1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Shear stress and pressure drop

  1. Feb 11, 2013 #1
    hello all... Please can i know physically why the pressure drop in a fluid when there's shear stress? what happens? thank you
  2. jcsd
  3. Feb 11, 2013 #2


    User Avatar
    Science Advisor
    Gold Member
    2017 Award

    Greetings Tonyik

    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

  4. Feb 11, 2013 #3
    Of course you can have shear stress in a fluid. According to Newton's law of viscosity, for a fluid that is being sheared between parallel plates, the shear stress is equal to the viscosity times the shear rate, with the shear rate equal to the relative velocity divided by the distance between the plates.

    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.

  5. Feb 12, 2013 #4


    User Avatar
    Science Advisor
    Gold Member
    2017 Award

    my geophysics lecturer at university would have severely debated that with you
    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.



    Attached Files:

    Last edited: Feb 12, 2013
  6. Feb 12, 2013 #5
    Who said anything about gasses and liquids supporting shear strain? You do know the difference between stress and strain, correct? The OP was asking about shear stresses in a liquid, and how they contribute to pressure drop. Either your lecturer didn't know much about fluid mechanics, or you misinterpreted what he was saying. Viscosity is precisely what we are talking about here. This is the fluid property that allows a fluid to develop shear stress. I hope you really didn't think that shear stresses could only exist in solids. In a liquid, the shear stress is proportional to the rate of strain, rather than the strain itself.

  7. Feb 12, 2013 #6
    Yea... viscous fluid shear is a pretty fundamental part of fluid dynamics and boundary layer theory.
  8. Feb 12, 2013 #7
    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 :
    i mean why the viscous forces causes pressure drop in a molecular view that's what i mean by physically... thanks again
  9. Feb 12, 2013 #8
    thanks a lot i think i will like it
  10. Feb 12, 2013 #9
    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?
  11. Feb 12, 2013 #10
    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.

  12. Feb 18, 2013 #11
    sorry for re-openning this thread but really im not getting it look what i found:
    Im confused about why they call it a pressure loss? if it's necessary for a pressure drop in a fluid to flow in a pipe than why they called it pessure loss? and why they say if there's no friction there's no pressure drop? so how can an ideal fluid even in theory flow without a pressure drop... thanks again

    http://www.uio.no/studier/emner/matnat/math/MEK4450/h11/undervisningsmateriale/modul-5/Pipeflow_intro.pdf even all the books they say its a pressure loss..
  13. Feb 18, 2013 #12
    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.
    Last edited: Feb 18, 2013
  14. Feb 18, 2013 #13
    Some people call pressure drop "pressure loss" because, if the pressure at the inlet is higher than the pressure at the outlet, there is a "pressure loss." When they talk about "friction", what they really mean, more precisely, is viscous resistance.

    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.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook