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Shear stress and pressure drop 
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#1
Feb1113, 05:45 PM

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hello all... Please can i know physically why the pressure drop in a fluid when there's shear stress? what happens? thank you



#2
Feb1113, 08:50 PM

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Greetings Tonyik
Firstly you cannot have shear stress in a fluid .... That is, fluids cannot support shear stress This is how we found out that the outer core of the earth is liquid, because earthquake shear waves will not propagate through that region knowing that, would you like to redefine your question and maybe give an example relating to your question cheers Dave 


#3
Feb1113, 11:02 PM

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In a fluid flowing through a pipe, there is a viscous shear stress at the wall that acts in the direction opposite to the direction of fluid motion. This integrates to a tangential force at the wall. In order to overcome this force, you need a higher pressure at the inlet of the pipe than at the exit. For pipe flow, the pressure drop in the pipe is equal to 4L/D times the shear stress at the wall, where L is the length of the pipe and D is the diameter. The shear stress at the wall for laminar flow is equal to 8V/D times the viscosity, where V is the volumetric average velocity of the fluid. If you want to learn more about this, see Transport Phenomena by Bird, Stewart, and Lightfoot. Chet 


#4
Feb1213, 12:36 AM

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Shear stress and pressure drop
and would have give you the inverted jug of beer experiment to prove otherwise if a fluid could support shearing then why do shear waves not travel through it ? it just doesnt happen, Shear modulus of a fluid = 0 shear modulus Shear Modulus (S) also known as the rigidity modulus Gases and liquids can not have shear moduli. They have viscosity instead. http://physics.info/elasticity/ Dave 


#5
Feb1213, 07:01 AM

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Chet 


#6
Feb1213, 09:34 AM

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Yea... viscous fluid shear is a pretty fundamental part of fluid dynamics and boundary layer theory.



#7
Feb1213, 12:55 PM

P: 107

hello again... yes im talking of the shear stress due to the viscosity of the fluid..i dont know but if you google it you will get thousands of links about it .. and yes im not an expert in fluid mechanics and still i didnt know why Physically there's drop of pressure i know its to overcome the energy loss due to friction ect quotation from the book Fundamentals of Fluid Mechanics by Bruce Munson :



#8
Feb1213, 01:01 PM

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#9
Feb1213, 01:31 PM

P: 107

ahh okk i think i understand it if we want to make a flow in a pipe we have to make a pressure difference ( drop of pressure between the entrance and the exit) so when fluid is flowing there's the viscosity of the fluid causing the viscous forces... but why we say an ideal fluid doesnt have viscosity so it has no pressure drop in a pipe? so how an ideal fluid can flow without pressure drop?



#10
Feb1213, 02:10 PM

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There's no such thing as an ideal fluid. It is only an approximation to the behavior of a real fluid in the limit of vanishingly small viscosity. As you make the viscosity of a real fluid lower and lower, the pressure drop in the pipe gets lower and lower. Thus, for the same axial velocity, water will have a lower pressure drop than molasses.
As to the question of what is happening on the molecular scale that gives rise to viscosity, see Transport Phenomena. Chet 


#11
Feb1813, 03:39 PM

P: 107

sorry for reopenning this thread but really im not getting it look what i found:
http://www.uio.no/studier/emner/matn...flow_intro.pdf even all the books they say its a pressure loss.. 


#12
Feb1813, 04:41 PM

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It is necessary for a pressure differential (i.e. the pressure at the inlet of the pipe is greater than the pressure at the outlet) for a fluid to flow in a pipe, not a pressure loss. Though all real fluids will have viscosity, and therefore some degree of shear, which results in pressure losses.



#13
Feb1813, 04:42 PM

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When you ask the question "how can an ideal fluid even in theory flow without a pressure drop," this is analogous to asking "how can electric current pass through a zero resistance wire without a voltage difference across it." Even the smallest voltage difference across the wire will result in a huge current. For a finite current, the voltage difference would approach zero. Even the smallest pressure drop across the pipe will result in a huge flow. For a finite fluid flow, the pressure drop would approach zero. We are talking about the limit as the resistance approaches zero or as the fluid viscosity becomes vanishingly small. 


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