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 A10: Jets I: Swirling, Mixing and Multiphase |
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Chair: Kenneth T. Kiger, University of Maryland Room: 334 |
Sunday, November 24, 2013 8:00AM - 8:13AM |
A10.00001: Stability of swirling coaxial jets Jessie Weller-Calvo, Laurent Joly, Jerome Fontane In order to improve the mixing properties of injectors, we investigate the potential synergy between azimuthal and axial shear. To this end, we examine the linear modal stability of a simplified analytical model which consists of a temporally evolving swirling jet surrounded by an annular jet with a different axial velocity. We denote $\Lambda = V_2/V_1$ the ratio between the axial velocity of the non-swirling outer jet $V_2$ and the axial velocity of the central jet $V_1$; and $q=\Omega_c r_1/V_1$ the swirl number of the central jet where $\Omega_c$ is the rotation rate on the axis and $r_1$ the central jet radius. The present study extends the results of Gallaire \& Chomaz (2003) where a single swirling jet was considered. For all values of the swirl number up to $q=2$, adding the outer non-swirling jet substantially increases the growth rate of the most amplified mode, which can be more than doubled when $\Lambda > 1$. This is the result of the collaborative axial and azimuthal shear instabilities localised in between the two jets. The mode selection of larger azimuthal wavenumbers with increasing $q$, identified by Gallaire \& Chomaz, is no longer observed when the outer jet is at least as fast as the central jet $\Lambda > 1$, the axisymmetric mode being the most amplified. [Preview Abstract] |
Sunday, November 24, 2013 8:13AM - 8:26AM |
A10.00002: Axial Evolution of Helical Modes in Reacting Swirl Flow Mike Aguilar, Benjamin Emerson, David Noble, Tim Lieuwen The swirling jet is a common method of flame stabilization for ground power and aviation combustors. The hydrodynamic stability characteristics of high Reynolds number, reacting, swirling jets are not well understood, but have important influences on the flame stability and combustion dynamics of these combustors. These systems exhibit a variety of unsteady motions associated with the globally unstable vortex breakdown region, as well as the convectively unstable shear layers. Depending upon whether the system is executing natural oscillations or is externally forced at some other frequency (such as during a combustion instability), the dominant shear layer modes can vary between symmetric, m$=$0 structures, to various helical m $= +$1 or $+$2 structures. In addition, these modes evolve axially in different ways. This study presents 10 kHz PIV measurements of such a reacting swirl flow, with and without harmonic forcing. Modal decomposition is used on the measured velocity fields to extract the dominant mode shape and its frequency and spatial growth. Next, hydrodynamic stability theory is used on the measured time averaged flow field to predict the dominant vortex shedding mode and its axial growthrate. The predictions are compared to the experimental results for various flow conditions and forcing arrangements. Finally, we comment on the utility of linear stability methods for predicting the dominant mode and its spatial growth in reacting, swirling jets. [Preview Abstract] |
Sunday, November 24, 2013 8:26AM - 8:39AM |
A10.00003: The coiling of electrified liquid jets Javier Rivero Rodriguez, Miguel P\'erez-Saborid We have carried out a numerical study of the coiling regime which takes place when an electrified liquid jet issuing from an orifice drilled in a metal plate electrode reaches the counter electrode. Based on the slenderness assumption, we have derived the set of one-dimensional dynamical equations by averaging the underlying balance laws over the jet cross sections (Cosserat rod model). Therefore, our equations and boundary conditions are related to those obtained by N.M. Ribe (Ann. Rev Fluid Mech., 2012) for the coiling of liquid ropes, but including electrostatic effects. In a first approach, we have simplified the electrical terms entering the problem by assuming a constant external electric field between electrodes, and that the charges are convected by the jet surface interacting electrostatically with each other via the local interaction approximation (Yarin et al., 2001). We have numerically investigated the problem in order to analize how the coiling regime depends on the dimensionless parameters of the problem, i.e., the Reynolds number, the electrical Bond number and the capillary number. In particular, we have found that both the displacement of the centerline of the jet and its cross-sectional stretching greatly depend on the electrostatic effects. [Preview Abstract] |
Sunday, November 24, 2013 8:39AM - 8:52AM |
