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Drag calculation in 3D, 'virtual wind tunnel'?

  1. Sep 28, 2006 #1
    Is there a (freeware?) program to calculate 3D lift and drag for simple flying bodies, e.g. ball, cylinder, cube, wing?

    There are programs to calculate lift and drag for foils (2D programs), e.g. javafoil of Martin Hepperle. Those
    programs could be used to study the flow in 2D only. E.g. airflow around an infinitely long cylinder or wing
    (coming perpendicular to the cylinder axis) could be calculated,
    but not around a ball.

    The drag force is of the form

    F= Cd * SOMEAREA * rho *v **2 /2

    where
    Cd= drag coefficient
    SOMEAREA = ... (!)
    rho= density of air
    v= velocity of air

    The formula for lift force is the except Cd is replaced with Cl, lift coefficient.

    If a symmetric foil ( eg. a NACA-foil) is rotated around its chord, one gets a 3D object, and I need to know its drag to design an object with low drag.

    Eg. if circle rotates, we get a sphere. But knowing the drag coefficient for a circle does not help in calculating the
    drag coefficient of a sphere, or does it?

    (ok, the drag for a sphere can be found in the literature, but what about an object with resembling a drop or almost a drop?)

    So, is there a 'virtual' windtunnel' to get the drag ?
     
    Last edited: Sep 28, 2006
  2. jcsd
  3. Sep 29, 2006 #2
    Hi optimistx:
    Oh if it were only that easy! The "virutal wind tunnel" programs you are asking about are actually called Computational Fluid Dynamic (CFD) programs. I know of no freeware CFD programs, and the ones used professionally are VERY expensive...for good reason. These CFD programs are VASTLY different than the 2-D approximations you refer to. The reason is because CFD programs are built upon the full DiffEq form of the Navier-Stokes fluid flow equations, and solving them numerically given a user-defined grid and boundary conditions. Even for steady and quasi-steady flowfields in 3-D, such programs are the only way you can get quasi-accurate estimates of aerodynamic parameters in 3-D.

    Rainman
     
  4. Sep 29, 2006 #3

    Q_Goest

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    Hi Opti. I'd suggest looking around the internet for graphs of Cd vs. Re, or just for generic data on coefficient of drag. Here's a few:
    http://www.lerc.nasa.gov/WWW/K-12/airplane/shaped.html
    http://www.aerospaceweb.org/question/aerodynamics/q0231.shtml
    http://en.wikipedia.org/wiki/Drag_coefficient
    http://www.princeton.edu/~asmits/Bicycle_web/blunt.html

    If you have a graph of Cd, you can always curve fit it using Excel. I've done that for a couple of common shapes, sphere and cylinder.

    Once you do that, it's easy enough just to create your own spread sheet that calculates drag as a function of fluid properties.
     
  5. Oct 2, 2006 #4
    Thank you RainmanAero and Q_Goest. Your answers helped me some steps forward. CFD and Navier-Stokes gave numerous interesting links in Google.

    Q_Goests links helped me to understand that the drag coefficients for 2D foils and 3D objects are totally different animals in spite of the same name. They are connected in
    different reference areas in the drag formulas (chord length times unit span vs frontal area)

    Bicycle pages might help me to explain more accurately my practical problem, e.g.

    http://www.fortebikes.com/Diablo.htm

    Those guys are biking over 80 mph ( about 130 km/h )!

    I would like to find a theoretically optimal or nearly optimal design (=the least drag force) for a similar vehicle with the following differences:

    - the person inside need not pedal, but can lay legs straight like in bed
    - no window (no windscreen, the enclosure is transparent)
    - if wheel calculation is too complicated, they can be replaced with 2 skate-like metal blades or totally ignored

    One has to enclose a person with certain height, thickness profile, width profile to 'something' (assumably near the form of a water drop?) and move her/him at the unknown height (which?) over the ice, legs forward.

    A 3D waterdrop has the 3d-drag coefficient of 0.050 , and that would be a starting point for the optimization. How much would be the optimal ground cleareance? (the 3D CFD-program should answer this)

    An axially symmetric water drop form has unnecessarily large frontal area, because a person's maximal thickness (at chest typically) might be half of maximal width (at shoulders), eg. 20 cm vs. 45 cm. How to flatten the drop optimally? (CFD ...)

    How to improve the form even more (e.g.if the drop is unnecessarily large) ? (CFD...)

    The speed for the best performance is about 60-80 km/h (40-50 mph), and thus
    the Reynolds number is around 3 000 000. The purpose is not to try to make the world record, but perhaps somw kind of record of this town :).

    ---

    It looks like one has all the necessary tools(=Hepperle's program) to solve this problem in a computer in 2 dimensions, but 3 D solution is needed. Experimenting only with real life models is too frustrating :)
     
  6. Oct 2, 2006 #5

    brewnog

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    There is an excellent freeware code for FEA. I can't remember what it's called. I'm sure I'll remember soon though. Or maybe Claus or Astro will. Hmmmn.
     
  7. Oct 5, 2006 #6
    It would be nice to know, which program you have in mind. There seems to be several (free) 3D Navier-Stokes solvers in the net, e.g.

    http://fun3d.larc.nasa.gov/chapter-1.html
    http://wissrech.iam.uni-bonn.de/research/projects/NaSt3DGP/index.htm
    http://www.fenics.org/
    http://www.opencfd.co.uk/openfoam/index.html#openfoam

    Installing and learning one of those programs is so big an effort for me that I hope to find the program which really is worth learning and not a dead end.

    ---

    Above I have been thinking of a very simple 'speeding bullet' on the ice.
    Ilan Kroo, who is a Professor of Aeronautics and Astronautics at Stanford University, has pages

    http://www.desktopaero.com/appliedaero/appliedaero.html

    and in chapter 9.3, 'Slender bodies' he describes some properties of a rotational body with elliptic nose, cylindrical center and parabolic tail. Almost as commercial airliners are. Perhaps these are quite near the optimum.
     
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