a contoured nozzle, displays no axis switching, but significantly increased spread in the major-axis plane. Its near-field mass entrainment is considerably higher (by as much as 70%) than that of a non-whistling jet. Flow visualization reveals unexpected dynamics of the elliptic vortical structures in the whistler jet compared to that in the non-whistling jet. Vortices rolled up from the lip of the elliptic pipe impinge onto the collar, producing secondary vortices; interaction of these two opposite-signed vortices is shown to cause the different behaviour of the whistler jet.

Sign in / Sign up

Export Citation Format

Share Document