CERES :: Browsing by Author "Thornber, Ben"

Browsing by Author "Thornber, Ben"

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Open Access
Accuracy of high order density based compressible methods in low mach vortical flows
(Cranfield University, 2013) Shanmuganathan, Sanjeev; Thornber, Ben
A new, well posed, two-dimensional two-mode incompressible Kelvin{Helmholtz instability test case has been chosen to explore the ability of a compressible algorithm, Godunov-type scheme with the low Mach number correction, which can be used for simulations involving low Mach numbers, to capture the observed vortex pairing process due to the initial Kelvin{Helmholtz instability growth on low resolution grid. The order of accuracy, 2nd and 5th , of the compressible algorithm is also highlighted. The observed vortex pairing results and the corresponding momentum thickness of the mixing layer against time are compared with results obtained using the same compressible algorithm but without the low Mach number correction and three other methods, a Lagrange remap method where the Lagrange phase is 2nd order accurate in space and time while the remap phase is 3rd order accurate in space and 2nd order accurate in time, a 5th order accurate in space and time nite di erence type method based on the wave propagation algorithm and a 5th order spatial and 3rd order temporal accurate Godunov method utilising the SLAU numerical ux with low Mach capture property. The ability of the compressible ow solver of the commercial software, ANSYS Fluent, in solving low Mach ows is also examined for both implicit and explicit methods provided in the compressible ow solver. In the present two dimensional two mode incompressible Kelvin{Helmholtz instability test case, the ow conditions, stream velocities, length-scales and Reynolds numbers, are taken from an experiment conducted on the observation of vortex pairing process. Three di erent values of low Mach numbers, 0:2, 0:02 and 0:002 have been tested on grid resolutions of 24 24, 32 32, 48 48 and 64 64 on all the di erent numerical approaches. The results obtained show the vortex pairing process can be captured on a low grid resolution with the low Mach number correction applied down to 0:002 with 2nd and 5th order Godunovtype methods. Results also demonstrate clearly that a speci cally designed low Mach correction or ux is required for all algorithms except the Lagrange-remap approach, where dissipation is independent of Mach number. ANSYS Fluent's compressible ow solver with the implicit time stepping method also captures the vortex pairing on low resolutions but excessive dissipation prevents the instability growth when explicit time stepping method is applied.
Open Access
An algorithm for LES of premixed compressible flows using the Conditional Moment Closure model
(Elsevier Science B.V., Amsterdam, 2011-08-20T00:00:00Z) Thornber, Ben; Bilger, R. W.; Masri, A. R.; Hawkes, E. R.
A novel numerical method has been developed to couple a recent high order accurate fully compressible upwind method with the Conditional Moment Closure combustion model. The governing equations, turbulence modelling and numerical methods are presented in full. The new numerical method is validated against direct numerical simulation (DNS) data for a lean premixed methane slot burner. Although the modelling approaches are based on non-premixed flames and hence not expected to be valid for a wide range of premixed flames, the predicted flame is just 10% longer than that in the DNS and excellent agreement of mean mass fractions, conditional mass fractions and temperature is demonstrated. This new numerical method provides a very useful framework for future application of CMC to premixed as well as non-premixed combustion.
