# Reynolds Number Regarding Turbulent Intensity

1. Mar 4, 2013

### Squires

Hello,

I have a question regarding Reynolds numbers.

As I understand, the higher a Reynolds number, the more turbulent a flow becomes, with Re>4000 considered turbulent.

I have a model where I've calculated a Reynolds number of 10^7, and when going to set my model boundary conditions I'm asked for a 'Turbulent Intensity' parameter, so I do some research and find the following equation:

I=0.16*(Re)^-1/8

So my question is, why does the turbulent intensity decrease as the Reynolds number increases, when a higher Reynolds number flow is meant to become more turbulent?

For my calculations I got a turbulent intensity of around 2%, which seemed far too low!

Last edited: Mar 4, 2013
2. Mar 5, 2013

### SteamKing

Staff Emeritus
In order to understand I, you must realize that it represents a ratio of the rms value of the velocity fluctuations in a stream to the average stream velocity.

Low turbulence flows have an I well below 1%. Medium turbulence flows have an I of about 5%, and high turbulence flows have an I between 5% and 20%.

The I factor is important in CFD calculations since the calculations converge faster with lower values of I.

3. Mar 5, 2013

### Squires

Good morning SteamKing my friend, and thank you for the reply!

Yeah I did some research, and I understood that part pretty well.

But that just raises my question, instead of solve it, because as you say:

The more turbulent a flow, the higher the turbulent intensity, I

And we also know that:

The more turbulent a flow, the higher the Reynolds number, Re

But with the equation supplied, a higher Reynolds number produces a lower I value, which I can't make any sense of :(

4. Mar 5, 2013