3. Kinematics
3.1 Basic Assumptions
Continuum mechanics is a combination of mathematics and physical laws that approximate the largescale behavior of matter that is subjected to mechanical loading. It is a generalization of Newtonian particle dynamics, and starts with the same physical assumptions inherent to Newtonian mechanics; and adds further assumptions that describe the structure of matter. Specifically:
The Newtonian reference frame: In classical continuum mechanics, the world is idealized as a three dimensional Euclidean space (a vector space consisting of all triads of real numbers ). A point in space is identified by a unique set of three real numbers. A Euclidean space is endowed with a metric, which defines the distance between points: . Vectors can be expressed as components in a basis  of mutually perpendicular unit vectors. Physical quantities such as force, velocity, acceleration are expressed as vectors in this space. A Cartesian Coordinate Frame is a fixed point O together with a basis. A Newtonian reference frame is a particular choice of Cartesian coordinate frame in which Newton’s laws of motion hold.
The Continuum: Matter is idealized as a continuum, which has two properties: (i) it is infinitely divisible (you can subdivide some region of the solid as many times as you wish); and (ii) it is locally homogeneous in other words if you subdivide it sufficiently many times, all subdivisions have identical properties (eg mass density). A continuum can be thought of as an infinite set of vanishingly small particles, connected together.
Both the existence of a Newtonian reference frame, and the concept of a continuum, are mathematical idealizations. Experimental evidence suggest that the laws of motion based on these assumptions accurately approximate the behavior of most solid and fluid materials at length scales of order mmkm or so in engineering applications. In some cases continuum models can also approximate behavior at much shorter length scales (for volumes of material containing a few 1000 atoms), but models at these length scales often require different relations between internal forces deformation measures in the solid to those used to model larger volumes.
3.2 Reference and deformed configuration of a solid
The configuration of a solid is a region of space occupied (filled) by the solid. When we describe motion, we normally choose some convenient configuration of the solid to use as reference  this is often the initial, undeformed solid, but it can be any convenient region that could be occupied by the solid. The material changes its shape under the action of external loads, and at some time t occupies a new region which is called the deformed or current configuration of the solid.
For some applications (fluids, problems with growth or evolving microstructures) a fixed reference configuration can’t be identified in this case we usually use the deformed material as the reference configuration.
Mathematically, we describe a deformation as a 1:1 mapping which transforms points from the reference configuration of a solid to the deformed configuration. Specifically, let be three numbers specifying the position of some point in the undeformed solid (these could be the three components of position vector in a Cartesian coordinate system, or they could be a more general ccoordinate system, such as polar coordinates). As the solid deforms, each the values of the coordinates change to different numbers. We can write this in general form as . This is called a deformation mapping.
To be a physically admissible deformation
(i) The coordinates must specify positions in a Newtonian reference frame. This means that it must be possible to find some coordinate transformation , such that are components in an orthogonal basis, which is taken to be ‘stationary’ in the sense of Newtonian dynamics.
(ii) The functions must be 1:1 on the full set of real numbers; and must be invertible
(iii) must be continuous and continuously differentiable (we occasionally relax these two assumptions, but this has to be dealt with on a casebycase basis)
(iv) The mapping must satisfy .
To begin with, we will describe all motions and deformations by expressing positions of points in both undeformed and deformed solids as components in a Cartesian reference frame (which is also taken to be an inertial frame). Thus will denote components of the position vector of a materal particle before deformation, and will be components of its position vector after deformation.
3.3 The Displacement and Velocity Fields
The displacement vector u(x,t) describes the motion of each point in the solid. To make this precise, visualize a solid deforming under external loads. Every point in the solid moves as the load is applied: for example, a point at position x in the undeformed solid might move to a new position y at time t. The displacement vector is defined as
We could also express this formula using index notation, as
Here, the subscript i has values 1,2, or 3, and (for example) represents the three Cartesian components of the vector y.
