Basic Equations
of Fluid Dynamics
Sergey Pankratov
A lecture to the Practical Course
“Scientific Computing and Visualization”
(June 17, 2004)
© Sergey Pankratov,
2004
TU München
1
Focus areas
• Euler and Lagrange descriptions of
fluid flows
• The continuity equation (CE)
• The Navier-Stokes (N.-S.) equation
2
Part1:
Kinematics of fluid flows
3
Mathematical description of a flow
•
The material fluid is considered to be:
a continuous substance having a positive volume (V > 0)
distributed over a domain Ω of the d = 3 Euclidean space
•
A fluid flow: is represented by a one-parameter family of
mappings of a 3d domain Ω, filled with fluid, into itself, Ωt → Ωt+τ
Properties of the mappings:
- smooth: twice continuously differentiable in all variables, 2;
usually mappings are assumed thrice continuously differentiable
everywhere, with the exception of some singular points, curves
or surfaces where a special analysis is required;
- bijective (one-to-one)
Geometrically: a diffeomorphism (invertible, differentiable), Ωt Ωt+τ
4
The fluid flow transform
•
The real parameter t designating the mapping of fluid domains
is identified with the time, - < t < +
•
Analytical and numerical description of the fluid motion:
1) To introduce a fixed rectangular coordinate system {xi}
2) A fluid particle – an infinitesimal element of a fluid distribution
3) Each coordinate triple {xi} denotes a position of a fluid particle
Without loss of generality, t = 0 – an arbitrary initial instant
Observation: the particle that was initially in the position {ξ j} has
moved to position {xi}
The fluid flow transformation: xi = xi (ξ j,t)
(*)
If ξ j is fixed while t varies, (*) specifies the path of a fluid particle
(a pathline or a particle line): xi = xi(t) = xi (ξ j,t)|ξ j=const , xi(0) = ξ i
5
Fluid flow – a mapping
• A fluid particle moving with the fluid
x3
M (t )  gt M
{x1 , x 2 , x3}
M (t )
{ 1 ,  2 ,  3}
M
M (t   )
gt :
3

3
Transformations gt make up
a one-parameter group
of diffeomorphisms, gt+τ=gt+gτ
x2
x1
6
Fluid flow is invertible
• If t is fixed, the transformation of the fluid domain, Ω0 → Ωt :
xi ( j )  xi ( j , t ) |t const , xi (0)   i
is exactly the fluid flow
xi (0)   i  xi ( j , t )
Functions xi (ξ j,t) are single-valued (1-to-1) and are assumed to be
thrice differentiable in all variables. Then
 xi 
( x1 , x 2 , x3 )
J
 det  j   0
1
2
3
( ,  ,  )
  
Hence, there exists a set of inverse functions:  j   j ( xi , t ) (**)
defining the initial position of a fluid particle, which occupies any
position {xi} at time t.
Such inverse transform has the same properties as the direct one
7
Two equivalent descriptions
• An important consequence of invertibility:
A fluid flow can be equivalently described by a set of initial positions
of particles as a function of their positions at later times
• Thus, there are two equivalent descriptions of the fluid motion:
- The Eulerian description – in terms of space-time variables (xi,t)
Throughout all time, a given set of coordinates {xi} remains
attached to a fixed position. The spatial coordinates {xi} are called
the Euler coordinates; they serve as an identification for a fixed point
- The Lagrangian description – chronicles the history of each fluid
particle uniquely identified by variables {ξ i}
Throughout all time, a given set of coordinates {ξ i} remains attached
to an individual particle. They are called the Lagrangian coordinates
8
Physical interpretation of the Euler and
Lagrange descriptions
• Different physical interpretations
In the Euler’s picture, an observer is always located at a given
position {xi} at a time t → observes fluid particles passing by
In the Lagrange picture, an observer is moving with the fluid
particle, which was initially at the position {ξ i} → views changes
in the flow co-moving with the observer’s particle
Euler
Lagrange
The Euler description –
important for CFD;
The Lagrange description –
important for ecological
modeling (passive tracers,
contamination spread, etc.)
9
What is a fluid particle?
• Fluid dynamics – the macroscopic discipline studying the
continuous media
• The meaning of the word continuous: any “small” fluid element –
a fluid particle – is still big enough to contain N >>1 molecules
• Dimensions of a fluid particle: a << δx << L
a – a typical intermolecular distance
δ x – a fluid particle diameter
L – a characteristical system dimension
L is an external parameter, e.g. the pipe diameter, the wing length,...
a ~ N-1/3, where N is the total number of molecules in the system
In gases, one more parameter is important: the free path length
10
Field description of fluid flows
The fluid motion is completely determined either by transform (*),
Slide 5 – the Euler picture, or by its inverse (**), Slide 7 – the
Lagrange picture. The crucial questions:
How to:
- describe the state of motion at a given position in the course of
time?
- characterize the fluid motion in terms of some mathematical
objects defined at {xi,t}?
The simplest object for this role is the instant fluid velocity ui
u-functions can be defined in both descriptions, ui(xj,t) and ui(ξ j,t)
A historical remark: though the descriptions of fluid flows in terms of spatial and material
coordinates are attributed, respectively, to Euler and Lagrange, both of them are, in fact,
known to be due to Euler (see J.Serrin in Handbuch der Physik, Band 8/1, Springer1959)
11
Objects for fluid fields
• Consider a function f characterizing a fluid in the Euler or
Lagrange frameworks. Any such quantity (e.g. the temperature)
may be viewed as a function of spatial variables, f = f(xi,t) and
also of material variables, f = f (ξ i,t) i.e.
f  f ( xi ( j , t ), t ) or f  f ( j ( xi , t ), t )
• Geometrical meaning:
- f = f(xi,t) is the value of f experienced by the fluid particles
instantaneously located at {xi}
- f = f (ξ i,t) is the value of f felt at time t by the fluid particle
initially (at t = 0) located at {ξ i}
Question: why do we need ui and other objects, if we have (*) and
(**)? Answer: in most situations the actual motion of fluid particles
is not given explicitly i.e. neither xi=xi (ξ j,t) nor ξ j=ξ j(xi,t) is known
12
Change of variables
• The variation of quantity f when the fluid flows – two different
time derivatives:
f f ( xi , t )
1)

