me 231 thermofluid mechanics i navier-stokes equations
TRANSCRIPT
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ME 231Thermofluid
Mechanics I
Navier-Stokes Equations
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Computational Fluid Dynamics (AE/ME 339) K. M. Isaac
MAEEM Dept., UMR
Equations of Fluid Dynamics, Physical Meaning of the terms.
Equations are based on the following physical principles:
• Mass is conserved
• Newton’s Second Law:
• The First Law of thermodynamics: e = q - w, for a system.
F = m a
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Computational Fluid Dynamics (AE/ME 339) K. M. Isaac
MAEEM Dept., UMR
Control Volume Analysis
The governing equations can be obtained in the integral form by choosing a control volume (CV) in the flow field and applying the principles of the conservation of mass, momentum and energy to the CV.
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Computational Fluid Dynamics (AE/ME 339) K. M. Isaac
MAEEM Dept., UMR
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Computational Fluid Dynamics (AE/ME 339) K. M. Isaac
MAEEM Dept., UMR
Consider a differential volume element dV in the flow field. dV is small enough to be considered infinitesimal but large enough to contain a large number of molecules for continuum approach to be valid.
dV may be:
• fixed in space with fluid flowing in and out of its surface or,
• moving so as to contain the same fluid particles all the time. In this case the boundaries may distort and the volume may change.
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Computational Fluid Dynamics (AE/ME 339) K. M. Isaac
MAEEM Dept., UMR
Substantial derivative
(time rate of change following a moving fluid element)
1
2
x
y
z
Fluid element attime t = t1
Fluid element attime t = t2
V1
V2
i
j
k
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Computational Fluid Dynamics (AE/ME 339) K. M. Isaac
MAEEM Dept., UMR
The velocity vector can be written in terms of its Cartesian components as:
where
u = u(t, x, y, z)
v = v(t, x, y, z)
w = w(t, x, y, z)
ˆˆ ˆ( , , , ) ( , , , ) ( , , , )V iu t x y z jv t x y z kw t x y z
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Computational Fluid Dynamics (AE/ME 339) K. M. Isaac
MAEEM Dept., UMR
@ time t1:
@ time t2:
Using Taylor series
1 1 1 1 1( , , , )t x y z
2 2 2 2 2( , , , )t x y z
2 1 2 1 2 1 2 1 2 11 1 11
( ) ( ) ( ) ( )t t x x y y z zt x y z
( )higher order terms
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Computational Fluid Dynamics (AE/ME 339) K. M. Isaac
MAEEM Dept., UMR
The time derivative can be written as shown on the RHS in the following equation. This way of writing helps explain the meaning of total derivative.
2 1 2 1 2 1 2 1
1 1 12 1 2 1 2 1 2 11
...........(2.1)x x y y z z
t t t x t t y t t z t t
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Computational Fluid Dynamics (AE/ME 339) K. M. Isaac
MAEEM Dept., UMR
We can also write
2 1
2 1
2 1
limt t
D
t t Dt
2 1
2 1
2 1
limt t
x xu
t t
2 1
2 1
2 1
limt t
y yv
t t
2 1
2 1
2 1
limt t
z zw
t t
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Computational Fluid Dynamics (AE/ME 339) K. M. Isaac
MAEEM Dept., UMR
where the operator can now be seen to be defined in the following manner.