The displacement field completely specifies the change in shape of the solid. The velocity field would describe its motion, as
We also define the acceleration field
Examples of some simple deformations
Volume preserving uniaxial extension



Simple shear



Rigid rotation through angle about axis


General rigid rotation about the origin
where R must satisfy , det(R)>0. (i.e. R is proper orthogonal). I is the identity tensor with components
Alternatively, a rigid rotation through angle (with right hand screw convention) about an axis through the origin that is parallel to a unit vector n can be written as
The components of R are thus
where is the permutation symbol, satisfying


General homogeneous deformation
or
where are constants. The physical significance of a homogeneous deformation is that all straight lines in the solid remain straight under the deformation. Thus, every point in the solid experiences the same shape change. All the deformations listed above are examples of homogeneous deformations.

3.4 Eulerian and Lagrangian descriptions of motion and deformation.
The displacement and velocity are vector valued functions. In any application, we have a choice of writing the vectors as functions of the position of material particles before deformation
This is called the lagrangean description of motion. It is usually the easiest way to visualize a deformation.
But in some applications (eg fluid flow problems, where it’s hard to identify a reference configuration) it is preferable to write the displacement, velocity and acceleration vectors as functions of the deformed position of particles.
These express displacement, velocity and displacement as functions of a particular point in space (visualize describing air flow, for example). This is called the Eulerian description of motion. Of course the functions of and are not the same we just run out of symbols if we introduce different variables in the Lagrangian and Eulerian descriptions.
The relationships between displacement, velocity, and acceleration are somewhat more complicated in the Eulerian description. In the laws of motion, we normally are interested in the velocity and acceleration of a particular material particle, rather the rate of change of displacement and velocity at a particular point in space. When computing the time derivatives, it is necessary to take into account that is a function of time. Thus, displacement, velocity and acceleration are related by
You can derive these results by a simple application of the chain rule.
3.5 The Displacement gradient and Deformation gradient tensors
These quantities are defined by
Displacement Gradient Tensor: is a tensor with components
Deformation Gradient Tensor:
where I is the identity tensor, with components described by the Kronekor delta symbol:
and represents the gradient operator. Formally, the gradient of a vector field u(x) is defined so that
but in practice the component formula is more useful.
Note also that
The concepts of displacement gradient and deformation gradient are introduced to quantify the change in shape of infinitesimal line elements in a solid body. To see this, imagine drawing a straight line on the undeformed configuration of a solid, as shown in the figure. The line would be mapped to a smooth curve on the deformed configuration. However, suppose we focus attention on a line segment dx, much shorter than the radius of curvature of this curve, as shown. The segment would be straight in the undeformed configuration, and would also be (almost) straight in the deformed configuration. Thus, no matter how complex a deformation we impose on a solid, infinitesimal line segments are merely stretched and rotated by a deformation.The infinitesimal line segments dx and dy are related by
Written out as a matrix equation, we have
To derive this result, consider an infinitesimal line element dx in a deforming solid. When the solid is deformed, this line element is stretched and rotated to a deformed line element dy. If we know the displacement field in the solid, we can compute dy=[x+dx+u(x+dx)][x+u(x)] from the position vectors of its two end points
Expand
as a
so that
We identify the term in parentheses as the deformation gradient, so
The inverse of the deformation gradient arises in many calculations. It is defined through
or alternatively
3.6 Deformation gradient resulting from two successive deformations
Suppose that two successive deformations are applied to a solid, as shown. Let
map infinitesimal line elements from the original configuration to the first deformed shape, and from the first deformed shape to the second, respectively, with
The deformation gradient that maps infinitesimal line elements from the original configuration directly to the second deformed shape then follows as
Thus, the cumulative deformation gradient due to two successive deformations follows by multiplying their individual deformation gradients.