Open Access
Computational analysis of A-Pillar vortex formation in automotive applications
(Cranfield University, 2013-01) Bhambra, Devinder Pal Singh; Thornber, Ben
The research focusses on computational analysis of vortex generation behind A-Pillar of simplified model derived from Jaguar XF that excludes air from the underside of vehicle. This vortex formation contributes in generating wall pressure fluctuations especially at speeds higher than 100km/hr. It is a collaborative work between Cranfield University and Jaguar Land Rover. Three dimensional pressure based incompressible flow using Large Eddy Simulation turbulence model is selected for computational analysis in FLUENT v14. This used high parallel computing systems available in Cranfield University. In the initial phase, three grid resolutions (coarse, medium and fine) were prepared in ICEM CFD with fine case consisting of 10 million cells. Qualitative analysis includes extraction of slices, 3-D and surface streamlines and pressure and velocity contours for capturing the unsteadiness due to the vortex formation over the front side glass surface. The iso-surface of Q captures the unsteadiness at the A-Pillar wake and side mirror wake over front side glass surface. It also reveals that the range of length scales captured were limited even at the finest grid resolution. Quantitative analysis compares the mean pressure (Cp) data with JLR results. Probes were located at 51 locations over the front side glass window that showed a good comparison; specifically for the fine grid; with maximum variation incurred at probes located in separation areas. For predicting the wall pressure fluctuations, a total of ten probes were located over the front side glass window surface. The surface pressure (static) data was recorded for 1 sec of flow-time and later imported in MATLAB for post-processing. The results obtained in 1/3rd octave band showed that the large scales were too energetic and small scales are not captured. However, comparing sound pressure levels with the Aero-acoustic Wind Tunnel (AWT); provided by JLR; it is concluded that either the grid is too coarse to resolve higher frequencies or the numerical modelling used is too dissipative to benefits the use of LES.
Open Access
Computational science of turbulent mixing and combustion
(Cranfield University, 2010-09) Shimada, Yosuke; Drikakis, Dimitris; Thornber, Ben
Implicit Large Eddy Simulation (ILES) with high-resolution and high-order computational modelling has been applied to flows with turbulent mixing and combustion. Due to the turbulent nature, mixing of fuel and air and the subsequent combustion still remain challenging for computational fluid dynamics. However, recently ILES, an advanced numerical approach in Large Eddy Simulation methods, has shown encouraging results in prediction of turbulent flows. In this thesis the governing equations for single phase compressible flow were solved with an ILES approach using a finite volume Godunov-type method without explicit modelling of the subgrid scales. Up to ninth-order limiters were used to achieve high order spatial accuracy. When simulating non chemical reactive flows, the mean flow of a fuel burner was compared with the experimental results and showed good agreement in regions of strong turbulence and recirculation. The one dimensional kinetic energy spectrum was also examined and an ideal k−5/ 3 decay of energy could be seen in a certain range, which increased with grid resolution and order of the limiter. The cut-off wavenumbers are larger than the estimated maximum wavenumbers on the grid, therefore, the numerical dissipation sufficiently accounted for the energy transportation between large and small eddies. The effect of density differences between fuel and air was investigated for a wide range of Atwood number. The mean flow showed that when fuel momentum fluxes are identical the flow structure and the velocity fields were unchanged by Atwood number except for near fuel jet regions. The results also show that the effects of Atwood number on the flow structure can be described with a mixing parameter. In combustion flows simulation, a non filtered Arrhenius model was applied for the chemical source term, which corresponds to the case of the large chemical time scale compared to the turbulent time scale. A methane and air shear flow simulation was performed and the methane reaction rate showed non zero values against all temperature ranges. Small reaction rates were observed in the low temperature range due to the lack of subgrid scale modelling of the chemical source term. Simulation was also performed with fast chemistry approach representing the case of the large turbulent time scale compared to the chemical time scale. The mean flow of burner flames were compared with experimental data and a fair agreement was observed.
Open Access
Growth of a Richtmyer-Meshkov turbulent layer after reshock
(2011-09-30T00:00:00Z) Thornber, Ben; Drikakis, Dimitris; Youngs, D. L.; Williams, R. J. R.
This paper presents a numerical study of a reshocked turbulent mixing layer using high-order accurate Implicit Large-Eddy-Simulations (ILES). Existing theoretical approaches are discussed, and the theory of Youngs (detailed in Ref. 1) is extended to predict the behaviour of a reshocked mixing layer formed initially from a shock interacting with a broadband instability. The theory of Mikaelian2 is also extended to account for molecular mixing in the single-shocked layer prior to reshock. Simulations are conducted for broadband and narrowband initial perturbations and results for the growth rate of the reshocked layer and the decay rate of turbulent kinetic energy show excellent agreement with the extended theoretical approach. Reshock causes a marginal decrease in mixing parameters for the narrowband layer, but a significant increase for the broadband initial perturbation. The layer properties are observed to be very similar post-reshock, however, the growth rate exponent for the mixing layer width is higher in the broadband case, indicating that the reshocked layer still has a dependence (although weakened) on the initial conditions. These results have important implications for Unsteady Reynolds Averaged Navier Stokes modelling of such instabilities.