...........(2.2)D
u v wDt t x y z
D
Dt
.........(2.3)D
u v wDt t x y z
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Computational Fluid Dynamics (AE/ME 339) K. M. Isaac
MAEEM Dept., UMR
The operator in vector calculus is defined as
which can be used to write the total derivative as
ˆˆ ˆ ...........(2.4)i j kx y z
...........(2.5)D
VDt t
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Computational Fluid Dynamics (AE/ME 339) K. M. Isaac
MAEEM Dept., UMR
Example: derivative of temperature, T
( ) ........(2.6)
convectivelocal derivativederivative
DT T T T T TV T u v w
Dt t t x y z
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Computational Fluid Dynamics (AE/ME 339) K. M. Isaac
MAEEM Dept., UMR
A simpler way of writing the total derivative is as follows:
............(2.7)d dt dx dy dzt x y z
............(2.8)d dx dy dz
dt t x dt y dt z dt
..........(2.9)d
u v wdt t x y z
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Computational Fluid Dynamics (AE/ME 339) K. M. Isaac
MAEEM Dept., UMR
The above equation shows that and have the same meaning,
and the latter form is used simply to emphasize the physical meaning that it consists of the local derivative and the convective derivatives. Divergence of Velocity (What does it mean?) Consider a control volume moving with the fluid. Its mass is fixed with respect to time.Its volume and surface change with time as it moves from one location to another.
d
dt
D
Dt
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Computational Fluid Dynamics (AE/ME 339) K. M. Isaac
MAEEM Dept., UMR
Insert Figure 2.4
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Computational Fluid Dynamics (AE/ME 339) K. M. Isaac
MAEEM Dept., UMR
The volume swept by the elemental area dS during time interval t can be written as
Note that, depending on the orientation of the surface element, v could be positive or negative. Dividing by t and letting 0 gives the following expression.
ˆ .........(2.10)V V t n dS V t dS
V
0
1..........(2.11)lim
t S S
DVV t dS V dS
Dt t
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Computational Fluid Dynamics (AE/ME 339) K. M. Isaac
MAEEM Dept., UMR
The LHS term is written as a total time derivative because the fluid element is moving with the flow and it would undergo both the local acceleration and the convective acceleration. The divergence theorem from vector calculus can now be used to transform the surface integral into a volume integral.
. ........(2.12)V
DVV dV
Dt
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Computational Fluid Dynamics (AE/ME 339) K. M. Isaac
MAEEM Dept., UMR
If we now shrink the moving control volume to an infinitesimal volume, , , the above equation becomes
When the volume integral can be replaced by on the RHS to get the following.
The divergence of is the rate of change of volume per unit volume.
δV
( )...............(2.13)
D VV dV
Dt
δV 0
VδV
V
1 ( ).........(2.14)
D VV
V Dt
δV
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Computational Fluid Dynamics (AE/ME 339) K. M. Isaac
MAEEM Dept., UMR
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Computational Fluid Dynamics (AE/ME 339) K. M. Isaac
MAEEM Dept., UMR
Continuity Equation
Consider the CV fixed in space. Unlike the earlier case the shape and size of the CV are the same at all times. The conservation of mass can be stated as: Net rate of outflow of mass from CV through surface S = time rate of decrease of mass inside the CV
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Computational Fluid Dynamics (AE/ME 339) K. M. Isaac
MAEEM Dept., UMR
The net outflow of mass from the CV can be written as
Note that by convention is always pointing outward. Therefore can be (+) or (-) depending on the directions of the velocity and the surface element.
..........(2.16)nV dS V dS
V dSdS
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Computational Fluid Dynamics (AE/ME 339) K. M. Isaac
MAEEM Dept., UMR
Total mass inside CV
Time rate of increase of mass inside CV (correct this equation)
Conservation of mass can now be used to write the following equation
See text for other ways of obtaining the same equation.