To see this, write the cumulative mapping as and apply the chain rule
3.7 The Jacobian of the deformation gradient change of volume
The Jacobian is defined as
It is a measure of the volume change produced by a deformation. To see this, consider the infinitessimal volume element shown with sides dx, dy, and dz in the figure above. The original volume of the element is
Here, is the permutation symbol. The element is mapped to a paralellepiped with sides dr, dv, and dw with volume given by
Recall that
so that
Recall that
so that
Hence
Observe that
For any physically admissible deformation, the volume of the deformed element must be positive (no matter how much you deform a solid, you can’t make material disappear). Therefore, all physically admissible displacement fields must satisfy J>0
If a material is incompressible, its volume remains constant. This requires J=1.
If the mass density of the material at a point in the undeformed solid is , its mass density in the deformed solid is
Derivatives of J. When working with constitutive equations, it is occasionally necessary to evaluate derivatives of J with respect to the components of F. The following result (which can be proved e.g. by expanding the Jacobian using index notation see HW1, problem 7, eg) is extremely useful
3.8 Transformation of internal surface area elements
When we deal with internal forces in a solid, we need to work with forces acting on internal surfaces in a solid. An important question arises in this treatment: if we identify an element of area with normal in the reference configuration, and then what are the area of and normal of this area element in the deformed solid?
The two are related through
To see this,
1. let be two infinitesimal material fibers with different orientations at some point in the reference configuration. These fibers bound a parallelapiped with area and normal
2. The vectors map to , in the deformed solid
3. In the deformed solid the area element is thus
4. Recall the identity  so
3.8 The Lagrange strain tensor
The Lagrange strain tensor is defined as
The components of Lagrange strain can also be expressed in terms of the displacement gradient as
The Lagrange strain tensor quantifies the changes in length of a material fiber, and angles between pairs of fibers in a deformable solid. It is used in calculations where large shape changes are expected.
To visualize the physical significance of E, suppose we mark out an imaginary tensile specimen with (very short) length on our deforming solid, as shown in the picture. The orientation of the specimen is arbitrary, and is specified by a unit vector m, with components . Upon deformation, the specimen increases in length to . Define the strain of the specimen as
Note that this definition of strain is similar to the definition you are familiar with, but contains an additional term. The additional term is negligible for small . Given the Lagrange strain components , the strain of the specimen may be computed from
We proceed to derive this result. Note that
is an infinitesimal vector with length and orientation of our undeformed specimen. From the preceding section, this vector is stretched and rotated to
The length of the deformed specimen is equal to the length of dy, so we see that
Hence, the strain for our line element is
giving the results stated.
3.9 The Eulerian strain tensor
The Eulerian strain tensor is defined as
Its physical significance is similar to the Lagrange strain tensor, except that it enables you to compute the strain of an infinitesimal line element from its orientation after deformation.
Specifically, suppose that n denotes a unit vector parallel to the deformed material fiber, as shown in the picture. Then
The proof is left as an exercise.
3.10 The Infinitesimal strain tensor
The infinitesimal strain tensor is defined as
where u is the displacement vector. Written out in full
The infinitesimal strain tensor is an approximate deformation measure, which is only valid for small shape changes. It is more convenient than the Lagrange or Eulerian strain, because it is linear.
Specifically, suppose the deformation gradients are small, so that all . Then the Lagrange strain tensor is
so the infinitesimal strain approximates the Lagrange strain. You can show that it also approximates the Eulerian strain with the same accuracy.
Properties of the infinitesimal strain tensor
For small strains, the engineering strain of an infinitesimal fiber aligned with a unit vector m can be estimated as
Note that
(see below for more details)
The infinitesimal strain tensor is closely related to the strain matrix introduced in elementary strength of materials courses. For example, the physical significance of the (2 dimensional) strain matrix
is illustrated in the figure.
To relate this to the infintesimal strain tensor, let be a Cartesian basis, with parallel to x and parallel to y as shown. Let denote the components of the infinitesimal strain tensor in this basis. Then
3.11 Engineering shear strains
For a general strain tensor (which could be any of , or , among others), the diagonal strain components are known as `direct’ strains, while the off diagonal terms are known as ‘shear strains’
The shear strains are sometimes reported as ‘Engineering Shear Strains’ which are related to the formal definition by a factor of 2 i.e.
