Cauchy momentum equation

The Cauchy momentum equation is a vector partial differential equation put forth by Cauchy that describes the non-relativistic momentum transport in any continuum.
Main equation.
In convective (or Lagrangian) form the Cauchy momentum equation is written as:
formula_1
where 
Commonly used SI units are given in parentheses although the equations are general in nature and other units can be entered into them or units can be removed at all by nondimensionalization.
Note that only we use column vectors (in the Cartesian coordinate system) above for clarity, but the equation is written using physical components (which are neither covariants ("column") nor contravariants ("row") ). However, if we chose a non-orthogonal curvilinear coordinate system, then we should calculate and write equations in covariant ("row vectors") or contravariant ("column vectors") form.
After an appropriate change of variables, it can also be written in conservation form:
formula_18
where is the momentum density at a given space-time point, is the flux associated to the momentum density, and contains all of the body forces per unit volume.
Differential derivation.
Let us start with the generalized momentum conservation principle which can be written as follows: "The change in system momentum is proportional to the resulting force acting on this system". It is expressed by the formula:
formula_19
where formula_20 is momentum in time , formula_21 is force averaged over formula_22. After dividing by formula_22 and passing to the limit formula_24 we get (derivative):
formula_25
Let us analyse each side of the equation above.
Right side.
We split the forces into body forces formula_26 and surface forces formula_27
formula_28
Surface forces act on walls of the cubic fluid element. For each wall, the "X" component of these forces was marked in the figure with a cubic element (in the form of a product of stress and surface area e.g. formula_29 with units formula_30).
Adding forces (their "X" components) acting on each of the cube walls, we get:
formula_31
After ordering formula_32 and performing similar reasoning for components formula_33 (they have not been shown in the figure, but these would be vectors parallel to the Y and Z axes, respectively) we get:
formula_34
We can then write it in the symbolic operational form:
formula_35
There are mass forces acting on the inside of the control volume. We can write them using the acceleration field formula_36 (e.g. gravitational acceleration):
formula_37
Left side.
Let us calculate momentum of the cube:
formula_38
Because we assume that tested mass (cube) formula_39 is constant in time, so
formula_40
Left and Right side comparison.
We have
formula_41
then
formula_42
then
formula_43
Divide both sides by formula_44, and because formula_45 we get:
formula_46
which finishes the derivation.
Integral derivation.
Applying Newton's second law (th component) to a control volume in the continuum being modeled gives:
formula_47
Then, based on the Reynolds transport theorem and using material derivative notation, one can write
formula_48
where represents the control volume. Since this equation must hold for any control volume, it must be true that the integrand is zero, from this the Cauchy momentum equation follows. The main step (not done above) in deriving this equation is establishing that the derivative of the stress tensor is one of the forces that constitutes .
Conservation form.
The Cauchy momentum equation can also be put in the following form:
simply by defining:
formula_49
where is the momentum density at the point considered in the continuum (for which the continuity equation holds), is the flux associated to the momentum density, and contains all of the body forces per unit volume. is the dyad of the velocity.
Here and have same number of dimensions as the flow speed and the body acceleration, while , being a tensor, has .
In the Eulerian forms it is apparent that the assumption of no deviatoric stress brings Cauchy equations to the Euler equations.
Convective acceleration.
A significant feature of the Navier–Stokes equations is the presence of convective acceleration: the effect of time-independent acceleration of a flow with respect to space. While individual continuum particles indeed experience time dependent acceleration, the convective acceleration of the flow field is a spatial effect, one example being fluid speeding up in a nozzle.
Regardless of what kind of continuum is being dealt with, convective acceleration is a nonlinear effect. Convective acceleration is present in most flows (exceptions include one-dimensional incompressible flow), but its dynamic effect is disregarded in creeping flow (also called Stokes flow). Convective acceleration is represented by the nonlinear quantity , which may be interpreted either as or as , with the tensor derivative of the velocity vector . Both interpretations give the same result.
Advection operator vs tensor derivative.
The convection term formula_50 can be written as , where is the advection operator. This representation can be contrasted to the one in terms of the tensor derivative. The tensor derivative is the component-by-component derivative of the velocity vector, defined by , so that
formula_51
Lamb form.
