Bulletin of the American Physical Society
71st Annual Meeting of the APS Division of Fluid Dynamics
Volume 63, Number 13
Sunday–Tuesday, November 18–20, 2018; Atlanta, Georgia
Session A30: Swirling Jets and Jets in Crossflow |
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Chair: Ann Karagozian, University of California, Los Angeles Room: Georgia World Congress Center B402 |
Sunday, November 18, 2018 8:00AM - 8:13AM |
A30.00001: Linear stability of reacting swirling shear flows. Christopher M Douglas, Benjamin Emerson, Timothy C Lieuwen Many modern combustion systems leverage the phenomenon of vortex breakdown for flame stabilization and mixing enhancement. Yet, vortex breakdown introduces shear flow features which may promote other flow instabilities, manifested by helical vortex shedding, precession of the swirling vortex core, and full-blown combustion instability. Theoretical studies of incompressible flows have revealed the nature of these bifurcations and used linear stability analysis to predict their occurrence. However, the role of combustion in these dynamics is not clear. Flame-induced baroclinity and compressibility explicitly modifies the flow’s vorticity distribution, alters interactions among regions of concentrated vorticity, and couples the thermodynamic and hydrodynamic states. This numerical investigation explores the linear stability of normal disturbance modes with respect to a model axisymmetric swirling base flow. The results are interpreted in terms of the asymptotic spatiotemporal behavior of the disturbance modes and a budget of terms in the vorticity equations is discussed and compared with the incompressible flow case. |
Sunday, November 18, 2018 8:13AM - 8:26AM |
A30.00002: Experimental study on the interaction between non-reacting swirling jets Yuvi Nanda, Lipika Kabiraj Modern gas turbine combustors employ swirl flows for flame stabilization. In such systems, the dynamic feature of the swirl flows govern critical processes such as mixing of the reactants, ignition dynamics, and the dynamics of flames. The latter in turn has a governing influence on the (thermoacoustic) stability of the combustor. Often combustor configurations employ an array of swirl-stabilized flames, where the interaction between flames has a critical |
Sunday, November 18, 2018 8:26AM - 8:39AM |
A30.00003: Abstract Withdrawn
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Sunday, November 18, 2018 8:39AM - 8:52AM |
A30.00004: Control of local vorticity generation and shear layer instabilities in transverse jets Elijah Weston Harris, Andrea C Besnard, Ann Karagozian The present experimental study examines the effects of strategic positioning of small tabs in the periphery of the exit plane of a gaseous jet in crossflow (JICF) as a means of controlling vorticity generation and mixing. Tab location can significantly influence the dynamics of the shear layer instability as quantified via acetone planar laser induced fluorescence (PLIF) imaging and stereo particle image velocimetry (PIV). When the JICF is created with a low jet-to-crossflow momentum flux ratio J, typically below 6-10, which in the absence of tabs is associated with a globally unstable upstream shear layer (USL), the tab can significantly weaken the instability. When the tab is placed in the jet's upstream region, for example, the USL can become convectively unstable. The effects of tabs on the JICF at a much higher values of J are not quite as significant, but further weakening of the convectively unstable USL does have an influence on cross-sectional symmetry and hence vorticity evolution and mixing. Proper orthogonal decomposition (POD) enables quantification of changes in the shear layer dynamics, providing evidence for the potential to tailor local vorticity and mixing via simple geometrical alterations. |
Sunday, November 18, 2018 8:52AM - 9:05AM |
A30.00005: Stability and sensitivity of the low-speed jet in cross-flow Marc Regan, Krishnan Mahesh The tri-global stability and sensitivity of the jet in cross-flow (JICF) is studied at $Re=2000$, and two jet-to-cross-flow ratios: $R=2$ and $R=4$. A novel capability is developed on unstructured grids and parallel platforms for this purpose. Linear stability analysis reveals that upstream shear-layer modes have frequencies that match simulation (Iyer & Mahesh, 2016) and experiment (Megerian et al., 2007). Asymmetric modes are more important to the overall dynamics at $R=4$. Low-frequency modes show a connection to wake vortices. For $R=4$ a downstream shear-layer mode is the most unstable. Adjoint modes show that the upstream shear-layer is most sensitive along the upstream side of the jet nozzle. Lower frequency downstream modes are sensitive in the cross-flow boundary layer. For $R=2$, optimal perturbations reveal that for short-time horizons, asymmetric perturbations dominate and grow along the counter-rotating vortex pair. However, as the time horizon increases, growth is optimal along the upstream shear-layer. When $R=4$, the optimal perturbations for short-time scales grow along the downstream shear-layer. For long-time horizons, they become hybrid modes that grow along both the upstream and downstream shear-layers. |
Sunday, November 18, 2018 9:05AM - 9:18AM |