..........(2.20)V
Mass dV
................(2.18)V
dVt
0.................(2.19)V S
dV V dSt
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Computational Fluid Dynamics (AE/ME 339) K. M. Isaac
MAEEM Dept., UMR
0........(2.21)V
DdV
Dt
Integral form of the conservation of mass equation thus becomes
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Computational Fluid Dynamics (AE/ME 339) K. M. Isaac
MAEEM Dept., UMR
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Computational Fluid Dynamics (AE/ME 339) K. M. Isaac
MAEEM Dept., UMR
An infinitesimally small element fixed in space
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Computational Fluid Dynamics (AE/ME 339) K. M. Isaac
MAEEM Dept., UMR
Net mass flow =
( ) uuu dx dydz u dydz dxdydz
x x
v vv dy dxdz v dxdz dxdydz
y y
w ww dz dxdy w dxdy dxdydz
z z
..........(2.22)
u v wdxdydz
x y z
Net outflow in x-direction
Net outflow in y-direction
Net outflow in z-direction
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Computational Fluid Dynamics (AE/ME 339) K. M. Isaac
MAEEM Dept., UMRvolume of the element = dx dy dz mass of the element = (dx dy dz)
Time rate of mass increase = ...........(2.23)dxdydzt
Net rate of outflow from CV = time rate of decrease of mass within CV
u v wdxdydz dxdydz
x y z t
0.......(2.24)
u v w
t x y z
or
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Computational Fluid Dynamics (AE/ME 339) K. M. Isaac
MAEEM Dept., UMR
0..........(2.25)Vt
Which becomes
The above is the continuity equation valid for unsteady flow Note that for steady flow and unsteady incompressible flow the first term is zero.
Figure 2.6 (next slide) shows conservation and non-conservation forms of the continuity equation. Note an error in Figure 2.6: Dp/Dt should be replace with D/Dt.
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Computational Fluid Dynamics (AE/ME 339) K. M. Isaac
MAEEM Dept., UMR
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Computational Fluid Dynamics (AE/ME 339) K. M. Isaac
MAEEM Dept., UMRMomentum equation
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Computational Fluid Dynamics (AE/ME 339) K. M. Isaac
MAEEM Dept., UMR
Consider the moving fluid element model shown in Figure 2.2bBasis is Newton’s 2nd Law which says F = m aNote that this is a vector equation. It can be written in terms of the three cartesian scalar components, thefirst of which becomes
Fx = m ax
Since we are considering a fluid element moving with the fluid, its mass, m,is fixed. The momentum equation will be obtained by writing expressions for theexternally applied force, Fx, on the fluid element and the acceleration, ax,of the fluid element. The externally applied forces can be divided into two types:
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Computational Fluid Dynamics (AE/ME 339) K. M. Isaac
MAEEM Dept., UMR
1. Body forces: Distributed throughout the control volume. Therefore,this is proportional to the volume. Examples: gravitational forces, magnetic forces, electrostatic forces.
2. Surface forces: Distributed at the control volume surface. Proportionalto the surface area. Examples: forces due to surface and normal stresses.These can be calculated from stress-strain rate relations.
Body force on the fluid element = fx dx dy dzwhere fx is the body force per unit mass in the x-direction
The shear and normal stresses arise from the deformation of the fluidelement as it flows along. The shape as well as the volume of the fluidelement could change and the associated normal and tangential stressesgive rise to the surface stresses.
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Computational Fluid Dynamics (AE/ME 339) K. M. Isaac
MAEEM Dept., UMR
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Computational Fluid Dynamics (AE/ME 339) K. M. Isaac
MAEEM Dept., UMRThe relation between stress and rate of strain in a fluid is known from thetype of fluid we are dealing with. Most of our discussion will relate to Newtonian fluids for which
Stress is proportional to the rate of strain
For non-Newtonian fluids more complex relationships should be used.
Notation: stress ij indicates stress acting on a plane perpendicular to the i-direction (x-axis) and the stress acts in the direction, j, (y-axis).
The stresses on the various faces of the fluid element can written asshown in Figure 2.8. Note the use of Taylor series to write thestress components.
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Computational Fluid Dynamics (AE/ME 339) K. M. Isaac
MAEEM Dept., UMR
The normal stresses also has the pressure term.
