Flow Structures of Coaxial Jets

Flow Structures of Coaxial Jets PDF Author: 區堪
Publisher: Open Dissertation Press
ISBN: 9781374759831
Category :
Languages : en
Pages :

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Book Description
This dissertation, "Flow Structures of Coaxial Jets" by 區堪, Hom, Au, was obtained from The University of Hong Kong (Pokfulam, Hong Kong) and is being sold pursuant to Creative Commons: Attribution 3.0 Hong Kong License. The content of this dissertation has not been altered in any way. We have altered the formatting in order to facilitate the ease of printing and reading of the dissertation. All rights not granted by the above license are retained by the author. DOI: 10.5353/th_b3123020 Subjects: Jets - Fluid dynamics Aerodynamics Turbulence

Flow Structures of Coaxial Jets

Flow Structures of Coaxial Jets PDF Author: 區堪
Publisher: Open Dissertation Press
ISBN: 9781374759831
Category :
Languages : en
Pages :

Get Book Here

Book Description
This dissertation, "Flow Structures of Coaxial Jets" by 區堪, Hom, Au, was obtained from The University of Hong Kong (Pokfulam, Hong Kong) and is being sold pursuant to Creative Commons: Attribution 3.0 Hong Kong License. The content of this dissertation has not been altered in any way. We have altered the formatting in order to facilitate the ease of printing and reading of the dissertation. All rights not granted by the above license are retained by the author. DOI: 10.5353/th_b3123020 Subjects: Jets - Fluid dynamics Aerodynamics Turbulence

Flow Structures of Coaxial Jets

Flow Structures of Coaxial Jets PDF Author: Hom Au
Publisher:
ISBN:
Category : Aerodynamics
Languages : en
Pages : 462

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Noise Source Distribution and Mean-flow Structure of Coaxial Jets

Noise Source Distribution and Mean-flow Structure of Coaxial Jets PDF Author: Sara Rostamimonjezi
Publisher:
ISBN: 9781124018096
Category :
Languages : en
Pages : 115

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Book Description
This study examines the noise and flow field characteristics of convergent coaxial jets with different diameter and velocity ratios. Experimental investigation of coaxial jet using two separate testing facilities enables us to seek connections among the various noise metrics, including sound pressure level (SPL), overall sound pressure level (OASPL), and noise source maps, and the flow field properties of the jet, including mean velocity, inflectional layers, centerline velocity, and primary and secondary core lengths. An anechoic acoustic testing facility is used to perform noise measurement tests and a Pitot rake velocity testing facility is used to obtain the mean velocity flow field of the jet plume. The results indicate that for zero velocity ratio (single-stream jet), the region near the nozzle emits strong high-frequency noise. Addition of a secondary flow reduces the convective velocity of the eddies in the primary shear layer. This is consistent with elimination of Mach waves in the region near the nozzle. The secondary flow also reduces the growth rate of the jet and elongates the primary potential core. Increasing the velocity ratio suppresses the near-nozzle noise and extends downstream the location of the peak noise. The location of peak noise tracks well the end of the primary core. The suppression of highfrequency noise is explained by the creation and elongation of the secondary core as the velocity ratio increases. The acoustic trends with velocity ratio are similar for polar angles ranging from 20± to 120±. This range of polar angles includes both large-scale and fine-scale turbulence noise which have been shown to radiate in downstream (small polar angles) and upstream (large polar angles) directions, respectively. The results indicate that the coaxial jet has a noise source distribution fundamentally different from that of a single-stream jet.

Transitional Coaxial Jets

Transitional Coaxial Jets PDF Author: Paolo Burattini (t.d.-)
Publisher:
ISBN:
Category :
Languages : en
Pages :

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Structure and Control of a Particle-laden Coaxial Jet with and Without Annular Swirl

Structure and Control of a Particle-laden Coaxial Jet with and Without Annular Swirl PDF Author: Stanford University. Thermosciences Division. Thermosciences Division
Publisher:
ISBN:
Category :
Languages : en
Pages : 284

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Experimental and Analytical Investigation of Near-Field Coaxial Mixing

Experimental and Analytical Investigation of Near-Field Coaxial Mixing PDF Author:
Publisher:
ISBN:
Category :
Languages : en
Pages : 136

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Book Description
In the near field of coaxial jets, the structure of the flow field undergoes rapid changes as it evolves from the boundary layers at the nozzle exit, through a quasi-two-dimensional shear-layer region, to an axisymmetric jet flow. These rapid changes in flow structure have raised questions concerning the applicability of turbulent flow models in the near field, but few experimental results are available to test predictive techniques. In this report, the results of a series of experiments concerned with the near field of coaxial, nearly constant density, air-air jets are described. The experiments were carried out at an external stream velocity of 30.5 m/sec, and at nominal inner jet-to-outer jet velocity ratios of 3:1, 2:1, and 1:1.

