# Publications

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## 2015 |

Mohanarangam, Krishna; Stephens, Darrin W; Cao, Xiaodong; Fawell, Phillip D; Simic, Kosta; Yang, William Experimental and numerical investigation of turbulent mixing fields behind bluff body jets Conference Eleventh International Conference on CFD in the Minerals and Process Industries, 2015. Abstract | BibTeX | Tags: OpenFOAM, PIV, Turbulence, Validation @conference{MOHANARANGAM2015, title = {Experimental and numerical investigation of turbulent mixing fields behind bluff body jets}, author = {Krishna Mohanarangam and Darrin W Stephens and Xiaodong Cao and Phillip D. Fawell and Kosta Simic and William Yang}, year = {2015}, date = {2015-12-07}, booktitle = {Eleventh International Conference on CFD in the Minerals and Process Industries}, abstract = {Bluff body shapes are used to promote better heat and mass transfer in various industrial flow situations, flow past them serving to enhance turbulence and thereby promote mixing, primarily due to the vortex shedding created. In this paper, the turbulent mixing fields for flow past a square sparge jet is investigated both experimentally and numerically. The geometry considered for this study was a square channel fitted with a square sparge located midway along its length. Time Resolved – Particle Image Velocimetry (TR-PIV) measurements were conducted to measure and quantify the mixing and the turbulent fields behind the sparge. Measurements were carried out at a frequency of 1 KHz to capture the unsteady behaviour of the flow past the sparge and its jet. Computational Fluid Dynamics (CFD) simulation of the set-up was carried out by solving Unsteady Reynolds Averaged Navier Stokes (URANS) equations with the k-ω model as turbulence closure. Various ratios between the bulk flow past the sparge and that of its jet were considered to investigate the turbulent behaviour of the sparge geometry, while also providing data for model development and calibration. The long-term aim is to identify sparge designs that provide enhanced mixing in industrial flow settings.}, keywords = {OpenFOAM, PIV, Turbulence, Validation}, pubstate = {published}, tppubtype = {conference} } Bluff body shapes are used to promote better heat and mass transfer in various industrial flow situations, flow past them serving to enhance turbulence and thereby promote mixing, primarily due to the vortex shedding created. In this paper, the turbulent mixing fields for flow past a square sparge jet is investigated both experimentally and numerically. The geometry considered for this study was a square channel fitted with a square sparge located midway along its length. Time Resolved – Particle Image Velocimetry (TR-PIV) measurements were conducted to measure and quantify the mixing and the turbulent fields behind the sparge. Measurements were carried out at a frequency of 1 KHz to capture the unsteady behaviour of the flow past the sparge and its jet. Computational Fluid Dynamics (CFD) simulation of the set-up was carried out by solving Unsteady Reynolds Averaged Navier Stokes (URANS) equations with the k-ω model as turbulence closure. Various ratios between the bulk flow past the sparge and that of its jet were considered to investigate the turbulent behaviour of the sparge geometry, while also providing data for model development and calibration. The long-term aim is to identify sparge designs that provide enhanced mixing in industrial flow settings. |

