# Reynolds Number and its Characteristic Length?

1. Jun 27, 2013

### blizzard12345

Hi im a little confused as to what length to use as the characteristic length of the Reynolds number for a body in and incompressible flow that has no boundary conditions(ie there all open).

im going to try and draw a few examples to help me out..(i have no idea why..) from what i have read it seems to vary from person to person as to what you choose for the characteristic length but if there is some sort of convention as to choosing it could you please clarify for me?

---> __
---> | . |
---> | . | Length D
---> |__ |

Length L

_
/ \
\ _ / Diameter D

2. Jun 27, 2013

### SteamKing

Staff Emeritus
The characteristic length will depend on several factors.

1. Is the flow oriented parallel to a certain direction relative to the body, i.e., is the flow lengthwise, or is the flow parallel to some other direction relative to the body?
2. Is the Reynolds number being used to estimate a certain characteristic of the flow or some other property of the body in the flow? If that is the case, then there should be some guidelines provided by the data you are attempting to utilize, or some calculation you are attempting to perform.

If, for example, you are attempting to calculated the resistance of a body to a flow running lengthwise, then the length of the body itself will be the characteristic length.

Of course, if you wish to provide a fuller description of what you want to use Rn for, a more accurate determination of the length can be provided.