Publications

@conference{Stephens2015b,
title = {A two equation VLES turbulence model with near-wall delayed behaviour},
author = {Darrin W Stephens and Chris Sideroff and Aleksandar Jemcov},
doi = {10.13140/RG.2.1.1791.1125},
year = {2015},
date = {2015-11-25},
booktitle = {7th Asia-Pacific International Symposium on Aerospace Technology, 25 – 27 November 2015, Cairns},
abstract = {Turbulence is a phenomenon that occurs frequently in nature and is present in almost all industrial applications. Despite significant increase in computational power in modern processors, Reynolds averaged Navier-Stokes (RANS) simulations are still the dominant approach to turbulence modelling of high Reynolds number flows. Hybrid LES/RANS approaches [1] are currently used to offset the cost of Large Eddy Simulation (LES) computations by retaining the RANS characteristics in boundary layers while using the LES model away from walls. The hybrid approach embodied in the Detached Eddy Simulation (DES) methodology has been used with success in industrial flow simulations. However, it should be noted that the DES approach still requires LES-like mesh resolution away from walls. This is a simple consequence of the fact that the DES model defaults to LES at large distances from the walls. This may prove prohibitively expensive in simulations where large turbulent structures persists over most of the computational domain.
In this work, a delayed two equation very large eddy simulation (VLES) model based on three length scales is introduced. The resolution control function used to rescale the Reynolds stresses is based on the ratio of the resolved to unresolved turbulence spectrum. The model constants are selected so that the Smagorinsky subgrid-scale model is recovered in the limit of grids approaching the resolution required for LES computations. The near wall RANS behaviour of the proposed model is obtained using blending functions. The objective was to implement this model using the open source library Caelus [2] and validate the results against two test cases involving turbulent vortex shedding from a bluff-body. The test cases used were flow past a square cylinder at a Reynolds number of 21,400 [3] and the Rudimentary Landing Gear benchmark case for Airframe Noise Computations (BANC) [4].
The numerical simulations were carried out using a transient solver based on the open source computational mechanics library Caelus. The pressure-based solver with second-order bounded spatial discretisation and second-order bounded implicit time marching scheme was applied to obtain a time-accurate solutions. Compressibility effects were negligible for the Mach numbers under consideration and the flow was treated as incompressible. Results from the simulations indicate close agreement between the proposed model and available experimental and numerical results.
},
keywords = {Caelus, LES, RANS, Rudimentary Landing Gear, Square cylinder, Turbulence, VLES, Vortex shedding},
pubstate = {published},
tppubtype = {conference}
}

@misc{Stephens2015b,
title = {A two equation VLES turbulence model with near-wall delayed behaviour},
author = {Darrin W Stephens and Chris Sideroff and Aleksandar Jemcov},
url = {http://www.appliedccm.com/wp-content/uploads/2015/11/APISAT-2015-PPT.pdf},
year = {2015},
date = {2015-11-25},
abstract = {Turbulence is a phenomenon that occurs frequently in nature and is present in almost all industrial applications. Despite significant increase in computational power in modern processors, Reynolds averaged Navier-Stokes (RANS) simulations are still the dominant approach to turbulence modelling of high Reynolds number flows. Hybrid LES/RANS approaches [1] are currently used to offset the cost of Large Eddy Simulation (LES) computations by retaining the RANS characteristics in boundary layers while using the LES model away from walls. The hybrid approach embodied in the Detached Eddy Simulation (DES) methodology has been used with success in industrial flow simulations. However, it should be noted that the DES approach still requires LES-like mesh resolution away from walls. This is a simple consequence of the fact that the DES model defaults to LES at large distances from the walls. This may prove prohibitively expensive in simulations where large turbulent structures persists over most of the computational domain.
In this work, a delayed two equation very large eddy simulation (VLES) model based on three length scales is introduced. The resolution control function used to rescale the Reynolds stresses is based on the ratio of the resolved to unresolved turbulence spectrum. The model constants are selected so that the Smagorinsky subgrid-scale model is recovered in the limit of grids approaching the resolution required for LES computations. The near wall RANS behaviour of the proposed model is obtained using blending functions. The objective was to implement this model using the open source library Caelus [2] and validate the results against two test cases involving turbulent vortex shedding from a bluff-body. The test cases used were flow past a square cylinder at a Reynolds number of 21,400 [3] and the Rudimentary Landing Gear benchmark case for Airframe Noise Computations (BANC) [4].
The numerical simulations were carried out using a transient solver based on the open source computational mechanics library Caelus. The pressure-based solver with second-order bounded spatial discretisation and second-order bounded implicit time marching scheme was applied to obtain a time-accurate solutions. Compressibility effects were negligible for the Mach numbers under consideration and the flow was treated as incompressible. Results from the simulations indicate close agreement between the proposed model and available experimental and numerical results.
},
keywords = {Caelus, LES, RANS, Rudimentary Landing Gear, Square cylinder, Turbulence, VLES, Vortex shedding},
pubstate = {published},
tppubtype = {presentation}
}