This factor of 2 is an endless source of confusion. Whenever someone reports shear strain to you, be sure to check which definition they are using. In particular, many commercial finite element codes output engineering shear strains.
3.12 Decomposition of infinitesimal strain into volumetric and deviatoric parts
The volumetric infinitesimal strain is defined as
The deviatoric infinitesimal strain is defined as
The volumetric strain is a measure of volume changes, and for small strains is related to the Jacobian of the deformation gradient by . To see this, recall that
The deviatioric strain is a measure of shear deformation (shear deformation involves no volume change).
3.13 The Infinitesimal rotation tensor
The infinitesimal rotation tensor is defined as
Written out as a matrix, the components of are
Observe that is skew symmetric: .
A skew tensor represents a rotation through a small angle. Specifically, the operation rotates the infinitesimal line element through a small angle about an axis parallel to the unit vector . (A skew tensor also sometimes represents an angular velocity).
To visualize the significance of , consider the behavior of an imaginary, infinitesimal, tensile specimen embedded in a deforming solid. The specimen is stretched, and then rotated through an angle about some axis q. If the displacement gradients are small, then .
The rotation of the specimen depends on its original orientation, represented by the unit vector m. One can show (although one would rather not do all the algebra) that represents the average rotation, over all possible orientations of m, of material fibers passing through a point.
As a final remark, we note that a general deformation can always be decomposed into an infinitesimal strain and rotation
Physically, this sum of and can be regarded as representing two successive deformations a small strain, followed by a rotation, in the sense that
first stretches the infinitesimal line element, then rotates it.
3.14 Principal values and directions of the infinitesimal strain tensor
The three principal values and directions of the infinitesimal strain tensor satisfy
Clearly, and are the eigenvalues and eigenvectors of . There are three principal strains and three principal directions, which are always mutually perpendicular.
Their significance can be visualized as follows.
1. Note that the decomposition can be visualized as a small strain, followed by a small rigid rotation, as shown in the picture.
2. The formula indicates that a vector n is mapped to another, parallel vector by the strain.
3. Thus, if you draw a small cube with its faces perpendicular to on the undeformed solid, this cube will be stretched perpendicular to each face, with a fractional increase in length . The faces remain perpendicular to after deformation.
4. Finally, w rotates the small cube through a small angle onto its configuration in the deformed solid.
3.15 Strain Equations of Compatibility for infinitesimal strains
It is sometimes necessary to invert the relations between strain and displacement that is to say, given the strain field, to compute the displacements. In this section, we outline how this is done, for the special case of infinitesimal deformations.
For infinitesimal motions the relation between strain and displacement is
Given the six strain componets (six, since ) we wish to determine the three displacement components . First, note that you can never completely recover the displacement field that gives rise to a particular strain field. Any rigid motion produces no strain, so the displacements can only be completely determined if there is some additional information (besides the strain) that will tell you how much the solid has rotated and translated. However, integrating the strain field can tell you the displacement field to within an arbitrary rigid motion.
Second, we need to be sure that the straindisplacement relations can be integrated at all. The strain is a symmetric second order tensor field, but not all symmetric second order tensor fields can be strain fields. The straindisplacement relations amount to a system of six scalar differential equations for the three displacement components u_{i}.
To be integrable, the strains must satisfy the compatibility conditions, which may be expressed as
Or, equivalently
Or, once more equivalently
It is easy to show that all strain fields must satisfy these conditions  you simply need to substitute for the strains in terms of displacements and show that the appropriate equation is satisfied. For example,
and similarly for the other expressions.
Not that for planar problems for which all of these compatibility equations are satisfied trivially, with the exception of the first:
It can be shown that
(i) If the strains do not satisfy the equations of compatibility, then a displacement vector can not be integrated from the strains.
(ii) If the strains satisfy the compatibility equations, and the solid simply connected (i.e. it contains no holes that go all the way through its thickness), then a displacement vector can be integrated from the strains.
