Now showing 1 - 10 of 30
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    Optimum air turbulence intensity for polydisperse droplet size growth
    (31-07-2019)
    Kumar, M. Shyam
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    The growth of the average size of liquid droplets suspended in a turbulent air flow is of paramount importance in several natural and engineering systems. Here we present an experimental study of the effects of air flow turbulent intensity on the size growth of water droplets in a polydisperse droplet field. For a given initial distribution of droplets in the size range of 0-120 μm diameter, we identify an optimum air turbulent intensity that maximizes the rate at which the average droplet diameter increases with time. The observed trend is understood in terms of droplet collision rate statistics, droplet clustering, and the existence of a crossover diameter, below and above which the number of droplets decreases and increases in time, respectively. We show that the onset of clustering suppresses the intuitive effect of an increase in droplet collision rate with air turbulent intensity, resulting in the existence of an optimum air turbulent intensity that maximizes the average droplet size growth rate due to droplet coalescence. Our results bear consequences for the understanding of warm rain initiation from clouds and the design of engines with improved combustion characteristics.
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    Bay of Bengal intraseasonal oscillations and the 2018 monsoon onset
    (01-10-2021)
    Shroyer, Emily
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    Tandon, Amit
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    Sengupta, Debasis
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    Fernando, Harindra J.S.
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    Lucas, Andrew J.
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    Farrar, J. Thomas
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    Chattopadhyay, Rajib
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    de Szoeke, Simon
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    Flatau, Maria
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    Rydbeck, Adam
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    Wijesekera, Hemantha
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    McPhaden, Michael
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    Seo, Hyodae
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    Subramanian, Aneesh
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    Venkatesan, R.
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    Joseph, Jossia
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    Ramsundaram, S.
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    Gordon, Arnold L.
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    Bohman, Shannon M.
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    Pérez, Jaynise
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    Simoes-Sousa, Iury T.
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    Jayne, Steven R.
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    Todd, Robert E.
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    Bhat, G. S.
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    Lankhorst, Matthias
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    Schlosser, Tamara
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    Adams, Katherine
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    Jinadasa, S. U.P.
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    Mohapatra, M.
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    Rao, E. Pattabhi Rama
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    Sahai, A. K.
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    Sharma, Rashmi
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    Lee, Craig
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    Rainville, Luc
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    Cherian, Deepak
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    Cullen, Kerstin
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    Centurioni, Luca R.
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    Hormann, Verena
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    MacKinnon, Jennifer
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    Send, Uwe
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    Anutaliya, Arachaporn
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    Waterhouse, Amy
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    Black, Garrett S.
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    Dehart, Jeremy A.
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    Woods, Kaitlyn M.
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    Creegan, Edward
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    Levy, Gad
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    Kantha, Lakshmi H.
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    Subrahmanyam, Bulusu
    In the Bay of Bengal, the warm, dry boreal spring concludes with the onset of the summer monsoon and accompanying southwesterly winds, heavy rains, and variable air-sea fluxes. Here, we summarize the 2018 monsoon onset using observations collected through the multinational Monsoon Intraseasonal Oscillations in the Bay of Bengal (MISO-BoB) program between the United States, India, and Sri Lanka. MISO-BoB aims to improve understanding of monsoon intraseasonal variability, and the 2018 field effort captured the coupled air-sea response during a transition from active-to-break conditions in the central BoB. The active phase of the ~20-day research cruise was characterized by warm sea surface temperature (SST > 30°C), cold atmospheric outflows with intermittent heavy rainfall, and increasing winds (from 2 to 15 m s−1). Accumulated rainfall exceeded 200 mm with 90% of precipitation occurring during the first week. The following break period was both dry and clear, with persistent 10-12 m s−1 wind and evaporation of 0.2 mm h−1. The evolving environmental state included a deepening ocean mixed layer (from ~20 to 50 m), cooling SST (by ~1°C), and warming/drying of the lower to midtroposphere. Local atmospheric development was consistent with phasing of the large-scale intraseasonal oscillation. The upper ocean stores significant heat in the BoB, enough to maintain SST above 29°C despite cooling by surface fluxes and ocean mixing. Comparison with reanalysis indicates biases in air-sea fluxes, which may be related to overly cool prescribed SST. Resolution of such biases offers a path toward improved forecasting of transition periods in the monsoon.
