HARP REU Program 2012 Susan T. Brown, Ph.D. ND Numbers in CFD
and Setting up a Problem
Slide 2
Non-Dimensional Numbers in CFD Ratio of forces or effects Have
no dimensions If properly conceived, will scale WHY IS THIS
IMPORTANT? Use to model experiments Find constants Determine
validity of equation usage
Slide 3
Most-used numbers in CFD Reynolds number = Re =
inertia/viscosity = vL/ Froude number = Fr = inertia/gravity = v 2
/Lg Peclet Number = Pe = convection/diffusion = vL/ Prandtl number
= Pr = viscosity/conduction = / Strouhal number =So =
characteristic length = fL/u distance travelled during period
t
Slide 4
Considerations in CFD, or Where do I start? The actual scenario
(it happens all the time): Um, okay, you are the CFD expert We have
this physical problem. Model it and give us the answer. And, more
often than not, we go away thinking, Yeah, I can do that! And we go
off and create a gorgeous and expensive 2- or 3-D CFD model of
whatever it was, and show them streaklines or velocity vectors that
are beautiful to behold in a 20-minute presentation in full
technicolor. Are they happy? NO! Why not? Because thats not what
they really wanted. They dont care a whit about the flow field.
(Practical people rarely do!)
Slide 5
A Better Place to Start So When you hear, Model this, the first
question should be: 1. What do you want to know? Pressure, Force,
Velocity, what exactly? And the second question should be: 2.How
accurate does it have to be? Answers to these two questions will
determine if you should do a Back-of-the-envelope calculation,
rough parameter model, or a full 3-D geometrical model.
Slide 6
What Physics are Involved? What is the fluid? Gas, liquid? Is
it non-compressible? Is it Newtonian? What flow regime are we
dealing with? Re? What other non-dimensional numbers are important?
Do we need the energy equation? Heat transfer?
Slide 7
What is the Geometry like? Simple, or Complex? 2D or 3D? Does
it matter for our problem?
Slide 8
Look in the Literature Has this problem or anything similar
been done before? Similar solution available, at least for
comparison? What software did they use? What software recommended
for these type of problems?
Slide 9
What software/hardware do you have available? Where will you
run your problem? What is available for this type of problem? Is it
freeware, or do you have to buy a license? Is it easy to learn, do
you already know how, or will you have to attend training? Is there
a resident expert? Which software fits best?
Slide 10
Now that you have chosen your software, what next? Build your
geometry Does the software have its own tool for that, or do you
have to use something else. (Oops, maybe I want to use that other
software that does) Build your computational grid NOT the same as
geometry! Can I use a uniform grid? May need to refine in places
where there are smaller scale phenomena occurring. Follow the rules
wrt gridding! NO sharp corners NO hi aspect ratios between
neighboring grids Square with gradual change leads to stable
computation
Slide 11
What next, 2 Determine your domain boundaries. Solution will be
inaccurate a priori at the boundary unless known, so set any
boundary that is not a wall far away from the action. Want it large
for solution accuracy, but small for computation time. Determine
boundary conditions. Must be known. If you dont know them, choose
boundaries where you do. Choose initial conditions. The closer to
the final solution you choose your ICs, the shorter your simulation
will have to run.
Slide 12
What next, 3.. Choose computational models. Do you need a
turbulence model? What is available in your code? What works best
for your type of problem? Dont get too fancy to start, use the
default. Do you have a choice of computational solvers? Use the
default to start unless your type of problem requires the special
option. Do you have multi-phase flow? Put in physical properties.
The code does not magically know you have ocean water instead of
koolaid!
Slide 13
What next, 4 Remember numerical parameters. Do you need to set
the time step? Are you running just a few time steps to start, to
see what happens, before you start a big batch job? How are you
outputting the results? Where are results being stored for viewing?
Are you ready to submit your job?
