9: Transport Phenomena
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Mathematical introduction
An important relation is: if X is a property of a volume element which travels from position →r to →r+d→r in a time dt, the total differential dX is then given by:
dX=∂X∂xdx+∂X∂ydy+∂X∂zdz+∂X∂tdt ⇒ dXdt=∂X∂xvx+∂X∂yvy+∂X∂zvz+∂X∂t
This in general leads to: dXdt=∂X∂x+(→v⋅▽)X.
From this follows that : ddt∫∫∫Xd3V=∂∂t∫∫∫Xd3V+∫∫X◯(→v⋅→n)d2A
where the volume V is surrounded by surface A. Some properties of the ∇ operator are:
div(ϕ→v)=ϕdiv→v+gradϕ⋅→vrot(ϕ→v)=ϕrot→v+(gradϕ)×→vrot gradϕ=→0div(→u×→v)=→v⋅(rot→u)−→u⋅(rot→v)rot rot→v=grad div→v−∇2→vdiv rot→v=0div gradϕ=∇2ϕ∇2→v≡(∇2v1,∇2v2,∇2v3)
Here, →v is an arbitrary vector field and ϕ an arbitrary scalar field. Some important integral theorems are:
Gauss:∫∫◯ (→v⋅→n)d2A=∫∫∫(div→v)d3VStokes for a scalar field:∮(ϕ⋅→et)ds=∫∫(→n×gradϕ)d2AStokes for a vector field:∮(→v⋅→et)ds=∫∫(rot→v⋅→n)d2AThis results in:∫∫◯ (rot→v⋅→n)d2A=0Ostrogradsky:∫∫◯ (→n×→v)d2A=∫∫∫(rot→v)d3A∫∫◯ (ϕ→n)d2A=∫∫∫(gradϕ)d3V
Here, the orientable surface ∫∫d2A is limited by the Jordan curve ∮ds.
Conservation laws
Two types of forces can do work on a volume:
- The force →f0 on each volume element. For gravity: →f0=ϱ→g.
- Surface forces working only on the boundaries: →t. For these: →t=→n T, where T is the stress tensor.
T can be split into a part pI representing the normal tension and a part T' representing the shear stress:T=T'+pI, where I is the unit tensor. When viscous aspects can be ignored: div T =−gradp.
When the flow velocity is →v at position →r holds at position →r+d→r:
→v(d→r)=→v(→r)⏟translation+d→r⋅(grad→v)⏟rotation, deformation, dilatation
The quantity L:=grad→v can be split in a symmetric part D and an antisymmetric part W. L= D + W with
Dij:=12(∂vi∂xj+∂vj∂xi) , Wij:=12(∂vi∂xj−∂vj∂xi)
When the rotation or vorticity →ω=rot→v is introduced: Wij=12εijkωk. →ω represents the local rotation velocity: →dr⋅ W =12ω×→dr.
For a Newtonian liquid : T' =2ηD. Here, η is the dynamical viscosity. This is related to the shear stress τ by:
τij=η∂vi∂xj
For compressible media it can be stated that: T'=(η′div→v) I+ 2η D. From equating the thermodynamic and mechanical pressure it follows: 3η′+2η=0. If the viscosity is constant: div(2D) =∇2→v+grad div→v.
The conservation laws for mass, momentum and energy for continuous media can be written in both integral and differential form. They are:
Integral notation
- Conservation of mass: ∂∂t∫∫∫ϱd3V+∫∫◯ ϱ(→v⋅→n)d2A=0
- Conservation of momentum: ∂∂t∫∫∫ϱ→vd3V+∫∫◯ ϱ→v(→v⋅→n)d2A=∫∫∫f0d3V+∫∫◯ →n⋅Td2A
- Conservation of energy: ∂∂t∫∫∫(12v2+e)ϱd3V+∫∫◯ (12v2+e)ϱ(→v⋅→n)d2A=
−∫∫◯ (→q⋅→n)d2A+∫∫∫(→v⋅→f0)d3V+∫∫◯ (→v⋅→n)Td2A
Differential notation:
- Conservation of mass: ∂ϱ∂t+div⋅(ϱ→v)=0
- Conservation of momentum: ϱ∂→v∂t+(ϱ→v⋅∇)→v=→f0+div T = →f0−gradp+divT'
- Conservation of energy: ϱTdsdt=ϱdedt−pϱdϱdt=−div→q+ T':D
Here, e is the internal energy per unit of mass E/m and s is the entropy per unit of mass S/m. →q=−κ→∇T is the heat flow. Further:
p=−∂E∂V=−∂e∂1/ϱ , T=∂E∂S=∂e∂s so CV=(∂e∂T)V and Cp=(∂h∂T)p
with h=H/m the enthalpy per unit of mass.