(iii) If the solid is not simply connected, a displacement vector can be calculated, but it may not be single valued i.e. you may get different solutions depending on how the path of integration encircles the holes.
Now, let us return to the question posed at the beginning of this section. Given the strains, how do we compute the displacements?
2D strain fields
For 2D (plane stress or plane strain) the procedure is quite simple and is best illustrated by working through a specific case
As a representative example, we will use the strain field in a 2D (plane stress) cantilever beam with Young’s modulus E and Poisson’s ratio loaded at one end by a force P. The beam has a rectangular crosssection with height 2a and outofplane width b. We will show later (Sect 5.2.4) that the strain field in the beam is
We first check that the strain is compatible. For 2D problems this requires
which is clearly satisfied in this case.
For a 2D problem we only need to determine and such that
.
The first two of these give
We can integrate the first equation with respect to and the second equation with respect to to get
where and are two functions of and , respectively, which are yet to be determined. We can find these functions by substituting the formulas for and into the expression for shear strain
We can rewrite this as
The two terms in parentheses are functions of and , respectively. Since the left hand side must vanish for all values of and , this means that
where is an arbitrary constant. We can now integrate these expressions to see that
where c and d are two more arbitrary constants. Finally, the displacement field follows as
The three arbitrary constants , c and d can be seen to represent a small rigid rotation through angle about the axis, together with a displacement (c,d) parallel to axes, respectively.
3D strain fields
For a general, three dimensional field a more formal procedure is required. Since the strains are the derivatives of the displacement field, so you might guess that we compute the displacements by integrating the strains. This is more or less correct. The general procedure is outlined below.
We first pick a point in the solid, and arbitrarily say that the displacement at is zero, and also take the rotation of the solid at to be zero. Then, we can compute the displacements at any other point x in the solid, by integrating the strains along any convenient path. In a simply connected solid, it doesn’t matter what path you pick.
Actually, you don’t exactly integrate the strains instead, you must evaluate the following integral
where
Here, are the components of the position vector at the point where we are computing the displacements, and are the components of the position vector of a point somewhere along the path of integration. The fact that the integral is pathindependent (in a simply connected solid) is guaranteed by the compatibility condition. Evaluating this integral in practice can be quite painful, but fortunately almost all cases where we need to integrate strains to get displacement turn out to be twodimensional.
3.16 CauchyGreen Deformation Tensors
There are two CauchyGreen deformation tensors defined through
The Right Cauchy Green Deformation Tensor
The Left Cauchy Green Deformation Tensor
They are called `left’ and `right’ tensors because of their relation to the `left’ and ‘right’ stretch tensors defined below. They can be regarded as quantifying the squared length of infinitesimal fibers in the deformed configuration, by noting that if a material fiber in the undeformed solid is stretched and rotated to in the deformed solid, then
3.17 Rotation tensor, and Left and Right Stretch Tensors
The definitions of these quantities are
The Right Stretch Tensor
The Left Stretch Tensor
The Rotation Tensor
To calculate these quantities you need to remember how to calculate the square root of a matrix. For example, to calculate the square root of C, you must
1. Calculate the eigenvalues of C we will call these , with n=1,2,3. Since C and B are both symmetric and positive definite, the eigenvalues are all positive real numbers, and therefore their square roots are also positive real numbers.
2. Calculate the eigenvectors of C and normalize them so they have unit magnitude. We will denote the eigenvectors by . They must be normalized to satisfy
3. Finally, calculate , where denotes a dyadic product (See Appendix B). In components, this can be written
4. As an additional bonus, you can quickly compute the inverse square root (which is needed to find R) as
To see the physical significance of these tensors, observe that
1. The definition of the rotation tensor shows that
2. The multiplicative decomposition of a constant tensor can be regarded as a sequence of two homogeneous deformations U, followed by R. Similarly, is R followed by V.
