MODELLING: AERODYNAMICS
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l 2-equation models, belonging to the RANS/ URANS category and still today’s industry standard. Ansys’ recommendation here is the SST (Shear Stress Transport) model, a combination of k-epsilon and k-omega turbulence models with a shear stress limiter and automatic wall treatment. Braune says this is the workhorse of many steady-state and unsteady-state simulations; it is efficient
and accurate for a wide range of applications; l Reynolds stress models, which are used less often, because they require significantly more computational efforts than 2-equation models and are less stable in terms of solver convergence. However they have advantages in some areas such as strong streamline
curvature and vorticities; l Scale Adaptive Simulation (SAS), an enhancement and extension of the SST model. It can provide a LES-like behaviour in detached flow regions, but it falls back to the URANS (SST) solution for areas where the requirements for mesh or time discretisation
to resolve the eddies are not fulfilled; l Detached Eddy Simulation (DES), a hybrid approach combining LES and RANS models, to improve prediction capabilities of turbulence models in regions with high
separation potentials; and l Large Eddy Simulation (LES).
‘The basic premise is that it is as easy to do the very complex as it is to do the very easy (and everything in between)’
Auto-selection option
On the specifics of turbulence modelling for aerodynamics in Star-CCM+, CD-adapco offers a wide selection of models ranging from the near ubiquitous k-epsilon model, through k-omega SST (widely used in vehicle aerodynamics) and on to the more sophisticated (and hence computationally expensive) Reynolds stress models (RSM), large eddy and detached eddy simulation (DES and LES) types. Also available are models for quite specific subsets of the aerodynamics market such as Spalart- Allmaras, which is commonly used in the aerospace industry. For each of these there are, of course, a range of options with different ‘flavours’ of each model type, which can be chosen to capture specific features within the flow field. Another option that can be critical in computational aerodynamics is the ability to model the transition from laminar flow to turbulence, and like many other packages Star-CCM+ can identify and model the onset of transition. While Star-CCM+ provides many options, which can be somewhat daunting to inexperienced users, the software can auto-select recommend models to help guide the setup. If the user is more experienced and has need of a specific model, this is also available. The basic premise is that it is as easy to do the very complex as it is to do the very simple (and everything in between). Even so, there is a crucial need for strong customer support says Joel Davison, Star- CCM+ solution champion. He suggests that users look for a company that provides a comprehensive support organisation with every user allocated a dedicated support engineer.
Fig 2: Turbulence models vary in the degree to which they can resolve small eddies, but the tradeoff is computational efforts. Here three methods (SAS, DES, URANS SST) for the same computational mesh and boundary conditions result in different flow fields (copyright Robert Bosch GmbH)
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Enhanced Turbulence Modelling
Aerodynamic testing is a classic flow- simulation application, and besides the ability to simulate airflow over the entire range of engineering-relevant Reynolds numbers – including subsonic, transitional, supersonic and even hypersonic flows – Mentor Graphic’s Concurrent FloEFD offers a range of additional physical models for
SCIENTIFIC COMPUTING WORLD JUNE/JULY 2010
characterisation of a design’s aerodynamic behaviour. This includes a special automatic laminar/turbulent modelling process, an innovative model created to simulate near- wall physics in a very efficient way. In addition, the firm’s FloEFD product supports a dual-wall function modified k-epsilon model that addresses the key weaknesses of standard k-epsilon models such as low Reynolds number flows and high curvature flows. The firm also provides what it terms Enhanced Turbulence Modelling (ETM), which consists of modifications of the k-epsilon model and modifications of the wall function approach to specifying the wall boundary conditions for the Navier- Stokes equations. In addition to turbulence, when simulating
fluid flows it is also necessary to simulate fluid boundary layers of these flows over solid bodies or walls, which is a substantial drawback also due to high velocity and temperature gradients across these layers. To solve the Navier-Stokes equations with conventional turbulence models without resolving the fluid boundary layer by the computation mesh, a wall functions approach can be used. The fluid wall friction and heat flux from the fluid to the wall serve as the wall boundary conditions for the Navier- Stokes equations. Naturally, the main flow’s properties are used as the boundary layer’s external boundary conditions. FloEFD employs Van Driest’s universal
profiles to describe turbulent boundary layers along with two approaches (2-scale wall functions) to fit the boundary layer calculation to the main flow properties depending on whether the fluid mass centres of the near-wall mesh cells are located inside or outside the boundary layer. These two approaches allow the software to overcome the restriction on the mesh density near the walls and use Cartesian meshes.
References
1. ScienceWatch, July/Aug 1995, Stanford University’s George Papanicolauo Seeks Order in Turbulence
www.scientific-computing.com
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