From this one can derive the Navier-Stokes equations for an incompressible, viscous and heat-conducting medium:
div→v=0ϱ∂→v∂t+ϱ(→v⋅∇)→v=ϱ→g−gradp+η∇2→vϱC∂T∂t+ϱC(→v⋅∇)T=κ∇2T+2ηD:D
with C the thermal heat capacity. The force →F on an object within a flow, when viscous effects are limited to the boundary layer, can be obtained using the momentum law. If a surface A surrounds the object outside the boundary layer:
→F=−∫∫◯ [p→n+ϱ→v(→v⋅→n)]d2A
Bernoulli’s equations
Starting with the momentum equation one can find that for a non-viscous medium for stationary flows, with
(→v⋅grad)→v=12grad(v2)+(rot→v)×→v
and the potential equation →g=−grad(gh):
12v2+gh+∫dpϱ=constant along a streamline
For compressible flows: 12v2+gh+p/ϱ=constant along a strreamline. If also rot→v=0 and the entropy is the same on each streamline 12v2+gh+∫dp/ϱ=constant everywhere. For incompressible flows this becomes: 12v2+gh+p/ϱ=constant everywhere. For ideal gases with constant Cp and CV, with γ=Cp/CV:
12v2+γγ−1pϱ=12v2+c2γ−1=constant
with a velocity potential defined by →v=gradϕ for instantaneous flows:
∂ϕ∂t+12v2+gh+∫dpϱ=constant everywhere
Characterising flows by dimensionless numbers
The advantage of dimensionless numbers is that they make model experiments possible if one makes the dimensionless numbers which are important for the specific experiment equal for both model and real situation. One can also deduce functional equalities without solving the differential equations. Some dimensionless numbers are given by:
Strouhal:Sr=ωLvFroude:Fr=v2gLMach:Ma=vcFourier:Fo=aωL2P\'eclet:Pe=vLaReynolds:Re=vLνPrandtl:Pr=νaNusselt:Nu=LακEckert:Ec=v2cΔT
Here, ν=η/ϱ is the kinematic viscosity, c is the speed of sound and L is a characteristic length of the system. α follows from the equation for heat transport κ∂yT=αΔT and a=κ/ϱc is the thermal diffusion coefficient.
These numbers can be interpreted as follows:
- Re: (stationary inertial forces)/(viscous forces)
- Sr: (non-stationary inertial forces)/(stationary inertial forces)
- Fr: (stationary inertial forces)/(gravity)
- Fo: (heat conductance)/(non-stationary change in enthalpy)
- Pe: (convective heat transport)/(heat conductance)
- Ec: (viscous dissipation)/(convective heat transport)
- Ma: (velocity)/(speed of sound): objects moving faster than approximately Ma = 0,8 produce shockwaves which propagate at an angle θ to the velocity of the object. For this angle Ma=1/arctan(θ).
- Pr and Nu are related to specific materials.
Now, the dimensionless Navier-Stokes equation, with x′=x/L, →v′=→v/V, grad′=Lgrad, ∇′2=L2∇2 and t′=tω becomes:
Sr∂→v′∂t′+(→v′⋅∇′)→v′=−grad′p+→gFr+∇′2→v′Re
Flow in tubes
Tube flow is laminar if Re<2300 across the diameter of the tube, and turbulent if Re is larger. For an incompressible laminar flow through a straight, circular tube the velocity profile is:
v(r)=−14ηdpdx(R2−r2)
For the volumetric flow: ΦV=R∫0v(r)2πrdr=−π8ηdpdxR4
The entrance length Le is given by:
- 500<ReD<2300: Le/2R=0.056ReD
- Re>2300: Le/2R≈50
For gas transport at low pressures (Knudsen-gas): ΦV=4R3α√π3dpdx
For flows at a small Re: ∇p=η∇2→v and div→v=0. For the total force on a sphere with radius R in a flow then: F=6πηRv. For large Re the force on a surface A is: F=12CWAϱv2.
