Pressure gradient in a smooth pipe
 

Hi,

I found in some article that the pressure gradient for a smooth pipe is dp/dx = 0 for fully developed laminar/turbulent flow. If we accept the fact then, there would be no friction at all because the pressure has not been dropped along the length. Can someone explain the theory to me please? May be I am misunderstanding some point.
What about the favourable pressure gradient, and I am talking about both laminar and turbulent flows.

Is "x" the main flow direction? Perhaps a body force instead of a pressure gradient was used to drive the flow. Hard to tell without knowing which article you are referring to.

Ok, I even do not remember exactly that where I read this. But, irrespective of this, what happens in reality, if dp/dx = 0 for fully developed laminar / turbulent. If yes, then why do we get a pressure drop along the length? (experimentally and numerically)

dp/dx is non-zero for laminar and turbulent fully developed flows in a pipe. What's there to explain? You should read carefully your article.

To reiterate LuckyTran,

The streamwise pressure gradient (dp/dx) is non-zero in pipe flow. In the case of fully developed pipe flow (laminar or turbulent), the pressure gradient balances the shear developed at the pipe walls. The wall shear is non-zero given that the velocity does vary across the pipe, i.e. du/dy is non-zero.

The condition for "fully-developed" is that the streamwise velocity gradient is zero (du/dx = 0).

Maybe this condition you read is prescribed at the outflow??