@conference{Stephens2015b,
title = {Euler-Lagrange Large Eddy Simulation of a Square Cross-Sectioned Bubble Column},
author = {Darrin W Stephens and Shannon Keough and Chris Sideroff},
doi = {10.13140/RG.2.1.4772.4003},
year = {2015},
date = {2015-09-29},
abstract = {Bubble columns are widely used in the chemical and biochemical process industries. In order to develop design tools for engineering purposes, a large amount of research has been carried out in the area of CFD of gas-liquid flows. In this paper a transient Euler-Lagrange solver developed using the open source Caelus library is used to simulate the gas-liquid flow in a 3D square cross-sectioned bubble column. The turbulence is modelled using large eddy simulation (LES). The results of the simulations are compared to published PIV measurements. It is found that, good quantitative agreement with experimental data is obtained when drag, lift and virtual mass forces are used.},
keywords = {Bubble column, Caelus, Euler-Lagrange, Gas-liquid flow, LES},
pubstate = {published},
tppubtype = {conference}
}

@misc{Stephens2015b,
title = {Euler-Lagrange Large Eddy Simulation of a Square Cross-Sectioned Bubble Column},
author = {Darrin W Stephens and Chris Sideroff},
url = {http://www.appliedccm.com/wp-content/uploads/2015/10/Chemeca2015.pdf},
year = {2015},
date = {2015-09-29},
abstract = {Bubble columns are widely used in the chemical and biochemical process industries. In order to develop design tools for engineering purposes, a large amount of research has been carried out in the area of CFD of gas-liquid flows. In this paper a transient Euler-Lagrange solver developed using the open source Caelus library is used to simulate the gas-liquid flow in a 3D square cross-sectioned bubble column. The turbulence is modelled using large eddy simulation (LES). The results of the simulations are compared to published PIV measurements. It is found that, good quantitative agreement with experimental data is obtained when drag, lift and virtual mass forces are used.},
keywords = {Bubble column, Caelus, Euler-Lagrange, Gas-liquid flow, LES, Validation},
pubstate = {published},
tppubtype = {presentation}
}

@conference{kellycomputation,
title = {Computation of the Aero-Optical Effect of a Helicopter Rotor Wake Using Unsteady RANS and LES},
author = { Ryan Kelly and Aleksandar Jemcov and R Mark Rennie and Kan Wang and Meng Wang5and Eric J Jumper and Matthew R Whitely and David Goorskey},
doi = {doi:10.2514/6.2015-0678},
year = {2015},
date = {2015-01-02},
booktitle = {53rd AIAA Aerospace Sciences Meeting},
abstract = {The aero-optical characteristics of the wake beneath a hovering helicopter was computed using Unsteady RANS (URANS) and Large-Eddy Simulations (LES). The computational methodology consisted of combining a URANS simulation with a vorticity-confinement method to produce a model of the large-scale vortex wake of the helicopter. An LES simulation of the local flow around a vortex segment was then performed and the solutions were superimposed onto the large-scale vortex structures computed using the URANS CFD, and time-resolved optical wavefronts were computed on a beam of light passing through the flow. The computed optical aberrations of the wake can be used to develop adaptive-optic systems or other beam-control approaches.},
keywords = {Computation, LES, RANS},
pubstate = {published},
tppubtype = {conference}
}

@article{jemcov2012comparative,
title = {Comparative Study of Reynolds Averaged and Embedded Large Eddy Simulations of a High Pressure Turbine Stage},
author = { Aleksandar Jemcov and Theodore Williams and Thomas Corke},
year = {2012},
date = {2012-01-01},
journal = {Bulletin of the American Physical Society},
volume = {57},
publisher = {APS},
abstract = {An Embedded Large Eddy Simulation (ELES) approach is used to simulate the flow path through a high pressure turbine stage. The turbine stage includes the entry duct, stationary inlet and exit guide vanes, and a rotor. The rotor blade design includes a squealer tip. The flowfield around the rotor is simulated using LES. A Reynolds Averaged Simulation (RAS) is used to simulate the rest of the flow domain. The interface between RAS and LES domains uses the RAS turbulence quantities as a means of obtaining length scales that are used in computing the vorticity that is required to trigger a proper energy cascade within the LES part of the flow field. The ELES approach allows for substantial computational savings since it allows for different mesh resolutions in various parts of the computational domain as needed. The objective of this work is to observe at a lower computational cost, the local flow features that cannot be resolved in a RAS approach. A comparative analysis between RAS and ELES approaches for this turbomachinery problem is then presented.},
keywords = {ELES, Flowfield, LES, RAS, Simulation},
pubstate = {published},
tppubtype = {article}
}