Transient energy growth of a swirling jet with vortex breakdown

2018 ◽  
Vol 856 ◽  
pp. 288-322 ◽  
Author(s):  
Gopalsamy Muthiah ◽  
Arnab Samanta
Keyword(s):  
High Speed ◽  
Vortex Breakdown ◽  
Swirling Flow ◽  
Azimuthal Velocity ◽  
Transient Growth ◽  
Transient Energy ◽  
Orr Mechanism ◽  
Streamwise Streaks ◽  
Short Time ◽  
Oseen Vortex

We investigate the existence of short-time, local transient growth in the helical modes of a rapidly swirling, high-speed jet that has transitioned into an axisymmetric bubble breakdown state. The time-averaged flow consisting of the bubble and its wake downstream constitute the base state, which we show to exhibit strong transient amplification owing to the non-modal behaviour of the continuous eigenspectrum. A pseudospectrum analysis mathematically identifies the so-called potential modes within this continuous spectrum and the resultant non-orthogonality between these modes and the existing discrete stable modes is shown to be the main contributor to such growth. As the swirling flow develops post the collapsed bubble, the potential spectrum moves further toward the unstable half-plane, which along with the concurrent weakening of exponential growth from the discrete unstable modes, increases the dynamic importance of transient growth inside the wake region. The transient amplifications calculated at several locations inside the bubble and wake confirm this, where strong growths inside the wake far outstrip the corresponding modal growths (if available) at shorter times, but especially at the higher helical orders and smaller streamwise wavenumbers. The corresponding optimal perturbations at initial times consist of streamwise streaks of azimuthal velocity, which if concentrated inside the core vortical region, unfold via the classical Orr mechanism to yield structures resembling core (or viscous) Kelvin waves of the corresponding Lamb–Oseen vortex. However, in contrast to that in Lamb–Oseen vortex flow, where critical-layer waves are associated with higher transient gains, here, such core Kelvin modes with the more compact spiral structure at the vortex core are seen to yield the maximum transient amplifications.

The Effect of Non-Normality of Navier-Stokes Operator Mechanism on Dynamics of an Axisymmetric Swirling Flow

2013 ◽  
Vol 681 ◽  
pp. 72-78
Author(s):  
Cheng Chen ◽  
Ya Yong Shi
Keyword(s):  
Swirling Flow ◽  
Initial Perturbation ◽  
Fluid Motion ◽  
Nonlinear Behavior ◽  
Stokes Operator ◽  
Navier Stokes ◽  
Transient Growth ◽  
Optimal Perturbation ◽  
Energy Growth ◽  
Oseen Vortex

The effect of non-normality of the Navier-Stokes operator on the dynamics of an axisymmetric swirling flow, namely, the Oseen vortex, has been investigated. The eigenvalue analysis and transient growth analysis have been employed in order to obtain the least stable eigenmode and the global optimal perturbation, which are both considered as the initial perturbation. Three stages of dynamic process have been put into evidence for the evolution of the optimal perturbation. The early (linear) stage is characterized by the amplification of radial perturbation, consistent with the prediction of transient growth theory. Having come into the nonlinear stage, the perturbation energy growth is suppressed by the interaction between the vortex ring and the Oseen vortex core. Finally, the phenomena of secondary energy growth are also observed. Compared with the results obtained by applying the least stable eigenmode as the initial disturbance, the nonlinear behavior of the optimal perturbation features radial fluid motion and the rapid production of small eddies, which are both thought to be beneficial to fluid entrainment or mixing. The effect of perturbation amplitude on the nonlinear evolution of flows is also studied.

Flame and Flow Topologies in an Annular Swirling Flow

2013 ◽  
Author(s):  
I. Chterev ◽  
C. W. Foley ◽  
S. Kostka ◽  
A. W. Caswell ◽  
N. Jiang ◽  
...  
Keyword(s):  
High Speed ◽  
Vortex Breakdown ◽  
Swirling Flow ◽  
Flow Fields ◽  
Shear Layers ◽  
Flame Stabilization ◽  
Preheat Temperature ◽  
Breakdown Region ◽  
Flame Shape ◽  
Annular Swirling Flow

A variety of different flame configurations and heat release distributions, with their associated flow fields, can exist in high swirl, annular flows. Each of these different configurations, in turn, has different thermoacoustic sensitivities and influences on combustor emissions, nozzle life, and liner heating. These different configurations arise because at least three flame stabilization locations are present, associated with the inner and outer shear layers of the annulus, and the stagnation point of the vortex breakdown region. This paper discusses the flame and flow topologies that exist in these flows. These results illustrate the importance of the sensitivity of flame configurations to geometric (such as centerbody size and shape, combustor diameter, exhaust contraction) and operational (e.g., bulkhead temperature, preheat temperature, fuel air ratio) parameters. We particularly emphasize the centerbody shape as differentiating between two different families of flame shapes. Results are shown illustrating the time averaged and instantaneous flame shape and flow fields, using high speed PIV, OH-PLIF, and luminosity imaging.

