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Author: Philip Robert Edward Cutler
Publisher:
ISBN:
Category : Jets
Languages : en
Pages : 480
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Book Description
Author: Philip Robert Edward Cutler
Publisher:
ISBN:
Category : Jets
Languages : en
Pages : 480
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Book Description
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.
Author: Joint Institute for Aeronautics and Acoustics. Joint Institute for Aeronautics and Acoustics
Publisher:
ISBN:
Category :
Languages : en
Pages : 22
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Author: W. L. Chow
Publisher:
ISBN:
Category : Thermodynamics
Languages : en
Pages : 40
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Book Description
Author: Ann R. Karagozian
Publisher: Springer
ISBN: 3709127920
Category : Technology & Engineering
Languages : en
Pages : 304
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Book Description
Fundamental Non-Reactive Jets in Crossflow and Other Jet Systems; Background on Modeling, Dynamical Systems, and Control; Reactive Jets in Crossflow and Multiphase Jets; Controlled Jets in Crossflow and Control via Jet Systems;
Author: Andrew William Cameron
Publisher:
ISBN:
Category : University of Ottawa theses
Languages : en
Pages : 302
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Author: Takeshi Shoji
Publisher:
ISBN:
Category :
Languages : en
Pages : 619
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Book Description
This dissertation describes an experimental exploration of structural and mixing characteristics of the unforced as well as forced jet in crossflow (JICF). For unforced experiments, variable jet-to-crossflow density ratios (0.35 S 1.00) and momentum flux ratios (5 J 41) were explored, for three alternative injectors with a fixed jet Reynolds number (Rej=1900). For forced jet experiments, an equidensity jet emanating from two alternative flush nozzles (Rej=1900; Rej=1500) were explored. The interplay between scalar and velocity fields for the JICF was studied using simultaneous PLIF/stereo PIV measurements in the jet's centerplane. POD analysis showed a transition from convective to absolutely instability in the upstream shear layer as J was reduced, consistent with hotwire measurements. Local strain rates were extracted from both PLIF and PIV data via the Howarth transformation. Mixing characterization with variable mixing scale lengths was explored via centerplane PLIF images. Mixing associated with stirring as well as molecular mixing could be distinguished, apparent especially in vortical structures for flush pipe injection. Sinusoidal forcing of the convectively unstable JICF demonstrated that relatively low amplitude forcing affected jet structures and mixing, especially close to the fundamental frequency. For the absolutely unstable JICF, a much higher forcing amplitude was required to alter jet characteristics, consistent with lock-in, leading to enhanced molecular mixing. Single-pulse square wave forcing created deeply-penetrating vortical structures for specific stroke ratios (L/D), even for the absolutely unstable JICF. L/D producing the greatest jet spread and penetration decreased with decreasing J, but L/D for optimal molecular mixing corresponded to different values and trends in J. Double-pulse square wave excitation, with two variable pulses within each temporal period, enabled the jet's nearfield vortex rings to interact and collide, with attendant effect on molecular mixing.
Author: David Charles Thoman (Jr)
Publisher:
ISBN:
Category :
Languages : en
Pages :
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Book Description
This thesis summaries the results of an experimental investigation of the near-field behavior and physics of the round buoyant turbulent jet in crossflow. In particular, the study centers on the physics associated with entrainment and mixing phenomena of the jet with the goal of better understanding the trends of trajectory and dilution behavior. The experiments involved a downward discharge of cold nitrogen gas (at about $-$85$spcirc$C) from a cylindrical structure placed in a wind tunnel of horizontally flowing ambient air. The jet was mapped using thermocouple measurements. For the purpose of studying jet/crossflow and jet/wake interactions, fog-oil smoke was used to mark parcels of fluid in the crossflow upstream of the jet and in the wake flow downstream of the discharge structure. Time-averaged, smoke concentrations were gathered through an aspirated sampling probe in conjunction with a calibrated, optical aerosol monitor. Smoke distributions were also photographed. Experiments were performed for four different values for the crossflow-to-exit velocity ratio k, namely, k = 0.7, 1.3, 2.1, and 3.5. The results of the experiments yield a comprehensive picture of the near-field flow patterns, flow interactions, and flow-transport physics for a buoyant jet in crossflow. Key, phenomenologically distinct zones of flow which comprise the near-exit structure of the jet are identified. The flow patterns within these zones, and thus the structure of the near-exit jet, are found to be extremely dependent on the value of the velocity ratio. Flow interactions in this region establish flow patterns which have a pronounced influence on the downstream development of the jet. A method is developed to fully document the trajectory and dilution behavior of the jet with the key parameters of influence. Trajectory and dilution are found to correlate with two parameters, namely, the velocity ratio and the density-difference ratio. Finally, unsteady, large-scale mixing motions within the near-exit flow zones are documented. Distinct modes of large-scale mixing which are responsible for the rapid and extensive dispersion observed in the jet are revealed by this study.
Author: Wayne Maurice VanLerberghe
Publisher:
ISBN:
Category :
Languages : en
Pages : 410
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Book Description
The large scale structure and mixing characteristics of the flowfield surrounding a single, underexpanded, sonic, transverse jet injected into a Mach 1.6 crossflow were experimentally studied. The flowfield was investigated at three distinct sets of flow conditions corresponding to a jet-to-crossflow momentum flux ratio of 1.2, 1.7, and 2.2. Planar laser-induced fluorescence from acetone molecules and planar Mie scattering from condensed ethanol droplets were used to obtain spatially and temporally resolved flowfield visualizations in various side, end, and top view image planes. Statistical processing of the image ensembles produced mean and standard deviation images, two-dimensional spatial autocovariance fields representing the characteristics of the dominant turbulent structures, and probability density functions representing the instantaneous state of scalar mixing in the flowfield. Time-averaged descriptions of the mixing characteristics of this transverse jet flowfield were shown to be inaccurate. Intermittent large scale structures of various sizes, shapes, and orientations strongly influence the distribution of the jet fluid and the crossflow fluid. Relatively high probabilities of unmixed fluid (whether jet or crossflow) persist in many of the mixing regions between the jet and crossflow. The most significant instantaneous mixing in this flowfield seems to occur in the center of wake region slightly below the jet centerline. A counter-rotating streamwise vortex pair in the jet cross section plays an important role in the scalar mixing processes, as it transports jet fluid down towards the wake and entrains crossflow fluid from below up into the jet. The streamwise vortex pair initially develops in an asymmetric and undulating manner, but gradually becomes more symmetric farther downstream of the Mach disk. A relatively large portion of the total jet fluid bypasses the Mach disk through the upstream edge of the barrel shock, thereby retaining a significant fraction of its momentum. This relatively high momentum jet fluid penetrates deeply into the crossflow, contributes to the formation of the largest turbulent structures, and affects the evolution of the streamwise vortex pair.
Author: Stephen B. Pope
Publisher: Cambridge University Press
ISBN: 9780521598866
Category : Science
Languages : en
Pages : 810
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Book Description
This is a graduate text on turbulent flows, an important topic in fluid dynamics. It is up-to-date, comprehensive, designed for teaching, and is based on a course taught by the author at Cornell University for a number of years. The book consists of two parts followed by a number of appendices. Part I provides a general introduction to turbulent flows, how they behave, how they can be described quantitatively, and the fundamental physical processes involved. Part II is concerned with different approaches for modelling or simulating turbulent flows. The necessary mathematical techniques are presented in the appendices. This book is primarily intended as a graduate level text in turbulent flows for engineering students, but it may also be valuable to students in applied mathematics, physics, oceanography and atmospheric sciences, as well as researchers and practising engineers.