Potential theory
The circulation Γ is defined as: Γ=∮(→v⋅→et)ds=∫∫(rot→v)⋅→nd2A=∫∫(→ω⋅→n)d2A
For non viscous media, if p=p(ϱ) and all forces are conservative, Kelvin’s theorem can be derived:
dΓdt=0
For rotationless flows a velocity potential →v=gradϕ can be introduced. In the incompressible case it follows from conservation of mass that ∇2ϕ=0. For a 2-dimensional flow a flow function ψ(x,y) can be defined: with ΦAB the amount of liquid flowing through a curve s between the points A and B:
ΦAB=B∫A(→v⋅→n)ds=B∫A(vxdy−vydx)
and the definitions vx=∂ψ/∂y, vy=−∂ψ/∂x then: ΦAB=ψ(B)−ψ(A). In general:
∂2ψ∂x2+∂2ψ∂y2=−ωz
In polar coordinates:
vr=1r∂ψ∂θ=∂ϕ∂r , vθ=−∂ψ∂r=1r∂ϕ∂θ
For source flows with power Q in (x,y)=(0,0): ϕ=Q2πln(r) so that vr=Q/2πr, vθ=0.
For a dipole of strength Q in x=a and strength −Q in x=−a it follows from the superposition that: ϕ=−Qax/2πr2 where Qa is the dipole strength. For a vortex: ϕ=Γθ/2π.
If an object is surrounded by an uniform main flow with →v=v→ex and such a large Re that viscous effects are limited to the boundary layer: Fx=0 and Fy=−ϱΓv. The statement that Fx=0 is d’Alembert’s paradox and originates from the neglect of viscous effects. The lift Fy is also created by η because Γ≠0 due to viscous effects. Hence rotating bodies also create a force perpendicular to their direction of motion: the Magnus effect.
Boundary layers
Flow boundary layers
If for the thickness of the boundary layer : δ≪L then: δ≈L/√Re. With v∞ the velocity of the main flow it follows that the velocity vy ⊥ at the surface will be: vyL≈δv∞. Blasius’ equation for the boundary layer is vy/v∞=f(y/δ): 2f‴+ff″=0 with boundary conditions f(0)=f′(0)=0, f′(∞)=1. From this follows: CW=0.664 Re−1/2x.
The momentum theorem of von Karman for the boundary layer is: ddx(ϑv2)+δ∗vdvdx=τ0ϱ
where the displacement thickness δ∗v and the momentum thickness ϑv2 are given by:
ϑv2=∞∫0(v−vx)vxdy , δ∗v=∞∫0(v−vx)dy and τ0=−η∂vx∂y|y=0
The boundary layer is released from the surface if (∂vx∂y)y=0=0. This is equivalent to dpdx=12ηv∞δ2.
Temperature boundary layers
If the thickness of the temperature boundary layer δT≪L then:
- If Pr≤1: δ/δT≈√Pr.
- If Pr≫1: δ/δT≈3√Pr.
Heat conductance
For non-stationairy heat conductance in one dimension without flow:
∂T∂t=κϱc∂2T∂x2+Φ
where Φ is a source term. If Φ=0 the solutions for harmonic oscillations at x=0 are:
T−T∞Tmax−T∞=exp(−xD)cos(ωt−xD)
with D=√2κ/ωϱc. At x=πD the temperature variation is in anti-phase with the surface. The one-dimensional solution at Φ=0 is
T(x,t)=12√πatexp(−x24at)
This is mathematically equivalent to the diffusion problem:
∂n∂t=D∇2n+P−A where P is the production of and A the discharge of particles. The flow density J=−D∇n.
Turbulence
The time scale of turbulent velocity variations τt is of the order of: τt=τ√Re/Ma2 with τ the molecular time scale. For the velocity of the particles: v(t)=⟨v⟩+v′(t) with ⟨v′(t)⟩=0. The Navier-Stokes equation now becomes:
∂⟨→v⟩∂t+(⟨→v⟩⋅∇)⟨→v⟩=−∇⟨p⟩ϱ+ν∇2⟨→v⟩+divSRϱ
where SRij=−ρ⟨vivj⟩ is the turbulent stress tensor. Boussinesq’s assumption is: τij=−ϱ⟨v′iv′j⟩. It is stated that, analogous to Newtonian media: S\(_R=2\varrho\nu_t\left\langle \mbox{\sfd D} \right\rangle\). Near a boundary: νt=0, far away from a boundary: νt≈νRe.
Self organization
For a (semi) two-dimensional flow: dωdt=∂ω∂t+J(ω,ψ)=ν∇2ω
With J(ω,ψ) the Jacobian. So if ν=0, ω is conserved. Further, the kinetic energy/mA and the enstrofy V are conserved: with →v=∇×(→kψ)
E∼(∇ψ)2∼∞∫0E(k,t)dk=constant , V∼(∇2ψ)2∼∞∫0k2E(k,t)dk=constant
From this follows that in a two-dimensional flow the energy flux goes towards large values of k: larger structures become larger at the expanse of smaller ones. In three-dimensional flows the situation is just the opposite.