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cfd
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• Lectures
• Computer sessions
• Assignments:
– Intro to CFD (CFX)
– Forward-facing step (CFX)
– 1D convection-diffusion transport (Matlab)
– RANS derivation
– Flow around simplified car model (CFX)
Organization
• Jonas Lantz: examiner, lecturer, labs
• Matts Karlsson: lecturer
• Magnus Andersson: course assistant
• Anna Wahlund: administration
• Invited Guests
Personnel
• An introduction to Computational Fluid Dynamics –The Finite Volume Method, HK Versteeg & W Malalasekera, PEARSON/PRENTICE HALL, 2nd Edition, 2007
ISBN: 978-0-13-127498-3
• Journal papers
• Assignment descriptions
• Tutorials, ANSYS material.
Literature
Course web page
http://www.iei.liu.se/mvs/utbildning/avancerade-kurser/tmmv08?l=en … Or google: TMMV08, first hit
Contains: assignments, tutorials, additional material, this lecture, etc…
Assignments Assignment Name Available from Due Points
1 Intro to CFX Now 27/1 PASS/FAIL
2 Forward-facing step 30/1 10/2 1,2
3 1D convection- diffusion 6/2 24/2 1,2
4 RANS derivation 20/2 3/3 PASS/FAIL
5 Ahmed car model 20/2 22/3* 3,6,9
*mandatory lecture 13/3 13.15-17.00 Points Grade
0-4 FAIL
5-7 3
8-9 4
10-13 5
Time budget:
Lectures 30 h + labs 56 h + own time = 160 h (6 ECTS credits)
Day Type,
Time
Teacher,
Room
Content
Tue
21/1
Lecture
10-12
JL
A33
Course intro, assignments, etc., Intro to CFX, assignment 1,
Classification of physical behaviors, chapter 1 and 2.6
Wed
22/1
Lecture
08-10
MK
A33
Governing equations, continuity, chapter 2
Thu
23/1
Lab
17-21
JL
VALHALL
Assignment 1: introduction to CFX, tutorials
Mon
27/1
Lab
17-21
JL
VALHALL
Assignment 1: introduction to CFX, tutorials
Tue
28/1
Lecture
10-12
MK
A33
Governing Equations, momentum, chapter 2
Wed
29/1
Lecture
8-10
JL
A34
CFD: best practice, verification and validation
tips and tricks, chapter 10
Thu
30/1
Lecture
13-17
JL
A33
CFD: Live demo, how-to
Introduction to Assignment 2
Thu
30/1
Lab
17-21
JL/MA
VALHALL
Assignment 2: Forward-facing step (CFX)
Mon
3/2
Lab
17-21
JL/MA
VALHALL
Assignment 2: Forward-facing step (CFX)
Thu
6/2
Lecture
13-17
JL
A34
Transport eqn. Basic numerics, Discretization schemes,
Diffusion + Convection/Diffusion, assignment 3, chapter 4 and 5
Thu
6/2
Lab
17-21
JL/MA
VALHALL
Assignment 2: Forward-facing step (CFX)
Mon
10/2
Lab
17-21
JL/MA
VALHALL
Assignment 3: 1D-CD (Matlab)
Wed
12/2
Lecture
8-10
JL
A33
Discretization schemes, Diffusion + Convection/Diffusion
Finite volumes , chapter 4 and 5
Thu
13/2
Lecture
13-17
JL
BL32
Turbulence, intro and modeling, RANS eqns. Tensor notation, chapter 3
Thu
13/2
Lab
17-21
JL/MA
ALFHEIM
Assignment 3: 1D-CD (Matlab)
Mon
17/2
Lab
17-21
JL/MA
ALFHEIM
Assignment 3: 1D-CD (Matlab)
Tue
18/2
Lecture
10-12
JL
A33
Turbulence, RANS
Law-of-the-wall, chapter 3
Thu
20/2
Lecture
13-17
JL
A33
Turbulence, RANS, Law-of-the-wall , chapter 3
Intro to Assignment 4 and 5
Thu
20/2
Lab
17-21
JL/MA
BIFROST
Assignment 3: 1D-CD (Matlab)
Mon
24/2
Lab
17-21
JL/MA
ALFHEIM
Assignment 5: Ahmed (CFX)
Tue
25/2
Lecture
10-12
<extra>
Wed
26/2
Lecture
8-10
<extra>
Thu
