Calculus of moving surfaces explained

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
whose original definition [2] was put forth by Jacques Hadamard. It plays the role analogous to that of the covariant derivative

\nabla\alpha

on differential manifolds in that it produces a tensor when applied to a tensor.

Suppose that

\Sigmat

is the evolution of the surface

\Sigma

indexed by a time-like parameter

t

. The definitions of the surface velocity

C

and the operator
\nabla
are the geometric foundations of the CMS. The velocity C is the rate of deformation of the surface

\Sigma

in the instantaneous normal direction. The value of

C

at a point

P

is defined as the limit

C=\limh\to

Distance(P,P*)
h

where

P*

is the point on

\Sigmat+h

that lies on the straight line perpendicular to

\Sigmat

at point P. This definition is illustrated in the first geometric figure below. The velocity

C

is a signed quantity: it is positive when

\overline{PP*

} points in the direction of the chosen normal, and negative otherwise. The relationship between

\Sigmat

and

C

is analogous to the relationship between location and velocity in elementary calculus: knowing either quantity allows one to construct the other by differentiation or integration.

The Tensorial Time Derivative

\nabla
for a scalar field F defined on

\Sigmat

is the rate of change in

F

in the instantaneously normal direction:
\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
in terms of elementary operations from calculus and differential geometry.

Analytical definitions

For analytical definitions of

C

and
\nabla
, consider the evolution of

S

given by

Zi=Zi\left(t,S\right)

where

Zi

are general curvilinear space coordinates and

S\alpha

are the surface coordinates. By convention, tensor indices of function arguments are dropped. Thus the above equations contains

S

rather than

S\alpha

. The velocity object
ibf{Z}
bf{V}=V
i
is defined as the partial derivative

Vi=

\partialZi\left(t,S\right)
\partialt

The velocity

C

can be computed most directly by the formula

C=ViNi

where

Ni

are the covariant components of the normal vector

\vec{N}

.

Also, defining the shift tensor representation of the Surface's Tangent Space

\alpha
Z
i
\alphabf{Z}
=bf{S}
i
and the Tangent Velocity as

V\alpha=Z

i
iV
, then the definition of the
\nabla
derivative for an invariant F reads
\nablaF=
\partialF\left(t,S\right)
\partialt
\alpha\nabla
-V
\alpha

F

where

\nabla\alpha

is the covariant derivative on S.

For tensors, an appropriate generalization is needed. The proper definition for a representative tensor

i\alpha
T
j\beta
reads
\nabla
i\alpha
T=
j\beta
\partial
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
are Christoffel symbols and
\Gamma
\alpha
\beta=\nabla

\betaV\alpha-C

\alpha
B
\beta
is the surface's appropriate temporal symbols (
\alpha
B
\beta
is a matrix representation of the surface's curvature shape operator)

Properties of the

\nabla
-derivative

The

\nabla
-derivative commutes with contraction, satisfies the product rule for any collection of indices
\nabla
i
(S
\alpha
\beta
T
j
\beta
)=T
j\nabla
i
S
\alpha

+

i
S
\alpha\nabla
\beta
T
j

and obeys a chain rule for surface restrictions of spatial tensors:

\nabla
j
F=
k(Z,t)
\partial
j
F
k
\partialt

+CNi\nablai

j
F
k

Chain rule shows that the

\nabla
-derivatives of spatial "metrics" vanishes
\nabla
i
\delta
j=0,\nabla

Zij=0,

\nabla

Zij=0,

\nabla

\varepsilonijk=0,

\nabla

\varepsilonijk=0

where

Zij

and

Zij

are covariant and contravariant metric tensors,

\delta

i
j
is the Kronecker delta symbol, and

\varepsilonijk

and

\varepsilonijk

are the Levi-Civita symbols. The main article on Levi-Civita symbols describes them for Cartesian coordinate systems. The preceding rule is valid in general coordinates, where the definition of the Levi-Civita symbols must include the square root of the determinant of the covariant metric tensor

Zij

.

Differentiation table for the

\nabla
-derivative

The

\nabla
derivative of the key surface objects leads to highly concise and attractive formulas. When applied to the covariant surface metric tensor

S\alpha

and the contravariant metric tensor

S\alpha

, the following identities result
\begin{align} \nabla

S\alpha&=0\\[8pt]

\nabla

S\alpha&=0 \end{align}

where

B\alpha

and

B\alpha

are the doubly covariant and doubly contravariant curvature tensors. These curvature tensors, as well as for the mixed curvature tensor
\alpha
B
\beta
, satisfy
\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
and the normal

Ni

satisfy
\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

and

\varepsilon\alpha

satisfy
\begin{align} \nabla

\varepsilon\alpha&=0\\[8pt]

\nabla

\varepsilon\alpha&=0 \end{align}

Time differentiation of integrals

The CMS provides rules for time differentiation of volume and surface integrals.

Notes and References

  1. Grinfeld, P. (2010). "Hamiltonian Dynamic Equations for Fluid Films". Studies in Applied Mathematics. . .
  2. J. Hadamard, Leçons Sur La Propagation Des Ondes Et Les Équations de l'Hydrodynamique. Paris: Hermann, 1903.