t
t
simply a partial derivative
x const
- the rate of change of f with regard to a fixed position {xi}
2)
material or convective derivative – the
df f ( i , t )

rate of change of f with regard to a
dt
t  const
moving fluid particle
i
i
j
For f = xi, by definition:
dx

x
(

, t)
u i ( j , t ) 

dt
t
 i  const
- the velocity of a fluid particle
i
i
13
The velocity field
• Usually it is the fluid velocity ui(xj,t) that is a measurable
quantity. The velocity identifies a fluid particle (in terms of its
initial position {ξ i})
An observer is
sitting in a position {xi}
to measure the speed
of moving particles
(initially were at {ξ i})
Polizei
• The fluid velocity is a vector field (transformation properties!).
Throughout all time, a set of velocity components {ui(xj,t)}
remains attached to a fixed position – the field variables
• In numerics or in experiment: a fluid velocity ui (1d, 2d, 3d, 4d)
for the time interval Δt → to determine xi ≈ xi (ξ j,t), Δ xi ≈ uiΔt
14
The velocity field (continued)
• Analytically:
dxi
 u i ( x j , t ), xi (0)   i
dt
(***)
- a dynamical system (the Cauchy problem)
A unique solution exists on U  T for ui  1 (Cauchy-LipschitzPeano), the solution depends continuously on initial values {ξ i}
• It means that the fluid motion is characterized completely, i.e.
the description in terms of fluid velocity is equivalent to the
Euler or Lagrange pictures
• Thus: the state of a fluid in motion is fully represented by the
velocity field {ui(xj,t)} of a fluid particle at {xj,t}
• Fundamental importance of the field description: providing the
possibility to formulate the fluid dynamics in terms of PDEs.
15
Other objects
How can one calculate the material derivative of any quantity f(xi,t)?
df f ( i )
f ( i ( x j , t ), t )
f ( x j ( i , t ), t )




dt
t  i const
t
t
 i const
 i const
f

t
 i  const
f x j ( i , t )
 j
x
t
 i  const
or
df f
j f

u
dt t
x j
1. This formula interconnects the material and spatial derivatives
through the velocity field
2. This formula expresses the change rate of any local parameter
f(xi,t) with respect to a moving fluid particle located at {xi,t}
Example: the material derivative – fluid acceleration ai(xi,t)
i
i
i
i
i
j
i
i
j i
du

u

u
i
j
Notation:
D
u
:


u

u

u

u
,

u
u, j
a 

u
t
t
j
t
dt
t
x j
16
A remark on acceleration
• In classical mechanics constructed by Newton, the acceleration
plays the central role – in distinction to the Aristotelian picture
• In fluid dynamics, although it is just a branch of mechanics,
acceleration is much less important
• The reason for it:
in most physical and engineering settings acceleration of a fluid
particle ai is uniquely determined by its coordinate xi and velocity ui
Therefore, acceleration is usually not considered a state parameter
So, we might formulate the following field description framework:
The fluid motion can be characterized in terms of “objects” –
local parameters of the media - defined at {xi,t}.
The simplest object is the fluid velocity ui
17
Streamlines and trajectories
• A streamline - a curve defined for a given t > 0, for which the
tangent is collinear to the velocity vector ui(xj,t):
dx1
dx 2
dx3
 2 i  3 i
1
i
u (x , t) u (x , t) u (x , t)
Streamlines depend on time t (parameter), for which they are written
• Trajectory of a fluid particle {ξ j,t} (path line, particle line) –
a locus of the particle positions for all moments t > 0
• For a stationary flow, ui(xj,t) = ui(xj), streamlines do not depend on
t and coincide with particle lines (trajectories)
• The inverse is not true: the flow may be non-stationary, however
streamlines can coincide with path lines, for example:
i
Q
(
t
)
x
i
1/ 2
ui 
,
R
:

(
x
x
)
i
4 R3
18
Compressible and incompressible fluids
• Incompressible: when the flow does not result in the change of
the fluid volume for all {xi,t} dJ
J is the Jacobian of
dt
0
the transform (*),
dV = JdV0
dV = dx1dx2dx3
- the particle volume
at {xi}
dV0 = dξ1 dξ2 dξ3
- the initial particle
volume
• Otherwise - compressible:
dJ
0
A reminder: the fluid motion
dt
is a diffeomorphism, i.e.
continuously differentiable
and invertible
This is sufficient for 0 < J < 
• J = dV/dV0 – the fluid dilatation
j
i
j
i


dJ


d

x



u
• How to calculate dJ/dt?
J i

 j J i
j
j
i
k
dt
x dt   
x 
 u x
i
J i
 J i u  Jdivu
k
j
x x 
19
Two fundamental statements
•
(I)
(II)
What is the rate of change:
of an infinitesimal volume of a fluid particle?
Of any local quantity f(xi,t) integrated over an arbitrary closed
fluid domain (volume) Ω(t) moving with the fluid?
Answer to (I): the Euler expansion theorem
d
ln J  divu
dt
Answer to (II): the Reynolds transport theorem
dF (t )
A volume integral:
F (t ) 
f ( xi , t )dV ,
?

 (t )
dt
The idea: to transform F(t) into an integral over the initial volume
Ω(0), which is fixed
dF d
 d
dJ 

i
j
i
j
i
j

f
(
x
(

,
t
),
t
)
JdV

f
(
x
(

,
t
),
t
)
J

f
(
x
(

,
t
),
t
)

 dV0
0



dt dt (0)
dt
dt 

 (0)  
20
The Reynolds transport theorem
• Using the Euler expansion theorem, we get
 df
dF (t )
dJ 
u k
 df
   Jf
 dV0     f k
dt
dt
dt 
dt
x
 (0) 
 (t ) 

 dV

• Another form of the Reynolds theorem: using the interconnection
between the material (Lagrange) and field (Euler) derivatives (see
Slide 16), we obtain
d
i
i
f
(
x
,
t
)
dV


f


(
fx
) dV

t
i


dt  (t )
(t )
• There exists one more form – useful for numerical methods (e.g.
for so called mimetic schemes):
d
Here 2-surface element
i
k
f
(
x
,
t
)
dV


f
dV

fu
dS
t
k


dt (t )
 (t )
 ( t )
is a vector normal to
∂Ω(t) enclosing Ω(t)
21
Physical interpretation of the
Reynolds theorem
• The total rate of change of any quantity f (integrated
over the moving volume) = the rate of change of f itself
+ the net flow of f over the surface enclosing the volume
• A moving (control or material) volume Ω(t) is a volume
of fluid that moves with the flow and permanently
consists of the same material particles
• Specific case: f = const – what is the fluid volume rate
of change?
dV (, t )
V (, t ) :  dV ;
   k u k dV   u k dSk
dt
 (t )
(t )
 ( t )
22
Geometry of fluid flows
• A fluid particle moving along its pathline changes its volume,
shape, and orientation (e.g. rotates) – these are geometric
transforms of a fluid particle (with respect to its initial position)
• From the geometrical viewpoint, such changes can be described
as the time evolution of the flow governed by the geodesic
equation on the group of volume-preserving diffeomorphisms.
So, fluid moves along a geodesic curve
• A dynamical system (***), Slide 15, corresponding to fluid
motion, can be visualized as a vector field in the phase space, on
which the solution is an integral curve
• Geometry enables us to describe global properties of the family of
solution curves filling up the entire phase space
• The geometric approach (originally suggested by V.I. Arnold) is a
natural framework to describe the fluid motion
23
Strain rate and vorticity
• The simplest geometrical object describing spatial variations of a
fluid particle is the tensor τij(xk, t) := ∂ui/∂xj constructed from the
fluid velocity ui = δijuj
• In general, τij has no symmetry, however it can be decomposed
into a symmetric and antisymmetric parts:
 ij   ij   ij ,  ij   ji ,  ij   ji
Here the strain rate θ and vorticity ζ are defined as
1  ui u j 
1  ui u j 

