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

In summary, there are no freeware programs specifically designed for calculating 3D lift and drag for simple flying bodies. However, there are 2D programs that can be used to study flow in 2D, and CFD programs which are more accurate but also more expensive. The drag and lift forces can be calculated using formulas that take into account factors such as drag coefficient, density of air, and velocity. Curve fitting and spreadsheet calculations can also be used to estimate drag. There are also various factors to consider when designing a vehicle for optimal drag, such as shape, ground clearance, and speed. There are several 3D Navier-Stokes solvers available for free, but it is important to research and choose the best one for
  • #1
optimistx
3
0
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 ?
 
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  • #2
Hi optimistx:
optimistx said:
Is there a (freeware?) program to calculate 3D lift and drag for simple flying bodies, e.g. ball, cylinder, cube, wing?

...(snip)...

So, is there a 'virtual' windtunnel' to get the drag ?

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
 
  • #3
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 [Broken]
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.
 
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  • #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 :)
 
  • #5
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.
 
  • #6
brewnog said:
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.

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/ [Broken]
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 [Broken]

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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1. How do you calculate drag in a 3D virtual wind tunnel?

Drag is calculated by determining the forces acting on an object in a virtual wind tunnel simulation. This is done by analyzing the air flow around the object and measuring the pressure changes. These pressure changes are then used to calculate the drag force using mathematical equations.

2. What factors affect drag in a 3D virtual wind tunnel?

There are several factors that can affect drag in a 3D virtual wind tunnel, including the shape and size of the object, the speed and direction of the air flow, and the viscosity of the fluid being simulated. Other factors such as surface roughness and turbulence can also have an impact.

3. How accurate are drag calculations in a 3D virtual wind tunnel?

The accuracy of drag calculations in a 3D virtual wind tunnel depends on several factors, such as the accuracy of the simulation software, the resolution of the virtual wind tunnel, and the complexity of the object being simulated. Generally, the results are quite accurate, but it is important to validate them with physical experiments.

4. Can a 3D virtual wind tunnel be used for all types of objects?

Yes, a 3D virtual wind tunnel can be used for a wide range of objects, including vehicles, aircraft, buildings, and even sports equipment. However, the accuracy of the results may vary depending on the complexity and size of the object being simulated.

5. What are the benefits of using a 3D virtual wind tunnel for drag calculations?

Using a 3D virtual wind tunnel for drag calculations has several benefits. It is a cost-effective and time-efficient way to test and analyze the aerodynamics of objects. It also allows for easy manipulation of variables such as air flow speed and direction, making it possible to quickly assess the impact of different design changes on drag. Additionally, it eliminates the need for physical prototypes, reducing the overall cost of testing and development.

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