Karl W. Jenkins

Direct Numerical Simulation of Turbulent Flame Kernels

A combustion DNS code has been developed to solve a fully compressible reacting flow and applied to studying the effects of a turbulent flame kernel. High accuracy numerical techniques have been employed which are 10th order explicit in space and a third order explicit Runge Kutta method in time. Parallel coding is achieved using the Message Passing Interface (MPI) and a performance test is presented showing efficiency and speed up factors. Turbulence is generated numerically for 64 independent simulations using the same laminar flame as an initial condition. Each initial turbulence field has been tested as a simulation of decaying isotropic turbulence without the inclusion of a flame. Initial results for the turbulent reacting simulations on a grid of 963 points are presented along with a laminar flame on a grid of 3843 points.

Curvature effects on flame kernels in a turbulent environment

This paper presents results from the direct numerical simulation (DNS) of flame kernels in various turbulent environments. The flames are fully premixed and propagate through a field of decaying isotropic turbulence. The DNS code solves the fully compressible reacting flow equations using high-order explicit finite differences in space and a third-order explicit Runge-Kutta method in time. Results have been obtained for three-dimensional time evolution of initially laminar flame kernels. The normalized turbulence intensity u ′/S L was varied from moderately low values that produce only minor flame-wrinkling effects to larger values that produce severe wrinkling. When the turbulence intensity is large in comparison with the laminar flame speed, holes begin to appear in the flame surface and evidence of local flame breakaway is observed. Flame kernels have significant mean curvature, and it is the curvature and other geometrical properties of the flame surface that form the main focus of the present work. Simulation results are presented for a number of different turbulence Reynolds numbers corresponding to computational grid sizes with up to 384 3 points, and a detailed analysis provides statistical data on flame curvature and local shape effects. Turbulence intensity is found to have a major influence on the extent and character of the observed flame wrinkling. In particular, effects due to the mean curvature of the kernel are shown to diminish strongly with increasing turbulence intensity.

Effects of initial radius on the propagation of premixed flame kernels in a turbulent environment

The effects of mean curvature on the propagation of turbulent premixed flames have been investigated using three-dimensional direct numerical simulations (DNS) with single step Arrhenius-type chemistry in the thin reaction zones regime. A number of spherical flame kernels with different initial radius have been studied under identical conditions of turbulence and thermochemistry. A statistically planar turbulent back-to-back flame has been simulated as a special case of a spherical kernel in the limit of infinite kernel radius. Statistical analysis in terms of standard and joint probability density functions (pdfs) clearly indicates that the mean curvature of the flame kernel configuration has a major influence on the propagation behavior of the flame. For the planar flame configuration the density-weighted displacement speed is found to be fairly constant throughout the flame brush, in good agreement with previous DNS results. By contrast, for the flame kernel configuration the density-weighted displacem...

Effects of Swirl on Intermittency Characteristics in Non-Premixed Flames

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.

Identification and analysis of instability in non-premixed swirling flames using LES

Large eddy simulations (LES) of turbulent non-premixed swirling flames based on the Sydney swirl burner experiments under different flame characteristics are used to uncover the underlying instability modes responsible for the centre jet precession and large scale recirculation zone. The selected flame series known as SMH flames have a fuel mixture of methane-hydrogen (50:50 by volume). The LES solves the governing equations on a structured Cartesian grid using a finite volume method, with turbulence and combustion modelling based on the localised dynamic Smagorinsky model and the steady laminar flamelet model respectively. The LES results are validated against experimental measurements and overall the LES yields good qualitative and quantitative agreement with the experimental observations. Analysis showed that the LES predicted two types of instability modes near fuel jet region and bluff body stabilised recirculation zone region. The mode I instability defined as cyclic precession of a centre jet is identified using the time periodicity of the centre jet in flames SMH1 and SMH2 and the mode II instability defined as cyclic expansion and collapse of the recirculation zone is identified using the time periodicity of the recirculation zone in flame SMH3. Finally frequency spectra obtained from the LES are found to be in good agreement with the experimentally observed precession frequencies.

A study of mixing and intermittency in a coaxial turbulent jet

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.

Influence of bluff-body and swirl on mixing and intermittency of jets

Abstract 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 with respect to bluff body and swirl at both upstream and downstream recirculation regions.

