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

2008 ◽  
Vol 602 ◽  
pp. 383-401 ◽  
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.

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.

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

2007 ◽  
Vol 593 ◽  
pp. 93-129 ◽  
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.

Large Scale Structures in Elevated Jet Normal to Crossflow

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.

Dynamics of Large-Scale Structures for Jets in a Crossflow

1999 ◽  
Vol 121 (3) ◽  
pp. 577-587 ◽  
Author(s):  
F. Muldoon ◽  
S. Acharya
Keyword(s):  
Shear Layer ◽  
Large Scale ◽  
Computational Study ◽  
Near Field ◽  
Velocity Ratio ◽  
Leeward Side ◽  
Dynamic Features ◽  
Mean Velocity ◽  
Large Scale Structures ◽  
Scale Structures

Results of a three-dimensional unsteady computational study of a row of jets injected normal to a crossflow are presented with the aim of understanding the dynamics of the large-scale structures in the region near the jet. The jet to crossflow velocity ratio is 0.5. A modified version of the computer program (INS3D), which utilizes the method of artificial compressibility, is used for the computations. Results obtained clearly indicate that the near-field large-scale structures are extremely dynamic in nature, and undergo breakup and reconnection processes. The dynamic near-field structures identified include the counterrotating vortex pair (CVP), the horseshoe vortex, wake vortex, wall vortex, and shear layer vortex. The dynamic features of these vortices are presented in this paper. The CVP is observed to be a convoluted structure interacting with the wall and horseshoe vortices. The shear layer vortices are stripped by the crossflow, and undergo pairing and stretching events in the leeward side of the jet. The wall vortex is reoriented into the upright wake system. Comparison of the predictions with mean velocity measurements is made. Reasonable agreement is observed.

The Effect of the Velocity Ratio on the Diffusion Inhibition of Coaxial Jet

2019 ◽  
Author(s):  
Fujio Akagi ◽  
Takumi Etou ◽  
Yuito Fukuda ◽  
Ryoya Yoshioka ◽  
Youichi Ando ◽  
...  
Keyword(s):  
Shear Layer ◽  
Energy Transport ◽  
Velocity Ratio ◽  
Streamwise Velocity ◽  
Inhibition Mechanism ◽  
Outer Diameter ◽  
Potential Core ◽  
Round Jet ◽  
Eddy Simulation ◽  
Core Length

Abstract The mixing characteristics of a coaxial round jet having inner to outer diameter ratio di/do = 0.67 was investigated to establish the method for mass and energy transport with inhibiting its diffusion. In the present study, the effect of the velocity ratio on the streamwise length of the potential core of the central round jet, the streamwise velocity distribution, and the evolution of vortcies of a coaxial jet was evaluated by using Large Eddy Simulation and experimental flow visualization to clarify diffusion inhibition mechanism of a central jet. Three velocity ratios γ = 0.25, 0.5, and 0.75 were conducted under a single condition of the central jet Reynolds number of 2000. The results show that the potential core length of a central jet become the longest on condition of the velocity ratio of 0.5, and the length was 15 times of the inner nozzle diameter. This value is equivalent to 3.8 times in the case of a general round jet. In other velocity ratio, it was also confirmed that the potential core length of a central jet becomes short by enhancement of vortices generation in the shear layer of annular and central jet. It is concluded that the velocity gradient in the shear layer is the important parameter in determining the diffusion of a central jet.

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.

Local stability analysis of an inviscid transverse jet

2007 ◽  
Vol 581 ◽  
pp. 401-418 ◽  
Author(s):  
LEONARDO S. de B. ALVES ◽  
ROBERT E. KELLY ◽  
ANN R. KARAGOZIAN
Keyword(s):  
Stability Analysis ◽  
Growth Rates ◽  
Near Field ◽  
Velocity Ratio ◽  
Three Dimensional ◽  
Constant Density ◽  
Jet Instability ◽  
Transverse Jet ◽  
Free Jet ◽  
Local Stability Analysis

A local linear stability analysis is performed for a round inviscid jet with constant density that is injected into a uniform crossflow of the same density. The baseflow is obtained from a modified version of the inviscid transverse jet near-field solution of Coelho & Hunt (J. Fluid Mech.vol. 200, 1989, p. 95) which is valid for small values of the crossflow-to-jet velocity ratio λ. A Fourier expansion in the azimuthal direction is used to couple the disturbances with the three-dimensional crossflow. The spatial growth rates of the modes corresponding to the axisymmetric and first helical modes of the free jet as λ → 0 increase as λ increases. The diagonal dominance of the dispersion relation matrix is used as a quantitative criterion to estimate the range of velocity ratios (0 < λ < λc) within which the transverse jet instability can be considered to have a structure similar to that of the free jet. Further, we show that for λ>0 positive and negative helical modes have different growth rates, suggesting an inherent weak asymmetry in the transverse jet.