3. R is proper orthogonal (it satisfies and det(R)=1), and therefore represents a rotation. To see this, note that U is symmetric, and therefore satisfies , so that
and det(R)=det(F)det(U^{1})=1
 U can be expressed in the form
where are the three (mutually perpendicular) eigenvectors of U. (By construction, these are identical to the eigenvectors of C). If we interpret as basis vectors, we see that U is diagonal in this basis, and so corresponds to stretching parallel to each basis vector, as shown in the figure below.
The decompositions
and
are known as the right and left polar decomposition of F. (The right and left refer to the positions of U and V). They show that every homogeneous deformation can be decomposed into a stretch followed by a rigid rotation, or equivalently into a rigid rotation followed by a stretch. The decomposition is discussed in more detail in the next section.
3.18 Principal stretches
The principal stretches can be calculated from any one of the following (they all give the same answer)
 The eigenvalues of the right stretch tensor U
 The eigenvalues of the left stretch tensor V
 The square root of the eigenvalues of the right CauchyGreen tensor C
 The square root of the eigenvalues of the left CauchyGreen tensor B
The principal stretches are also related to the eigenvalues of the Lagrange and Eulerian strains. The details are left as an exercise.
There are two sets of principal stretch directions, associated with the undeformed and deformed solids.
 The principal stretch directions in the undeformed solid are the (normalized) eigenvectors of U or C. Denote these by .
 The principal stretch directions in the deformed solid are the (normalized) eigenvectors of V or B. Denote these by .
To visualize the physical significance of principal stretches and their directions, note that a deformation can be decomposed as into a sequence of a stretch followed by a rotation.
Note also that
 The principal directions are mutually perpendicular. You could draw a little cube on the undeformed solid with faces perpendicular to these directions, as shown above.
 The stretch U will stretch the cube by an amount parallel to each . The faces of the stretched cube remain perpendicular to .
 The rotation R will rotate the stretched cube so that the directions rotate to line up with .
 The faces of the deformed cube are perpendicular to
The decomposition can be visualized in much the same way. In this case, the directions are first rotated to coincide with . The cube is then stretched parallel to each to produce the same shape change.
We could compare the undeformed and deformed cubes by placing them side by side, with the vectors and parallel, as shown in the figure.
3.19 Generalized strain measures
The polar decompositions and provide a way to define additional strain measures. Let denote the principal stretches, and let and denote the normalized eigenvectors of U and V. Then one could define strain tensors through
The correspoinding Eulerian strain measures are
Another strain measure can be defined as
This can be computed directly from the deformation gradient as
and is very similar to the Lagrangean strain tensor, except that its principal directions are rotated through the rigid rotation R.
3.20 Measure of rate of deformation  the velocity gradient
We now list several measures of the rate of deformation. The velocity gradient is the basic measure of deformation rate, and is defined as
It quantifies the relative velocities of two material particles at positions y and y+dy in the deformed solid, in the sense that
The velocity gradient can be expressed in terms of the deformation gradient and its time derivative as
To see this, note that
and recall that , so that
3.21 Stretch rate and spin (vorticity) tensors
The stretch rate tensor is defined as
The spin tensor or Vorticity tensor is defined as
A general velocity gradient can be decomposed into the sum of stretch rate and spin, as
The stretch rate quantifies the rate of stretching of material fibers in the deformed solid, in the sense that
is the rate of stretching of a material fiber with length l and orientation n in the deformed solid. To see this, let , so that
By definition,
Hence
Finally, take the dot product of both sides with n, note that since n is a unit vector must be perpendicular to n and therefore . Note also that , since W is skewsymmetric. It is easiest to show this using index notation: . Therefore
The spin tensor W can be shown to provide a measure of the average angular velocity of all material fibers passing through a material point.
The vorticity vector is another measure of the angular velocity. It is defined as
It is related to the spin tensor as
Where dual (W) denotes the dual vector of the skew tensor W.
The vorticity vector has the property that, for any vector g, .
A motion satisfying W=curl(v)= 0 is said to be irrotational such motions are of interest in fluid mechanics.
