Bulletin of the American Physical Society
65th Annual Meeting of the APS Division of Fluid Dynamics
Volume 57, Number 17
Sunday–Tuesday, November 18–20, 2012; San Diego, California
Session L10: Instability: Jets, Wakes and Shear Layers VI: Jets |
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Chair: Ann Karagozian, University of California, Los Angeles Room: 25C |
Monday, November 19, 2012 3:35PM - 3:48PM |
L10.00001: Experimental Study of Axial Forcing on a Swirling Jet Amy McCleney, Philippe Bardet An experimental swirling jet is created by independently controlling the axial and angular momentum injection with the resulting water jet discharging freely into a large tank. The jet is excited to enhance mixing in low to high swirl number regimes. Axial forcing on the jet is imposed for Strouhal number ranging from 0 to 15, Reynolds number from 1,000 to 10,000, and Swirl number from 0 to 1.3, where limited experimental data exists. The forcing amplitude is changed from 5 to 20 percent of the axial flow rate, while the azimuthal momentum injection stays constant; the resulting forcing creates a jet with varying swirl number. Swirling jets enhance the growth and mixing of fluids compared to non-swirling jets; this can lead to shorter combustion chambers and increased combustion efficiency. This mixing can be enhanced further by forcing natural instabilities in the jet. These imposed disturbances are either axial, which generates vortex rings, or angular, which create more complex structures. Past research involving forcing with swirling jets resulted in limited findings due to the concentration of forcing in either axial or angular directions. The flow structures of forced and steady jets are observed using PLIF through azimuthal dye injection. [Preview Abstract] |
Monday, November 19, 2012 3:48PM - 4:01PM |
L10.00002: Unsteady Surface-Pressure Measurements and Time-Resolved Flow Visualization of a Normally and Obliquely Impinging Jet Malek Al-Aweni, Ahmed Naguib In comparison to heat transfer and flow field data, there is little information regarding the space-time characteristics of the surface pressure fluctuations in impinging jets. Moreover, when available, such measurements are rarely accompanied by flow field information, making it difficult to identify the mechanisms/flow structures leading to pressure generation. The current work employs an axisymmetric air jet impinging on a flat wall to identify the flow features associated with significant wall-pressure signatures. To this end, unsteady surface-pressure measurements, using an embedded electret microphone array in the impingement plate, and time-resolved smoke-wire flow visualization are carried out for normal and oblique incidence of the jet. The results show the presence of two dominant modes of pressure oscillation, one is observed throughout the measurement domain and the other found primarily within the wall-jet zone. Links are drawn between these modes and the flow structures, yielding information useful for constructing structure-based models of the surface pressure in impinging jets. [Preview Abstract] |
Monday, November 19, 2012 4:01PM - 4:14PM |
L10.00003: Convective and Absolute Instability of Liquid Jets under Gravity Effects Ghobad Amini, Matthias Ihme, Ali Dolatabadi The break-up of liquid jets is of practical importance for several applications, including liquid-fuel-injection and ink-jet printing. In this work, the effect of gravity on the onset and growth rate of absolute and convective instabilities in liquid jets is studied. The mathematical problem is formulated in terms of quasi-one-dimensional equations, and the linearized stability equations are solved using a first-order perturbation method. An analytic form of the dispersion equation is derived, and the variation of the growth rate is investigated for a range of positive and negative Bond numbers, corresponding to downward-pointing and rising liquid jet. The critical Weber number, demarcating the transition between convective and absolute instability is determined as function of Reynolds and Froude numbers. Model-results for the limiting case of zero gravity are compared with classical results of Chandrasekhar and Leib \& Goldstein, confirming the validity of this approach. [Preview Abstract] |
Monday, November 19, 2012 4:14PM - 4:27PM |
L10.00004: ABSTRACT WITHDRAWN |
Monday, November 19, 2012 4:27PM - 4:40PM |
L10.00005: Preferred modes in jets: comparison between different measures of the receptivity Xavier Garnaud, Lutz Lesshafft, Peter J. Schmid, Patrick Huerre The response of jets to frequency forcing is usually measured experimentally in terms of the maximum amplitude of velocity fluctuations reached along the axis (Crow \& Champagne (1971)). In the present work, the preferred mode of isothermal jets is discussed in terms of the linear flow response to time-harmonic forcing (Trefethen \emph{et al.} (1993)). The optimal frequency response is computed for different choices of the objective functional: the usual energy ($L^2$) norm and the maximum amplitude over the entire domain ($L^\infty$ norm). The relevance and limitations of the different objective functionals are critically analyzed. Although the dominant flow structures are robustly identified in all cases, the measure of the flow response in terms of the maximum amplitude does not suffer from the continually slow axial growth of low frequency perturbations. [Preview Abstract] |
Monday, November 19, 2012 4:40PM - 4:53PM |
