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ND Numbers in CFD and Setting up a Problem

ND Numbers in CFD and Setting up a Problem. HARP REU Program 2012 Susan T. Brown, Ph.D. 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

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ND Numbers in CFD and Setting up a Problem

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  1. ND Numbers in CFD and Setting up a Problem HARP REU Program 2012 Susan T. Brown, Ph.D.

  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

  3. Most-used numbers in CFD Reynolds’ number = Re = inertia/viscosity = ρvL/μ Froude number = Fr = inertia/gravity = v2/Lg Peclet Number = Pe = convection/diffusion = vL/к Prandtl number = Pr = viscosity/conduction = μ/ρк Strouhal number =So = characteristic length = fL/u distance travelled during period t

  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 that’s not what they really wanted. They don’t care a whit about the flow field. (Practical people rarely do!)

  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.

  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?

  7. What is the Geometry like? • Simple, or Complex? • 2D or 3D? • Does it matter for our problem?

  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?

  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?

  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

  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 don’t 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.

  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? • Don’t 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!

  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?

  14. Have I thought of everything??

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