Open Access
High-order Implicit Large Eddy Simulation of gaseous fuel injection and mixing of a bluff body burner
(Elsevier Science B.V., Amsterdam., 2011-05-31T00:00:00Z) Shimada, Yosuke; Thornber, Ben; Drikakis, Dimitris
Implicit Large Eddy Simulation (ILES) with high-resolution and high-order computational modelling was applied to a turbulent mixing fuel injector flow. In the ILES calculation, the governing equations for three dimensional, non- reactive, multi-species compressible flows were solved using a finite volume Godunov-type method. Up to ninth-order spatial accurate reconstruction methods were examined with a second order explicit Runge-Kutta time integration. Mean and root mean square velocity and mixture fraction profiles showed good agreement with experimental data, which demonstrated that ILES using high-order methods successfully captured complex turbulent flow structure without using an explicit subgrid scale model. The effects of grid resolution and the influence of order of spatial accuracy on the resolution of the kinetic energy spectrum were investigated. An k(-5/3) decay of energy could be seen in a certain range and the cut-off wavenumbers increased with grid resolution or order of spatial accuracy. The effective cut-off wavenumbers are shown to be larger than the maximum wavenumbers appearing on the given grid for all test cases, implying that the numerical dissipation represents sufficiently the energy transport between resolved and unresolved eddies. The fifth-order limiter with a 0.6 million grid points was found to be optimal in terms of the resolution of kinetic energy and reasonable computational time. (C) 2011 Elsevier Ltd. All rights reserved.
Open Access
Implicit large eddy simulation for unsteady multi-component compressible turbulent flows
(Cranfield University, 2007) Thornber, Ben; Drikakis, Dimitris
Numerical methods for the simulation of shock-induced turbulent mixing have been investigated, focussing on Implicit Large Eddy Simulation. Shock-induced turbulent mixing is of particular importance for many astrophysical phenomena, inertial confinement fusion, and mixing in supersonic combustion. These disciplines are particularly reliant on numerical simulation, as the extreme nature of the flow in question makes gathering accurate experimental data difficult or impossible. A detailed quantitative study of homogeneous decaying turbulence demonstrates that existing state of the art methods represent the growth of turbulent structures and the decay of turbulent kinetic energy to a reasonable degree of accuracy. However, a key observation is that the numerical methods are too dissipative at high wavenumbers (short wavelengths relative to the grid spacing). A theoretical analysis of the dissipation of kinetic energy in low Mach number flows shows that the leading order dissipation rate for Godunov-type schemes is proportional to the speed of sound and the velocity jump across the cell interface squared. This shows that the dissipation of Godunov-type schemes becomes large for low Mach flow features, hence impeding the development of fluid instabilities, and causing overly dissipative turbulent kinetic energy spectra. It is shown that this leading order term can be removed by locally modifying the reconstruction of the velocity components. As the modification is local, it allows the accurate simulation of mixed compressible/incompressible flows without changing the formulation of the governing equations. In principle, the modification is applicable to any finite volume compressible method which includes a reconstruction stage. Extensive numerical tests show great improvements in performance at low Mach compared to the standard scheme, significantly improving turbulent kinetic energy spectra, and giving the correct Mach squared scaling of pressure and density variations down to Mach 10−4. The proposed modification does not significantly affect the shock capturing ability of the numerical scheme. The modified numerical method is validated through simulations of compressible, deep, open cavity flow where excellent results are gained with minimal modelling effort. Simulations of single and multimode Richtmyer-Meshkov instability show that the modification gives equivalent results to the standard scheme at twice the grid resolution in each direction. This is equivalent to sixteen times decrease in computational time for a given quality of results. Finally, simulations of a shock-induced turbulent mixing experiment show excellent qualitative agreement with available experimental data.
