Bulletin of the American Physical Society
68th Annual Meeting of the APS Division of Fluid Dynamics
Volume 60, Number 21
Sunday–Tuesday, November 22–24, 2015; Boston, Massachusetts
Session G5: Jets I: Mixing, Stability and Turbulence |
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Chair: Akinori Muramatsu, College of Science and Technology, Nihon University Room: 104 |
Monday, November 23, 2015 8:00AM - 8:13AM |
G5.00001: Generation of a flapping motion in a two-dimensional jet at low Reynolds numbers Akinori Muramatsu, Hokuto Tsuchiya In the development of a turbulent two-dimensional jet, a varicose mode generates initially and the sinusoidal mode appears after that. While, it is found that the two-dimensional jet at low Reynolds number does not have the varicose mode. An experiment for the two-dimensional jet at low Reynolds numbers was performed. The Reynolds number was varied from 10 to 200. The jet is laminar when the Reynolds number is less than 60. If the Reynolds number exceeds 60, the jet is flapping. If the Reynolds number exceeds 150, the irregular fluctuations are generated in the jet after the jet is flapping. Next, a numerical simulation was performed for the two-dimensional jet at the Reynolds number of 300. When the spatio-temporally random disturbance is given at the issuing velocity, the flapping motion is generated. If the spatially symmetrical disturbance is given, the velocity fluctuation is generated at peak frequency corresponding to instability in the share layer. However, the velocity fluctuation decays immediately. If the spatially asymmetrical disturbance is given, the velocity fluctuation is generated at peak frequency of the order of 1/100 of the above frequency. The asymmetrical fluctuation rapidly grows and the higher frequencies are generated in the fluctuation. [Preview Abstract] |
Monday, November 23, 2015 8:13AM - 8:26AM |
G5.00002: Mapping the Interactions between Shocks and Mixing Layers in a 3-Stream Supersonic Jet Jacques Lewalle, Christopher Ruscher, Pinqing Kan, Andrew Tenney, Sivaram Gogineni, Barry Kiel Pressure is obtained from an LES calculation of the supersonic jet ($Ma_1 = 1.6$) issuing from a rectangular nozzle in a low-subsonic co-flow; a tertiary flow, also rectangular with $Ma_3 = 1$ insulates the primary jet from an aft-deck plate. The developing jet exhibits complex three-dimensional interactions between oblique shocks, multiple mixing layers and corner vortices, which collectively act as a skeleton for the flow. Our study is based on several plane sections through the pressure field, with short signals (0.1 s duration at 80 kHz sampling rate). Using wavelet-based band-pass filtering and cross-correlations, we map the directions of propagation of information among the various ``bones'' in the skeleton. In particular, we identify upstream propagation in some frequency bands, 3-dimensional interactions between the various shear layers, and several key bones from which the pressure signals, when taken as reference, provide dramatic phase-locking for parts of the skeleton. [Preview Abstract] |
Monday, November 23, 2015 8:26AM - 8:39AM |
G5.00003: Jet-mixing of initially stratified flows Stuart Wright, Christos Markides, Omar Matar Low pipeline velocities in the oil-and-gas industry generally lead to liquid-liquid flows stratifying due to density differences. Pipeline stratified flows inherently have no single point for sub-sampling and phase slip leads to in situ phase fractions differing from input volume fractions. Establishing representative or average properties and phase fractions is therefore difficult for industry. This leads to sampling errors through measurement uncertainty. In-line mixing overcomes liquid-liquid stratification, establishing a liquid-liquid dispersion that minimises slip between phases. Here, we use jets-in-crossflow (JICF) as a means of mixing. We present results of CFD simulations using the volume-of- fluid method that demonstrate the breakup of stratification as a result of the application of JICF. A number of simple jet configurations are described, and their effectiveness in generating dispersions is compared. We also present preliminary experimental results based on the use of a matched-refractive-index method, laser-induced fluorescence, particle-tracking- and particle-image-velocimetry. [Preview Abstract] |
Monday, November 23, 2015 8:39AM - 8:52AM |
G5.00004: CFD simulation of boundary effects on closely spaced jets Ishita Shrivastava, Eric Adams In coastal areas characterized by shallow water depth, industrial effluents are often diluted using multiple closely spaced jets. Examples of such effluents include brine from desalination plants, treated wastewater from sewage treatment plants and heated water from thermal power plants. These jets are arranged in various orientations, such as unidirectional diffusers and rosette groups, to maximize mixing with ambient water. Due to effects of dynamic pressure, the jets interact with each other leading to mixing characteristics which are quite different from those of individual jets. The effect of mutual interaction is exaggerated under confinement, when a large number of closely spaced jets discharge into shallow depth. Dilution through an outfall, consisting of multiple jets, depends on various outfall and ambient parameters. Here we observe the effects of shoreline proximity, in relation to diffuser length and water depth, on the performance of unidirectional diffusers discharging to quiescent water. For diffusers located closer to shore, less dilution is observed due to the limited availability of ambient water for dilution. We report on the results of Computational Fluid Dynamics (CFD) simulations and compare the results with experimental observations. [Preview Abstract] |
Monday, November 23, 2015 8:52AM - 9:05AM |
