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CONSIDERATIONS ABOUT THE ROLE OF ATMOSPHERIC STABILITY IN CFD MODELS

CONSIDERATIONS ABOUT THE ROLE OF ATMOSPHERIC STABILITY IN CFD MODELS. C. Montavon, ANSYS UK. Contents. Stable standard atmosphere in free stream Surface heat fluxes. Mountain waves Possible changes in flow topology Affects Mixing Boundary layer height Boundary layer regeneration.

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CONSIDERATIONS ABOUT THE ROLE OF ATMOSPHERIC STABILITY IN CFD MODELS

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  1. CONSIDERATIONS ABOUT THE ROLE OF ATMOSPHERIC STABILITY IN CFD MODELS C. Montavon, ANSYS UK

  2. Contents • Stable standard atmosphere in free stream • Surface heat fluxes • Mountain waves • Possible changes in flow topology • Affects • Mixing • Boundary layer height • Boundary layer regeneration • Buoyancy terms in momentum and turbulence equations • Low Froude number • Always, • More or less so depending on surface stability

  3. How: Stability implementation Continuity Momentum Turbulence quantities (e.g. k-e) Energy (potential temperature) gravity waves mixing, boundary layer height

  4. Effects: Mountain waves Potential temperature

  5. Effect: change in flow topology Neutral Stable • Low Froude number • High terrain elevation • Strong stability • Low wind speed

  6. Effect: reduced boundary layer height, reduced mixing • Equilibrium profiles on flat terrain, roughness z0, Coriolis parameter f Eddy diffusivity Turbulence kinetic energy

  7. Effect: reduced boundary layer height and momentum fluxes • Equilibrium profiles on flat terrain, roughness z0, Coriolis parameter f • Reduction of fluxes  expected slower boundary layer regeneration downstream of forests or large arrays Increasing free stream stability Vertical momentum fluxes

  8. Effect: Boundary layer height 25 75 100 Increasing free stream stability Increasing surface stability Source: C. Montavon, 1998, Simulation of atmospheric flows over complex terrain for wind power potential assessment, Ph D thesis, EPF Lausanne, http://library.epfl.ch/en/theses/?nr=1855

  9. Example of An Suidhe Normalised wind speed Normalised TI 50m 30m 10m

  10. Stability in ANSYS CFD • Free stream stability tested as part of WindModeller developments • Preliminary results • Validation successful of mountain waves from 2D theoretical test case (witch of Agnesi mountain profile) • Application on real terrain (An Suidhe, Scotland), see poster session for details • inclusion of stability helped improve modelling accuracy, particularly on the turbulence intensity predictions • also observed strong sensitivity of results to domain vertical extent when including stability. More work required to find best configuration for top boundary condition.

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