MER331 – Week 6

1. MER331 – Week 6 An Introduction to Computational Fluids Dynamics Professor Anderson

2. What is CFD? • the analysis of systems involving fluid flow, heat transfer,chemical reactions, by means of computer based simulation • 17th century the “hydraulicists” • 18th and 19th - the “mathematicians” • 1960 - two fold approach – theory and experiments • Now we have CFD as a tool that complements theory and experiments (it will never replace these!) • CFD is both a research tool and design tool

3. What is CFD? • CFD has lagged behind developments in stress analysis codes • Powerful computers now make CFD more accessible • Advantages of CFD over experiments • reduction in lead time and costs of new designs • ability to study systems where controlled experiments are difficult to perform (large systems) • ability to study systems under hazardous conditions at and beyond normal performance limits • detail of results

4. Applications • Aerospace • Automobile and Engine Applications • Appliances • Boats • Computers

5. How Does a CFD Code Work? • CFD codes are structured around the numerical algorithms that can tackle fluid flow problems • Three main elements: • Pre Processor • Solver • Post Processor

6. 1) Pre-Processor • Consists of the input of a flow problem to CFD • User Activities: • define geometry & generate grid (50% time) • selection of phenomena to be modeled • definition of fluid properties • specification of boundary and initial conditions

7. 2) Solver Three primary numerical solution techniques • finite difference, finite element, finite control volume The numerical method performs the following: • Approximates the unknown variables by simple functions • Discretization by substitution of the approx-imations into the governing flow equations and subsequent mathematical manipulations • Solution of the algebraic equations

8. Solver - Finite Difference Method • Finite difference methods describe the unknowns f of the flow problems by means of point samples at the node points of a grid co-ordinate lines • Truncated Taylor series expan-sions are used to generate finite difference approximations of the derivatives of f in terms of point samples of f at each grid point and its immediate neighbors

9. Solver - Finite Volume Method • Based on control volume formulation of analytical fluids • The domain is divided into a number of control volumes (aka cells, elements) - the variable of interest is located at the centroid of the control volume. • The differential form of the governing equations are integrated over each control volume. • Finite difference approximations are substituted for the terms in the integrated equations (discretization) converts the integral equations into a system of algebraic equations. • Set of algebraic equations are solved by an iterative method.

10. 3) Post Processor • Provides a “user friendly” (??) way to look at the results of a simulation • Domain geometry and grid display • Vector Plots • Contour Plots • Particle Tracking

11. Problem Solving With CFD • Results of CFD are at best as good as the physics embedded in it as at worst as good a its operator • THESE PROBLEMS ARE COMPLEX • Prior to running a simulation there is a stage of identification and formulation of the flow problem in terms of the physical and chemical phenomena that need to be considered. • A successful simulation has • converged solution • grid independence

12. In Lab Today • We will use StarCCM which is a one of the major commercial CFD codes. • Complete the tutorial on Introduction to STARCCM (handout of online) • Geometry File is in: C:\Program Files\CD-adapco\STAR-CCM+7.06.012\DOC\startutorialsdata\introductions\data • To Pass In: Nothing!