- #1

- 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

You are using an out of date browser. It may not display this or other websites correctly.

You should upgrade or use an alternative browser.

You should upgrade or use an alternative browser.

- Thread starter skaboy607
- Start date

- #1

- 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

- 7,774

- 13

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

- #3

- 108

- 0

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

thanks

thanks

- #4

minger

Science Advisor

- 1,495

- 2

- #5

- 108

- 0

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

- #6

mgb_phys

Science Advisor

Homework Helper

- 7,774

- 13

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

- 108

- 0

Thanks

- #8

minger

Science Advisor

- 1,495

- 2

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

- 108

- 0

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

- 4,662

- 5

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

Share:

- Replies
- 9

- Views
- 27K