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    An analytical criterion for centrifugal instability in non-axisymmetric vortices
    (01-01-2015)
    Nagarathinam, David
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    Non-axisymmetric vortices are ubiquitous in nature; examples include polar vortices in planets, the giant red spot in Jupiter, tornadoes and cyclones on Earth, mesoscale eddies in the ocean. Turbulent flows are furthermore known to be dominated by small- and large-scale vortex structures. Owing to the wide range of applications, knowledge of conditions under which a given vortex becomes unstable is beneficial. Here, the centrifugal instability of two-dimensional, non-axisymmetric vortices in the presence of an axial flow (w) and a background rotation (Ωz) is studied using the local stability approach. The local stability approach, based on geometric optics and similar in formulation to the rapid distortion theory [2], considers the evolution of shortwavelength perturbations along streamlines in the base flow. This approach, developed by Lifschitz & Hameiri [4], is particularly useful for base flows for which a global stability analysis is computationally expensive. A sufficient criterion for centrifugal instability in an axisymmetric vortex with (w) and (Ωz) is first derived by analytically solving the local stability equations for wave vectors that are periodic upon evolution around a closed streamline. This criterion is then heuristically extended to non-axisymmetric vortices and written in terms of integral quantities on a streamline. The criterion is then shown to be accurate in describing centrifugal instability over a reasonably large range of parameters that specify Stuart vortices and Taylor-Green vortices.
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    Topographic scattering of the low-mode internal tide in the deep ocean
    (01-01-2014) ;
    Carter, Glenn S.
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    Peacock, Thomas
    We investigate the role of deep-ocean topography in scattering energy from the large spatial scales of the low-mode internal tide to the smaller spatial scales of higher modes. The complete Green function method, which is not subject to the restrictions of the WKB approximation, is used for the first time to study the two-dimensional scattering of a mode-1 internal tide incident on subcritical and supercritical topography of any form in arbitrary stratifications. For an isolated Gaussian ridge in a uniform stratification, large amplitude critical topography is the most efficient at mode-1 scattering and small amplitude topography scatters with an efficiency on the order of 5-10%. In a nonuniform stratification with a pycnocline, the results are qualitatively the same as for a constant stratification, albeit with the key features shifted to larger height ratios. Having validated these results by direct comparison with the results of nonlinear numerical simulations, and in the process demonstrated that WKB results are not appropriate for reasonable ocean predictions, we proceed to use the Green function approach to quantify the role of topographic scattering for the region of the Pacific Ocean surrounding the Hawaiian Islands chain. To the south, the Line Islands ridge is found to scatter ∼40% of a mode-1 internal tide coming from the Hawaiian Ridge. To the north, realistic, small-amplitude, rough topography scatters ∼5-10% of the energy out of mode 1 for transects of length 1000-3000 km. A significant finding is that compared to large extents of small-amplitude, rough topography a single large topographic feature along the path of a mode-1 internal tide plays the dominant role in scattering the internal tide. Key Points Quantitative estimates of internal tide scattering by deep-ocean topography The analytical Green function method is validated by numerical simulations Tall topographic features play a dominant role in internal tide scattering © 2014. American Geophysical Union. All Rights Reserved.
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    Lagrangian coherent structures during combustion instability in a premixed-flame backward-step combustor
    (14-12-2016)
    Sampath, Ramgopal
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    This paper quantitatively examines the occurrence of large-scale coherent structures in the flow field during combustion instability in comparison with the flow-combustion-acoustic system when it is stable. For this purpose, the features in the recirculation zone of the confined flow past a backward-facing step are studied in terms of Lagrangian coherent structures. The experiments are conducted at a Reynolds number of 18600 and an equivalence ratio of 0.9 of the premixed fuel-air mixture for two combustor lengths, the long duct corresponding to instability and the short one to the stable case. Simultaneous measurements of the velocity field using time-resolved particle image velocimetry and the CH∗ chemiluminescence of the flame along with pressure time traces are obtained. The extracted ridges of the finite-time Lyapunov exponent (FTLE) fields delineate dynamically distinct regions of the flow field. The presence of large-scale vortical structures and their modulation over different time instants are well captured by the FTLE ridges for the long combustor where high-amplitude acoustic oscillations are self-excited. In contrast, small-scale vortices signifying Kelvin-Helmholtz instability are observed in the short duct case. Saddle-type flow features are found to separate the distinct flow structures for both combustor lengths. The FTLE ridges are found to align with the flame boundaries in the upstream regions, whereas farther downstream, the alignment is weaker due to dilatation of the flow by the flame's heat release. Specifically, the FTLE ridges encompass the flame curl-up for both the combustor lengths, and thus act as the surrogate flame boundaries. The flame is found to propagate upstream from an earlier vortex roll-up to a newer one along the backward-time FTLE ridge connecting the two structures.
