- #1

skaboy607

- 108

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In a Drag problem, I'm trying to calculate the drag force but I dont know the drag coefficient? Is there any way to calculate it?

Thanks

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- Thread starter skaboy607
- Start date

- #1

skaboy607

- 108

- 0

In a Drag problem, I'm trying to calculate the drag force but I dont know the drag coefficient? Is there any way to calculate it?

Thanks

- #2

mgb_phys

Science Advisor

Homework Helper

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Generally for real shapes you have to measure it, either in a wind tunnel or a computer simulation (CFD)

- #3

skaboy607

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Ok, im trying to do a tutorial sheet on it, how do I calculate it say for sphere?

thanks

thanks

- #4

minger

Science Advisor

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- #5

skaboy607

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Yea I have used that but it doesnt give me the required answer.

- #6

mgb_phys

Science Advisor

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The drag equation ( propertional Area * velocity^2) is an approximation for high Reynolds number flow (eg air) it isn't necessarily correct for low speed or high viscosity cases.

- #7

skaboy607

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Thanks

- #8

minger

Science Advisor

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The drag equation ( propertional Area * velocity^2) is an approximation for high Reynolds number flow (eg air) it isn't necessarily correct for low speed or high viscosity cases.

The drag coefficient is quite a function of Reynolds, and potentially other factors. Man, I must be in a good mood today. Let's see what I can find. For REALLY low Reynolds numbers, (Re < 1), we have

[tex] C_f = \frac{24}{R^*} \left( 1 + \frac{3}{16}R^* - \frac{7k}{48}R^* \right)\,\,R^*=2R [/tex]

Not sure why it's written like that, but oh well. [tex]R\equiv [/tex] Reynolds number of course. [tex] k = V^* / U_\infty[/tex] where V* is the radial velocity of blowing through the surface...which I assume you can take to be zero in your case.

There is also a "famous" Oseen's (1910) drag coefficient forumula for a sphere in uniform stream:

[tex] C_D = \frac{24}{{Re}_D}\left(1+\frac{3}{16}{Re}_D\right)[/tex]

Stokes gave an exact solution in the limit as Re->0, such as creeping flow, where:

[tex] C_D = \frac{24}{{Re}_D}[/tex]

However, that's only valid where Reynolds is less than 0.2.

What type of Reynolds are you looking at?

- #9

skaboy607

- 108

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Also from the drag equation, where I use area, will it be the surface area of the sphere, i.e. d^2*pi.

Thanks

- #10

Bob S

- 4,662

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The area you use is the frontal area, which is pi r^2 or (pi/4) d^2.Also from the drag equation, where I use area, will it be the surface area of the sphere, i.e. d^2*pi.

Thanks

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