Open Access
Implicit large eddy simulation of ship airwakes
(Royal Aeronautical Society, 2010-12-31T00:00:00Z) Thornber, Ben; Starr, M.; Drikakis, Dimitris
Implicit large eddy simulations (ILES) of two different Royal Navy ships have been conducted as part of the UK Ship/Air Interface Frame-work project using a recently developed very high order accuracy numerical method. Time-accurate CFD data for fourteen flow angles was produced to incorporate into flight simulators for definition of safe helicopter operating limits (SHOLs). This paper discusses the flow phenomenology for the different wind directions and where possible reports on the validation of the ILES results for mean and fluctuating velocity components and spectra against experimental data.
Open Access
Implicit LES of turbulent compressible high-speed flows with transverse jet injection
(Cranfield University, 2011) Rana, Zeeshan Ahmed; Thornber, Ben; Drikakis, Dimitris
Implicit Large Eddy Simulation (ILES) has rapidly emerged as a powerful technique which is utilised to explore the unsteady compressible turbulent flows. Apart from o ering accuracy in numerical simulations, ILES is also computationally e cient compared to Direct Numerical Simulations or conventional Large Eddy Simulations. This report focuses on the validation of the existing high-resolution methods within the framework of ILES and explores its applications to the high-speed compressible turbulent flows such as a typical flow field inside a scramjet engine. The methodology applied in the current work employs a fifth-order MUSCL scheme with a modified variable extrapolation and a three-stage second-order Runge-Kutta scheme for temporal advancement. In order to simulate a realistic and accurate supersonic turbulent boundary layer (STBL) a synthetic turbulent inflow data generation method based upon digital filters has been implemented. This technique has been validated and compared against various other turbulent inflow data generation methods in order to find the most accurate, reliable and computationally e cient technique. The high-speed complex multi-species flow of a transverse sonic jet injection into a supersonic crossflow (JISC), which is typical fuel injection strategy inside a scramjet engine, has been investigated for time-averaged and instantaneous flow. It has been demonstrated that the incoming STBL plays a vital role in establishing the correct flow dynamics in JISC study as it enhances the KH instabilities in the flow field. Thermally perfect gas formulation has been implemented according to the NACA- 1135 report to study the e ects of high temperatures on the ratio of specific heats ( ). Using this, the full geometry of the HyShot-II scramjet engine is investigated to obtain the inflow conditions for the HyShot-II combustion chamber. Although the design of HyShot-II allowed to disgorge the shock and boundary layer which could otherwise enter the combustion chamber, but, it has been demonstrated that the flow field inside the combustion chamber still consists of a weak shock-train. Finally, the hydrogen injection is analysed inside the HyShot-II combustion chamber, with the shock-train travelling inside and the incoming STBL using digital filters based technique, to explore various time-averaged and instantaneous flow structures and parameters with a view to enhance the understanding of the complex flow field inside the combustion chamber. It is demonstrated from the detailed investigations of a complex high-speed flow that ILES methodology has the potential to develop the understandings of the high-speed compressible turbulent flows using comparatively less computational resources.
Open Access
The influence of initial conditions on turbulent mixing due to Richtmyer-Meshkov instability
(Cambridge University Press, 2010-07-10T00:00:00Z) Thornber, Ben; Drikakis, Dimitris; Youngs, D. L.; Williams, R. J. R.
This paper investigates the influence of different three-dimensional multi-mode initial conditions on the rate of growth of a mixing layer initiated via a Richtmyer-Meshkov instability through a series of well-controlled numerical experiments. Results are presented for large-eddy simulation of narrowband and broadband perturbations at grid resolutions up to 3 x 10(9) points using two completely different numerical methods, and comparisons are made with theory and experiment. It is shown that the mixing-layer growth is strongly dependent on initial conditions, the narrowband case giving, a power-law exponent theta approximate to 0.26 at low Atwood and theta approximate to 0.3 at high Atwood numbers. The broadband case uses a perturbation power spectrum of the form P(k) proportional to k(-2) with a proposed theoretical growth rate of theta = 2/3. The numerical results confirm this; however, they highlight the necessity of a very fine grid to capture an appropriately broad range of initial scales. In addition, an analysis of the kinetic energy decay rates, fluctuating kinetic energy spectra, plane-averaged volume fraction profiles and mixing parameters is presented for each case.