3.22 Spatial (Eulerian) description of acceleration
The acceleration of a material particle is, by definition
In fluid mechanics, it is often convenient to use a spatial description of velocity and acceleration that is to say the velocity field is expressed as a function of position y in the deformed solid as . The acceleration of the material particle with instantaneous position y in the deformed solid can be expressed as
3.23 Acceleration  spin vorticity relations
In fluid mechanics, equations relating the acceleration to the spatial velocity field are useful. In particular, it can be shown that
Deriving these relations is left as an exercise.
3.24 Rate of change of volume
We have seen that
quantifies the volume change associated with a deformation, in that
In rate form:
.
The trace of D, trace of L or the trace of div(v) are therefore measures of rate of change of volume.
3.25 Infinitesimal strain rate and rotation rate
For small strains the rate of deformation tensor can be approximated by the infinitesimal strain rate, while the spin can be approximated by the time derivative of the infinitesimal rotation tensor
Similarly, you can show that
3.26 Other deformation rate measures
The rate of deformation tensor can be related to time derivatives of other strain measures. For example the time derivative of the Lagrange strain tensor can be shown to be
Other useful results are
For a pure rotation , or equivalently . To see this, recall that and evaluate the time derivative.
If the deformation gradient is decomposed into a stretch followed by a rotation as then and
For small strains the rate of change of Lagrangian strain E is approximately equal to the rate of change of infinitesimal strain :
3.27 Path lines, streamlines, and vortex lines
Path lines, streamlines, and vortex lines are useful concepts in fluid mechanics.
A path line is the curve traced by a material particle as it moves through space. If the curve is described in parametric form by , with a scalar, then the curve satisfies
A stream line is a curve that is everywhere tangent to the spatial velocity vector. In general, streamlines may be functions of time. If is the parametric representation of the curve, at time t , then is a member of the family of solutions to the differential equation
For the particular case of a steady flow, the spatial velocity field is (by definition) independent of time, and therefore the curves are fixed in space.
A vortex line is a curve that is everywhere tangent to the vorticity vector. These curves satisfy the differential equation
Again, for the special case of a steady flow the vortex lines are independent of time.
3.27 Reynolds Transport Relation
The Reynolds transport theorem is a useful way to calculate the rate of change of a quantity inside a volume that deforms with a solid (e.g. the total mass of a volume). Let be any scalar valued property of a material particle at position y in the deformed solid. The Reynolds transport relation states that rate of change of the total value of this property within a volume V of a deformed solid can be calculated as
Note that the material volume V and surface S convect with the deforming solid they are not control volumes.
To see this, note that we can’t take the time derivative inside the integral because the volume changes with time as the solid deforms. But we can map the integral back to the reference configuration, which is time independent the derivative can then be taken inside the integral.
The last result follows by noting that . Then note that and apply the divergence theorem to this term.
3.28 Transport Relations for material curves and surfaces
Similar transport relations can be derived for material curves and surfaces which convect with a deformable solid or fluid.
Let C be a material curve in a deformable solid; and let S be an interior surface with normal vector n. Let be any scalar valued property of a material particle at position y in the deformed solid. Then
To show the first result, start by mapping the integral to the reference configuration, then take the time derivative, and map back to the current configuration, as follows
To show the second, apply the same process to the surface integral. The details are left as an exercise…
3.28 Circulation and the circulation transport relation
The circulation of the velocity field around a closed curve C is defined as
,
where is a unit vector tangent to the curve. If C is a reducible curve (i.e. if there is a regular, open surface S bounded by C that lies within the configuration) then Stokes theorem shows that
The circulation transport relation states that
for any material curve (i.e. a curve that convects with material particles within a body). To see this recall the transport relation for a material curve, and set
Note that
and hence
because C is a closed curve.
Kelvin’s circulation theorem is a direct consequence of this result. The theorem states that if the acceleration is the gradient of a potential, then the circulation around any closed material curve remains constant. To see this, let