Water Turbines

Computational Fluid Dynamics (CFD)
Commercial Codes • ANSYS CFX: Fluid Dynamics • CFD ACE, COMSOL: Multi-physics OpenSource Code (Code Development) • OpenFOAM Meshing • ICEM CFD • Pointwise

Thomas Staubli, Product and Application Training, Rittmeyer, 20. Feb. 2012

Computational Fluid Dynamics (CFD)
• Theoretical foundations of CFD • How to setup a CFD Project • Use of CFD for optimizationof the Acoustic Transit Time (ATT) measurement • Use of CFD for uncertainty estimation • Challenges in CFD at high Reynolds numbers • Some examples

Thomas Staubli, Product and Application Training, Rittmeyer, 20. Feb. 2012

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Theoretical foundations of CFD
The physics of classical fluid mechanics: • Continuity equation (mass conservation) • Momentum equation (force balance) • Energyequation (energy conservation) The system of momentum equations for unsteady viscous flow was formulated in the mid-19th century and is since known as the Navier-Stokes equations

Thomas Staubli, Product and Application Training, Rittmeyer, 20. Feb. 2012

Theoretical foundations of CFD
Navier-Stokes equations: • Partial differential equations • Numerical solutions  Prediction velocity fields Prediction pressure fields  (Temperature fields)

Thomas Staubli, Product and Application Training, Rittmeyer, 20. Feb. 2012

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Theoretical foundations of CFD
Turbulence: • Statistical methods (k-, k-, SST-model) • High dissipation  Constant supply of energy needed  Turbulent kinetic energy

Thomas Staubli, Product and Application Training, Rittmeyer, 20. Feb. 2012

Theoreticalfoundations of CFD
Quality: • Mesh quality (finite volumes)  Structured - unstructured  Size (number of elements), angles, aspect ratios  Boundary layer resolution • Discretization scheme (second order)  Iterative solution  Convergence

Thomas Staubli, Product and Application Training, Rittmeyer, 20. Feb. 2012

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Theoretical foundations of CFD
Mode of CFD-simulation: • stationary(steady) • transient (unsteady)

Thomas Staubli, Product and Application Training, Rittmeyer, 20. Feb. 2012

Best practice guidelines for CFD
A report on a CFD study should contain a section on “Numerical Method and Chosen Parameters”: • Selection of the simulation domain • Assumptions and simplifications of the geometry • Mesh (type of mesh, number of elements, resolution in flow direction andin the flow cross section, min/max y+, min/max angle, max aspect ratio, max volume ratio, …) • Assumptions of boundary conditions (constant values, assumed distributions) • Positions and properties of interfaces • Applied models (turbulence, cavitation, …) • Numerical parameter settings (procedures, turbulence model, wall functions) • Time steps for transient simulations, number of inner loops •Convergence (local and global) • Calculation of output data • Mesh study/error estimation
Thomas Staubli, Product and Application Training, Rittmeyer, 20. Feb. 2012

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Best practice guidelines for CFD
A report on a CFD study should contain: • Choice of boundary conditions (constant or distribution) • Choice of turbulence model • The numerical advection scheme should be of second order •Convergence (residuals) should be reported in terms of  mean values (RMS)  maximum residuals and their location

Thomas Staubli, Product and Application Training, Rittmeyer, 20. Feb. 2012

Review of a CFD study
The mesh should be judged with respect to: • Resolution in streamwise and transversal direction, • resolution in areas of high acceleration or deceleration • resolution in zones of flowseparation and zones of high gradients • Resolution in curved flow channels (secondary flow prediction) • Resolution of the boundary layer flow (y+ max Application of CFD as tool can be only justified on basis of accuracy and level of confidence on results

Best Practice in CFD computations is a key competence

Thomas Staubli, Product and Application Training, Rittmeyer, 20. Feb. 2012

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