Flashback in a Swirl Burner With Cylindrical Premixing Zone

2001 ◽  
Author(s):  
Jassin Fritz ◽  
Martin Kröner ◽  
Thomas Sattelmayer
Keyword(s):  
Flame Propagation ◽  
Gas Turbines ◽  
High Speed ◽  
Vortex Breakdown ◽  
Swirling Flow ◽  
Laser Doppler Anemometry ◽  
Burning Velocity ◽  
Turbulent Flame ◽  
Measurement Techniques ◽  
Mean Flow

Flame flashback from the combustion chamber into the mixing zone is one of the inherent problems of lean premixed combustion and essentially determines the reliability of low NOx burners. Generally, flashback can be initiated by one of the following four phenomena: flashback due to the conditions in the boundary layer, flashback due to turbulent flame propagation in the core flow, flashback induced by combustion instabilities and flashback caused by combustion induced vortex breakdown. In this study, flashback in a swirling tubular flow was investigated. In order to draw maximum benefit from the tests with respect to the application in gas turbines, the radial distribution of the axial and circumferential momentum in the tube was selected such that the typical character of a flow in mixing zones of premix burners without centerbody was obtained. A single burner test rig has been designed to provoke flashback with the preheating temperature, the equivalence ratio and the mean flow rate being the influencing parameters. The flame position within the mixing section is detected by a special optical flame sensor array, which allows the control of the experiment and furthermore the triggering of the measurement techniques. The burning velocity of the fuel has been varied by using natural gas or hydrogen. The characteristics of the flashback, the unsteady swirling flow during the flame propagation, the flame dynamics and the reaction zones have been investigated by applying High Speed Video recordings, the Laser Doppler Anemometry and the Laser Induced Fluorescence. The presented results show that a combustion induced vortex breakdown is the dominating mechansim of the observed flashback. This mechanism is very sensitive to the momentum distribution in the vortex core. By adding axial momentum around the mixing tube axis, the circumferential velocity gradient is reduced and flashback can be prevented.

An experimental study of the effect of external turbulence on the decay of a single vortex and a vortex pair

2011 ◽  
Vol 670 ◽  
pp. 214-239 ◽  
Author(s):  
J. P. J. van JAARSVELD ◽  
A. P. C. HOLTEN ◽  
A. ELSENAAR ◽  
R. R. TRIELING ◽  
G. J. F. van HEIJST
Keyword(s):  
Vortex Breakdown ◽  
Symmetry Plane ◽  
Vortex Pair ◽  
Azimuthal Velocity ◽  
Single Vortex ◽  
Image Velocimetry ◽  
Vortex Centre ◽  
Observed Behaviour ◽  
External Turbulence ◽  
Oseen Vortex

This study is concerned with the effect of external turbulence on the decay of vortices. Single vortices and vortex pairs were generated with wing(s) mounted in the sidewalls of a wind tunnel. The distance between the two vortices could be adjusted such that they just touched each other or overlapped. The intensity of the turbulence could be controlled with a turbulence grid. The development of the vortex was measured at a number of downstream stations with particle image velocimetry for a range of wing settings. The results indicate that the single vortex can be described by the ‘two length scales’ model of Jacquin, Fabre & Geffroy (AIAA, vol. 1038, 2001, p. 1). A vortex core, which decays like a Lamb–Oseen vortex, is embedded in a region with an almost constant radius and a much lower azimuthal velocity that obeys a ~r−β power law, with r being the radius measured from the vortex centre. For the turbulence levels and the test section length used in this study, the single-vortex behaviour is independent of the external turbulence and in contrast with the decay of the vortex pair that strongly depends on the external turbulence. In the initial stages of the vortex pair evolution, the vortices decay due to cancellation of vorticity at the symmetry plane. At a later stage, Crow oscillations are observed, followed by a breakdown of the vortices. This vortex breakdown might be due to direct turbulent action. The observed behaviour is in agreement with the theoretical model of Crow & Bate (J. Aircraft, vol. 13, 1976, p. 476).

