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We will consider the case of single-phase flow with conservative body forces (e.g., gravitational) and density a single valued function of pressure. The pressure and potential from the body force can be combined into a single potential.

f - 1 ρ p = - Ω w h e r e Ω = p d p ρ - g z

If the change in density is small enough, the potential can be approximated by potential that has the units of pressure.

Ω P ρ , small change in density where P = p - ρ g z

Suppose that the flow is characterized by a certain linear dimension, L , a velocity U , and a density ρ . For example, if we consider the steady flow past an obstacle, L may be it's diameter and U and ρ the velocity and density far from the obstacle. We can make the variables dimensionless with the following

v * = v U , x * x L , t * = U L t , P * = P ρ U 2 * = L , * 2 = L 2 2

The conservative body force, Navier-Stokes equation is made dimensionless with these variables.

ρ D v D t = - P + ( λ + μ ) Θ + μ 2 v ρ U 2 L D v * D t * = - ρ U 2 L * P * + μ U L 2 ( λ / μ + 1 ) * Θ * + μ U L 2 * 2 v * ρ U L μ D v * D t * + * P * = ( λ / μ + 1 ) * Θ * + * 2 v * N R e D v * D t * + * P * = ( λ / μ + 1 ) * Θ * + * 2 v * where N R e = ρ U L μ = ρ U 2 μ U / L

The Reynolds number partitions the Navier -Stokes equation into two parts. The left side or inertial and potential terms, which dominates for large NRe and the right side or viscous terms, which dominates for small NRe. The potential gradient term could have been on the right side if the dimensionless pressure was defined differently, i.e., normalized with respect to ( μ U ) / L , the shear stress rather than kinetic energy. Note that the left side has only first derivatives of the spatial variables while the right side has second derivatives. We will see later that the left side may dominate for flow far from solid objects but the right side becomes important in the vicinity of solid surfaces.

The nature of the flow field can also be seen form the definition of the Reynolds number. The second expression is the ratio of the characteristic kinetic energy and the shear stress.

The alternate form of the dimensionless Navier-Stokes equation with the other definition of dimensionless pressure is as follows.

N R e D V * D t * = - * P * * + ( λ / μ + 1 ) * Θ * + * 2 v * P * * = P μ U / L

Dissipation of energy by viscous forces

If there was no dissipation of mechanical energy during fluid motion then kinetic energy and potential energy can be exchanged but the change in the sum of kinetic and potential energy would be equal to the work done to the system. However, viscous effects result in irreversible conversion of mechanical energy to internal energy or heat. This is known as viscous dissipation of energy. We will identify the components of mechanical energy in a flowing system before embarking on a total energy balance.

The rate that work W is done on fluid in a material volume V with a surface S is the integral of the product of velocity and the force at the surface.

d W d t = s v t ( n ) d S = s v T n d S = v ( v T ) d V

The last integrand is rather complicated and is better treated with index notation.

( v i T i j ) , j = T i j v i , j + v i T i j , j = T i j v i , j + v i ρ D v i D t - ρ f i = T i j v i , j + 1 2 ρ D v 2 D t - ρ f i v i ( v t ) = T : v + 1 2 ρ D v 2 D t - ρ f v

We made use of Cauchy's equation of motion to substitute for the divergence of the stress tensor. The integrals can be rearranged as follows.

Questions & Answers

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Maciej
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s. Reply
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Devang Reply
are you nano engineer ?
s.
fullerene is a bucky ball aka Carbon 60 molecule. It was name by the architect Fuller. He design the geodesic dome. it resembles a soccer ball.
Tarell
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Damian
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Tarell
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Mostly, they use nano carbon for electronics and for materials to be strengthened.
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is Bucky paper clear?
CYNTHIA
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s. Reply
Yeah, it is a pain to say the least. You basically have to heat the substarte up to around 1000 degrees celcius then pass phosphene gas over top of it, which is explosive and toxic by the way, under very low pressure.
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Do you know which machine is used to that process?
s.
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SUYASH Reply
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SUYASH
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s. Reply
of graphene you mean?
Ebrahim
or in general
Ebrahim
in general
s.
Graphene has a hexagonal structure
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Cied
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abeetha Reply
I start with an easy one. carbon nanotubes woven into a long filament like a string
Porter
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Porter
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AMJAD
what is system testing
AMJAD
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Stotaw
In this morden time nanotechnology used in many field . 1-Electronics-manufacturad IC ,RAM,MRAM,solar panel etc 2-Helth and Medical-Nanomedicine,Drug Dilivery for cancer treatment etc 3- Atomobile -MEMS, Coating on car etc. and may other field for details you can check at Google
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anybody can imagine what will be happen after 100 years from now in nano tech world
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after 100 year this will be not nanotechnology maybe this technology name will be change . maybe aftet 100 year . we work on electron lable practically about its properties and behaviour by the different instruments
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Source:  OpenStax, Transport phenomena. OpenStax CNX. May 24, 2010 Download for free at http://cnx.org/content/col11205/1.1
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