Jemcov, Aleksandar; Gonzalez-Juez, Esteban D A Finite Volume Time-Domain Solver to Estimate Combustion Instabilities Conference 53rd AIAA Aerospace Sciences Meeting, 2015, At Kissimmee, FL, 2015. Abstract | Links | BibTeX | Tags: Combustion, Modelling, OpenFOAM, Thermoacoustic @conference{jemcov2015finite, title = {A Finite Volume Time-Domain Solver to Estimate Combustion Instabilities}, author = { Aleksandar Jemcov and Esteban D Gonzalez-Juez}, doi = {10.2514/6.2015-1567}, year = {2015}, date = {2015-01-01}, booktitle = {53rd AIAA Aerospace Sciences Meeting, 2015, At Kissimmee, FL}, abstract = {Even though the damaging effect of thermoacoustic combustion instabilities on combustion systems is well documented, estimating the occurrence of these undesirable phenomena is still very difficult. Modeling tools used for this purpose include classic network models, 3D frequency-domain acoustic solvers, and computational fluid dynamics (CFD). Motivated by the large gap in both computational cost and predictive capability between the first two tools and CFD, the present work discusses an approach that bridges this gap: a 3D acoustic solver in the time domain. Unique aspects of the present solver include the ability to handle both linear and nonlinear acoustics and the use of a solution algorithm based on an approximate Riemann solver. Another notable feature, critical for industrial applications, is the use of a finite volume discretization that can be applied to unstructured meshes of arbitrary shape and complexity. This new acoustic solver is based on the C++ library OpenFOAM. Predictions with this solver are in good agreement with analytical solutions for a 2D isothermal cavity, a 1D Rijke tube, and a 1D model of a reheat buzz.}, keywords = {Combustion, Modelling, OpenFOAM, Thermoacoustic}, pubstate = {published}, tppubtype = {conference} } Even though the damaging effect of thermoacoustic combustion instabilities on combustion systems is well documented, estimating the occurrence of these undesirable phenomena is still very difficult. Modeling tools used for this purpose include classic network models, 3D frequency-domain acoustic solvers, and computational fluid dynamics (CFD). Motivated by the large gap in both computational cost and predictive capability between the first two tools and CFD, the present work discusses an approach that bridges this gap: a 3D acoustic solver in the time domain. Unique aspects of the present solver include the ability to handle both linear and nonlinear acoustics and the use of a solution algorithm based on an approximate Riemann solver. Another notable feature, critical for industrial applications, is the use of a finite volume discretization that can be applied to unstructured meshes of arbitrary shape and complexity. This new acoustic solver is based on the C++ library OpenFOAM. Predictions with this solver are in good agreement with analytical solutions for a 2D isothermal cavity, a 1D Rijke tube, and a 1D model of a reheat buzz. |

Gonzalez-Juez, Esteban D; Jemcov, Aleksandar A Finite Volume Time-Domain Solver to Estimate Combustion Instabilities Journal Article Journal of Propulsion and Power, 31 (2), pp. 632–642, 2015. Abstract | Links | BibTeX | Tags: Combustion, Modelling, OpenFOAM, Thermoacoustic @article{gonzalez2015finite, title = {A Finite Volume Time-Domain Solver to Estimate Combustion Instabilities}, author = { Esteban D Gonzalez-Juez and Aleksandar Jemcov}, doi = {10.2514/1.B35488}, year = {2015}, date = {2015-01-01}, journal = {Journal of Propulsion and Power}, volume = {31}, number = {2}, pages = {632--642}, publisher = {American Institute of Aeronautics and Astronautics}, abstract = {Modeling tools used to estimate thermoacoustic combustion instabilities include classic network models, three-dimensional frequency-domain acoustic solvers, and computational fluid dynamics. Motivated by the large gap in both computational cost and predictive capability between the first two tools and computational fluid dynamics, the present work discusses and tests an approach that bridges this gap: a three-dimensional finite volume, acoustic solver in the time domain. Distinguishing features of the newly developed solver include the ability to capture both linear and nonlinear acoustics, the use of a solution algorithm based on an approximate Riemann solver, and the ability to handle complex geometries with unstructured meshes. This new solver produces results that agree well with analytical solutions for a two-dimensional isothermal cavity and a one-dimensional Rijke tube, as well as with the experimental data of a reheat buzz. For this last problem, a limit cycle is produced with a physical model that bounds heat-release fluctuations. In addition, results from the new acoustic solver for an annular combustor compare well with those of a three-dimensional frequency-domain acoustic solver, demonstrating the capabilities of the new solver to capture multidimensional acoustics.}, keywords = {Combustion, Modelling, OpenFOAM, Thermoacoustic}, pubstate = {published}, tppubtype = {article} } Modeling tools used to estimate thermoacoustic combustion instabilities include classic network models, three-dimensional frequency-domain acoustic solvers, and computational fluid dynamics. Motivated by the large gap in both computational cost and predictive capability between the first two tools and computational fluid dynamics, the present work discusses and tests an approach that bridges this gap: a three-dimensional finite volume, acoustic solver in the time domain. Distinguishing features of the newly developed solver include the ability to capture both linear and nonlinear acoustics, the use of a solution algorithm based on an approximate Riemann solver, and the ability to handle complex geometries with unstructured meshes. This new solver produces results that agree well with analytical solutions for a two-dimensional isothermal cavity and a one-dimensional Rijke tube, as well as with the experimental data of a reheat buzz. For this last problem, a limit cycle is produced with a physical model that bounds heat-release fluctuations. In addition, results from the new acoustic solver for an annular combustor compare well with those of a three-dimensional frequency-domain acoustic solver, demonstrating the capabilities of the new solver to capture multidimensional acoustics. |

