Bulletin of the American Physical Society
66th Annual Meeting of the APS Division of Fluid Dynamics
Volume 58, Number 18
Sunday–Tuesday, November 24–26, 2013; Pittsburgh, Pennsylvania
Session H1: Jets III: Round, Liquid and Impinging |
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Chair: James J. Feng, University of British Columbia Room: 323 |
Monday, November 25, 2013 10:30AM - 10:43AM |
H1.00001: Bifurcation and Turbulent transitions of round jets Philippe Bardet, Amy McCleney An experimental round jet is created by controlling the axial momentum injection rate, with the resulting water jet discharging freely into a large tank. The evolution and the transition to turbulence of round jets into a quiescent fluid are examined for Reynolds number ranging from 1,000 to 10,000. The boundary layer roll-up, merging of vortices, and turbulence transition of the jets natural instabilities are tracked from the nozzle exit. Near field measurements of the flow structures are observed using PLIF through azimuthal dye injection and PIV. These types of jets are observed both in nature and industrial applications. Understanding the early transition steps of a jet in detail is important in the view of efficient turbulence control. Modification of the natural fluid flow can lead to the delay in transition when turbulence is harmful or to promote instability when better mixing is beneficial. This study focuses on improving the understanding of jet flow structures for the development of flow manipulation. [Preview Abstract] |
Monday, November 25, 2013 10:43AM - 10:56AM |
H1.00002: The effect of viscosity gradients on the stability of the turbulent round jet Ryan Keedy, James Riley, Alberto Aliseda The effect of viscosity differences between the ambient and injected fluids on a high Reynolds number round jet is poorly understood and has been largely ignored in stability analyses of this canonical shear flow. When viscosity gradients are present at the mixing interface between the two fluids, the jet behavior can be significantly affected. A new set of linear stability equations, which account for differences in jet and ambient viscosities, have been developed to study the growth of spatial disturbances. The equations are shown to reproduce results found in the literature for constant viscosity. Eigenvalue analysis is carried out to evaluate the predicted growth rates and unstable wavelengths as a function of the dominant variables: frequency, momentum thickness, etc. Experimental results obtained in a high Reynolds number ($10^5$) round jet facility, with a submerged jet issuing vertically in a large, essentially unconfined water tank were compared with the parametric study of the linear stability analysis results. Jet viscosity in our experiments was modified by altering the viscosity of the miscible injected fluid over a range of the viscosity ratio, $\mu_{jet}/\mu_{ambient} > 1$ (by adding glycerol) and $\mu_{jet}/\mu_{ambient} < 1$ (by raising the temperature). [Preview Abstract] |
Monday, November 25, 2013 10:56AM - 11:09AM |
H1.00003: Time-resolved imaging studies of adjacent liquid jet formation Julia Yang, Frederik Brasz, Craig Arnold Laser-induced jetting of liquids is an area of interest in fluid dynamics due to its versatile range of applications in printing and patterning. In this work, we use time-resolved imaging to examine the formation of two adjacent liquid jets produced by separate laser pulses of similar energy. A laser pulse is absorbed within a polymer layer coated with ink, forming a rapidly expanding blister that induces a liquid jet. The time delay and spatial separation between pulses are varied, and for close enough proximities, the second jet exhibits changes in propagation direction. As the separation between pulses decreases, the jets intertwine and form one long, twisted jet or a single large jet. The time-resolved images are also compared with simulation results, which reveal similar trends. This scenario of two adjacent liquid jets provides insight into high repetition rate printing and the limitations of separation and time delay between pulses. [Preview Abstract] |
Monday, November 25, 2013 11:09AM - 11:22AM |
H1.00004: Global stability of gravitationally stretched capillary jets Mariano Rubio-Rubio, Alejandro Sevilla, Jos\'e Manuel Gordillo We analyze the global linear stability of capillary jets stretched by gravity both experimentally and theoretically, extending the work by Sauter \& Buggisch (J. Fluid Mech. vol. 533, 2005, pp. 237-257). Our results reveal the essential stabilizing role played by the axial curvature of the jet, the latter effect being especially relevant for injectors with a large diameter. The theoretical description, based on the one-dimensional mass and momentum equations retaining the exact expression for the interfacial curvature, accurately predicts the onset of jet self-excited oscillations experimentally observed for wide ranges of liquid viscosity and injector diameter. The marginal self-sustained oscillations observed in the experiments are shown to correspond to the excitation of the leading global mode of the jet. The model developed in the present work shows better agreement with the experimental jetting-dripping transition events than those available in the literature, thus allowing us to conclude that, surprisingly, the size of the steady threads produced at a given distance from the exit can be reduced by increasing the nozzle diameter. The proposed formulation allows to describe the inviscid limit, and experiments are being performed to study this distinguished case. [Preview Abstract] |
Monday, November 25, 2013 11:22AM - 11:35AM |
