Browsing by Author "Ranga Dinesh, K. K. J."

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    Effects of swirl on intermittency characteristics in non-premixed flames
    (Taylor & Francis, 2012-05-31T00:00:00Z) Ranga Dinesh, K. K. J.; Jenkins, Karl W.; Kirkpatrick, M. P.; Malalasekera, W.
    Swirl effects on velocity, mixture fraction, and temperature intermittency have been analyzed for turbulent methane flames using large eddy simulation (LES). The LES solves the filtered governing equations on a structured Cartesian grid using a finite volume method, with turbulence and combustion modeling based on the localized dynamic Smagorinsky and the steady laminar flamelet models, respectively. Probability density function (PDF) distributions demonstrate a Gaussian shape closer to the centerline region of the flame and a delta function at the far radial position. However, non-Gaussian PDFs are observed for velocity and mixture fraction on the centerline in a region where center jet precession occurs. Non-Gaussian behavior is also observed for the temperature PDFs close to the centerline region of the flame. Due to the occurrence of recirculation zones, the variation from turbulent to nonturbulent flow is more rapid for the velocity than the mixture fraction and therefore indicates how rapidly turbulence affects the molecular transport in these regions of the flame
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    External Intermittency Simulation in Turbulent Round Jets
    (Springer Science Business Media, 2012-09-30T00:00:00Z) Gilliland, T.; Ranga Dinesh, K. K. J.; Fairweather, M.; Falle, S. A. E. G.; Jenkins, Karl W.; Savill, Mark A.
    Abstract to study passive scalar mixing and intermittency in turbulent round jets. Both simulation techniques are applied to the case of a low Reynolds number jet with Re between time-averaged results for the scalar field of the low Re case demonstrate reasonable agreement between the DNS and LES, and with experimental data and the predictions of other authors. Scalar probability density functions (pdfs) for this jet derived from the simulations are also in reasonable accord, although the DNS results demonstrate the more rapid influence of scalar intermittency with radial distance in the jet. This is reflected in derived intermittency profiles, with LES generally giving profiles that are too broad compared to equivalent DNS results, with too low a rate of decay with radial distance. In contrast, good agreement is in general found between LES predictions and experimental data for the mixing field, scalar pdfs and external intermittency in the high Reynolds number jet. Overall, the work described indicates that improved sub-grid scale modelling for use with LES may be beneficialDirect numerical and large eddy simulation (DNS and LES) are applied= 2,400, whilst LES is also used to predict a high Re = 68,000 flow. Comparison
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    Influence of bluff-body and swirl on mixing and intermittency of jets
    (2010-09-30T00:00:00Z) Ranga Dinesh, K. K. J.; Jenkins, Karl W.; Savill, Mark; Kirkpatrick, M. P.
    In this paper we present the modelled results of turbulence, scalar mixing and intermittency for three different basic fluid dynamical problems using large eddy simulation (LES). The modelled problems are a turbulent round jet, a bluff body stabilised jet, and a bluff body stabilised swirl jet in a co-flow environment. Both instantaneous and time averaged results along with the probability density functions (pdf) and intermittency of velocity and passive scalar are presented. Simulations well captured the flow features of jet, bluff body stabilised jet and bluff body stabilised swirl jet. The instantaneous and time averaged data show the differences in turbulence and mixing and also an improvement of mixing in the presence of a bluff body and swirl. The addition of bluff body and swirl affect the structure of pdfs for both velocity and passive scalar at different axial and radial locations. The radial variation of intermittency at locations close to the centreline indicates turbulent to non- turbulent phenomena respect to bluff body and swirl at both upstream and downstream recirculation regions.
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    Investigation of the influence of swirl on a confined coannular swirl jet
    (Elsevier Science B.V., Amsterdam., 2010-05-31T00:00:00Z) Ranga Dinesh, K. K. J.; Kirkpatrick, M. P.; Jenkins, Karl W.
    Large Eddy Simulations are used to model a turbulent confined coannular combustor and examine the effects of swirl on the flow field and mixing. Three separate simulations with relatively high mesh resolutions and different swirl numbers have been carried out using a finite volume method on a Cartesian non-uniform structured grid. A localised dynamic Smagorinsky model is used to parameterize the subgrid scale turbulence. The snapshots of the axial and swirl velocities and velocity vector fields show the complex flow patterns developing with increased swirl number and the rapid decay of axial momentum. Precessing vortex cores (PVC) were identified for all three cases and the mean axial velocity plots indicate that the upstream extremity of the vortex breakdown bubble shifts towards the inlet as the swirl number increases. The calculated power spectra indicate the distinct precession frequency for high swirl number. Probability density functions of axial velocity showed the changes of their distributions from approximately Gaussian to non-Gaussian with increased swirl number. The swirl has a large effect on the rate of decay of the axial velocity throughout the domain, whereas only has a significant effect on the decay of swirl velocity in the near field close to the jet inlet. The relation between swirl number and the axial extent of the recirculation zone is approximately linear. Radial plots of mean passive scalar and its variance also demonstrate an increase in the rate of mixing with increasing swirl number.