27/2
Lecture
13-17
BL32
<extra>
Thu
27/2
Lab
17-21
JL/MA
BIFROST
Assignment 5: Ahmed (CFX)
Mon
3/3
Lab
17-21
JL/MA
ALFHEIM
Assignment 5: Ahmed (CFX)
Tue
4/3
Lecture
10-12
<extra>
Wed
5/3
Lecture
8-10
<extra>
Thu
6/3
Lecture
13-17
A35
<extra>
Thu
6/3
Lab
17-21
JL/MA
BIFROST
Assignment 5: Ahmed (CFX)
Mon
10/3
Lab
17-21
JL/MA
ALFHEIM
Assignment 5: Ahmed (CFX)
Tue
11/3
Lecture
10-12
<extra>
Wed
12/3
Lecture
8-10
<extra>
Thu
13/3
Lecture
13-17
JL
A33
ANSYS Presentation + other companies
Poster presentation Ahmed, closing
Note: lectures with content <extra> will not be given, unless stated otherwise.
Assignment 1, intro to CFX
Intro to programs Tutorials Easy questions
Assignment 2, Forward-facing step First ‘real’ simulation Compare with measurements Report
1p: perform simulations, discuss results
2p: above + excellent discussion and convincing arguments that your simulations are accurate.
Assignment 3, Convection-diffusion
Your own implementation Follow example in course book Numerical schemes Report
1p: working code and discussion on numerical schemes
2p: above + quantification of accuracy
Assignment 4, RANS derivation (in x)
No scheduled time, do it at home Hand in derivation
Assignment 5: vehicle aerodynamics Ahmed Body – simplified car model
Assignment 5: vehicle aerodynamics
Compare with wind tunnel data Design challenge - spoiler Present own poster for invited guests + report 3p: basic simulation + spoiler 6,9p: dig deeper into a topic of your own choice.
So…. Lets start!
(with a brief introduction to CFD)
• Stationary / Transient • Laminar / Turbulent • Compressible / Incompressible • Subsonic / Supersonic • Inviscid / Viscous • Internal / External • Heat transfer
Conduction Convection Radiation
Recap: Fluid flow characteristics
Recap: Fluid Mechanics (more in next lecture)
Conservation of mass and momentum:
The Navier-Stokes equations
(stationary, incompressible,…)
CFD: trying to solve the Navier-Stokes equations
• No known analytical solution exist (?) • Resort to numerical methods to solve the PDEs:
• Finite differences • Finite elements • Finite volumes
Wikipedia: CFD is a branch of fluid mechanics that uses numerical methods and algorithms to solve and analyze problems that involve fluid flows.
Approximations of the governing equations
CFD - ColorFul Display?
Space shuttle re-entry
Turbulent blood flow in the human body
Jonas Lantz 18
Weather prediction
Jonas Lantz 19
CFD - ColorFul Display?
Limitations of CFD
• Physical models CFD solutions rely upon physical models of real world processes (turbulence, compressibility, chemistry, multiphase flow, etc.). The CFD solutions can only be as accurate as the physical models on they are based • Boundary conditions The accuracy of the CFD solution is only as good as the initial/boundary conditions provided to the numerical model (type/value) • Numerical errors Discretization errors due to approximations in the numerical models. (goes to zero as the grid is refined) Convergence errors Round off errors • User Errors
You! Knowledge of CFD and fluid mechanics is very important in order to get an accurate simulation.