Development of Swirling Flow in a Rod Bundle Subchannel

2002 ◽  
Vol 124 (3) ◽  
pp. 747-755 ◽  
Author(s):  
Heather L. McClusky ◽  
Mary V. Holloway ◽  
Donald E. Beasley ◽  
Michael E. Conner
Keyword(s):  
Angular Momentum ◽  
Swirling Flow ◽  
Axial Direction ◽  
Azimuthal Velocity ◽  
Lateral Velocity ◽  
Full Field ◽  
Rod Bundle ◽  
Image Velocimetry ◽  
Oseen Vortex ◽  
Support Grid

Experimental measurements of the axial development of swirling flow in a rod bundle subchannel are presented. Swirling flow was introduced in the subchannel from a split vane pair located on the downstream edge of the support grid. Particle image velocimetry using an optical borescope yielded full-field lateral velocity data. Lateral flow fields and axial vorticity fields at axial locations ranging from 4.2 to 25.5 hydraulic diameters downstream of the support grid were examined for a Reynolds number of 2.8×104. The lateral velocity fields show that the swirling flow was initially centered in the subchannel. As the flow developed in the axial direction, the swirling flow migrated away from the center of the subchannel. Radial distributions of azimuthal velocity and circulation are presented relative to the centroid of vorticity, and are compared to that of a Lamb-Oseen vortex. The angular momentum decreased as the flow developed in the axial direction. The spatial decay rate of the angular momentum is compared to that of decaying, swirling flow in a pipe.

Flashback in a Swirl Burner With Cylindrical Premixing Zone

2004 ◽  
Vol 126 (2) ◽  
pp. 276-283 ◽  
Author(s):  
J. Fritz ◽  
M. Kro¨ner ◽  
T. Sattelmayer
Keyword(s):  
Flame Propagation ◽  
Gas Turbines ◽  
High Speed ◽  
Vortex Breakdown ◽  
Swirling Flow ◽  
Laser Doppler Anemometry ◽  
Burning Velocity ◽  
Turbulent Flame ◽  
Measurement Techniques ◽  
Mean Flow

Flame flashback from the combustion chamber into the mixing zone is one of the inherent problems of lean premixed combustion and essentially determines the reliability of low NOx burners. Generally, flashback can be initiated by one of the following four phenomena: flashback due to the conditions in the boundary layer, flashback due to turbulent flame propagation in the core flow, flashback induced by combustion instabilities and flashback caused by combustion induced vortex breakdown. In this study, flashback in a swirling tubular flow was investigated. In order to draw maximum benefit from the tests with respect to the application in gas turbines, the radial distribution of the axial and circumferential momentum in the tube was selected such that the typical character of a flow in mixing zones of premix burners without centerbody was obtained. A single burner test rig has been designed to provoke flashback with the preheating temperature, the equivalence ratio and the mean flow rate being the influencing parameters. The flame position within the mixing section is detected by a special optical flame sensor array, which allows the control of the experiment and furthermore the triggering of the measurement techniques. The burning velocity of the fuel has been varied by using natural gas or hydrogen. The characteristics of the flashback, the unsteady swirling flow during the flame propagation, the flame dynamics and the reaction zones have been investigated by applying high-speed video recordings, the laser Doppler anemometry and the laser induced fluorescence. The presented results show that a combustion induced vortex breakdown is the dominating mechanism of the observed flashback. This mechanism is very sensitive to the momentum distribution in the vortex core. By adding axial momentum around the mixing tube axis, the circumferential velocity gradient is reduced and flashback can be prevented.

Designing a Radial Swirler Vortex Breakdown Burner

2006 ◽  
Author(s):  
Stephan Burmberger ◽  
Christoph Hirsch ◽  
Thomas Sattelmayer
Keyword(s):  
Flow Field ◽  
Vortex Breakdown ◽  
Swirling Flow ◽  
Azimuthal Velocity ◽  
Design Guidelines ◽  
Flame Stabilization ◽  
Fuel Flexibility ◽  
Primary Zone ◽  
Large Eddy ◽  
Flame Flashback

