Introduction to Fluent

1. Introduction to Fluent

2. What CFD packages do • Aim is to solve, numerically, the equations of motion (continuity and motion) for a fluid for a given flow geometry: • Plus transport equations for any additional models such as heat transfer and/or turbulence models

3. What CFD packages provide • Offer a CAD/GUI based facility for generating meshes or grids for a flow geometry • Provide a GUI based facility to set up flow problems using the grid • Can enable established models without hard coding • Provide reasonably robust solvers to get converged numerical solutions • Provide post processing tools to analyse results • Provide a facility to implement user defined models • The UQ licence is for Fluent

4. Why are you doing CFD? • If you have an accurate CFD simulation then • You will gain a detailed understanding of the flow physics. This should enable you to understand why particular things happen, which may be difficult or impossible to verify experimentally • You can use the simulations to quickly assess geometric and flow modifications for design or R&D purposes without resorting to prototyping • This is CFD from an industrial perspective as it should be

5. Why are you doing CFD? • If you don’t have an accurate simulation then • You are doing CFD research • You need to think about experimental verification • You need to get a competent grasp of the physical assumptions behind the models so that you find out what is going wrong and propose modifications • This is a PhD project • This is also as it should be

6. In Practice • Industry may see CFD as a quick fix • Some dope who doesn’t really understand the physics of flows will cook up an apparent solution that looks pretty but results in a total stuff up when implemented. This is easy to do

7. The aim of this course • To teach you not to be that dope (hopefully)

8. A warning • CFD is a very young branch of engineering • Even though the modeling capabilities of CFD packages appear to be immense, the modeling still grossly simplifies the behavior of turbulent flows • Most (All??) turbulence models are empirical and it may come as a surprise but are based on some very smelly assumptions (particularly the k-e model!!!!) • Advanced turbulence modeling (RSM or LES) is still under development • CFD is only now being applied to multiphase flows

9. Questionable assumptions of the Standard k-e turbulence model • Makes the Bousinessq approximation • treats the turbulence only as a turbulent kinetic energy and makes no attempt to model scales of turbulence • this intrinsically assumes that turbulence is iso-tropic • the model is an empirical model tuned for high Re flows • but it wasn’t until the mid 1990’s that computers came into wide spread use that could even use this model for engineering flows !!!! • but for a lot of problems it gives accurate results • for others it gives poor results

10. Example of what to expect • Cyclone separator for mineral processing • concentration of mineral slurries on density or particle size • swirl induced due to tangential feed • flow reversal • air core forms due to negative pressure induced by swirl

11. CFD modeling of cyclone separators • Swirl and flow reversal results in highly strained flows • air core makes modeling at least a two phase flow problem • if you consider the solids it is a three phase flow problem

12. Air core for a DMC with a poor mesh • t-grid mesh

13. Air core for a DMC with a better mesh • cooper mesh

14. but….. • Even with the better mesh the flow out the underflow is too large • Experimental underflows are ~15% of feed flow rate. I am getting ~40% reporting to underflow • This happens with the RSM turbulence model which is supposed to be the best model for swirling flows if you survey the literature

15. so... • I have have to think fairly hard about the physics over the next few months • and also think about the funding for project

16. Basic Steps in CFD are • Mesh Generation - Gambit • Problem set up - partly Gambit, partly Fluent • Initialisation and solution - Fluent • Validation • Maybe start again

17. Gambit • This is the meshing tool used by Fluent • Geometric model of the flow domain is drawn using the CAD functionality (points, lines, faces, volumes) • Mesh is generated (line, face and volume meshes) • and mesh is checked • Nature of the flow domain is specified

18. Gambit • Boundary condition types are specified (wall, velocity inlet, outlet) • Solver type is specified (Fluent 4, 5, 6) • Mesh is exported to a file *.msh • Mesh and geometry is stored in a database *.dbs

19. Fluent • Mesh is read into Fluent • Mesh is checked by Fluent and scaled • Type of fluid is specified in Fluent (eg water air) • If flow is turbulent then specify a turbulence model • Boundary conditions are chosen • Other models are enabled depending on the nature of the problem

20. Fluent • Implement user designed functions • Discretization methods are chosen • Problem is initialized • Solver is started • Once convergence is reached (non-trivial), the solution is checked for validity • Intermediate cases should be saved along the way (you’ll learn this the hard way)