,

:

 j i
ij
j
i 
2  x
x 
2  x
x 
ij : 
Frequently the vorticity vector is defined as
1
 i   ijk jk ,  ijk is the unit antisymmetric pseudotensor of rank 3
2
These definitions are used when treating the Navier-Stokes equation
24
Transition to physics ρ(xi,t)
The continuity equation
∂ρ/∂t + div ρu = 0
The Reynolds transport
theorem
d/dt∫Ω(t)fdV =
∫Ω(t)(df/dt+fdivu)dV
The Euler expansion
J-1dJ/dt = divu
IVP for fluid motion
dxi/dt = ui(xj,t), xi(0)= ξi
The fluid velocity
ui(ξ j,t) = ∂txi(ξj,t)
The law of fluid motion
xi = xi(ξ j,t)
The constitution of fluid kinematics
25
Control questions on fluid kinematics
1. Can one find the law of motion of a continuous
medium, if particle lines are known?
2. Can one find the velocity field, if streamlines are
given?
3. Can the particles move with acceleration, if
a) velocities of all particles are equal?
b) the velocity does not change with time in each point
xi  Ω ?
4. The density of each individual particle of an
incompressible medium is constant. Can the density
vary with time in some point xi  Ω ?
26
Answers to control questions on fluid
kinematics
1.
Can one find the law of motion of a continuous medium, if particle lines
are known?
No, since although a curve (trajectory), along which a particle
moves, is known for each particle, but the velocity of the motion
may be different (is not defined)
2.
Can one find the velocity field, if streamlines are given?
No, in each point the straight line, along which the velocity is
directed is known, but the velocity value may be different
3. Can the particles move with acceleration, if:
a)
velocities of all particles are equal? Yes
b)
the velocity does not change with time in each point
xi  Ω ? Yes
4.
The density of each individual particle of an incompressible medium is
constant. Can the density vary with time in some point xi  Ω ? Yes
27
Fluid kinematics – summing it up
• All particles are individualized (classical mechanics!)
• Lagrange coordinates {ξ i} are particle identifiers
• Motion of continuous media and accompanying processes are
described by physical fields (velocity, pressure, temperature,
etc.). In case these fields are considered as functions of ξ i, such
description is called Lagrangian or material
• Events in the Lagrangian picture occur with individual particles
• The law of motion of a continuous medium: xi = xi (ξ j,t)
• Velocity of particles: ui(ξ i, t) = ∂txi(ξ i, t) – the velocity field, i.e.
the velocity of particles located in {xi} at time t
• Acceleration of particles: ai(ξ i, t) = ∂tui(ξ i, t)
• Fields considered as functions of {xi, t} – the Euler description
28
Kinematics – summing up (cont’d)
df f
f

 ui i   t  uii  f
dt t
x
• Transition from the Lagrangian to the Eulerian description:
• Time derivative:


xi  xi  j , t    i   i  x j , t   f  i  x j , t  , t    x j , t 
• Transition from the Eulerian to the Lagrangean description to solve the Cauchy problem
dxi
 u i  x j , t  , xi   i
t 0
(***), Slide 15:
dt
i
j
The solution (if it could be found): x  , t
Then for any f(xi, t) whose Euler’s description is known:




f x i  j , t  , t    j , t 
29
Part2:
Dynamics of fluid flows
30
Transition to fluid dynamics
• No physics so far - no information about the nature or structure of
a fluid was required
• How to find the equations governing the fluid behavior?
1) Mathematical axioms and theorems (mostly kinematics)
2) Physical hypotheses (structure, interaction law, etc.)
3) Experimental evidence
4) Mathematical and computational models
• Transition to dynamics: taking inertia into account (recall the
Newton’s law!)
• Fluid specificity: liquid mass is contained within the domain Ω(t):
M (t )   dV  0 for any domain Ω(t).
i
i
 (t )
Hence  ( x , t )  0, {x , t} (t )
31
The continuity equation (CE)
• Extremely important!
• The underlying physical principle – conservation of mass
(strictly speaking, not a physical conservation law – not
associated with any symmetry, a phenomenological principle)
• Leads to a very profound question – what is a mass?
dM (t ) d
(in fact along the fluid particle paths)