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    Three-dimensional small-scale instabilities of plane internal gravity waves
    (25-03-2019)
    Ghaemsaidi, Sasan John
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    We study the evolution of three-dimensional (3-D), small-scale, small-amplitude perturbations on a plane internal gravity wave using the local stability approach. The plane internal wave is characterised by its non-dimensional amplitude, , and the angle the group velocity vector makes with gravity, . For a given , the local stability equations are solved on the periodic fluid particle trajectories to obtain growth rates for all two-dimensional (2-D) and 3-D perturbation wave vectors. For small , the local stability approach recovers previous results of 2-D parametric subharmonic instability (PSI) while offering new insights into 3-D PSI. Higher-order triadic resonances, and associated deviations from them, are also observed at small . Moreover, for small , purely transverse instabilities resulting from parametric resonance are shown to occur at select values of . The possibility of a non-resonant instability mechanism for transverse perturbations at finite allows us to derive a heuristic, modified gravitational instability criterion. We then study the extension of small to finite internal wave instabilities, where we recover and build upon existing knowledge of small-scale, small-amplitude internal wave instabilities. Four distinct regions of the -plane based on the dominant instability modes are identified: 2-D PSI, 3-D oblique, quasi-2-D shear-aligned, and 3-D transverse. Our study demonstrates the local stability approach as a physically insightful and computationally efficient tool, with potentially broad utility for studies that are based on other theoretical approaches and numerical simulations of small-scale instabilities of internal waves in various settings.
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    Estimation of myocardial deformation using correlation image velocimetry
    (05-04-2017)
    Jacob, Athira
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    Background: Tagged Magnetic Resonance (tMR) imaging is a powerful technique for determining cardiovascular abnormalities. One of the reasons for tMR not being used in routine clinical practice is the lack of easy-to-use tools for image analysis and strain mapping. In this paper, we introduce a novel interdisciplinary method based on correlation image velocimetry (CIV) to estimate cardiac deformation and strain maps from tMR images. Methods: CIV, a cross-correlation based pattern matching algorithm, analyses a pair of images to obtain the displacement field at sub-pixel accuracy with any desired spatial resolution. This first time application of CIV to tMR image analysis is implemented using an existing open source Matlab-based software called UVMAT. The method, which requires two main input parameters namely correlation box size (C B ) and search box size (S B ), is first validated using a synthetic grid image with grid sizes representative of typical tMR images. Phantom and patient images obtained from a Medical Imaging grand challenge dataset ( http://stacom.cardiacatlas.org/motion-tracking-challenge/ ) were then analysed to obtain cardiac displacement fields and strain maps. The results were then compared with estimates from Harmonic Phase analysis (HARP) technique. Results: For a known displacement field imposed on both the synthetic grid image and the phantom image, CIV is accurate for 3-pixel and larger displacements on a 512 × 512 image with (C B ,S B )=(25,55) pixels. Further validation of our method is achieved by showing that our estimated landmark positions on patient images fall within the inter-observer variability in the ground truth. The effectiveness of our approach to analyse patient images is then established by calculating dense displacement fields throughout a cardiac cycle, and were found to be physiologically consistent. Circumferential strains were estimated at the apical, mid and basal slices of the heart, and were shown to compare favorably with those of HARP over the entire cardiac cycle, except in a few (-4) of the segments in the 17-segment AHA model. The radial strains, however, are underestimated by our method in most segments when compared with HARP. Conclusions: In summary, we have demonstrated the capability of CIV to accurately and efficiently quantify cardiac deformation from tMR images. Furthermore, physiologically consistent displacement fields and circumferential strain curves in most regions of the heart indicate that our approach, upon automating some pre-processing steps and testing in clinical trials, can potentially be implemented in a clinical setting.