The Cambridge CFD grid for large-scale distributed CFD applications

The Cambridge CFD (computational fluid dynamics) Grid is a distributed problem-solving environment for large-scale CFD applications set up between the Cambridge eScience Centre and the CFD Laboratory in the Engineering Department at the University of Cambridge. A Web portal, the Cambridge CFD Web Portal (CamCFDWP) has been developed to provide transparent integration of CFD applications to non-computer scientist end users. In addition to the basic services provided of authentication, job submission and file transfer, the CamCFDWP makes use of XML (extensible markup language) techniques which make it possible to easily share datasets between different groups of users. A Web service interface has recently been implemented for a CFD database which could be integrated in the CamCFDWP in the near future. We also review how this Web service can be made secure using SSL, XML signatures and XML encryption.

Complete Body Aerodynamic Study of three Vehicles

Cooling drag, typically known as the difference in drag coefficient between open and closed cooling configurations, has traditionally proven to be a difficult flow phenomenon to predict using computational fluid dynamics. It was seen as an academic yardstick before the advent of grille shutter systems. However, their introduction has increased the need to accurately predict the drag of a vehicle in a variety of different cooling configurations during vehicle development. This currently represents one of the greatest predictive challenges to the automotive industry due to being the net effect of many flow field changes around the vehicle. A comprehensive study is presented in the paper to discuss the notion of defining cooling drag as a number and to explore its effect on three automotive models with different cooling drag deltas using the commercial CFD solvers; STARCCM+ and Exa PowerFLOW. The notchback DrivAer model with under-hood cooling provides a popular academic benchmark alongside two fully-engineered production cars; a large saloon (Jaguar XJ) and an SUV (Land Rover Range Rover). Initially three levels of spatial discretization were used with three steady-state RANS solvers (k-ɛ realizable, k-ω SST and Spalart-Allmaras) to ascertain whether previous work using RANS on the large saloon studying cooling flows could be replicated on other vehicle shapes. For both the full-production vehicles, all three turbulence models were capable of predicting the cooling drag delta within 5 counts (0.005 Cd). However, the DrivAer model was much more sensitive to both changes in turbulence models and mesh sizes. For the SA turbulence model only the drag coefficient was well predicted, for the other two RANS models no amount of grid refinement allowed the models to correctly predict the flow field. It was seen when comparing the k-ɛ realizable and SA turbulence models the difference in cooling drag was attributed to the rear of the vehicle. This highlighted that despite similar drag values from the cooling package, the cooling deltas were very different, suggesting that cooling drag cannot be thought of as open-closed drag with the addition of drag due to the cooling package. Further work on the DrivAer model expanded on the RANS simulations utilizing the eddy-resolving methods, IDDES and LBM, as validation cases. Oscillations which were seen in the SA and k-ω SST RANS turbulence models were shown to be of similar levels to those in the transient methods indicating a pseudo-unsteadiness present in the steady-state solvers and the importance of resolving it. Drag and lift coefficient absolute values were compared showing that only the IDDES method with sliding wheels and LBM method could obtain physical results for the majority of the tested criteria. Introduction Current and future regulations in the automotive industry place a high importance on the environmental impact of vehicles. It is becoming increasingly important to be able to calculate the drag of each vehicle specification and the affect of each changeable component on the final configuration. Wind tunnels can obtain all this information but it is difficult to create representative prototypes early enough in the development process. With ever increasing computational power available Computational Fluid Dynamics (CFD) provides the ability to simulate and calculate the drag configurations for a variety of models, while state-of-the-art multi-physics simulations with coupling of aerodynamics, thermal management and power-train simulations provide the potential to model real-time simulations. This paper focuses on increasing understanding of so-called cooling air drag, as applied to a fully engineered production large saloon and SUV, this is achieved by discussing the concept of cooling drag vs. complete body aerodynamic simulations and comparing against the popular benchmark DrivAer model.

The Cambridge CFD Grid Portal for Large-Scale Distributed CFD Applications

The Cambridge CFD (computational fluid dynamics) Web Portal (CamCFDWP) has been set up in the Cambridge eScience Centre to provide transparent integration of CFD applications to non-computer scientist end users who have access to the Cambridge CFD Grid. Besides the basic services provided as other web portals such as authentication, job submission and file transfer through a web browser, the CamCFDWP makes use of the XML (extensible markup language) techniques which make it possible to easily share datasets between different groups of users.