G5.00005: Entrainment in a Free Surface Plunging Jet Syed Harris Hassan, Pavlos P. Vlachos Ambient fluid entrainment and near-field flow characteristics of a free surface plunging jet are investigated using time resolved particle image velocimetry. The plunging height is about twice the nozzle diameter and we study five different Reynolds numbers ranging from Re$_{\mathrm{FS}}$ 5000 to Re$_{\mathrm{FS}}$ 13,070. We found that the near-field entrainment in the Re$_{\mathrm{FS\thinspace }}$5000 and Re$_{\mathrm{FS\thinspace }}$6086 jets is enhanced significantly than the latter cases due to mixing transition that occurs at about 2-3D$_{\mathrm{N}}$ below the free surface resulting in the breakdown of laminar vortices into smaller secondary structures. With an increase in the Reynolds number, mixing transition declines in the latter three cases as they have a more homogenous and turbulent flow structure. In addition, plunging jets show a reduction in the penetration depth of about 20-30{\%} and the length of Zone of Flow Establishment (ZFE) of about 40-60{\%} when compared to free jets at the same Reynolds numbers. Finally, we look into the distribution of coherent structures in the near-field over time in order to find the depth of mixing transition. The depth where primary structures breakdown into smaller secondary structures decreases as the Reynolds number is increased and is consistent with the reduction in the length of ZFE. [Preview Abstract] |
Monday, November 23, 2015 9:05AM - 9:18AM |
G5.00006: The development of a turbulent jet issuing from an annular source with an open centre Shahid Padhani, Tim Jukes, Gary R. Hunt Using particle image velocimetry (PIV) and flow visualisation we investigate the dynamics of a turbulent jet that issues from a thin slot annular source with an open centre - by `thin' we refer to a slot width that is at least an order of magnitude smaller than the diameter of the annulus. For this geometry, entrainment into the near-field region of the jet results in an induced flow in the environment that passes through the open annulus. We explore the streamwise development of the jet towards an approximately self-similar form and identify the key regions of the flow. For the far-field, we show that the flow approximates to a round jet from a circular source and we use our results to estimate the location of the origin for the virtual point source of a round jet that matches the far-field flow produced by an annular source. Finally, we consider some practical implications of our results for inducing air movement in rooms. [Preview Abstract] |
Monday, November 23, 2015 9:18AM - 9:31AM |
G5.00007: Large Eddy Simulations and an Analysis of the Flow Field of a Radially Lobed Nozzle Noushin Amini, Aarthi Sekaran Lobed nozzles have been a subject of regained interest over the past couple of decades owing to their established mixing capabilities. Despite experimental (Hu et al, 1999 and Hu et al, 2008) and limited numerical studies (Boulenouar et al 2011 and Cooper et al, 2005), the exact nature of the jet ensuing from this nozzle is yet to be completely understood. The present numerical study is intended to complement prior experimental investigation, involving the analysis of the flow field downstream of a six lobed nozzle (Amini et al, 2012). Preliminary results (presented at DFD 2014, Amin and Sekaran), which involved three dimensional simulations of the full domain via URANS and Large Eddy Simulations (LES) were used to assess the domain extents and simulation technique. Based on these results it was seen that LES were able to capture the region of interest satisfactorily and a qualitative corroboration with previous studies was obtained. The study is thus extended to analyzing the flow originating from within the nozzle, following it downstream in order to confirm the vortical interaction mechanisms inside the lobed nozzle. [Preview Abstract] |
Monday, November 23, 2015 9:31AM - 9:44AM |
G5.00008: Modal and non-modal evolution of perturbations for parallel round jets Jose Ignacio Jimenez-Gonzalez, Pierre Brancher, Carlos Martinez-Bazan We investigate the local modal and non-modal stability of round jets for varying aspect ratios and Reynolds numbers. The competition between axisymmetric (azimuthal wavenumber m $=$ 0) and helical (m $=$ 1) perturbations depending on the jet aspect ratio, $\alpha =$R/$\theta $, where R is the jet radius and $\theta $ the shear layer momentum thickness, is quantified at different time horizons. Optimal excitation and optimal perturbation analyses allow us to characterize the transient dynamics of jets, showing that two mechanisms may cause large energy gains, namely the Orr mechanism at small wavelengths and the ``shift-up'' mechanism, in the long wavelength limit, which is found to shift the jet as a whole in a way that resembles the classical lift-up effect active in wall shear flows. The ``shift-up'' mechanism is found to be especially efficient for vanishing perturbations axial wavenumbers. Furthermore, it has been found that adjoint modes drive the transient process at relatively short temporal horizons, in such a way that, for large aspect ratios, an optimal excitation analysis might suffice to properly characterize transient dynamics. [Preview Abstract] |
Monday, November 23, 2015 9:44AM - 9:57AM |
G5.00009: Numerical simulation of air-blast atomization of a liquid layer G. Gilou Agbaglah, Jeremy McCaslin, Olivier Desjardins Numerical simulations of a planar co-flowing air/water airblast atomization is performed using an in-house multiphase Navier-Stokes solver based on a semi-lagrangian geometric Volume of Fluid (VOF) method to track the position of the interface. This solver conserves mass and momentum exactly within each phase. Excellent agreement with recent experiments is obtained when comparing physical quantities, such as the liquid cone length, the maximum wave frequency and the spatial growth rate of the primary instability. A full three dimensional simulation is used to analyze the turbulence in the gas phase. The gas layer is laminar close to the injector and becomes turbulent at downstream positions. The transition to the turbulence is shown to increase first as an exponential function of the downstream positions and then reach a statistically stable regime where the liquid wave crests expand in a thin sheet which breaks into secondary droplets. [Preview Abstract] |
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