Net surface force acting in x direction =
pp p dx dydz
x
yxxxxx xx yx yxdx dydz dy dxdz
x y
.............(2.46)zxzx zxdz dxdy
z
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Computational Fluid Dynamics (AE/ME 339) K. M. Isaac
MAEEM Dept., UMR
...........(2.47)yxxx zxx x
pF dxdydz f dxdydz
x x y z
........(2.48)m dxdydz
...........(2.49)x
Dua
Dt
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Computational Fluid Dynamics (AE/ME 339) K. M. Isaac
MAEEM Dept., UMR
.........(2.50 )yxxx zxx
Du pf a
Dt x x y z
.........(2.50 )xy yy zyy
Dv pf b
Dt y x y z
.........(2.50 )yzxz zzz
Dw pf c
Dt z x y z
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Computational Fluid Dynamics (AE/ME 339) K. M. Isaac
MAEEM Dept., UMR
....................(2.51)Du u
V uDt t
u uu
t t t
.............(2.52)
uuu
t t t
........(2.53)V u uV u V
uDuu u V uV
Dt t t
.............(2.54)u
u V uVt t
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Computational Fluid Dynamics (AE/ME 339) K. M. Isaac
MAEEM Dept., UMR
The term in the brackets is zero (continuity equation)The above equation simplifies to
...........(2.55)uDu
uVDt t
Substitute Eq. (2.55) into Eq. (2.50a) shows how the following equationscan be obtained.
..........(2.56 )yxxx zxx
u puV f a
t x x y z
..........(2.56 )xy yy zyy
v pvV f b
t y x y z
..........(2.56 )yzxz zzz
w pwV f c
t z x y z
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Computational Fluid Dynamics (AE/ME 339) K. M. Isaac
MAEEM Dept., UMRThe above are the Navier-Stokes equations in “conservation form.”
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Computational Fluid Dynamics (AE/ME 339) K. M. Isaac
MAEEM Dept., UMRFor Newtonian fluids the stresses can be expressed as follows
2 ...........(2.57 )xx
uV a
x
2 ...........(2.57 )yy
vV b
y
2 ...........(2.57 )zz
wV c
z
...........(2.57 )xy yx
v ud
x y
...........(2.57 )xz zx
u we
z x
...........(2.57 )yz zy
w vf
y z
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Computational Fluid Dynamics (AE/ME 339) K. M. Isaac
MAEEM Dept., UMR
In the above is the coefficient of dynamic viscosity and is the second viscosity coefficient.Stokes hypothesis given below can be used to relate the above two coefficients
= - 2/3 The above can be used to get the Navier-Stokes equations in the followingfamiliar form
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Computational Fluid Dynamics (AE/ME 339) K. M. Isaac
MAEEM Dept., UMR
2
2uu uv uw p u
Vt x y z x x x
.........(2.58 )x
v u u wf a
y x y z z x
2
2vv uv vw p v
Vt y x z y y y
.........(2.58 )y
v u v wf b
x x y z z y
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Computational Fluid Dynamics (AE/ME 339) K. M. Isaac
MAEEM Dept., UMR
2
2ww uw vw p w
Vt z x y z z z
.........(2.58 )z
w v u wf c
y y z x z x
The energy equation can also be derived in a similar manner.
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Computational Fluid Dynamics (AE/ME 339) K. M. Isaac
MAEEM Dept., UMR
The above equations can be simplified for inviscid flows by droppingthe terms involving viscosity.
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Summary
• Apply conservation equations to a control volume (CV)• As the CV shrinks to infinitesimal volume, the resulting partial
differential equations are the Navier-Stokes equations• Taylor series can be used to write the variables• The total derivative consists of local time derivative and convective
derivative terms• In incompressible flow, divergence of velocity is a statement of
the conservation of volume• Need surface and body forces to write the momentum equation• Surface forces are: pressure forces, forces due to normal stresses
and forces due to shear stresses• Body forces are due to weight, magnetism and electrostatics • Momentum equation is a vector equation. Can be written in terms
of its components.
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Summary
• Stresses are indicated by a plane and a direction, respectively, by two subscripts• For Newtonian fluids, stresses are proportional to the rates of strain• Stokes hypothesis is used to relate the first and second coefficients of viscosity• The resulting equations are the Navier-Stokes equations• In order to solve the equations, they must be simplified for the problem
you are considering (e. g., boundary layer, jet, airfoil flow, nozzle flow)