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Author: Sean Clees
Publisher:
ISBN:
Category :
Languages : en
Pages :
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Book Description
Swirling jets are commonly used in combustion applications to stabilize flames and improve emissions. Thus, their dynamics play an important role in combustor design. Despite the prevalence of swirling flows in industrial applications that involve highly turbulent flow fields, the majority of experimental and numerical studies consider only laminar conditions. In this study, the dynamics of the vortex core are investigated in a swirling, turbulent jet at swirl numbers in the range of the critical swirl number for vortex breakdown. Vortex breakdown, a bifurcation in the structure of a swirling jet, results in the establishment of a stagnation point and recirculation region along the centerline of the jet. To study these dynamics, dynamic mode decomposition, an order-reduction technique used to extract coherent structures from flow data, is implemented. Investigation of time-averaged velocity fields and profiles leads to the identification of three flow regimes: pre-breakdown, near-breakdown, and post-breakdown. Velocity fields in these regimes are further analyzed using dynamic mode decomposition, Rankine-vortex fitting, and proper orthogonal decomposition to characterize jet dynamics with a particular focus on the development of the recirculation region characteristic of vortex breakdown. A precessing vortex core is also identified in the post-breakdown regime and its behavior is discussed.
Author: Sean Clees
Publisher:
ISBN:
Category :
Languages : en
Pages :
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Book Description
Swirling jets are commonly used in combustion applications to stabilize flames and improve emissions. Thus, their dynamics play an important role in combustor design. Despite the prevalence of swirling flows in industrial applications that involve highly turbulent flow fields, the majority of experimental and numerical studies consider only laminar conditions. In this study, the dynamics of the vortex core are investigated in a swirling, turbulent jet at swirl numbers in the range of the critical swirl number for vortex breakdown. Vortex breakdown, a bifurcation in the structure of a swirling jet, results in the establishment of a stagnation point and recirculation region along the centerline of the jet. To study these dynamics, dynamic mode decomposition, an order-reduction technique used to extract coherent structures from flow data, is implemented. Investigation of time-averaged velocity fields and profiles leads to the identification of three flow regimes: pre-breakdown, near-breakdown, and post-breakdown. Velocity fields in these regimes are further analyzed using dynamic mode decomposition, Rankine-vortex fitting, and proper orthogonal decomposition to characterize jet dynamics with a particular focus on the development of the recirculation region characteristic of vortex breakdown. A precessing vortex core is also identified in the post-breakdown regime and its behavior is discussed.
Author:
Publisher:
ISBN:
Category :
Languages : en
Pages : 173
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Author: Aran Fitzgerald
Publisher:
ISBN:
Category : Axial flow
Languages : en
Pages : 196
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Author: National Aeronautics and Space Administration (NASA)
Publisher: Createspace Independent Publishing Platform
ISBN: 9781722385910
Category :
Languages : en
Pages : 48
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Book Description
Passive and active control of swirling turbulent jets is experimentally investigated. Initial swirl distribution is shown to dominate the free jet evolution in the passive mode. Vortex breakdown, a manifestation of high intensity swirl, was achieved at below critical swirl number (S = 0.48) by reducing the vortex core diameter. The response of a swirling turbulent jet to single frequency, plane wave acoustic excitation was shown to depend strongly on the swirl number, excitation Strouhal number, amplitude of the excitation wave, and core turbulence in a low speed cold jet. A 10 percent reduction of the mean centerline velocity at x/D = 9.0 (and a corresponding increase in the shear layer momentum thickness) was achieved by large amplitude internal plane wave acoustic excitation. Helical instability waves of negative azimuthal wave numbers exhibit larger amplification rates than the plane waves in swirling free jets, according to hydrodynamic stability theory. Consequently, an active swirling shear layer control is proposed to include the generation of helical instability waves of arbitrary helicity and the promotion of modal interaction, through multifrequency forcing. Farokhi, Saeed and Taghavi, R. Unspecified Center NCC3-56...
Author: Ing Kiet Francis Toh
Publisher:
ISBN:
Category : Jets
Languages : en
Pages : 247
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The mean flow properties and the instantaneous flow structures of the jets are then investigated. The introduction and increase of swirl result in a higher jet growth, decay and turbulent intensity, the formation of an off-axis axial velocity maximum and the occurrence of vortex breakdown. The swirl-induced jet growth enhancement can be categorized into three regimes: a low swirl regime in which there is a limited enhancement, a moderate swirl regime in which the enhancement scales with swirl, and a high swirl regime in which vortex breakdown dominates the process. The jet growth behavior in the high swirl regime is found to be dependent on the structure of the jet and the vortex breakdown configuration. A significant change in the flow structures and certain mean flow properties including the centerline and local maximum axial velocity decay are observed in the low and the moderate swirl regimes. These properties appear to be less sensitive to swirl in the high swirl regime after the occurrence of vortex breakdown. The effects of Reynolds number on swirling jets may not be identical to that on a non-swirling jet. The statistical characteristics measured show that low velocity occasions in the vicinity of the jet centerline start to appear at a sufficiently high degree of swirl. These occasions increase with swirl until the eventual occurrence of vortex breakdown. The statistical characteristics of jets that have undergone vortex breakdown are very similar, as they are dominated by the vortex breakdown phenomenon. The mean flow and the statistical measurements also reveal the presence of an unstable vortex breakdown that is difficult to detect using flow visualization and instantanous velocity field measurements.
