The calculus of moving surfaces (CMS) [1] is an extension of the classical tensor calculus to deforming manifolds. Central to the CMS is the Tensorial Time Derivative
\nabla |
\nabla\alpha
Suppose that
\Sigmat
\Sigma
t
C
\nabla |
\Sigma
C
P
C=\limh\to
Distance(P,P*) | |
h |
where
P*
\Sigmat+h
\Sigmat
C
\overline{PP*
\Sigmat
C
The Tensorial Time Derivative
\nabla |
\Sigmat
F
\deltaF | |
\deltat |
=\limh\to
F(P*)-F(P) | |
h |
This definition is also illustrated in second geometric figure.
The above definitions are geometric. In analytical settings, direct application of these definitions may not be possible. The CMS gives analytical definitions of C and
\nabla |
For analytical definitions of
C
\nabla |
S
Zi=Zi\left(t,S\right)
where
Zi
S\alpha
S
S\alpha
ibf{Z} | |
bf{V}=V | |
i |
Vi=
\partialZi\left(t,S\right) | |
\partialt |
The velocity
C
C=ViNi
where
Ni
\vec{N}
Also, defining the shift tensor representation of the Surface's Tangent Space
\alpha | |
Z | |
i |
\alpha ⋅ bf{Z} | |
=bf{S} | |
i |
V\alpha=Z
i | |
iV |
\nabla |
\nabla | F= |
\partialF\left(t,S\right) | |
\partialt |
\alpha\nabla | |
-V | |
\alpha |
F
where
\nabla\alpha
For tensors, an appropriate generalization is needed. The proper definition for a representative tensor
i\alpha | |
T | |
j\beta |
\nabla |
i\alpha | ||
T | = | |
j\beta |
| |||||||||
\partialt |
-Vη\nablaη
i\alpha | |
T | |
j\beta |
+Vm\Gamma
i | |
mk |
k\alpha | |
T | |
j\beta |
-Vm\Gamma
k | |
mj |
i\alpha | ||
T | + | |
k\beta |
\Gamma |
\alpha | |
η |
iη | ||
T | - | |
j\beta |
\Gamma |
η | |
\beta |
i\alpha | |
T | |
jη |
where
k | |
\Gamma | |
mj |
\Gamma |
\alpha | |
\beta=\nabla |
\betaV\alpha-C
\alpha | |
B | |
\beta |
\alpha | |
B | |
\beta |
\nabla |
The
\nabla |
\nabla |
i | |
(S | |
\alpha |
\beta | |
T | |
j |
\beta | |||
)=T | |||
|
i | |
S | |
\alpha |
+
i | |||
S | |||
|
\beta | |
T | |
j |
and obeys a chain rule for surface restrictions of spatial tensors:
\nabla |
j | ||
F | = | |
k(Z,t) |
| |||||||||
\partialt |
+CNi\nablai
j | |
F | |
k |
Chain rule shows that the
\nabla |
\nabla |
i | |||
\delta | |||
|
Zij=0,
\nabla |
Zij=0,
\nabla |
\varepsilonijk=0,
\nabla |
\varepsilonijk=0
where
Zij
Zij
\delta
i | |
j |
\varepsilonijk
\varepsilonijk
Zij
\nabla |
The
\nabla |
S\alpha
S\alpha
\begin{align} \nabla |
S\alpha&=0\\[8pt]
\nabla |
S\alpha&=0 \end{align}
where
B\alpha
B\alpha
\alpha | |
B | |
\beta |
\begin{align} \nabla |
B\alpha&=\nabla\alpha\nabla\betaC+CB\alpha
\gamma | ||
B | \\[8pt] | |
\beta |
\nabla |
\alpha | |
B | |
\beta& |
=
\alpha | |
\nabla | |
\beta\nabla |
C+
\alpha | |
CB | |
\gamma |
\gamma | ||
B | \\[8pt] | |
\beta |
\nabla |
B\alpha&=\nabla\alpha\nabla\betaC+CB\gamma
\beta | |
B | |
\gamma \end{align} |
The shift tensor
i | |
Z | |
\alpha |
Ni
\begin{align} \nabla |
i | |
Z | |
\alpha |
&=
i\nabla | |
N | |
\alpha |
C\\[8pt]
\nabla |
Ni&=
i | |
-Z | |
\alpha |
\nabla\alphaC \end{align}
Finally, the surface Levi-Civita symbols
\varepsilon\alpha
\varepsilon\alpha
\begin{align} \nabla |
\varepsilon\alpha&=0\\[8pt]
\nabla |
\varepsilon\alpha&=0 \end{align}
The CMS provides rules for time differentiation of volume and surface integrals.