Open Access
Large eddy simulation using high-resolution and high-order methods
(Royal Society, 2009-07-31T00:00:00Z) Drikakis, Dimitris; Hahn, Marco; Mosedale, Andrew; Thornber, Ben
Restrictions on computing power make direct numerical simulation too expensive for complex flows; thus, the development of accurate large eddy simulation (LES) methods, which are industrially applicable and efficient, is required. This paper reviews recent findings about the leading order dissipation rate associated with high-resolution methods and improvements to the standard schemes for use in highly turbulent flows. Results from implicit LES are presented for a broad range of flows and numerical schemes, ranging from the second-order monotone upstream-centered schemes for conservation laws to very high-order (up to ninth-order) weighted essentially non-oscillatory schemes.
Open Access
Large-eddy simulation of multi-component compressible turbulent flows using high resolution methods
(Elsevier Science B.V., Amsterdam., 2008-08-31T00:00:00Z) Thornber, Ben; Drikakis, Dimitris; Youngs, D.
The ability of a finite volume Godunov and a semi-Lagrangian large-eddy simulation (LES) method to predict shock induced turbulent mixing has been examined through simulations of the half-height experiment [Holder and Barton. In: Proceedings of the international workshop on the physics of compressible turbulent mixing, 2004]. Very good agreement is gained in qualitative comparisons with experimental results for combined Richtmyer-Meshkov and Kelvin- Helmholtz instabilities in compressible turbulent multi-component flows. It is shown that both numerical methods can capture the size, location and temporal growth of the main flow features. In comparing the methods, there is variability in the amount of resolved turbulent kinetic energy. The semi-Lagrangian method has constant dissipation at low Mach number, thus allowing the initially small perturbations to develop into Kelvin-Helmholtz instabilities. These are suppressed at the low Mach stage in the Godunov method. However, there is an excellent agreement in the final amount of fluid mixing when comparing both numerical methods at different grid resolutions.
Open Access
Modelling and simulation of turbulence in unsteady separated and suddenly-expanded flows
(Cranfield University, 2011-11) Karantonis, Konstantinos; Drikakis, Dimitris; Thornber, Ben
The scope of this PhD thesis is the simulation of turbulence in time-dependent, separated and suddenly-expanded channel flows. High-resolution and very high-order numerical methods have been employed in the framework of Implicit Large Eddy Simulation (ILES) to elucidate open questions about the physics in flows with sudden expansion. It is well known that the planar sudden expansion (PSE), despite its simple and symmetric geometry it produces a very complex behaviour and a distinctly asymmetric flow pattern ascribed mainly to the Coanda effect. Such flows are encountered in a wide range of practical engineering applications, such as combustion, hydraulic and fluidic devices, air ducts, and mixing equipments. It is of great importance, therefore, to understand the mechanisms that dominate flows with separation and reattachment of the shear layers, as well as flows with regions of strong reversed motion. This thesis has for the first time analysed in detail the turbulent kinetic energy budget (TKEB) for the PSE. This analysis has been extended to examine the influence of Mach number on each individual component of the TKEB. The resulting data can be used as reference for further development of turbulence models capable of accurately resolving the flow behaviour in suddenly-expanded flows. Cont/d.
