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- Thread starter tonyjk
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davenn

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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

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Chestermiller

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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

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.

Chet

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davenn

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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.

Chet

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 (S) also known as the rigidity modulus

Gases and liquids can not have shear moduli. They have viscosity instead.

http://physics.info/elasticity/

Dave

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Chestermiller

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Who said anything about gasses and liquids supporting shearmy 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.

http://physics.info/elasticity/

Dave

Chet

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

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i mean why the viscous forces causes pressure drop in a molecular view that's what i mean by physically... thanks again"The need for the pressure drop can be viewed from two different standpoints. In terms of a force balance, the pressure force is needed to overcome the viscous forces generated. In terms of an energy balance, the work done by the pressure force is needed to overcome the viscous dis- sipation of energy throughout the fluid"

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If you want to learn more about this, see Transport Phenomena by Bird, Stewart, and Lightfoot.

Chet

thanks a lot i think i will like it

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- #10

Chestermiller

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As to the question of what is happening on the molecular scale that gives rise to viscosity, see Transport Phenomena.

Chet

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But in fluid flow, DP is used to designate pressure drop, and thus it is P1 - P2

.A pressure drop due to viscous

effects represents an irreversible pressure loss, and it is called pressure loss

DPL to emphasize that it is a loss (just like the head loss hL

Note from Eq. 8–20 that the pressure drop is proportional to the viscosity

u of the fluid, and DP would be zero if there were no friction. Therefore,

the drop of pressure from P1 to P2 in this caseis due entirely to viscous

effects

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..

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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.

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- #13

Chestermiller

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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..

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.

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