Structure, Mixing, and Dynamics of Controlled Single and Coaxial Jets in Crossflow

Structure, Mixing, and Dynamics of Controlled Single and Coaxial Jets in Crossflow PDF Author: Elijah Weston Harris
Publisher:
ISBN:
Category :
Languages : en
Pages : 493

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Book Description
This dissertation describes an experimental study of the instability, structural, dynamical, and mixing characteristics of jets in crossflow (JICF). Constituent species of the jet fluid were nitrogen and helium, with additional seeding of tracer particulates for implementation in non-intrusive laser diagnostics of acetone planar laser induced fluorescence (PLIF) imaging, and particle image velocimetry (PIV). Various jet-to-crossflow momentum flux ratios in the range of $61\leq J\leq5$ were investigated for three alternative flush mounted circular contracting nozzle injector configurations: a small nozzle ($D=4.04~mm$) passively augmented by a small triangular tab placed about the jet exit periphery, a large nozzle ($D=7.59~mm$) actively augmented by axisymmetric excitation of the jet flow, and a coaxial nozzle ($D=3.81~mm$) with varying degrees of counterflow applied in the outer annulus. Hotwire anemometry was implemented for investigations of the instabilities along the upstream and downstream shear layers of the jet flow, in addition to more in depth analysis of the dynamics of the flow from application of time series analysis techniques to the signal. PLIF imaging served to characterize the JICF by providing qualitative visualizations of the flowfield, and quantitative measurements of the scalar field concentrations and mixing metrics, along both the centerplane and cross-sectional planes of the developing jet. PIV provided determination of the velocity and vorticity fields, along with further investigation of the flow dynamics from proper orthogonal decomposition (POD) analysis, again from both the centerplane and cross-sectional planes of the developing jet. Flow conditions corresponding to a naturally absolutely unstable (AU) upstream shear layer (USL) ($J = 7$) and a convectively unstable (CU) condition ($J = 61$) with jet Reynolds numbers of $Re_j=2300$ were explored for PLIF imaging of the tabbed JICF. Tab location was seen in some cases to significantly alter shear layer instabilities, especially for the case with $J = 7$. Yet acetone planar laser induced fluorescence (PLIF) imaging revealed that more substantive structural changes could be realized with tab placement for the case where $J = 61$. Tab locations with the greatest influence appeared to be consistent with wavemaker regions predicted in numerical simulations of the round transverse jet by \cite{Regan}, providing evidence for the potential to tailor local shear layer rollup, jet structure, and mixing via simple passive geometrical alterations. For the PIV imaging different $J$ values were explored, ranging from naturally AU USL, for $J = 5$ and 8, to naturally CU conditions, for $J = 20$ and 41, with $Re_j=1900$. Placement of the tab at or near the upstream region of the jet exit caused a delay in shear layer rollup and, as quantified from the PIV, a reduction in USL vorticity associated with a thickening of the upstream jet momentum thickness. Tab placement was observed to have a symmetrizing influence on nearfield cross-sectional vorticity dynamics at high and low $J$ values, though specific tab locations had differing degrees of influence for different flow conditions. Proper orthogonal decomposition (POD) modes extracted from centerplane velocity field measurements showed significant influence of tab placement on jet upstream as well as wake structures, depending on $J$. Phase portraits extracted from POD mode coefficient plots produced periodic (circular) shapes for tab placement corresponding to conditions for which the USL was determined to be absolutely unstable. Flow conditions corresponding to a naturally AU USL ($J = 7$) and a CU condition ($J = 10$) with $Re_j=1800$ were explored for PLIF imaging of the axisymmetrically excited JICF. Implementation of a novel double-pulse waveform demonstrated significant enhancement of the quantified jet mixing, where the most significant alterations was seen for forcing waveforms which generated nearfield vortical interactions and breakdown. The same forcing waveform yielded differences between the AU and CU jets resulting from changes to the formation number of the vortex rings as suggested by \cite{Sau_10}, which resulted in alterations in the celerity, circulation, and nearfield interaction of said vortex rings. Separate investigations treated a jet with a stronger AU USL ($J=6$) to comparing the novel double-pulse forcing with sinusoidal and square wave excitation of the jet. Synchronization analysis demonstrated dramatic improvement in the ability of the jet to lock-in to the forcing when a square or double-pulse waveform was implemented. Additional Van der Pol oscillator modeling of Fourier approximated square wave forcing suggested greater significance was seen in the proximity of the harmonics to the natural instability of the jet compared to the actual coherence of the waveform, analogoous to findings by \cite{Sau_10}. Interestingly, PIV-based POD further suggested the nearfield dynamics and efficacy of mixing were heavily dependent upon the vortex rings which were formed, in some cases quite independent of the state of synchronization to the applied forcing. Application of a quasiperiodic forcing significantly improved the mixing without significantly altering the jet structure. For the coaxial JICF, a single flow condition corresponding to a CU USL with a naturally highly asymmetric cross-section ($J = 41$), at $Re_j=1900$, was explored with PLIF and PIV imaging. Suction was applied locally in the upstream and downstream edges of the jet in order to alter the jet shear layer instabilities and vortex dynamics. Indeed, hotwire based spectral characteristics along the USL demonstrated the jet transitioned to an AU flow with strong suction upstream. PIV-based POD also depicted significant enhancement of mode structures along the USL of the jet. Hotwire spectral measurements detected little alteration to the USL with suction applied in the downstream of the jet, where the jet remained CU even with strong suction. However, PIV-based POD dynamics depicted significant enhancement of structures downstream which resembled upright wake vortices, and appeared coupled with the vortex rollup along the downstream of the jet. Interestingly, the corresponding cross-sectional CVP structure was made quite symmetric with the suction applied both upstream or downstream of the jet, suggestive that the suction applied along the symmetry plane of the jet was able to overcome the jet's natural susceptibility to asymmetric perturbations (\cite{Alves_2}), and further supported the suggested wavemaker region for a CU jet purported by \cite{Regan}. Mixing metrics determined significant enhancement in mixing due to the applied suction, further establishing agreement in the correlation between the strength of the shear layer dynamics, symmetry of the cross-sectional CVP, and the resulting jet mixing.

The Developing Region of a Turbulent Coaxial Jet

The Developing Region of a Turbulent Coaxial Jet PDF Author: Gilbert Milanovic
Publisher:
ISBN:
Category :
Languages : en
Pages :

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" Scalar mixing in the developing region of a turbulent coaxial jet was investigated experimentally. The primary objective of this work was to perform single scalar measurements at three downstream locations in a coaxial jet of water. The secondary objective was to determine radial positions within the jet structure where multi-scalar measurements should be made, as part of a bigger study involving multi-scalar mixing in turbulent flows. Experiments were conducted in a momentum-driven, axisymmetric turbulent coaxial jet of water containing a passive scalar in the form of a fluorescent dye. Using planar laser-induced fluorescence (PLIF), planar scalar measurements were obtained at three downstream locations (x/D_in=2,5 and 10) from the jet exit. The Reynolds number of the inner (jet) flow was 6700, the Reynolds number of the outer (annular) flow was 5500, the Schmidt number was 2000, the jet area ratio was 2.54, the velocity ratio was 1, and the jet exit velocity profile was fully developed. Planar concentration measurements were also obtained at the same downstream locations for a simple jet, with a Reynolds number of 6700. Radial profiles of the mean, root-mean-square (rms) and skewness of the concentration, as well as probability density functions, were computed and discussed. The results supported the established trend that higher mean velocity gradients lead to more mixing and faster jet development. The results also implied the possible presence of energetic coherent structures formed in the outer shear layer of the coaxial jet (between the outer flow and the ambient fluid). The experimental conditions used during the coaxial jet tests are novel in the literature; in addition this work presents the first radial profiles of the skewness of the concentration in a coaxial jet. Furthermore, the first set of planar scalar measurements in a simple jet for downstream distances of x/D_in

Some Observations of Flow Structure in Multiple Jet Mixing

Some Observations of Flow Structure in Multiple Jet Mixing PDF Author: Joint Institute for Aeronautics and Acoustics. Joint Institute for Aeronautics and Acoustics
Publisher:
ISBN:
Category :
Languages : en
Pages : 22

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Fluid-Structure-Sound Interactions and Control

Fluid-Structure-Sound Interactions and Control PDF Author: Yu Zhou
Publisher: Springer Science & Business Media
ISBN: 3642403719
Category : Technology & Engineering
Languages : en
Pages : 409

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Book Description
With rapid economic and industrial development in China, India and elsewhere, fluid-related structural vibration and noise problems are widely encountered in many fields, just as they are in the more developed parts of the world, causing increasingly grievous concerns. Turbulence clearly has a significant impact on many such problems. On the other hand, new opportunities are emerging with the advent of various new technologies, such as signal processing, flow visualization and diagnostics, new functional materials, sensors and actuators, etc. These have revitalized interdisciplinary research activities, and it is in this context that the 2nd symposium on fluid-structure-sound interactions and control (FSSIC) was organized. Held in Hong Kong (May 20-21, 2013) and Macau (May 22-23, 2013), the meeting brought together scientists and engineers working in all related branches from both East and West and provided them with a forum to exchange and share the latest progress, ideas and advances and to chart the frontiers of FSSIC. The Proceedings of the 2nd Symposium on Fluid-Structure-Sound Interactions and Control largely focuses on advances in the theory, experimental research and numerical simulations of turbulence in the contexts of flow-induced vibration, noise and their control. This includes several practical areas for interaction, such as the aerodynamics of road and space vehicles, marine and civil engineering, nuclear reactors and biomedical science etc. One of the particular features of these proceedings is that it integrates acoustics with the study of flow-induced vibration, which is not a common practice but is scientifically very helpful in understanding, simulating and controlling vibration. This offers a broader view of the discipline from which readers will benefit greatly. These proceedings are intended for academics, research scientists, design engineers and graduate students in engineering fluid dynamics, acoustics, fluid and aerodynamics, vibration, dynamical systems and control etc. Yu Zhou is a professor in Institute for Turbulence-Noise-Vibration Interaction and Control at Harbin Institute of Technology. Yang Liu is an associate professor at The Hong Kong Polytechnic University. Lixi Huang, associate professor, works at the University of Hong Kong. Professor Dewey H. Hodges works at the School of Aerospace Engineering, Georgia Institute of Technology.