Open Access
Numerical modelling of compressible turbulent premixed hydrogen flames
(Cranfield University, 2016-05) Turquand D'Auzay, Charles; Aspden, Andrew; Moulitsas, Irene; Thornber, Ben
Turbulent combustion has a profound effect on the way we live our lives; homes and businesses predominantly rely on power generated by burning some form of fuel, and the vast majority of transport of passengers and cargo are driven by combustion. Fossil fuels remain readily available and relatively cheap, and so will continue to power the modern world for the foreseeable future. Combustion of fossil fuels produces emissions that detrimentally affect air quality, particularly in highly-populated cities, and are also widely believed to be contributing to global climate change. Consequently, increasing attention is being focused on alternative fuels, increased efficiency and reduced emissions. One alternative fuel is hydrogen, which introduces challenges in end-usage, storage and safety that are not encountered with more conventional fuels. Advances in computational power and software technology means that numerical simulation has a growing role in the development of combustors and safety evaluation. Despite these advances, many challenges remain; the broad range of time and length scales involved are coupled with complex thermodynamics and chemistry on top of turbulent fluid mechanics, which means that detailed simulations of even relatively-simple burners are still prohibitively expensive. Engineering turbulent flame models are required to reduce computational expense, and the challenge is to retain as much of the flow physics as possible. Furthermore, the choice of numerical approach has a significant effect on the quality of simulation, and different target applications place different demands on the numerical scheme. In the case of hydrogen explosion, the approach needs to be able to capture a range of physical behaviours including turbulence, low-speed deflagration, high-speed shock waves and potentially detonations. One such numerical approach that has enjoyed widespread success is finite volumes schemes based on the Godunov method. These methods perform well at all speeds, and have positive shock-capturing capability, but recent studies have demonstrated difficulties with numerical stability for more complex thermodynamics, specifically in the case of fully-conservative methods for multi-component fluids with varying thermodynamic properties. A recent development is the so-called double-flux method, which retains many of the positive properties of the fully-conservative approaches and does not suffer from the same numerical instabilities, but is quasi-conservative and involves additional computational expense. The present work consolidates the state-of-the-art in the literature, and considers two equation sets, based on mass fraction and volume fraction, respectively, along with fully-conservative and quasiconservative schemes. Comprehensive validation and evaluation of the different approaches is presented. It was found that both quasi-conservative approaches performed well, with a better conservative behaviour for the quasi-conservative volume fraction, but a better stability for the quasi-conservative mass fraction. Finally, the numerical tool developed is applied to turbulent combustion of premixed hydrogen in the context of the semi-confined experiments from the University of Sydney. The LES results showed an good overall agreement with the experimental data, and the critical parameters such as overpressure and flame speed where globally well captured, highlighting the large potential of LES for safety analysis.
Metadata only
Shock-induced turbulent mixing at high density ratios.
(Cranfield University, 2015-05) Probyn, Michael; Thornber, Ben; Aspden, Andrew; Drikakis, Dimitris
Restricted under Crown Copyright. Only available to MOD students/employees with MOD ID (alternatively written permission from Supervisor or MOD).
Open Access
Supersonic combustion modelling using the conditional moment closure approach
(Cranfield University, 2014) Picciani, Mark; Thornber, Ben; Drikakis, Dimitris
This work presents a novel algorithm for supersonic combustion modelling. The method involved coupling the Conditional Moment Closure (CMC) model to a fully compressible, shock capturing, high-order flow solver, with the intent of modelling a reacting hydrogen-air, supersonic jet. Firstly, a frozen chemistry case was analysed to validate the implementation of the algorithm and the ability for CMC to operate at its frozen limit. Accurate capturing of mixing is crucial as the mixing and combustion time scales for supersonic flows are on the order of milliseconds. The results of this simulation were promising even with an unexplainable excess velocity decay of the jet core. Hydrogen mass fractions however, showed fair agreement to the experiment. The method was then applied to the supersonic reacting case of ONERA. The results showed the method was able to successfully capture chemical non-equilibrium effects, as the lift-off height and autoignition time were reasonably captured. Distributions of reactive scalars were difficult to asses as experimental data was deemed to be very inaccurate. As a consequence, published numerical results for the same test case were utilised to aid in analysing the results of the presented simulations. Due to the primary focus of the study being to assess non-equilibrium effects, the clustering of the computational grid lent itself to smeared and lower magnitude wall pressure distributions. Nevertheless, the wall pressure distributions showed good qualitative agreement to experiment. The primary conclusions from the study were that the CMC method is feasible to model supersonic combustion. However, a more detailed analysis of sub-models and closure assumptions must be conducted to assess the feasibility on a more fundamental level. Also, from the results of both the frozen chemistry and the reacting case, the effects of assuming constant species Lewis number was visible.

Browsing by Author "Thornber, Ben"

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