L10.00006: Structural Variation in Convectively and Absolutely Unstable Jets in Crossflow Daniel Getsinger, Levon Gevorkyan, Owen Smith, Ann Karagozian This experimental study explores the behavior of both unforced and acoustically forced variable density transverse jets, via acetone PLIF and stereo PIV measurements. Jets composed of mixtures of helium and nitrogen are injected normally from a converging nozzle into an air crossflow, for a range of jet-to-crossflow momentum flux ratios $J$ and density ratios $S$. A recent study\footnote{Getsinger, et al., {\bf Expts in Fluids}, 2012} determined, based on hotwire-based spectral characteristics and excitation response, that transverse jet shear layers transition to global instability in response to sufficient lowering of $S$ (below 0.45-0.40) and/or sufficient lowering of $J$ (below 10). The changes in flow structure during such transitions are documented in the present study, where alterations in the transverse jet's vorticity field, cross-sectional symmetry or asymmetry, and dynamic flow features are affected. Both absolutely unstable transverse jets and forced transverse jets are observed to have more symmetric cross-sections than those for convectively unstable jets at higher $J$ values, the latter of which also can exhibit the presence of tertiary vortex structures first identified by Kuzo.\footnote{Kuzo, D., Ph.D. thesis, Caltech, 1995} [Preview Abstract] |
Monday, November 19, 2012 4:53PM - 5:06PM |
L10.00007: Mixing Characteristics of Convectively and Absolutely Unstable Jets in Crossflow Levon Gevorkyan, Daniel Getsinger, Owen Smith, Ann Karagozian This experimental study explores the mixing characteristics of both unforced and acoustically forced variable density transverse jets via acetone PLIF measurements. A range of jet-to-crossflow momentum flux ratios $J$ and density ratios $S$ are explored in this study, spanning conditions for which the jet shear layer transitions from being convectively to absolutely unstable. While there are clear differences in the flow structure among convectively unstable, absolutely unstable, and externally forced jets in crossflow, it is of interest in the present study to explore the implications of such differences for jet mixing. A range of metrics is used to quantify jet mixing, including jet centerline concentration decay and spread,\footnote{Smith, S. H. and Mungal, M. G., {\bf JFM}, 357, 83-122, 1998} and the spatial evolution of jet cross-sectional Unmixedness, Spatial Mixing Deficiency, and Scale of Segregation.\footnote{D. Bothe in {\bf Micro and Macro Mixing}, 17-35, Springer 2010} It is found that the cross-sectional jet fluid distribution can be affected significantly by unforced shear layer stability characteristics as well as the nature of jet forcing, and hence can affect mixing in ways that are not evident from centerplane mixing metrics alone. [Preview Abstract] |
Monday, November 19, 2012 5:06PM - 5:19PM |
L10.00008: Effect of dilatant additive on stability of waterjet Nariman Ashrafi Effect of addition of dilatant cornstarch on the stability and precision enhancement of the abrasive waterjet is studied. It is shown that the normal stresses developed in the nonlinear viscoelastic additive remains substantially unchanged throughout effective length of the jet resulting in an almost completely prismatic jet, applicable for precision and straight machining. Furthermore, the jet becomes more stable upon increasing the cornstarch percentage. Clearly, there is restriction of the pump delivery upon adding the dilatant cornstarch. Different percentages of the additive are therefore examined. It is found that, a {\%}22 additive results in the best performance based on the precision, required pump power and stability of the jet. Simulation of the problem is in good agreement with the experimental observations. The additive also appears to produce less friction with the surrounding air avoiding possible jet disintegration [Preview Abstract] |
Monday, November 19, 2012 5:19PM - 5:32PM |
L10.00009: Vortex breakdown in a coaxial swirling jet with a density difference: an experimental study Ahmad Adzlan Fadzli Bin Khairi, Hiroshi Gotoda The vortex breakdown (VB) in a coaxial swirling jet with a density difference has been experimentally investigated in this work, focusing on how the Reynolds number of the outer jet affects the presence and height of the stagnation point in the swirling jet undergoing VB. When the Reynolds number of the outer jet is increased, the degree of flow divergence of both air and CO$_{2}$ jets decrease, with the degree of flow divergence of the air jet decreasing more compared to the CO$_{2}$ jet at the same Reynolds number of the outer jet. The stagnation point for the air jet was also found to disappear more easily compared to the CO$_{2}$ jet, and its height of the stagnation point increases more compared to that of the CO$_{2}$ jet. Investigation of the velocity field revealed that increasing the Reynolds number of the outer jet during VB induces a decrease in the gradient of the axial velocity distribution along the perpendicular line connecting the stagnation point and the $r$ = 0 plane, causing the height of the stagnation point to increase. When the distribution no longer tends to zero, the stagnation point disappears. A physical model is considered in this presentation to explain these observations by estimating the momentum balance in the flow, based on a simplified Navier-Stokes equation (A. Adzlan, H. Gotoda, Chemical Engineering Science, 2012) -- the first of its kind reported in the field of fluid dynamics. [Preview Abstract] |
Monday, November 19, 2012 5:32PM - 5:45PM |
L10.00010: DNS of the transition from absolute to convective instability for transverse jets Erin Mussoni, Prahladh Iyer, Krishnan Mahesh Direct numerical simulations are performed to study the transition from absolute to convective instability for incompressible jets in crossflow using an unstructured finite volume solver. Flow conditions are based on experimental conditions of Megerian {\it et al.} (2007) for a flush injected jet. Results obtained from simulated jet velocity ratios of 2 and 4 are compared with experimental data (Getsinger {\it et al.} 2011 and Megerian {\it et al.} 2007) and show good agreement in instantaneous and time averaged flow characteristics as well as velocity spectra. Behavior of fluid flow in the near field, such as the recirculation region inside the nozzle for low jet velocity ratios, is discussed to examine the stability transition with increasing jet velocity ratio. Global stability analysis is used to further explore the absolute to convective instability transition. [Preview Abstract] |
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