dV

0
dt
dt (t )
i
d


u
• From here – the continuity equation:
 i 0
dt
x
- a necessary and sufficient condition for the mass of any fluid
domain to be conserved
Proof is based on the Reynolds transport theorem (see next slide)
32
The continuity equation - proof
• Sufficiency: to substitute the continuity equation into the
Reynolds theorem
d
dM (t )
u i 
  
  i  dV
dt
dt
x 
 (t ) 
dM (t )
0
dt
• Necessity:
for any arbitrary Ω(t), hence the integrand must be zero.
A more customary form:
i
t   i ( u )  0
• The continuity equation, despite its simplicity, brings about many
nontrivial results, e.g.:
- How to find the rate of change of a quantity f(xi,t) averaged with
the medium density distribution?
Q(t ) :

 (t )
 f dV
33
Some consequences of the continuity equation
• The rate of change of the weighted function f:
We have
d
i
i
Q(t ) 

(
x
,
t
)
f
(
x
, t )dV  ?

dt  (t )
 d ( f )
d
u k 
  f k dV 
  f dV   
dt  (t )
 (t )

dt
x 
 df d 

  

f   fdivu dV 
dt dt

 (t ) 
 df
df
 d

  
f
  divu  dV    dV
dt
dt
 dt

 (t ) 
 (t )
The result – a very simple formula:
dQ
df

 dV
Due to CE, the differentiation operator
dt  (t ) dt
ignores the density distribution.

34
Some more dynamics
• Note: although the medium density is not a kinematic
object, the continuity equation may be derived from
kinematics
• Given a fluid density distribution, one can determine a
velocity field ui(xj,t) for the fluid motion, which would
conserve the mass in each domain Ω(t)
• In such formulation, the continuity equation is the firstorder PDE with respect to ui(xj,t) – not sufficient to find
3 unknown functions
Fluid dynamics cannot be described by the
continuity equation alone – one needs more physics
35
Forces in fluid media
• Physical forces, both internal and external, are acting on fluid
particles to set them in motion
• The Cauchy model: two types of forces
 internal – contact forces, act on a fluid particle over its surface
(e.g. stress, pressure)
 external – body forces, act over a medium (e.g. gravity,
electromagnetic field)
External forces – described per unit mass by a vector Fi(xj,t,uk)
depending on position, time, and state of fluid motion (e.g. magnetic
field, relativism, etc.)
• Cauchy: internal forces are more interesting - for any closed
surface ∂Ω(t), there exists a stress vector distribution si(xj,t,nk)
depending on the normal nk
36
The Cauchy stress
• The idea: the stress action exercised by the stress vector si(xj,t,nk)
is equivalent to the action of internal forces P
The Cauchy stress
P
s  lim
 0 
vector
s
Δσ
The model: si is a linear function
n
of the normal vector ni(xj,t):
si  Tji n j
• So, the stress vector for a surface element with the normal n can
be calculated as sn = sn
• The coefficients Tij - the fluid stress tensor
• In general, no symmetry
37
The Cauchy equation of motion
• An adaptation of the Newton’s law to fluid flows:
ij
du i

T

 Fi  j
dt
x
acceleration of
a fluid particle
external force
density
internal force
density
A system of 1st-order PDE with respect to fluid velocity components
• Includes the density distribution satisfying the continuity equation
• External forces Fi(xk,t) and the fluid stress tensor Tij (xk,t) are
considered to be known, together with initial and boundary
conditions. In this case the solution can be obtained
38
The general system of equations in fluid
dynamics
• If the external forces and the stress tensor components are known,
initial and boundary conditions are given, then the fluid velocity
ui(xj,t) can be obtained from the following system:
1