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    Large Eddy Simulation of a turbulent swirling coaxial jet
    (Inderscience, 2010-12-31T00:00:00Z) Ranga Dinesh, K. K. J.; Savill, Mark A.; Jenkins, Karl W.; Kirkpatrick, M. P.
    This work uses the Large Eddy Simulation (LES) technique to study velocity and passive scalar mixing along with intermittency of a spatially evolving turbulent coaxial swirl jet. The simulations captured the potential core and also predicted high level turbulence intensities in the inner mixing regions. The Probability Density Functions (PDFs) and radial intermittency plots revealed an intermittent mixing behaviour especially in the outer region of the flow where the fluctuations of velocity rapidly change from rotational to irrotational and vice versa. The PDF and radial intermittency profiles exhibit Gaussian and non- Gaussian distributions close to the jet centreline and away from the centreline, respectively.
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    LES Of intermittency in a turbulent round jet with different inlet conditions
    (Elsevier, 2010-10) Ranga Dinesh, K. K. J.; Savill, Mark A.; Jenkins, Karl W.; Kirkpatrick, M. P.
    Large eddy simulation (LES) is a promising technique for accurate prediction of turbulent free shear flows in a wide range of applications. Here the LES technique has been applied to study the intermittency in a high Reynolds number turbulent jet with and without a bluff body. The objective of this work is to study the turbulence intermittency of velocity and scalar fields and its variation with respect to different inlet conditions. Probability density function distributions (pdf) of instantaneous mixture fraction and velocity have been created from which the intermittency has been calculated. The time averaged statistical results for a round jet are first discussed and comparisons of velocity and passive scalar fields between LES calculations and experimental measurements are seen to be good. The calculated probability density distributions show changes from a Gaussian to a delta function with increased radial distance from the jet centreline. The effect of introducing a bluff body into the core flow at the inlet changes the structure of pdfs, but the variation from Gaussian to delta distribution is similar to the jet case. However, the radial variation of the intermittency indicates differences between the results with and without a bluff body at axial locations due the recirculation zone created by the bluff body.
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    Study of jet precession, recirculation and vortex breakdown in turbulent swirling jets using LES.
    (Elsevier, 2009-06) Ranga Dinesh, K. K. J.; Kirkpatrick, M. P.
    Large eddy simulations (LES) are used to investigate turbulent isothermal swirling flows with a strong emphasis on vortex breakdown, recirculation and instability behaviour. The Sydney swirl burner configuration is used for all simulated test cases from low to high swirl and Reynolds numbers. The governing equations for continuity and momentum are solved on a structured Cartesian grid, and a Smagorinsky eddy viscosity model with the localised dynamic procedure is used as the sub-grid scale turbulence model. The LES successfully predicts both the upstream first recirculation zone generated by the bluff body and the downstream vortex breakdown bubble. The frequency spectrum indicates the presence of low frequency oscillations and the existence of a central jet precession as observed in experiments. The LES calculations well captured the distinct precession frequencies. The results also highlight the precession mode of instability in the center jet and the oscillations of the central jet precession, which forms a precessing vortex core. The study further highlights the predictive capabilities of LES on unsteady oscillations of turbulent swirling flow fields and provides a good framework for complex instability investigations.
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    A study of mixing and intermittency in a coaxial turbulent jet
    (Elsevier Science B.V., Amsterdam., 2010-04-30T00:00:00Z) Ranga Dinesh, K. K. J.; Savill, Mark A.; Jenkins, Karl W.; Kirkpatrick, M. P.
    A large eddy simulation study of mixing and intermittency of a coaxial turbulent jet discharging into an unconfined domain has been conducted. The work aims to gain insight into the mixing and intermittency of turbulent coaxial jet configurations. The coaxial jet considered has relatively high jet velocities for both core and annular jets with an aspect ratio (core jet to annular jet) of 1.48. The computations resolved the temporal development of large-scale flow structures by solving the transport equations for the spatially filtered mass, momentum and passive scalar on a non-uniform Cartesian grid and employed the localized dynamic Smagorinsky eddy viscosity as a sub-grid scale turbulence model. The results for the time-averaged mean velocities, associated turbulence fluctuations and mean passive scalar fields are presented. The initial inner and outer potential cores and the shear layers established between two cores have been resolved, together with the establishment of high turbulence regions between the shear layers. The passive scalar fields developing from the core and the bypass flow were found to exhibit differences at near and far field locations. Probability density distributions of instantaneous mixture fraction and velocity have been created from which intermittency has been calculated and the development of intermittency from the probability density distributions for instantaneous velocity follows similar variations as for the passive scalar fields.