Most gas turbine premix burners without centrebody employ the breakdown of a swirling flow at the transition between the mixing section and the combustor for aerodynamic flame stabilization [1]. As the formation of the desired vortex breakdown pattern depends very sensibly on the distribution of axial and azimuthal velocity in the mixing section, the design of suitable swirlers is usually a cumbersome iterative process. The presented burner design was found through the implementation of design guidelines derived from CFD-calculations and on the basis of analytical considerations [5]. The swirling flow is generated by a radial swirler with tangential inlets. In order to stabilize the flow pattern, the swirling flow confines a slow non-swirling flow on the centreline. The centre flow being set into azimuthal motion creates increasing azimuthal velocity in streamwise direction in the vortex core. This process is reinforced by a conical nozzle and leads to the production of positive azimuthal vorticity inside the nozzle which stabilizes the flow field. First atmospheric test runs and Large Eddy Simulations of the isothermal as well as reactive flow field prove that the design goals have been reached: The burner creates stable vortex breakdown in the primary zone of the combustion chamber without flame flashback or backflow on the centreline over the entire operating range and even for difficult fuels like hydrogen containing gases. This finding indicates that reliable vortex breakdown burners with remarkable fuel flexibility can be designed using the guidelines presented in [5].

scholarly journals Neuro-Inspired Computing with Spin-VCSELs

2021 ◽  
Vol 11 (9) ◽  
pp. 4232
Author(s):  
Krishan Harkhoe ◽  
Guy Verschaffelt ◽  
Guy Van der Sande
Keyword(s):  
Processing Speed ◽  
Delay Time ◽  
High Speed ◽  
State Of The Art ◽  
Polarization Modulation ◽  
Time Interval ◽  
Computing Technique ◽  
High Modulation ◽  
Short Time ◽  
Modulation Speed

Delay-based reservoir computing (RC), a neuromorphic computing technique, has gathered lots of interest, as it promises compact and high-speed RC implementations. To further boost the computing speeds, we introduce and study an RC setup based on spin-VCSELs, thereby exploiting the high polarization modulation speed inherent to these lasers. Based on numerical simulations, we benchmarked this setup against state-of-the-art delay-based RC systems and its parameter space was analyzed for optimal performance. The high modulation speed enabled us to have more virtual nodes in a shorter time interval. However, we found that at these short time scales, the delay time and feedback rate heavily influence the nonlinear dynamics. Therefore, and contrary to other laser-based RC systems, the delay time has to be optimized in order to obtain good RC performances. We achieved state-of-the-art performances on a benchmark timeseries prediction task. This spin-VCSEL-based RC system shows a ten-fold improvement in processing speed, which can further be enhanced in a straightforward way by increasing the birefringence of the VCSEL chip.

scholarly journals Vortex breakdown of the swirling flow in a Lean Direct Injection burner

2020 ◽  
Vol 32 (12) ◽  
pp. 125118
Author(s):  
Yazhou Shen ◽  
Mohamad Ghulam ◽  
Kai Zhang ◽  
Ephraim Gutmark ◽  
Christophe Duwig
Keyword(s):  
Vortex Breakdown ◽  
Swirling Flow ◽  
Direct Injection ◽  
Lean Direct Injection ◽  
Injection Burner

Transition of streamwise streaks in zero-pressure-gradient boundary layers

2002 ◽  
Vol 472 ◽  
pp. 229-261 ◽  
Author(s):  
LUCA BRANDT ◽  
DAN S. HENNINGSON
Keyword(s):  
Boundary Layer ◽  
Pressure Gradient ◽  
High Speed ◽  
Plate Boundary ◽  
Transition Process ◽  
Free Stream Turbulence ◽  
Tollmien Schlichting ◽  
Streamwise Streaks ◽  
The Stability ◽  
Optimal Disturbances

A transition scenario initiated by streamwise low- and high-speed streaks in a flat-plate boundary layer is studied. In many shear flows, the perturbations that show the highest potential for transient energy amplification consist of streamwise-aligned vortices. Due to the lift-up mechanism these optimal disturbances lead to elongated streamwise streaks downstream, with significant spanwise modulation. In a previous investigation (Andersson et al. 2001), the stability of these streaks in a zero-pressure-gradient boundary layer was studied by means of Floquet theory and numerical simulations. The sinuous instability mode was found to be the most dangerous disturbance. We present here the first simulation of the breakdown to turbulence originating from the sinuous instability of streamwise streaks. The main structures observed during the transition process consist of elongated quasi-streamwise vortices located on the flanks of the low-speed streak. Vortices of alternating sign are overlapping in the streamwise direction in a staggered pattern. The present scenario is compared with transition initiated by Tollmien–Schlichting waves and their secondary instability and by-pass transition initiated by a pair of oblique waves. The relevance of this scenario to transition induced by free-stream turbulence is also discussed.

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