j
i
i
ij

u

u

u

F


T
j
j
 t


    (  u j )  0
j
 t
- four first-order PDEs over four unknown functions
Once the solution is obtained, the law of motion xi = xi (ξ j,t) can be
found from ui by solving the system of first-order ODEs (***):
dxi
 u i ( x j , t ), xi (0)   i
dt
39
The total acceleration
• Thus, in principle, the problem of fluid motion can be solved
(theoretically, in practice this is rather difficult)
• A reduced form of fluid motion equation – analogous to the
D’Alembert principle in classical mechanics:
i
du
- the total acceleration, a combined
Ai ( x j , t ) :
 Fi
dt
response to inertial and external forces
Physical interpretation: total acceleration is
 Ai   jT ij
determined by internal forces (acting on a
fluid particle)
• One needs physical assumptions regarding the nature of internal
forces (acting across fluid surface elements)
• Such assumptions would specify the Tij tensor components
40
Specific forms of motion equations
No a priori properties of Tij(xk,t): no algebraic symmetry (indices),
no geometric symmetry ( xk  Rlk xl ), no physical symmetry (t→-t)
•
An example of a physical assumption – the Boltzmann
postulate:
T ij  T ji
– an immediate consequence: conservation of angular momentum
Holds not for all fluids: e.g. for polar fluids the asymmetry of Tij
may be essential
•
Now, we restrict ourselves to fluids without heat transfer:
the temperature T = const, the heat flux qi(xi,t) = 0
Tree main fluid equations:
1. The Euler equation
2. The Stokes equation;
3. The Navier-Stokes equation
41
The Euler equation
• The perfect fluid, Tij = -pδij:
t u i  u j  ju i  F i 
1
 ij  j p

p(xj,t) is the fluid pressure – depends on the thermodynamic state
A control question: can the pressure be < 0?
Answer: yes – expansion of a fluid particle (in fact instability)
What is a perfect fluid? (“Nobody‘s perfect!“)
• No dissipation, no internal friction, no internal heat transfer
Heat conductivity and viscosity are neglected – adiabatic motion:
ds s
s
  ui i  0
dt t
x
Combining with the continuity equation:
(  s) 
 i (  su i )  0
t
x
the entropy flux
42
Another form of the Euler equation
Introducing h – enthalpy, dh = Tds + Vdp:
j
u i
h
i u
u
 i
i
t
x
x
Using the formula:
we can write
or (applying curl):
(for ds = 0, (1/ρ)∂ρ/∂xi = ∂h/∂xi)
1
(u)u  (u 2 )  [u curlu]
2
2
u
u
 [u curlu]  (h  )
t
2

curlu  curl[u curlu]
t
- only the velocity is present
Boundary condition: un = 0 (fixed boundary)
In general, 5 variables: ui, ρ, p; ρ = ρ(p,T) – the state equation
43
The Stokes equation
T ij  ( p   ) ij   ij   kl ik jl
• The model:
The viscosities:
α(xi,t), β(xi,t), γ(xi,t) – scalar functions, in general depend on the
thermodynamical state of the fluid
   t u i  u j  j u i    F i   ij  j  
 j   ij    kl  j  ik jl 
- a very complex expression.
The Stokes equation is used to model non-elastic fluids
44
The Navier-Stokes equation
• The model:
T ij  (  p) ij  2 ij
   t u i  u j  j u i    F i   ij  j    ij  j ( )  2 j (  ij )
Here λ(xi,t), μ(xi,t) are scalar functions - the first and the second
viscosities. For λ = const and μ = const:
i
2 i
u i
1 ij p    ij  2u k
j u
i
kl  u
u
F  


 
j
j
j
k
t
x
 x

x x
x k xl
For ρ = const (incompressible),
div u = 0. Then

xi
 
u
p
 (u)u  F 
 u  = 
t



 u k 
 k
 x 
- the main equation of
the classical fluid
dynamics
45
Another form of the Navier-Stokes equation
• Similarly to the Euler equation – an alternative form
• Incompressible fluid div u = 0 → the velocity field is rotational or
transversal, u = u. Applying curl (or rot) to the N.-S. equation:

uF
curlu  curl[u curlu]  curlu  curl
t
u·curlu - helicity

or
curlu   u  curlu   curlu  u  curlu  curlF
t
This is a closed equation with respect to u – no pressure involved
 2 (u i u j )
On the contrary, equation for the pressure:
p   
xi x j
or, transforming,
u i u j
p    j i    ij i j , ij := j u i
x x
46
Derivation of the pressure equation
• Applying div to the Navier-Stokes equation:
  u i  
 i  i
t  x  x
 j 
u
j

x

p

 i
 i
u  
 x


2
 j
 x x j
 i F i
 u  i
x

Assuming div F =0 (no charges). The second term in LHS:
2 i
j
i
i
j
i
  j   i u j u j u i