Jonas Lantz 20
Theoretical Fluid dynamics The foundation. Everyone HAS to learn it Experimental Fluid dynamics (XFD) Usually, everyone believes it except the person that conducted the experiment Computational Fluid dynamics (CFD) Usually, no one believes it except the person that performed the calculations As an engineer you should know the pro’s and con’s of all three methods, and should be in a position to assess which one is best under the circumstances.
Words of wisdom
Jonas Lantz 21
Jonas Lantz 22
Words of wisdom (2)
CFD requires:
Knowledge
CFD requires:
Knowledge, skill
CFD requires:
Knowledge, skill and gut feeling
CFD in Practice
Drawing/CAD Create geometry
Mesh
Solver settings: Boundary conditions Numerical accuracy Flow models Fluid …
Run the simulation
Post-process results Report
How to
Jonas Lantz 27
Already know?
CFD
http://www.bakker.org/
Mesh Fill the computational domain with mesh cells, where the governing equations are solved. The mesh represents the spatial resolution of the simulation
Jonas Lantz 28
Mesh terminology
Jonas Lantz 29
http://www.bakker.org/
Tetrahedrons
Polyhedrons
Running the simulation
Convergence criteria, “accuracy tolerance” (here 1e-4)
The lines represent the solution of conservation of mass (1 eqn) and conservation of momentum (3 eqns)
Jonas Lantz 31
Running the simulation (2)
Simulation time: 15 min on a laptop - several weeks (months) on > 1000 CPUs Advanced fluid models (turbulence/chemical) require more CPU & memory compared to simpler models (laminar flow). Normally have to make a tradeoff between the spatial and temporal accuracy (mesh and time step) with available computational resources (and license cost (!))
http://www.nsc.liu.se
CPU Power ↔ Resolution
Jonas Lantz 32
Post-processing Obtained directly: Stream lines Path lines Velocity vectors Pressure contours Wall shear stress Forces on/in walls Wall motion Concentration Shock waves Temperature … Derived parameters: pressure drop loss coefficients turbulent quantities Cd, Cl, Cm,… … Animations
Jonas Lantz 33
Drawing/CAD Create geometry
Mesh
Solver settings: Boundary conditions Numerical accuracy Flow models Fluid …
Run the simulation
Post-process results Report
Mesh independency study
Verification: Do the results make sense? Are the trends right? Does it agree with previous calculations on similar configurations?
Validation: Does the result agree with theory or experiment?
How to (2)
Jonas Lantz 34
Mesh independency study
Must always be done in order to ensure that the spatial resolution is sufficient. Evaluate parameter of interest on each mesh (e.g. drag coefficient of car, Cd) Never compare with experimental data until you have a mesh independent result
Mesh 1 Mesh 2 Mesh 3
Cd = 0.40 Cd = 0.32 Cd = 0.31
20% difference (!?)
3 % difference (ok?) 35
Fluid-structure interaction (FSI)
Simulations commonly assumes rigid walls – valid assumption?
start Fluid solver Results
convergence
advance in time
Normal CFD simulation with rigid walls
Jonas Lantz 36
Fluid-structure interaction (FSI)
Simulations commonly assumes rigid walls – valid assumption? In FSI simulations, the wall motion due to flow motion and pressure is computed. Obtains both motion and strain/stress in the wall Need two solvers: one for fluid simulation and one for solid (wall) simulation
start Fluid solver Results
convergence
advance in time
Solid solver
convergence
combined convergence
FSI simulation Adds approx. 10-100x more simulation time ( = weeks!)
Jonas Lantz 37
Multiphysics! Require both solid and fluid mechanics
What is the drag coefficient Cd of a T-Rex dinosaur ?
With CFD we can finally compute things that have troubled mankind for centuries…
http://www.bakker.org/
http://www.bakker.org/
http://www.bakker.org/
http://www.bakker.org/