Transition to global instability in transverse-jet shear layers

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.

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Transverse-jet shear-layer instabilities. Part 1. Experimental studies

Journal of Fluid Mechanics ◽

10.1017/s0022112007008385 ◽

2007 ◽

Vol 593 ◽

pp. 93-129 ◽

Cited By ~ 97

Author(s):

S. MEGERIAN ◽

J. DAVITIAN ◽

L. S. DE B. ALVES ◽

A. R. KARAGOZIAN

Keyword(s):

Shear Layer ◽

Near Field ◽

Experimental Studies ◽

Single Mode ◽

Tunnel Wall ◽

Low Level ◽

Transverse Jet ◽

Free Jet ◽

Instability Modes ◽

Crossflow Velocity

This study provides a detailed exploration of the near-field shear-layer instabilities associated with a gaseous jet injected normally into crossflow, also known as the transverse jet. Jet injection from nozzles which are flush as well as elevated with respect to the tunnel wall are explored experimentally in this study, for jet-to-crossflow velocity ratiosRin the range 1 ≲R≤ 10 and with jet Reynolds numbers of 2000 and 3000. The results indicate that the nature of the transverse jet instability is significantly different from that of the free jet, and that the instability changes in character as the crossflow velocity is increased. Dominant instability modes are observed to be strengthened, to move closer to the jet orifice, and to increase in frequency as crossflow velocity increases for the regime 3.5 <R≤ 10. The instabilities also exhibit mode shifting downstream along the jet shear layer for either nozzle configuration at these moderately high values ofR. WhenRis reduced below 3.5 in the flush injection experiments, single-mode instabilities are dramatically strengthened, forming almost immediately within the shear layer in addition to harmonic and subharmonic modes, without any evidence of mode shifting. Under these conditions, the dominant and initial mode frequencies tend to decrease with increasing crossflow. In contrast, the instabilities in the elevated jet experiments are weakened as R is reduced below about 4, probably owing to an increase in the vertical coflow magnitude exterior to the elevated nozzle, untilRfalls below 1.25, at which point the elevated jet instabilities become remarkably similar to those for the flush injected jet. Low-level jet forcing has no appreciable influence on the shear-layer response when these strong modes are present, in contrast to the significant influence of low-level forcing otherwise. These studies suggest profound differences in transverse-jet shear-layer instabilities, depending on the flow regime, and help to explain differences previously observed in transverse jets controlled by strong forcing.

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Transverse-jet shear-layer instabilities. Part 2. Linear analysis for large jet-to-crossflow velocity ratio

Journal of Fluid Mechanics ◽

10.1017/s002211200800102x ◽

2008 ◽

Vol 602 ◽

pp. 383-401 ◽

Cited By ~ 31

Author(s):

LEONARDO S. DE B. ALVES ◽

ROBERT E. KELLY ◽

ANN R. KARAGOZIAN

Keyword(s):

Shear Layer ◽

Near Field ◽

Velocity Ratio ◽

Base Flow ◽

Experimental Results ◽

Dimensional Parameter ◽

Potential Core ◽

Transverse Jet ◽

Spatial Growth ◽

Crossflow Velocity

The dominant non-dimensional parameter for isodensity transverse jet flow is the mean jet-to-crossflow velocity ratio,R. In Part 1 (Megerianet al.,J. Fluid Mech., vol. 593, 2007, p. 93), experimental results are presented for the behaviour of transverse-jet near-field shear-layer instabilities for velocity ratios in the range 1 <R≤ 10. A local linear stability analysis is presented in this paper for the subrangeR>4, using two different base flows for the transverse jet. The first analysis assumes the flow field to be described by a modified version of the potential flow solution of Coelho & Hunt (J. Fluid Mech., vol. 200, 1989, p. 95), in which the jet is enclosed by a vortex sheet. The second analysis assumes a continuous velocity model based on the same inviscid base flow; this analysis is valid for the larger values of Strouhal number expected to be typical of the most unstable disturbances, and allows prediction of a maximum spatial growth rate for the disturbances. In both approaches, results are obtained by expanding in inverse powers ofRso that the free-jet results are obtained asR→∞. The results from both approaches agree in the moderately low-frequency regime. Maximum spatial growth rates and associated Strouhal numbers extracted from the second approach both increase with decreasing velocity ratioR, in agreement with the experimental results from Part 1 in the range 4<R≤10. The nominally axisymmetric mode is found to be the most unstable mode in the transverse-jet shear-layer near-field region, upstream of the end of the potential core. The overall agreement of theoretical and experimental results suggests that convective instability occurs in the transverse-jet shear layer for jet-to-crossflow velocity ratios above 4, and that the instability is strengthened asRis decreased.