u

u

u


u

u

u
j
j
u
u



u


u
 i
i 
j 
i
j
i
j
i
j
j
i
x  x 
x
x x
x x
x x
x x
x x j
The second term in the RHS:
Thus:

xi
u i u j
p    j i    ij i j , ij := j u i
x x
2
i j
i
since   u u    i u j

u
j

u

u

xi x j
x i  x j
x j
  2u i
 j
 x x j

 2 u i
0
   j
i
x x j x

or, in an equivalent form,
 u j u i
 i
j

x

x

 2 (u i u j )
p   
xi x j
47
A closed system of equations of classical
fluid dynamics
• A closed system of equations consists of:
- The Navier-Stokes (N.-S.) equation
- The continuity equation
- The thermodynamic state equation ρ = ρ(p,T)
5 equations for unknown functions: ui(xj,t), p(xj,t), ρ(xj,t)
In principle, the system is closed, provided the external forces Fi(xj,t)
and the viscosity coefficients λ, μ are given
• Initial and boundary conditions must be imposed to select an
appropriate solution
• Although considerable difficulties do exist in solving the N.-S.
equation, it has been successfully applied to described various
regimes of fluid motion (including turbulence – to some extent)
48
Elementary model of the viscosity
• Many practically important problems cannot be solved without
viscosity being taken into account, e.g.
- motion of bodies in fluids
- fluid flow through channels, tubes, etc.
du x
- flows past an obstacle
x
F ( y  dy )   dxdz
dy y  dy
F
y
du x
F ( y )    dxdz
dy
x
dy
y
force applied to a unit volume:
F x ( y  dy)  F x ( y)
 2u x
f 
 2
dxdydz
y
x
A 2d shear flow
x
To N.-S.equation
49
Newtonian fluids
• When a solid is slightly deformed so that it is strained, a
restoring force opposing the strain is observed; for small strain it
is proportional to the strain (the Hooke’s law)
• Fluids also resist strains, but for a fluid it is not only the strain
magnitude that is important, but the strain rate as well
• Fluids responding to a sheer stress independent of its rate are
known as Newtonian fluids. More generally, in a Newtonian
fluid the sheer stress is proportional to the velocity gradient (see
Slide 49), i.e. components of the stress tensor are linear functions
of τij = ∂ui/∂xj
• For Newtonian fluids, the viscosity is constant (does not depend
on how fast the upper plate is sliding past the bottom)
• For non-Newtonian fluids, viscosity depends on relative velocity.
Such fluids (studied in rheology) may have no defined viscosity
50
The physical meaning of viscosity
• Viscosity is a macroscopic manifestation of intermolecular
interactions. Viscosity always leads to energy dissipation
• All phenomena involving viscosity are irreversible
• We can calculate the energy dissipation rate in an incompressible
viscous fluid – no work is spent on compression (or rarefaction)
of fluid particles. The rate of change of kinetic energy inside the
control volume Ω:
i
2 i

u i

u

p

u
E ()   ui
dV   ui   u k k 
 k
t
x xi
x xk



Since
u

ui k
 k
x xk x
2 i
we get
 u
 ui k
 x
i
2
 k u i 1 p  
  u 


   k  , ui  u
k
 xi  xi
  x 
 x
i

dV


 u 2 p 
 ui    
 2  


 u2 p 
 u i 
u i 
E ()    d   uk      ui k      k  dV
x 
x 
 2 



2
k
51
The physical meaning of viscosity (cont’d)
• The surface integral in E () describes the energy change due to
the fluid inflow/outflow (the term ρuku2/2) and with the work
produced by surface stresses
• The dissipated power is given by the volume integral, which is
always positive
i 2
 u 
ik
Q




