The standard, ReNormalisation Group RNG and realisable kemodel

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The standard, ReNormalisation Group (RNG) and
realisable k-e-model

• Stéphanie Gagné, Kimmo Neitola, Hanna Manninen
  and Anne Hirsikko

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Introduction I

• Two-equation models solution of two transport
  equations (for k and e) allows to determine the
  turbulent velocity and length scales
  independently
• k the turbulent kinetic energy
• e dissipation rate of kinetic energy

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Introduction II

• Models are semi-empirical
• Used especially in industrial flow and heat
  transfer simulations
• Robust, economy, reasonable accuracy for a wide
  range of turbulent flows

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Standard k-e model

Gk generation of turbulent kinetic energy due to
the mean velocity gradients Gb generation of
turbulent kinetic energy due to the buoyancy YM
the contribution of the fluctuating dilatation in
compressible turbulence to the overall
dissipation rate S Source s Prandtl number
Fluent Inc. January 11, 2005
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Standard k-e model

• High Reynolds number required -gt valid only for
  fully turbulent flows!

wiki.elearning.ubc.ca
www.qinetiq.com
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RNG k-e model

• Following improvements compared to standard k-e
  model
• Additional term in e equation improves the
  accuracy when modelling rapidly strained flows
• The effect of swirl turbulence is included in the
  RNG model
• The RNG model enables to use Prandtl numbers
  (turbulent viscosity/turbulent heat conductivity)
  as variables
• Enables to use lower Reynolds numbers than in
  standard model

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Effect of turbulence model on spray spreading
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Realisable k-e model

• Compared to standard model
• New formulation for the turbulent viscosity
• New equation for e derived from an exact equation
  for the transport of the mean square vorticity
  fluctuation

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Realisable k-e model

• Predicts more accurately the spreading rate of
  planar and round jets than standard one
• Provides the best performance of all the k-e
  model versions for separated flows and flows with
  complex secondary flow features
• May produce non-physical turbulent viscosities in
  some situations

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Thank you!