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Response of the shear layers separating from a circular cylinder to small-amplitude rotational oscillations

Journal of Fluid Mechanics ◽

10.1017/s0022112091003476 ◽

1991 ◽

Vol 231 ◽

pp. 481-499 ◽

Cited By ~ 79

Author(s):

J. R. Filler ◽

P. L. Marston ◽

W. C. Mih

Keyword(s):

Circular Cylinder ◽

Shear Layer ◽

Small Amplitude ◽

Shear Layers ◽

Forced Oscillations ◽

Near Resonance ◽

Lock In ◽

Hot Film ◽

Rotational Oscillations ◽

Frequency Curves

The frequency response of the shear layers separating from a circular cylinder subject to small-amplitude rotational oscillations has been investigated experimentally in water for the Reynolds number (Re) range 250 to 1200. A hot-film anemometer was placed in the separated shear layers from 1 to 1.5 diameters downstream of the cylinder, and connected to a lock-in analyser. by referencing the lock-in analyser to the cylinder oscillations, the amplitude and phase of the response to different frequency oscillations were measured directly. It is shown that rotational oscillations corresponding to cylinder peripheral speeds between 0.5 and 3% of the free stream can be used to influence the primary (Kármán) mode of vortex generation. For Re greater than ≈ 500, such oscillations can also force the shear-layer vortices associated with the instability of the separating shear layers. Corresponding to the primary and shear-layer modes are two distinct peaks in response amplitude versus frequency curves, and two very different phase versus frequency curves. The response of the shear layers (and near wake) in the range of Kármán frequency suggests qualitative similarities with the response of an oscillator near resonance. Forced oscillations in the higher-frequency shear-layer mode range are simply convected by the shear layers. Close to the cylinder, the shear-layer response is shown to be comparable to that of generic free shear layers studied by others.

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Effect of Multi-Modal Forcing on Thermal Transport in a Co-Axial Combustor Geometry

10.1115/imece2001/htd-24102 ◽

2001 ◽

Author(s):

P. R. Chandra ◽

Chun L. Lau ◽

S. Acharya

Keyword(s):

Shear Layer ◽

Coherent Structures ◽

Near Field ◽

Single Mode ◽

Spreading Rate ◽

Open Loop ◽

Velocity Shear ◽

Scalar Mixing ◽

Jet Streams ◽

Loop Control

Abstract This paper investigates open-loop control of the mixing between an inner stream and an outer stream in a coaxial combustor geometry. The inner and outer air streams enter the combustor geometry at different temperatures and mimic a gaseous fuel and combustion air stream respectively. Of specific interest in this study is the behavior of the coherent structures in the coaxial jet streams, and the manipulation of these coherent structures with controlled perturbation to enhance jet-spreading and scalar-mixing. The spectra of the unforced flow reveal the presence of a dominant coherent mode at 100 Hz (identified as the fundamental mode fo), as well as 150 Hz (3 fo/2) and 300 Hz (3 fo). Single-mode forcing of both the inner-jet and the outer-jet at 100 Hz, 150 Hz, 300 Hz, and 1 kHz is explored, and attention is focused on the spreading of the inner-jet shear layer and the outer-jet shear layer. It is observed that the 300 Hz mode shows the greatest enhancement in the spreading rate of the velocity shear-layer in the near field (x/D < 1), while downstream of x/D = 1, the 100 Hz appears to show the most significant effect. Scalar mixing is also significantly enhanced, with the 300 Hz forcing showing the largest enhancement. Dual-mode forcing is also investigated with 100 Hz inner-jet, 300 Hz outer-jet forcing and 300 Hz inner-jet, 100 Hz outer-jet forcing. The 100 Hz inner-jet, 300 Hz outer-jet forcing is shown to lead to greater enhancements in scalar mixing man all other cases considered.

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Large Scale Structures in Elevated Jet Normal to Crossflow

10.1115/fedsm2021-66037 ◽

2021 ◽

Author(s):

Jyoti Gupta ◽

Arun K. Saha

Keyword(s):

Shear Layer ◽

Large Scale ◽

Velocity Ratio ◽

Deep Ocean ◽

Sewage Water ◽

External Diameter ◽

Hairpin Vortices ◽

Large Scale Structures ◽

Transverse Jet ◽

Crossflow Velocity

Abstract Transverse jet from elevated source is found in various environmental and industrial field which include smoke exhausting from stack into atmosphere and sewage water disposal in deep-ocean. The experiment is carried out in water tunnel using flow visualization and Laser Doppler Velocimetry. Analysis has been performed for axisymmetric round jet of aspect ratio of 9.0 with the velocity ratio varying up to 2.5 at Reynolds number (based on free stream crossflow velocity and jet external diameter) of 1000. Result shows the formation of different jet shear layer vortices with varying velocity ratio are: (i) clockwise-downwash vortices (velocity ratio less than 0.3), (ii) delayed-regular-clockwise vortices (between 0.3 to 0.7), (iii) regular-clockwise vortices (between 0.7 to 1.4) at the lee side of the jet shear layer, (iv) irregular-anticlockwise vortex at the upstream along with clockwise vortices at the lee side of the jet shear layer that together forms mushroom vortices (between 1.4 to 1.9) and (v) regular-mushroom vortices (above 1.9). The other vortices found are stack-end vortex (less than 0.9) in the wake near free end, upright vortices (above 0.9) in jet-wake and hairpin vortices (between 0.3 to 0.6) in downstream which is the stretched part of evolving shear layer.