H1.00005: Global frequency response analysis of gravitationally stretched liquid jets Paula Consoli-Lizzi, Wilfried Coenen, Alejandro Sevilla The convective capillary break-up of freely falling axisymmetric jets of Newtonian liquid is theoretically studied with a one-dimensional description of the mass and momentum conservation equations. Instead of using the classical quasi-parallel assumption in the stability analysis, here we compute the global linear response of the flow to harmonic inputs at the exit of the jet, allowing us to predict its break-up length in cases where the base flow is not slender. Our theory compares favourably with recent experiments by Javadi \emph{et al.} (PRL 110, 144501, 2013), who measured the break-up length of unforced liquid jets of several viscosities. From the physical point of view, our main finding is that the meniscus region near the injector outlet, where the jet experiences the strongest axial stretching, delays the growth of capillary disturbances due to a \emph{spatial} counterpart of the kinematic stabilizing mechanism firstly described by Tomotika (Proc. Roy. Soc. 153, 1936) in a \emph{temporal} setting. [Preview Abstract] |
Monday, November 25, 2013 11:35AM - 11:48AM |
H1.00006: Auto-ejection of liquid jets and drops from capillary tubes Hadi Mehrabian, James J. Feng Capillary imbibition through a tube and nozzle assembly can built enough momentum to eject droplets. Such an auto-ejection process is studied using Cahn-Hilliard diffuse-interface simulations to capture the dynamic contact angle, interface deformation and drop pinch-off. The breakup process is studied and a criterion for ejection is proposed. We investigate the dependence of this criterion on geometric parameters and fluid properties. Finally, we estimate the size of the produced droplets and compare the numerical results with experiments. [Preview Abstract] |
Monday, November 25, 2013 11:48AM - 12:01PM |
H1.00007: Surface Pressure Fluctuations Produced by an Axisymmetric Impinging Jet: Generation Mechanisms Malek Al-Aweni, Ahmed Naguib This study is motivated by understanding the mechanisms leading to unsteady surface pressure generation in impinging jet flows. Employing an extensive database of concurrent time-resolved flow visualization and time series from a surface-embedded microphone array, two dominant mechanisms are found to affect the space-time evolution of the pressure within the wall-jet zone: vortex-wall and vortex-vortex interaction. To gain deeper insight into these mechanisms, two closely-related model problems are studied computationally using Fluent. The problems involve the impingement of a single or two axisymmetric vortex rings on a flat wall. The resulting spatio-temporally resolved computations are used in conjunction with Possion's equation for pressure to investigate the nature of the pressure-generating sources, their relative importance, and their relation to the observed surface pressure signature. The findings provide significant information towards realization of efficient, structure-based models for computing the unsteady wall pressure in impinging jets. [Preview Abstract] |
Monday, November 25, 2013 12:01PM - 12:14PM |
H1.00008: Surface Pressure Fluctuations Produced by an Axisymmetric Impinging Jet: Spatio-Temporal Characteristics Ahmed Naguib, Malek Al-Aweni This is the second of a sequence of two presentations concerned with understanding the nature and generation mechanisms of the unsteady surface pressure in impinging jet flows. In the first presentation, the mechanisms influencing the evolution of the surface pressure are studied by examining instantaneous realizations obtained from time-resolved flow visualization and concurrent surface-embedded microphone array measurements; along with numerical simulations of related model problems. In this presentation, the focus is on examining the statistical importance and persistence of these mechanisms by comparing knowledge obtained from the instantaneous analysis to that resulting from inspection of conditional spatio-temporal surface-pressure behaviors, frequency-wavenumber spectra and other statistical measures. Results are presented for surface-pressure measurements at a Reynolds number based on jet diameter of approximately 7000. Dependence of the results on the spacing between the impingement wall and the jet as well as the jet impingement angle is also considered. [Preview Abstract] |
Monday, November 25, 2013 12:14PM - 12:27PM |
H1.00009: Effects of density, velocity gradient, and compressibility on side-jet formation in round jets with variable density Akinori Muramatsu When a low density gas compared with the ambient gas is discharged from a round nozzle, side jets that are radial ejections of jet fluid are generated at the initial region of the jet. The density ratio between the jet fluid and the ambient fluid is a main parameter for the side-jet formation. Since the side-jet formation is also related to the instability of shear layer, it depends on the velocity gradient of the shear layer in the jet. The velocity gradient is evaluated by a ratio of the momentum thickness and the nozzle diameter at the nozzle exit. Compressibility suppresses the instability and the generation of the side jets. The compressibility is evaluated by a Mach number, which is a ratio defined by an issuing velocity of the jet and a sound velocity in the ambient fluid. Influence of these three parameters on the side-jet formation was examined experimentally. The density ratio and momentum thickness ratio were varied from 0.14 to 1.53, and from 14 to 155, respectively. The Mach number was varied to 0.7. Existence of side jets was confirmed by flow visualization using a laser sheet. Domains for the side-jet formation by the density ratio, the momentum thickness ratio, and the Mach number were determined. [Preview Abstract] |
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