scholarly journals Vortical structure in the wake of a transverse jet

1994 ◽  
Vol 279 ◽  
pp. 1-47 ◽  
Author(s):  
T. F. Fric ◽  
A. Roshko
Keyword(s):  
Boundary Layer ◽  
Velocity Ratio ◽  
Adverse Pressure Gradient ◽  
Reynolds Numbers ◽  
Structural Features ◽  
Bluff Bodies ◽  
Wake Vortices ◽  
Transverse Jet ◽  
Wall Boundary Layer ◽  
Crossflow Velocity

Structural features resulting from the interaction of a turbulent jet issuing transversely into a uniform stream are described with the help of flow visualization and hot-wire anemometry. Jet-to-crossflow velocity ratios from 2 to 10 were investigated at crossflow Reynolds numbers from 3800 to 11400. In particular, the origin and formation of the vortices in the wake are described and shown to be fundamentally different from the well-known phenomenon of vortex shedding from solid bluff bodies. The flow around a transverse jet does not separate from the jet and does not shed vorticity into the wake. Instead, the wake vortices have their origins in the laminar boundary layer of the wall from which the jet issues. It is argued that the closed flow around the jet imposes an adverse pressure gradient on the wall, on the downstream lateral sides of the jet, provoking 'separation events’ in the wall boundary layer on each side. These result in eruptions of boundary-layer fluid and formation of wake vortices that are convected downstream. The measured wake Strouhal frequencies, which depend on the jet-crossflow velocity ratio, match the measured frequencies of the separation events. The wake structure is most orderly and the corresponding wake Strouhal number (0.13) is most sharply defined for velocity ratios near the value 4. Measured wake profiles show deficits of both momentum and total pressure.

Dynamics of Large-Scale Structures for Jets in a Crossflow

1998 ◽  
Author(s):  
Frank Muldoon ◽  
Sumanta Acharya
Keyword(s):  
Shear Layer ◽  
Large Scale ◽  
Computational Study ◽  
Near Field ◽  
Velocity Ratio ◽  
Cross Flow ◽  
Leeward Side ◽  
Mean Velocity ◽  
Large Scale Structures ◽  
Scale Structures

Results of a three dimensional unsteady computational study of a row of jets injected normal to a cross-flow are presented with the aim of understanding the dynamics of the large scale structures in the region near the jet. The jet to cross-flow velocity ratio is .5. A modified version of the computer program (INS3D) which utilizes the method of artificial compressibility is used for the computations. Results obtained clearly indicate that the near field large scale structures are extremely dynamical in nature, and undergo breakup and reconnection processes. The dynamical near field structures identified include the counter rotating vortex pair (CVP), the horseshoe vortex, wake vortex, wall vortex and the shear layer vortex. The dynamical features of these vortices are presented in this paper. The CVP is observed to be a convoluted structure interacting with the wall and horseshoe vortices. The shear layer vortices are stripped by the crossflow, and undergo pairing and stretching events in the leeward side of the jet. The wall vortex is reoriented into the upright wake system. Comparison of the predictions with mean velocity measurements is made. Reasonable agreement is observed.

An Experimental Investigation of the Pressure-Velocity Cross-Correlation in an Axisymmetric Jet

2005 ◽  
Author(s):  
Andre´ M. Hall ◽  
Mark N. Glauser ◽  
Charles E. Tinney
Keyword(s):  
Velocity Field ◽  
Shear Layer ◽  
Cross Correlation ◽  
Near Field ◽  
Mixing Layer ◽  
Ambient Conditions ◽  
Potential Core ◽  
Axisymmetric Jet ◽  
Exit Nozzle ◽  
The Cross

This study investigates the strength of the pressure-velocity correlations of a Mach 0.6, axisymmetric jet, with an exit nozzle diameter of 50.8mm. Experiments are conducted at a constant exit temperature of 25°C, and exit pressure and temperature are balanced with ambient conditions. The instantaneous velocity measurements are acquired using a multi-component LDA system who’s measurement volume is traversed along several radial and streamwise locations within the potential core, and mixing layer regions of the flow. The fluctuating lip pressure is simultaneously sampled by an azimuthal array of (15) dynamic transducers, evenly spaced at 24°. These are positioned just outside the shear layer near the jet exit at z/D = 0.875, and 1.75R from the centerline, where the pressure field has been shown to be hydrodynamic. From this multi-point evaluation, the cross-correlations between the near-field pressure array (fixed), and streamwise component of the velocity field (traversed) are examined as a function of radial, streamwise, and also azimuthal separation. The results illustrate a remarkable coherence between the near field pressure and the velocity field, on the order of 25%. Streamwise convection velocities of 0.77Uj and 0.73Uj are calculated within the potential core and shear layer, respectively. Analysis of the coherency spectra illustrates the frequency band of the correlations and suggest that the potential core and mixing layer regions of the flow are, in general, governed by the high and low frequency motions of the flow, respectively. The azimuthal modal distribution of the cross-correlation shows the dominance of the column mode of the jet, with no higher modes exhibited within the potential core region, and only modes 1 & 2 within the shear layer.

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