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Vorticity structure and evolution in a transverse jet

Journal of Fluid Mechanics ◽

10.1017/s0022112006004411 ◽

2007 ◽

Vol 575 ◽

pp. 267-305 ◽

Cited By ~ 39

Author(s):

YOUSSEF M. MARZOUK ◽

AHMED F. GHONIEM

Keyword(s):

Shear Layer ◽

Near Field ◽

Mean Field ◽

Three Dimensional ◽

Oil Field ◽

Large Reynolds Number ◽

Streamwise Vorticity ◽

Transverse Jet ◽

Jet Trajectory ◽

The Mean

Transverse jets arise in many applications, including propulsion, effluent dispersion, oil field flows, and V/STOL aerodynamics. This study seeks a fundamental, mechanistic understanding of the structure and evolution of vorticity in the transverse jet. We develop a high-resolution three-dimensional vortex simulation of the transverse jet at large Reynolds number and consider jet-to-crossflow velocity ratiosrranging from 5 to 10. A new formulation of vorticity-flux boundary conditions accounts for the interaction of channel wall vorticity with the jet flow immediately around the orifice. We demonstrate that the nascent jet shear layer contains not only azimuthal vorticity generated in the jet pipe, but wall-normal and azimuthal perturbations resulting from the jet–crossflow interaction. This formulation also yields analytical expressions for vortex lines in the near field as a function ofr.Transformation of the cylindrical shear layer emanating from the orifice begins with axial elongation of its lee side to form sections of counter-rotating vorticity aligned with the jet trajectory. Periodic roll-up of the shear layer accompanies this deformation, creating complementary vortex arcs on the lee and windward sides of the jet. Counter-rotating vorticity then drives lee-side roll-ups in the windward direction, along the normal to the jet trajectory. Azimuthal vortex arcs of alternating sign thus approach each other on the windward boundary of the jet. Accordingly, initially planar material rings on the shear layer fold completely and assume an interlocking structure that persists for several diameters above the jet exit. Though the near field of the jet is dominated by deformation and periodic roll-up of the shear layer, the resulting counter-rotating vorticity is a pronounced feature of the mean field; in turn, the mean counter-rotation exerts a substantial influence on the deformation of the shear layer. Following the pronounced bending of the trajectory into the crossflow, we observe a sudden breakdown of near-field vortical structures into a dense distribution of smaller scales. Spatial filtering of this region reveals the persistence of counter-rotating streamwise vorticity initiated in the near field.

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Transverse jet lock-in and quasiperiodicity

Physical Review Fluids ◽

10.1103/physrevfluids.5.013901 ◽

2020 ◽

Vol 5 (1) ◽

Author(s):

Takeshi Shoji ◽

Elijah W. Harris ◽

Andrea Besnard ◽

Stephen G. Schein ◽

Ann R. Karagozian

Keyword(s):

Transverse Jet ◽

Lock In

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Dual-position excitation technique in flow control over an airfoil at low speeds

International Journal of Numerical Methods for Heat &amp Fluid Flow ◽

10.1108/hff-05-2018-0195 ◽

2018 ◽

Vol 30 (9) ◽

pp. 4141-4154

Author(s):

Abbas Ebrahimi ◽

Majid Hajipour ◽

Kamran Ghamkhar

Keyword(s):

Flow Control ◽

Shear Layer ◽

Control Method ◽

Lift Coefficient ◽

Shear Layers ◽

Control Flow ◽

Plasma Actuators ◽

Dbd Plasma ◽

Surface Shear ◽

Content Type

PurposeThe purpose of this paper is to control flow separation over a NACA 4415 airfoil by applying unsteady forces to the separated shear layers using dielectric barrier discharge (DBD) plasma actuators. This novel flow control method is studied under conditions which the airfoil angle of attack is 18°, and Reynolds number based on chord length is 5.5 × 105.Design/methodology/approachLarge eddy simulation of the turbulent flow is used to capture vortical structures through the airfoil wake. Power spectral density analysis of the baseline flow indicates dominant natural frequencies associated with “shear layer mode” and “wake mode.” The wake mode frequency is used simultaneously to excite separated shear layers at both the upper surface and the trailing edge of the airfoil (dual-position excitation), and it is also used singly to excite the upper surface shear layer (single-position excitation).FindingsBased on the results, actuations manipulate the shear layers instabilities and change the wake patterns considerably. It is revealed that in the single-position excitation case, the vortices shed from the upper surface shear layer are more coherent than the dual-position excitation case. The maximum value of lift coefficient and lift-to-drag ratio is achieved, respectively, by single-position excitation as well as dual-position excitation.Originality/valueThe paper contributes to the understanding and progress of DBD plasma actuators for flow control applications. Further, this research could be a beneficial solution for the promising design of advanced low speed flying vehicles.

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