CFD: What is it good for? Tom O Mahoney TNO Fluid Dynamics
Introduction to CFD
CFD - Computational Fluid Dynamics Computational the using of computers to simulate the physics of fluids Fluid Either gas or liquids Dynamics changes in time Usually CFD is used to refer to the simulation of fluid dynamics using finite volume methods (described in next slide) and not more reduced or simplified models
Computational Fluid Dynamics finite volume methods Take the simulation area and divide it up into small cells / boxes (called volumes) just like in FEM (but for where the fluid is) On the boundaries of the whole area say what the fluid is doing Say under what conditions the fluid starts Use the principles of conservation of mass, momentum and energy to determine how the fluid changes with time Mass Momentum - in Energy Mass Momentum - out Energy + change
What makes a CFD person? Knowledge of fluid dynamics Equations of motion Dominant forces for the situation at hand Knowledge of numerical methods Accuracy, stability of numerical algorithms Numerical treatment of physical models Knowledge of the programs / CFD software FLUENT, StarCCM+, CFX, OpenFoam, Xstream, Comsol CAD and meshing software
First application Internal flows in turbomachinery Large-Eddy Simulation of Ingestion of Hot Annulus Gas in Turbine Rim Seals
Rolls-Royce - Trent 1000 Bypass Fan Low pressure turbine Intermediate pressure turbine Intermediate pressure compressor High pressure compressor Combustion chamber High pressure turbine
What is the problem? Temperatures in the turbine are very high (900-1100ºC) The higher the temperature the more efficient the turbine But the temperature is higher than the melting point of the metal!! Metal has to be cooled, reduce thermal stress because the mechanical stress is already high (12,000-15,000rpm) Plane doesn t fall out of the sky! (the turbine last longer without having to be maintained / replaced)
Simulation of a turbine cavity for a jet engine Stationary Rotating
The actual CFD domain Stationary Rotating The cavity region between the discs
Setting the boundary conditions Inlet NGV Hot flow Rotor Blade Outlet Ingress flow Egress flow STATIONARY PART ROTATING PART TURBINE DISC Inlet Cool flow Too much ingestion has severe detrimental effect on turbine disc life
Results - turbulence Frames at same time but position changes in flow direction
Results - ingestion Model name Large-Eddy Simulation Unsteady RANS Two different models give very different answers
Second application Multiphase flows in process machinery Slug flow in a pipe with bends
Shell - Process installation There are lots of pipes
Experiments at TNO Sharp bend Water / air system Measuring pressure along the pipe Measuring force on the bend Visualise flow regime
Creating a CFD mesh
Results volume fraction of water It is very difficult to model the fast changing interface using CFD
Results effect on the pipe structure Also calculate the displacement or bending of the pipe
Result comparison of forces Blue is the experiment and red the simulations
CFD: Do it - yourself CFD with open source software OpenFoam
Download and install Ubuntu This is best www.ubuntu.com
Run a terminal Type the commands in Useful commands: ls list directory contents mkdir make directory cd <name> - go to <name> cd../ - go down a directory rm <name> remove a file rm r <name> - remove folder cp <name> <new-name> - copy file
Download and install OpenFoam (and parafoam) Follow these steps in an Ubuntu terminal www.openfoam.com
Go to a tutorial directory and run a case A case directory has folders 0, system and constant All tutorials are set up so that you should be able to create the mesh: Type blockmesh <enter> And then to run the solver Type icofoam <enter> Finally open the results in paraview: Type parafoam <enter> Note: the solvers vary, could be simplefoam, pisofoam etc.. Note also: some tutorials don t work like this, best to look in the incompressible folder
Look at results in parafoam www.paraview.org
That s CFD in a nutshell