student profile: Mr Asiful Islam


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Thesis work

Thesis title: Proposal and implementation of a novel high-order hybrid turbulence modelling algorithm for automotive aerodynamics

Supervisors: Ben THORNBER , Gareth VIO

Thesis abstract:

Hybrid turbulence modelling strategies have recently grown in popularity amongst the wider Computational Fluid Dynamics (CFD) community, especially with the increasing number of commercial codes offering variants such as Detached-Eddy Simulation. The limitations of Reynolds-Averaged Navier Stokes (RANS) are often exposed in flow configurations which are inherently unsteady and such transient information is often crucial in studies of aero-acoustics or fluid-structure interaction.

Turbulent flow fields around bluff bodies are of great interest both for fundamental and applied fluid dynamics research. Bluff bodies generate complex flow features such as regions of recirculation, reattachment and massively separated flows. Although unsteady RANS is typically used for such industrial-scale applications, it often suppresses turbulent content required for more detailed studies and the limited capability to capture massively separated flow is widely accepted. Detached-Eddy Simulation (DES) combines RANS and Large-Eddy Simulation (LES), as it essentially applies an eddy-viscosity based turbulence model, acting in RANS mode in the near-wall regions and as a subgrid model to a Large-Eddy Simulation (LES) in the free-stream.

The novel hybrid turbulence algorithm defined here combines stability-enhanced Spalart-Allmaras model and an Implicit Large-Eddy Simulation. Implicit-LES eliminates the need for an additional filtering operation, which separates the resolved from the unresolved scales in conventional LES. The numerical schemes used in Implicit-LES resolve the inviscid energy cascade however, beyond the inertial range, they provide the required level of dissipation at the grid-scale cut-off. Non-scillatory finite volume methods originally used to capture shock flows are often employed for their non-linear stability, efficiency and ease of implementation. Implicit-LES has now been used to simulate a wide range of different flows in engineering and the effectiveness of this approach mainly arises from the dissipative behaviour of leading-order truncation terms which serve as a built-in subgrid-scale model.

For conventional LES the computational expenses and the grid resolution requirements scale with the Reynolds number for all types of flow regimes. However this scaling exhibits a strong dependency in viscosity-affected regions of wall-bounded flows and this makes fully-resolved LES impractical for aerodynamic studies. Development and implementation of wall-modelling for LES also led to some challenges which are still being explored.


There are several mechanisms which combine to establish this proposed turbulence model. The first is the increase in the dissipation term for the Spalart-Allmaras eddy-viscosity transport equation, which is affected by the characteristic length. Secondly, the modelled turbulent contribution to the total effective viscosity is also multiplied by a blending function which operates at the interface between the RANS and ILES regions. Finally, the separated and turbulent flow regions are identified using a supplementary transport equation for momentum Reynolds number, which restricts the activation of the blending function based on a particular threshold relative to the freestream. Hence, the blending function operates in regions of attached and laminar flow and minimisation of the modelled contribution beyond these regions effectively transfer to ILES mode.

Selected publications

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Journals

  • Islam, A., Thornber, B. (2017). High-order detached-eddy simulation of external aerodynamics over an SAE notchback model. Aeronautical Journal, 121(1243), 1342-1367. [More Information]
  • Nader, A., Islam, A., Thornber, B. (2016). A Comparative Aerodynamic Investigation of a 20 degree SAE Notchback Model. Applied Mechanics and Materials, 846, 79-84. [More Information]

Conferences

  • Islam, A., Thornber, B. (2016). Development and Application of a novel RANS and Implicit LES Hybrid Turbulence Model for Automotive Aerodynamics. SAE 2016 World Congress and Exhibition, Detroit: SAE International. [More Information]
  • Islam, A., Thornber, B. (2015). High-order detached-eddy simulation of external aerodynamics over a SAE notchback model. 7th Asia-Pacific International Symposium on Aerospace Technology (APISAT 2015), Canberra: Engineers Australia.
  • Islam, A., Thornber, B. (2014). A Hybrid RANS-Implicit LES Method for External Aerodynamics. 19th Australasian Fluid Mechanics Conference, Melbourne: RMIT University.

2017

  • Islam, A., Thornber, B. (2017). High-order detached-eddy simulation of external aerodynamics over an SAE notchback model. Aeronautical Journal, 121(1243), 1342-1367. [More Information]

2016

  • Nader, A., Islam, A., Thornber, B. (2016). A Comparative Aerodynamic Investigation of a 20 degree SAE Notchback Model. Applied Mechanics and Materials, 846, 79-84. [More Information]
  • Islam, A., Thornber, B. (2016). Development and Application of a novel RANS and Implicit LES Hybrid Turbulence Model for Automotive Aerodynamics. SAE 2016 World Congress and Exhibition, Detroit: SAE International. [More Information]

2015

  • Islam, A., Thornber, B. (2015). High-order detached-eddy simulation of external aerodynamics over a SAE notchback model. 7th Asia-Pacific International Symposium on Aerospace Technology (APISAT 2015), Canberra: Engineers Australia.

2014

  • Islam, A., Thornber, B. (2014). A Hybrid RANS-Implicit LES Method for External Aerodynamics. 19th Australasian Fluid Mechanics Conference, Melbourne: RMIT University.

Note: This profile is for a student at the University of Sydney. Views presented here are not necessarily those of the University.