Pressure gradient in a smooth pipe

Shamoon Jamshed · June 21, 2017, 18:50

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.

flotus1 · June 22, 2017, 01:00

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.

Shamoon Jamshed · June 22, 2017, 02:04

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)

LuckyTran · June 22, 2017, 02:23

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.

tas38 · June 22, 2017, 10:30

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).

FMDenaro · June 22, 2017, 12:20

Quote:

Originally Posted by Shamoon Jamshed

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.

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

June 21, 2017, 18:50	Pressure gradient in a smooth pipe	#1
Shamoon Jamshed Senior Member Shamoon Jamshed Join Date: Apr 2009 Location: Karachi Posts: 377 Rep Power: 18	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.

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June 22, 2017, 01:00		#2
flotus1 Super Moderator Alex Join Date: Jun 2012 Location: Germany Posts: 3,399 Rep Power: 46	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.

June 22, 2017, 02:04		#3
Shamoon Jamshed Senior Member Shamoon Jamshed Join Date: Apr 2009 Location: Karachi Posts: 377 Rep Power: 18	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)

June 22, 2017, 02:23		#4
LuckyTran Senior Member Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,675 Rep Power: 66	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.

June 22, 2017, 10:30		#5
tas38 Senior Member Troy Snyder Join Date: Jul 2009 Location: Akron, OH Posts: 219 Rep Power: 18	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).