# Help with Reynolds number

1. Jul 6, 2009

### eddierolling

hi

i have to find out the reynolds number for a missile launched from a sub.

i only have the temperature of the water, the dimensions of the missile and the velovity of a model of the of the actual missile at a scale of 1:10

i havn't given any quantities because i don't want a solution.

i need to know if i can solve Reynolds number from the information given in the question and if so where to find the information i need so i can study it for my self.

any help any one can give will be greatly appriciated.

thanks

2. Jul 7, 2009

### minger

Recall that Reynolds number is
$$Re = \frac{Vl}{\nu}$$
Where l is a rather arbitrary characteristic length. From the temperature of the water, you can calculate the viscosity, and you say that you're given the velocity of the missile.

That only leaves the length. Remember this is problem dependent and doesn't necessarily have to be the standard diameter that's used for pipe flow. Use a characteristic length that you think makes sense.

3. Jul 10, 2009

### plimph

I'm confused about this as well. I have a very odd "horseshoe" shaped design. I would like to calculate the drag under the ocean surface at a speed of 20 knots. I have a 1:10 scale model, but no good way of testing it at speed. Do I test it in water at the appropriate temperature going at only 2 knots and then scale the Reynolds number equation by multiplying the length by 10? How does mass affect the Reynolds number? I've watched Ascher Shapiro's videos at length and I still don't get it!

4. Jul 13, 2009

### minger

No, your scale is already there, 1:10. You don't need a characteristic length because you have the scale change. See
http://en.wikipedia.org/wiki/Similitude_(model [Broken])