## 2013 |

Martin, Scott; Jemcov, Aleksandar; de Ruijter, Bj"orn Modeling an Enclosed, Turbulent Reacting Methane Jet With the Premixed Conditional Moment Closure Method Inproceedings ASME Turbo Expo 2013: Turbine Technical Conference and Exposition, pp. V01BT04A011–V01BT04A011, American Society of Mechanical Engineers 2013. Abstract | Links | BibTeX | Tags: CMC, Methane, Mixing, Modelling, OpenFOAM, Turbulence @inproceedings{martin2013modeling, title = {Modeling an Enclosed, Turbulent Reacting Methane Jet With the Premixed Conditional Moment Closure Method}, author = { Scott Martin and Aleksandar Jemcov and Bj"orn de Ruijter}, doi = {10.1115/GT2013-95092}, year = {2013}, date = {2013-01-01}, booktitle = {ASME Turbo Expo 2013: Turbine Technical Conference and Exposition}, pages = {V01BT04A011--V01BT04A011}, organization = {American Society of Mechanical Engineers}, abstract = {Here the premixed Conditional Moment Closure (CMC) method is used to model the recent PIV and Raman turbulent, enclosed reacting methane jet data from DLR Stuttgart [1]. The experimental data has a rectangular test section at atmospheric pressure and temperature with a single inlet jet. A jet velocity of 90 m/s is used with an adiabatic flame temperature of 2,064 K. Contours of major species, temperature and velocities along with velocity rms values are provided. The conditional moment closure model has been shown to provide the capability to model turbulent, premixed methane flames with detailed chemistry and reasonable runtimes [2]. The simplified CMC model used here falls into the class of table lookup turbulent combustion models where the chemical kinetics are solved offline over a range of conditions and stored in a table that is accessed by the CFD code. Most table lookup models are based on the laminar 1-D flamelet equations, which assume the small scale turbulence does not affect the reaction rates, only the large scale turbulence has an effect on the reaction rates. The CMC model is derived from first principles to account for the effects of small scale turbulence on the reaction rates, as well as the effects of the large scale mixing, making it more versatile than other models. This is accomplished by conditioning the scalars with the reaction progress variable. By conditioning the scalars and accounting for the small scale mixing, the effects of turbulent fluctuations of the temperature on the reaction rates are more accurately modeled. The scalar dissipation is used to account for the effects of the small scale mixing on the reaction rates. The original premixed CMC model used a constant value of scalar dissipation, here the scalar dissipation is conditioned by the reaction progress variable. The steady RANS 3-D version of the open source CFD code OpenFOAM is used. Velocity, temperature and species are compared to the experimental data. Once validated, this CFD turbulent combustion model will have great utility for designing lean premixed gas turbine combustors.}, keywords = {CMC, Methane, Mixing, Modelling, OpenFOAM, Turbulence}, pubstate = {published}, tppubtype = {inproceedings} } Here the premixed Conditional Moment Closure (CMC) method is used to model the recent PIV and Raman turbulent, enclosed reacting methane jet data from DLR Stuttgart [1]. The experimental data has a rectangular test section at atmospheric pressure and temperature with a single inlet jet. A jet velocity of 90 m/s is used with an adiabatic flame temperature of 2,064 K. Contours of major species, temperature and velocities along with velocity rms values are provided. The conditional moment closure model has been shown to provide the capability to model turbulent, premixed methane flames with detailed chemistry and reasonable runtimes [2]. The simplified CMC model used here falls into the class of table lookup turbulent combustion models where the chemical kinetics are solved offline over a range of conditions and stored in a table that is accessed by the CFD code. Most table lookup models are based on the laminar 1-D flamelet equations, which assume the small scale turbulence does not affect the reaction rates, only the large scale turbulence has an effect on the reaction rates. The CMC model is derived from first principles to account for the effects of small scale turbulence on the reaction rates, as well as the effects of the large scale mixing, making it more versatile than other models. This is accomplished by conditioning the scalars with the reaction progress variable. By conditioning the scalars and accounting for the small scale mixing, the effects of turbulent fluctuations of the temperature on the reaction rates are more accurately modeled. The scalar dissipation is used to account for the effects of the small scale mixing on the reaction rates. The original premixed CMC model used a constant value of scalar dissipation, here the scalar dissipation is conditioned by the reaction progress variable. The steady RANS 3-D version of the open source CFD code OpenFOAM is used. Velocity, temperature and species are compared to the experimental data. Once validated, this CFD turbulent combustion model will have great utility for designing lean premixed gas turbine combustors. |

