Default friction coefficient?
 

Hello.

I have made a pipe model in which air will flow. I calculated dynamic pressure from the velocity I want to achieve and set it on the inlet.

I set roughness height 0 as default, and I thought as there is no friction within pipe, the velocity should be the same regardless the length of pipe.

However, I found that the velocity varies due to the length.

Is there any other reason for the pressure lose in pipe with default setting? I wonder if there is default friction coefficient with selected material.

I would be very much appreciated if anyone helps.

Thanks!