Dear Kaviarasu Chinnaiyan,

Once you define top & bottom surface of 2D pipe as "wall" type boundary condition,

You will get option to add wall roughness height & roughness constant under wall boundary condition.

Following link may be useful to understand different available inputs at wall boundary condition in Fluent.

https://www.sharcnet.ca/Software/Fluent6/html/ug/node253.htm

Happy to see your reply sir, but when i choose LES model in viscous , those options are locked. I couldn't able to enter the values.... 

I am not sure you can do a roughness modelling for LES implemented in Fluent....

However, concerning LES, in case of resolved boundary layer the filtered field, for very small cell size close to the wall, tends to the DNS variable. That means that theoretically you should prescribe a very complicated tangential condition in terms of the roughness geometry... I doubt that can be accomplished easily.

Alternatively, you could set a stress-model boundary condition for LES, this time the stress at the wall is prescribed and you could model the global effect of roughness into it.

have a look here http://www.tandfonline.com/doi/abs/10.1080/14685248.2016.1215604?journalCode=tjot20

but you can find many papers with google

Following link may be useful to understand different available inputs at wall boundary condition in Fluent.

https://www.sharcnet.ca/Software/Fluent6/html/ug/node253.htm