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    Effects of Schmidt number on the short-wavelength instabilities in stratified vortices
    (25-05-2019)
    Singh, Suraj
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    We present a local stability analysis to investigate the effects of differential diffusion between momentum and density (quantified by the Schmidt number ) on the three-dimensional, short-wavelength instabilities in planar vortices with a uniform stable stratification along the vorticity axis. Assuming small diffusion in both momentum and density, but arbitrary values for , we present a detailed analytical/numerical analysis for three different classes of base flows: (i) an axisymmetric vortex, (ii) an elliptical vortex and (iii) the flow in the neighbourhood of a hyperbolic stagnation point. While a centrifugally stable axisymmetric vortex remains stable for any , it is shown that can have significant effects in a centrifugally unstable axisymmetric vortex: the range of unstable perturbations increases when is taken away from unity, with the extent of increase being larger for than for . Additionally, for 1$]]>, we report the possibility of oscillatory instability. In an elliptical vortex with a stable stratification, is shown to non-trivially influence the three different inviscid instabilities (subharmonic, fundamental and superharmonic) that have been previously reported, and also introduce a new branch of oscillatory instability that is not present at . The unstable parameter space for the subharmonic (instability IA) and fundamental (instability IB) inviscid instabilities are shown to be significantly increased for , respectively. Importantly, for sufficiently small and large and 1$]], respectively, the maximum growth rate for instabilities IA and IB occurs away from the inviscid limit. The new oscillatory instability (instability III) is shown to occur only for sufficiently small
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    Centrifugal instability in non-axisymmetric vortices
    (01-01-2015)
    Nagarathinam, David
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    We study the centrifugal instability of non-axisymmetric vortices in the presence of an axial flow (w) and a background rotation (Ωz) using the local stability approach. Analytically solving the local stability equations for an axisymmetric vortex with w and Ωz, growth rates for wave vectors that are periodic upon evolution around a closed streamline are calculated. The resulting sufficient criterion for centrifugal instability in an axisymmetric vortex is then heuristically extended to non-axisymmetric vortices and written in terms of integral quantities and their derivatives with respect to the streamfunction on a streamline. The new criterion for non-axisymmetric vortices, which converges to the exact criterion of Bayly (Phys. Fluids, vol. 31, 1988, pp. 56-64) in the absence of background rotation and axial flow, is validated by comparisons with numerically calculated growth rates for two different anticyclonic vortices: the Stuart vortex (specified by the concentration parameter ρ, 0 < ρ ≤ 1) and the Taylor-Green vortex (specified by the aspect ratio E, 0 < E ≤ 1). With no axial velocity and finite background rotation, the criterion predicts a lower and an upper threshold of |Ωz| between which centrifugal instability is present. We further demonstrate that the criterion represents an improvement over the criterion of Sipp & Jacquin (Phys. Fluids, vol. 12, 2000, pp. 1740-1748). Finally, in the presence of both axial velocity and background rotation, the criterion is shown to be accurate for large enough ρ and E.
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    Local stability analysis of homogeneous and stratified Kelvin-Helmholtz vortices
    (25-07-2022)
    Aravind, H. M.
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    Dubos, Thomas
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    We perform a three-dimensional short-wavelength linear stability analysis of numerically simulated two-dimensional Kelvin-Helmholtz vortices in homogeneous and stratified environments at a fixed Reynolds number of. For the homogeneous case, the elliptic instability at the vortex core dominates at early times, before being taken over by the hyperbolic instability at the vortex edge. For the stratified case of Richardson number, the early-time instabilities comprise a dominant elliptic instability at the core and a hyperbolic instability influenced strongly by stratification at the vortex edge. At intermediate times, the local approach shows a new branch of (convective) instability that emerges at the vortex core and subsequently moves towards the vortex edge. A few more convective instability bands appear at the vortex core and move away, before coalescing to form the most unstable region inside the vortex periphery at large times. In addition, the stagnation point instability is also recovered outside the periphery of the vortex at intermediate times. The dominant instability characteristics from the local approach are shown to be in good qualitative agreement with the results based on global instability studies for both homogeneous and stratified cases. A systematic study of the dependence of the dominant instability characteristics on is then presented. While is identified as most unstable (with convective instability being dominant), another growth rate maximum at is not far behind (with the hyperbolic instability influenced by stratification being dominant). Finally, the local stability approach is shown to predict the potential orientation of the flow structures that would result from hyperbolic and convective instabilities, which is found to be consistent with three-dimensional numerical simulations reported previously.