 ik
• The viscous dissipation rate per unit volume is
 k
 x 
i.e. proportional to the dynamic viscosity
• The viscous coefficient μ ≥ 0 (this is the consequence of the
second law of thermodynamics)
When can one neglect viscosity?
• The viscous force μΔu ~ μu/L2 must be <<p ~ p/L ~ ρu2/L or
μ << ρuL (here L is the system‘s characteristic size, see Slide 10)
This criterion can be written as 1/Re <<1, Re:= ρuL/μ = uL/ν
52
The Reynolds number
• In 1883 Reynolds made the systematic study of of the water flow
through a circular tube and found that the flow pattern changes
from a “laminar“ to a “turbulent“ state at a certain value of uL/ν
• Later, this quantity was called after him the Reynolds number, Re
Using the Reynolds number, one can write the N.-S. equation in
i
dimensionless form:
Du
  ij  j p  Re 1  j  j u i  F i
Dt
(D:= ∂t + uj∂j, see Slide 16 – in fact a covariant derivative)
• Physically, Re is a measure of the ratio between inertal and
viscous forces in the flow; for Re>>1 inertia prevails, for Re<<1
friction (viscosity) prevails
• Both forces are balanced by the pressure gradient
• Simple steady flows at small values of Re vs. complicated nonstationary flows at large values of Re
53
Flow past an obstacle
• The transition from a laminar to a turbulent mode can be observed
in the flow past an obstacle, the simplest obstacle being a sphere
Re1
Re~10
Re~10 -100
Re~102 -105
Re > 105
54
Description of flow patterns
• Re  1: The flow is stationary and passes regularly around the
sphere (without separation)
• 1  Re  10: The flow is still stationary but separates at the back
side of the sphere. A ring vortex is formed, which increases with Re
• 10  Re  102: The ring vortex deforms into a helical vortex,
which rotates about the flow axis. The flow becomes non-stationary,
but periodic (pulsations)
• 102  Re  105: The helical vortex breaks up and is replaced by a
turbulent wake. The flow is totally aperiodic in the wake
• Re > 105: Not only the wake, but the entire flow (including the
boundary layer) becomes turbulent. The flow is completely
random everywhere. The wake is thinner and the drag reduced
55
Mathematical status of the N.-S. equation
• No existence theorem is known as yet – for a smooth and
physically reasonable solution to a 3d Navier-Stokes equation
• Two major sources of mathematical difficulties:
1) a nonlinear (quasilinear) system of equation – due to the
convection (inertial) term uj∂jui
This nonlinearity is of kinematic (geometrical) origin, arising
from mathematical derivation (the chain rule), not from physical
considerations. A unique case in physics
2) viscosity terms introduce the second-order operator into the
first-order equation – leads to singular perturbations
The question: would the solutions to the N.-S. equations in
general converge to the solutions of the Euler equations in a
domain Ω with boundary ∂Ω, as the viscosity tends to zero?
56
Physical status of the N.-S. equation
• A phenomenological macroscopic equation – to describe the
dynamics of fluids viewed as continuous media
• The fluid flow at the microscopic level is treated in nonequilibrium statistical mechanics and physical kinetics
• Physics of fluids can be reconstructed from the first principles
only for very special cases (e.g. dilute gases)
• The N.-S. equation, the Euler equation, etc. can be obtained from
the Boltzmann equation (the moments of Boltzmann equation)
• The Boltzmann equation is also a special case of a more general
dissipative kinetic equation, the latter being in its turn a specific
case of a more general equation (the Liouville equation), etc.
• So, the Navier-Stokes equation is accepted only as a reasonable
mathematical model for fluid flows
57
Phenomenology of fluid motion
• From the physical viewpoint, the fluid in motion is a macroscopic
non-equilibrium system
• When physicists are studying such systems, they apply two
distinct approaches: phenomenological and microscopic
• In the phenomenological method, the problem is reduced to
establishing the relationship between macroscopic parameters,
without referring to atomic or molecular considerations
• The concept of continuous medium is an idealization. When the
atomic structure is taken into account, fluctuations become
essential. One can consider their influence only within the
framework of kinetic theory.
• The meaning of stress tensor, viscosity, heat conductivity, etc.,
as well as the applicability of the N.-S. equation can be also
elucidated only within the framework of kinetic theory
58
Examples of using the distribution function
• The kinetic equation is with respect to distribution (one-particle)
function, PDF: f(r, v, t) = f(xi,vi,t)
• Definition of fluid dynamics quantities in terms of PDF:
  x i , t    f  x i , v i , t  d 3v
• For an incompressible fluid:

where
f  x i , v i  u i , t  d 3v  const
u i   v i f  x i , v i , t  d 3v
The Boltzmann kinetic
equation serves as the
base for a so-called lattice
Boltzmann numerical
approach
the average flow velocity (the main quantity in the fluid dynamics)
m
The flow temperature:
i
i 2
i
i
i
3
T
(v

2
 u ) f  x ,v  u ,t  d v
We assumed it to be constant, thus disregarding the heat transfer
59
What else?
Physical fluid dynamics:
•
•
•
•
•
•
•
Heat transfer
Thermodynamics of continuous media
Surface phenomena and free boundaries
Dimensional analysis, scaling, and self-similarity
Boundary layers
Comressible fluids and gas dynamics
Turbulence
Computational fluid dynamics (CFD) and
numerical modeling
60