A10.00004: Study of the turbulent diffusive flux for tracer dispersion in quasi-two-dimensional turbulent jets Julien R. Landel, Dimitry Foures, C.P. Caulfield The study of turbulent jets in relatively enclosed geometries is relevant to rivers flowing into lakes. In the event of a spillage of pollutants into a river, it is critical to understand how these agents disperse with the flow in order to assess damage to the environment. Using the ensemble-averaged time-dependent advection-diffusion equation, we obtain an equation for the temporal and spatial evolution of the ensemble-averaged concentration of passive tracers injected in a steady turbulent quasi-two-dimensional jet. The turbulent flux of the concentration fluctuations is responsible for the lateral and streamwise dispersion in the jet. We reconstruct this second-order turbulent flux using two independent experimental measurements of the concentration field and of the time-averaged velocity field. We present results in the case of constant-flux releases of tracers in the jets. We find that the dispersion properties differ significantly between the lateral and streamwise direction. Due to a large streamwise dispersion a significant amount of tracers can be transported faster than the speed predicted by a simple top-hat advection model in the jet. [Preview Abstract] |
Sunday, November 24, 2013 8:52AM - 9:05AM |
A10.00005: Optimum viscous flow in pressure-swirl atomizers Ghobad Amini, Aaron Pereira, Sangsig Yun, Xianguo Li Due to their simple configuration and reliable operation, pressure-swirl atomizers are widely used in applications such as combustion, painting, humidification, and sprinkling. The liquid is swirled by entering into the atomizer tangentially and its surface area is increased as discharges in a large spray angle. Understanding the effects of nozzle geometry and inlet flow condition on the discharge coefficient and spray angle is very important in nozzle design. To this end, the flow field inside a pressure-swirl atomizer has been studied theoretically. The main body of the liquid is taken to be moving in circles round the axis. Within the boundary layer, containing transverse and longitudinal velocity components, the retarded liquid is slowed down by viscosity and driven towards the exit orifice by pressure gradient. The swirling motion of liquid creates a low pressure zone near the nozzle axis and leads to the formation of a helical air-core. Through studying the growth of the boundary layer from nozzle entry to the orifice exit, the portions of the outflow exits the orifice from boundary layer current and also from the main body of the swirling liquid are specified. For a given range of pressure drop values, the optimum nozzle geometry and liquid flowrate are predicted. Additionally, the reason of increasing the flow by increasing liquid viscosity or decreasing orifice diameter is explained. A series of experiments and numerical modeling have also been carried out to support the theoretical results. [Preview Abstract] |
Sunday, November 24, 2013 9:05AM - 9:18AM |
A10.00006: Structure, Mixing, and Stability of Flush and Elevated Jets in Crossflow Levon Gevorkyan, Daniel Getsinger, Terry Wen Yu Peng, Owen Smith, Ann Karagozian The present experiments explore the characteristics of equidensity and variable density transverse jets using acetone PLIF, stereo PIV, and hot wire anemometry. Jets composed of mixtures of helium and nitrogen are injected normally from different types of nozzles (flush and elevated with respect to the wind tunnel wall, and converging as well as straight shapes) into an air crossflow. A range of jet-to-crossflow momentum flux ratios $J$ and density ratios $S$ is examined, within which previous studies\footnote{Megerian, et al., {\bf JFM}, 2007; Davitian, et al., {\bf JFM}, 2010; Getsinger, et al., {\bf Expts in Fluids}, 2012} have identified conditions for upstream shear layer transition from convective to absolute instability. The present study examines the relationships among transverse jet structure, including vortical rollup and cross-sectional symmetry/asymmetry, jet mixing characteristics, and shear layer stability characteristics. The role of the crossflow boundary layer as well as jet injection systems for structure, mixing, and stability is evaluated and related to prior observations on vorticity evolution for jets in crossflow. [Preview Abstract] |
Sunday, November 24, 2013 9:18AM - 9:31AM |
A10.00007: Dynamics of viscous fluid jets containing solid particles at low Reynolds number Michael Norton, Teresa Brugarolas, Jonathan Chou, Daeyeon Lee, Haim Bau Using a high-speed camera and a glass capillary flow focusing device, we observe the effects of suspended, elongated particles (aspect ratios 1 - 10) on the dynamics of low Reynolds number water jets ejected into oil (containing surfactant) and the size distribution of the droplets resulting from the jet breakdown. We report on the interaction between the jets and the particles in both the absolutely unstable (dripping) regime and the convectively unstable (jetting) regime. In the former, particles induced coalescence of droplets. In the jetting mode, in addition to coalescence, the presence of particles in the jet caused variations in droplets' sizes both upstream and downstream of the droplet that houses a particle. In the jetting regime, particles circulating in the cone of the jet upstream of the nozzle excited periodic disturbances in the jet that induced variations in droplet sizes. [Preview Abstract] |
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