## 2012 |

Jemcov, Aleksandar; Stephens, Darrin W Topological derivative formulation for shape sensitivity in incompressible turbulent flows Conference Ninth International Conference on CFD in the Minerals and Process Industries, 2012. Abstract | Links | BibTeX | Tags: Adjoint, OpenFOAM, Shape Sensitivity @conference{jemcov2012topological, title = {Topological derivative formulation for shape sensitivity in incompressible turbulent flows}, author = {Aleksandar Jemcov and Darrin W Stephens}, url = {http://www.researchgate.net/publication/265160757_TOPOLOGICAL_DERIVATIVE_FORMULATION_FOR_SHAPE_SENSITIVITY_IN_INCOMPRESSIBLE_TURBULENT_FLOWS}, year = {2012}, date = {2012-01-01}, booktitle = {Ninth International Conference on CFD in the Minerals and Process Industries}, abstract = {Shape derivative based on topological consideration is presented in this work. The resulting derivative has simpler form compared to corresponding classical shape derivative due to topological derivative formulation. Shape derivative is based on topological derivative in the limit of infinitesimally weak source terms in momentum equation approaching the boundary of the computational domain. The consistency of two derivatives is demonstrated and computational example of the flow in curved duct is used for the illustration of the derivative computations. }, keywords = {Adjoint, OpenFOAM, Shape Sensitivity}, pubstate = {published}, tppubtype = {conference} } Shape derivative based on topological consideration is presented in this work. The resulting derivative has simpler form compared to corresponding classical shape derivative due to topological derivative formulation. Shape derivative is based on topological derivative in the limit of infinitesimally weak source terms in momentum equation approaching the boundary of the computational domain. The consistency of two derivatives is demonstrated and computational example of the flow in curved duct is used for the illustration of the derivative computations. |

## 2007 |

Morgans, Rick C; Doolan, Con J; Stephens, Darrin W Derivative free global optimisation of CFD simulations Conference 16th Australasian Fluid Mechanics Conference, 2007. Abstract | BibTeX | Tags: Algorithm, EGO, OpenFOAM, Simulation @conference{morgans2007derivative, title = {Derivative free global optimisation of CFD simulations}, author = {Rick C Morgans and Con J Doolan and Darrin W Stephens}, year = {2007}, date = {2007-01-01}, booktitle = {16th Australasian Fluid Mechanics Conference}, abstract = {This work reports on the use of numerical optimisation techniques to optimise objective functions calculated by Computational Fluid Dynamics (CFD) simulations. Two example applications are described, the first being the shape optimisation of a low speed wind tunnel contraction. A potential flow and viscous flow solver have been coupled to produce a robust computational tool, with the contraction shape defined by a two parameter B´ezier curve. The second application is a simplified test case with a known minimum calculated using a commercial CFD code. For the optimisation of complex CFD simulations, it is sometimes advantageous to use an efficient derivative free global optimisation algorithm because of potentially long simulation times, the objective function may contain multiple local minima and it is often difficult to evaluate analytical or numerical gradients. The Efficient Global Optimisation (EGO) algorithm sequentially samples results from an expensive calculation, does not require derivative information, uses an inexpensive surrogate to search for a global optimum, and is used in this current work. For both applications, the EGO algorithm is able to efficiently and robustly find a global optimum that satisfies any constraints.}, keywords = {Algorithm, EGO, OpenFOAM, Simulation}, pubstate = {published}, tppubtype = {conference} } This work reports on the use of numerical optimisation techniques to optimise objective functions calculated by Computational Fluid Dynamics (CFD) simulations. Two example applications are described, the first being the shape optimisation of a low speed wind tunnel contraction. A potential flow and viscous flow solver have been coupled to produce a robust computational tool, with the contraction shape defined by a two parameter B´ezier curve. The second application is a simplified test case with a known minimum calculated using a commercial CFD code. For the optimisation of complex CFD simulations, it is sometimes advantageous to use an efficient derivative free global optimisation algorithm because of potentially long simulation times, the objective function may contain multiple local minima and it is often difficult to evaluate analytical or numerical gradients. The Efficient Global Optimisation (EGO) algorithm sequentially samples results from an expensive calculation, does not require derivative information, uses an inexpensive surrogate to search for a global optimum, and is used in this current work. For both applications, the EGO algorithm is able to efficiently and robustly find a global optimum that satisfies any constraints. |