1 © 2013 ANSYS, Inc. February 28, 2014 ANSYS Confidential 15.0 Release Lecture 7: Turbulence Modeling Introduction to ANSYS Fluent
2 © 2013 ANSYS, Inc. February 28, 2014 ANSYS Confidential Lecture Theme: The majority of engineering flows are turbulent. Simulating turbulent flows in Fluent requires activating a turbulence model, selecting a near-wall modeling approach and providing inlet boundary conditions for the turbulence model. Learning Aims: You will learn: •How to use the Reynolds number to determine whether the flow is turbulent •How to select the turbulence model •How to choose which approach to use for modeling flow near walls •How to specify turbulence boundary conditions at inlets Learning Objectives: You will be able to determine whether a flow is turbulent and be able to set up and solve turbulent flow problems Introduction Introduction Reynolds Number Models Near-Wall Treatments Inlet BCs Summary
3 © 2013 ANSYS, Inc. February 28, 2014 ANSYS Confidential •Flows can be classified as either : Laminar (Low Reynolds Number) Transitional (Increasing Reynolds Number) Turbulent (Higher Reynolds Number) Observation by Osborne Reynolds Introduction Reynolds Number Models Near-Wall Treatments Inlet BCs Summary
4 © 2013 ANSYS, Inc. February 28, 2014 ANSYS Confidential Reynolds Number •The Reynolds number is the criterion used to determine whether the flow is laminar or turbulent •The Reynolds number is based on the length scale of the flow: •Transition to turbulence varies depending on the type of flow: •External flow •along a surface : ReX> 500 000 •around on obstacle : ReL> 20 000 •Internal flow : ReD> 2 300 ..ReLU Letc.,dd,x,LhydIntroduction Reynolds Number Models Near-Wall Treatments Inlet BCs Summary
5 © 2013 ANSYS, Inc. February 28, 2014 ANSYS Confidential Overview of Computational Approaches •Three basic approaches can be used to calculate a turbulent flow DNS (Direct Numerical Simulation) •Numerically solving the full unsteady Navier-Stokes equations •Resolves the whole spectrum of scales •No modeling is required •But the cost is too prohibitive! Not practical for industrial flows! •Solves the filtered N-S equations •Some turbulence is directly resolved •Less expensive than DNS, but the efforts and computational resources needed are still too large for most practical applications •Available in Fluent but not discussed in Introductory Training •Solve time-averaged N-S equations •All turbulent motion is modeled •For most problems the time-averaged flow (and level of turbulence) are all that is needed •Many different models are available LES (Large Eddy Simulation) RANS (Reynolds Averaged Navier-Stokes Simulation) Introduction Reynolds Number Models Near-Wall Treatments Inlet BCs Summary
6 © 2013 ANSYS, Inc. February 28, 2014 ANSYS Confidential Turbulence Models Available in Fluent RANS based models One-Equation Model Spalart-Allmaras Two-Equation Models Standard k–εRNG k–εRealizable k–ε*Standard k–ωSST k–ω*Reynolds Stress Model k–kl–ωTransition Model SST Transition Model Detached Eddy Simulation
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