A30.00006: High-Frequency Imaging of a Reacting Jet in Crossflow Vedanth Nair, Christopher M Douglas, Benjamin Wilde, Benjamin Emerson, Timothy C Lieuwen The Reacting Jet in Crossflow (RJICF) is a canonically important flowfield in chemically reacting flows. The high-frequency (typically over 10 kHz) shear layer instabilities are of particular significance, since their behavior governs the near field mixing and entrainment. In this study, 10 kHz stereo PIV and OH-PLIF measurements were performed on a reacting hydrogen jet in a hot vitiated crossflow. By themselves, these measurements were not sufficient to resolve the high-frequency shear layer dynamics but were supplemented by 50 kHz OH* chemiluminescence. The signal intensity power spectrum indicates a peak frequency of these instabilities at 19 kHz, which lies in the dimensionless frequency range typical of non-reacting JICF studies. The spatial variation of the instability amplitude and phase was analyzed using Fourier analysis, and Proper Orthogonal Decomposition (POD) and Dynamic Mode Decomposition (DMD) were utilized to further isolate and understand the complex spatiotemporal dynamics. These methods succeeded in decoupling the lower-frequency jet flapping dynamics from the higher-frequency shear layer features, yielding new insight into the rich dynamics of the RJICF problem. |
Sunday, November 18, 2018 9:18AM - 9:31AM |
A30.00007: Turbulent mixing dynamics of radially injected jets into cross-flows Avick Sinha, Shivasubramanian Gopalakrishnan The turbulent mixing of jets especially in cross-flows is of significant importance for engineering applications. The break-up of a liquid jet in a cross-flow occurs due to the instabilities along the surface of the jet. However, the physics behind the mixing process is more complicated when multiple jets are injected radially and the complex nature of the flow has not been studied adequately. In the present study, we present three dimensional numerical simulations carried out for multiple radially located nozzles injecting water into a chamber at atmospheric pressure conditions. The velocity of the cross-flow at the inlet was maintained at 100 m/s, and the velocity of the nozzle inlet was varied to study the effect of jet-cross flow ratio. Two numerical approaches, one a Volume of fluid (VOF) method was employed along with RAS k-epsilon model to simulate the flow physics. A second, mixed Eulerian (cross-flow fluid) - Lagrangian (jet-fluid) technique was employed along with primary and secondary atomization models to capture the flow physics. A comparison of the two numerical approaches along with available experimental results in literature is presented. |
Sunday, November 18, 2018 9:31AM - 9:44AM |
A30.00008: Large Eddy Simulation of Transverse Jets into Crossflow with Mixing-Enhancement Wells Zhaoyi Xu, Liwei Zhang, Vigor Yang Transverse jets into crossflows (JICF) have wide applications in natural sciences and engineering. The present work proposed the use of a mixing well to enhance the mixing of a gaseous jet into crossflow. A compressible-flow Navier-Stokes solver is applied to examine flow structures and mixing efficiency by using a large-eddy-simulation technique and dynamic stress and scalar flux models, Two different well configurations and two jet-to-crossflow velocity ratios, 2 and 4, are considered at a jet velocity of 160 m/s. The jet trajectory, counter-rotating vortex pair (CVP), and near-field vortical structures are discussed. Scalar mixing efficiency is quantified by the maximum concentration decay and the spatial mixing deficiency (SMD) along the crossflow direction. Results show that in the presence of a mixing well, the strengths of horseshoe, shear layer, and wake vortices, and CVP instability and asymmetry are enhanced noticeably. Furthermore, SMD results show the nearfield scalar mixing rate is significantly strengthened by intensified vortex motions. The jet trajectory and penetration is affected by the mixing well. Such effect is sensitive to the velocity ratio. An optimization scheme can be applied to search for the optimal mixing well configuration for mixing control. |
Sunday, November 18, 2018 9:44AM - 9:57AM |
A30.00009: Modulation of the liquid flow rate and the gas swirl ratio as an actuation strategy for control of a two-fluid coaxial atomizer spray Alberto Aliseda, Peter Dearborn Huck, Nathanael Machicoane, Rodrigo Osuna Orozco We study the effect of harmonic modulation of the liquid flow rate and the the gas swirl ratio on the droplet size and number density distributions. A canonical coaxial two-fluid atomizer with laminar liquid and turbulent gas streams has been designed to investigate feedback control on spray. High-speed stereo visualization of the primary instability that drives the atomization will be presented, along with droplet size and velocity interferometry measurements in the region where the break-up spray is finished and the spray structure is developing. The liquid mass loading is kept smaller than 1, while the gas momentum ratio is varied from 5 to 125, and the swirl ratio ranges from 0-1 (equal stream-wise and cross-stream flow rates). Harmonic low frequency/ high amplitude (50% of maximum) forcing of the control valves and the nozzle back-pressure modulate the gas and liquid injection rates. The effect of the frequency and amplitude of the modulation on the spray primary instabilities is characterized to enable feedback control of the droplet break-up. Modulating the liquid injection produces larger drops than steady injection, while modulating swirl significantly homogenizes the droplet size and number density distributions. |
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