Author: Sharath Girimaji
Publisher: Springer
ISBN: 331915141X
Category : Technology & Engineering
Languages : en
Pages : 481
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This book gathers the proceedings of the Fifth Symposium on Hybrid RANS-LES Methods, which was held on March 19-21 in College Station, Texas, USA. The different chapters, written by leading experts, reports on the most recent developments in flow physics modelling, and gives a special emphasis to industrially relevant applications of hybrid RANS-LES methods and other turbulence-resolving modelling approaches. The book addresses academic researchers, graduate students, industrial engineers, as well as industrial R&D managers and consultants dealing with turbulence modelling, simulation and measurement, and with multidisciplinary applications of computational fluid dynamics (CFD), such as flow control, aero-acoustics, aero-elasticity and CFD-based multidisciplinary optimization. It discusses in particular advanced hybrid RANS-LES methods. Further topics include wall-modelled Large Eddy Simulation (WMLES) methods, embedded LES, and a comparison of the LES methods with both hybrid RANS-LES and URANS methods. Overall, the book provides readers with a snapshot on the state-of-the-art in CFD and turbulence modelling, with a special focus to hybrid RANS-LES methods and their industrial applications.
Author: National Aeronautics and Space Administration (NASA)
Publisher: Createspace Independent Publishing Platform
ISBN: 9781724700643
Category :
Languages : en
Pages : 230
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Book Description
An existing cold-jet facility at NASA Lewis Research Center was modified to produce swirling flows with controllable initial tangential velocity distribution. Two extreme swirl profiles, i.e., one with solid-body rotation and the other predominated by a free-vortex distribution, were produced at identical swirl number of 0.48. Mean centerline velocity decay characteristics of the solid-body rotation jet flow exhibited classical decay features of a swirling jet with S - 0.48 reported in the literature. However, the predominantly free-vortex distribution case was on the verge of vortex breakdown, a phenomenon associated with the rotating flows of significantly higher swirl numbers, i.e., S sub crit greater than or equal to 0.06. This remarkable result leads to the conclusion that the integrated swirl effect, reflected in the swirl number, is inadequate in describing the mean swirling jet behavior in the near field. The relative size (i.e., diameter) of the vortex core emerging from the nozzle and the corresponding tangential velocity distribution are also controlling factors. Excitability of swirling jets is also investigated by exciting a flow with a swirl number of 0.35 by plane acoustic waves at a constant sound pressure level and at various frequencies. It is observed that the cold swirling jet is excitable by plane waves, and that the instability waves grow about 50 percent less in peak r.m.s. amplitude and saturate further upstream compared to corresponding waves in a jet without swirl having the same axial mass flux. The preferred Strouhal number based on the mass-averaged axial velocity and nozzle exit diameter for both swirling and nonswirling flows is 0.4. Taghavi, Rahmat and Farokhi, Saeed Unspecified Center NASA-CR-180895, NAS 1.26:180895 NCC3-56; RTOP 505-62-21...
Author: Carl Fredrik Stein
Publisher:
ISBN:
Category :
Languages : en
Pages : 66
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Author: Michael S. Feyedelem
Publisher:
ISBN:
Category :
Languages : en
Pages : 175
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Book Description
The turbulent flow field of a swirling jet issuing from a nozzle, beneath and parallel to a free surface has been studied in as much detail as possible using a three-component laser Doppler velocimeter and flow visualization. The results have shown that the swirl leads to the faster spreading and quicker mixing of the jet. For strongly swirling jets (S = 0.522), the similiarity is not reached within ten diameters downstream. The results have also shown that both the acial and tangential velocity components decrease outward from the jet axis, naturall leading to centrifugal instabilities. This, in turn, leads to the creation of large scale coherent structures at the periphery of the jet, particularly when it is in the vicinity of the free surface. The turbulent shear stresses exhibit anisotropic behavior, the largest always being in the plane passing through the jet axis. The change of TKE with S is not monotonic. It is maximum for S - 0.265, smallest for S = 0.50, and has an intermediate value for S - 0.522. This is due to the occurrence of vortex breakdown and the resulting intensification of the turbulence within the jet prior to its exit from the nozzle.
Author: National Aeronautics and Space Administration (NASA)
Publisher: Createspace Independent Publishing Platform
ISBN: 9781722220938
Category :
Languages : en
Pages : 42
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Book Description
An experimental project was undertaken to investigate the character of vortex breakdown with particular regard to the waveguide theories of vortex breakdown. A rectangular wing based on the NACA 0012 airfoil was used to produce a trailing vortex which convected downstream without undergoing breakdown. Dye marked the vortex location. A disturbance was then introduced onto the vortex using a small moving wire to 'cut' the vortex. The development of upstream and downstream propagating disturbance waves was observed and the propagation velocities measured. The downstream traveling wave produced a structure similar in appearance to a vortex breakdown. The upstream wave produced a moving, swirling, turbulent region that was not a vortex breakdown. The waves moving in either direction have the same swirl velocity profiles but quite different axial velocity profiles. The upstream disturbance (turbulence) moved into a flow with an axial velocity profile that had a wake-like defect in the core region. The downstream moving vortex breakdown moved into a flow with a jet-like overshoot in the core region. The fact that no breakdown was observed for the wake-like defect and breakdown was observed for the jet-like overshoot is not consistent with computational fluid dynamics (CFD) calculations. Although there are not a lot of examples, CFD results show breakdown for both types of profiles. The longitudinal and swirl velocity profiles were documented by Laser Doppler Velocimeter (LDV) measurement. Wave velocities, swirl angles, and swirl parameters are reported. Panton, Ronald L. and Stifle, Kirk E. Unspecified Center NAG2-389...
