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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
Fan
Intermediate pressure compressor
High pressure compressor
Combustion chamber
High pressure turbine
Bypass
Intermediate pressure turbine
Low 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
The cavity region between the discs
Stationary Rotating
Setting the boundary conditions
STATIONARY
PART
ROTATING
PART
TURBINE DISC
Cool flow
Hot flow
Egress flow
Ingress flow
NGV Rotor Blade
Too much ingestion has severe detrimental effect on turbine disc life
Inlet
Inlet
Outlet
Results - turbulence
Frames at same time but position changes in flow direction
Results - ingestion
Unsteady RANS Large-Eddy Simulation
Two different models give very different answers Model name
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