The vector calculus identity of the cross product of a curl holds:
formula_52
where the Feynman subscript notation is used, which means the subscripted gradient operates only on the factor .
Lamb in his famous classical book Hydrodynamics (1895), used this identity to change the convective term of the flow velocity in rotational form, i.e. without a tensor derivative:
formula_53
where the vector formula_54 is called the Lamb vector. The Cauchy momentum equation becomes:
formula_55
Using the identity:
formula_56
the Cauchy equation becomes:
formula_57
In fact, in case of an external conservative field, by defining its potential :
formula_58
In case of a steady flow the time derivative of the flow velocity disappears, so the momentum equation becomes:
formula_59
And by projecting the momentum equation on the flow direction, i.e. along a "streamline", the cross product disappears due to a vector calculus identity of the triple scalar product:
formula_60
If the stress tensor is isotropic, then only the pressure enters: formula_61 (where is the identity tensor), and the Euler momentum equation in the steady incompressible case becomes:
formula_62
In the steady incompressible case the mass equation is simply:
formula_63
that is, "the mass conservation for a steady incompressible flow states that the density along a streamline is constant". This leads to a considerable simplification of the Euler momentum equation:
formula_64
The convenience of defining the total head for an inviscid liquid flow is now apparent:
formula_65
in fact, the above equation can be simply written as:
formula_66
That is, "the momentum balance for a steady inviscid and incompressible flow in an external conservative field states that the total head along a streamline is constant".
Irrotational flows.
The Lamb form is also useful in irrotational flow, where the curl of the velocity (called vorticity) is equal to zero. In that case, the convection term in formula_50 reduces to
formula_68
Stresses.
The effect of stress in the continuum flow is represented by the and terms; these are gradients of surface forces, analogous to stresses in a solid. Here is the pressure gradient and arises from the isotropic part of the Cauchy stress tensor. This part is given by the normal stresses that occur in almost all situations. The anisotropic part of the stress tensor gives rise to , which usually describes viscous forces; for incompressible flow, this is only a shear effect. Thus, is the deviatoric stress tensor, and the stress tensor is equal to:
formula_69
where is the identity matrix in the space considered and the shear tensor.
All non-relativistic momentum conservation equations, such as the Navier–Stokes equation, can be derived by beginning with the Cauchy momentum equation and specifying the stress tensor through a constitutive relation. By expressing the shear tensor in terms of viscosity and fluid velocity, and assuming constant density and viscosity, the Cauchy momentum equation will lead to the Navier–Stokes equations. By assuming inviscid flow, the Navier–Stokes equations can further simplify to the Euler equations.
The divergence of the stress tensor can be written as
formula_70
The effect of the pressure gradient on the flow is to accelerate the flow in the direction from high pressure to low pressure.
As written in the Cauchy momentum equation, the stress terms and are yet unknown, so this equation alone cannot be used to solve problems. Besides the equations of motion—Newton's second law—a force model is needed relating the stresses to the flow motion. For this reason, assumptions based on natural observations are often applied to specify the stresses in terms of the other flow variables, such as velocity and density.
External forces.
The vector field represents body forces per unit mass. Typically, these consist of only gravity acceleration, but may include others, such as electromagnetic forces. In non-inertial coordinate frames, other "inertial accelerations" associated with rotating coordinates may arise.
Often, these forces may be represented as the gradient of some scalar quantity , with in which case they are called conservative forces. Gravity in the direction, for example, is the gradient of . Because pressure from such gravitation arises only as a gradient, we may include it in the pressure term as a body force . The pressure and force terms on the right-hand side of the Navier–Stokes equation become
formula_71
It is also possible to include external influences into the stress term formula_12 rather than the body force term. This may even include antisymmetric stresses (inputs of angular momentum), in contrast to the usually symmetrical internal contributions to the stress tensor.
Nondimensionalisation.
In order to make the equations dimensionless, a characteristic length and a characteristic velocity need to be defined. These should be chosen such that the dimensionless variables are all of order one. The following dimensionless variables are thus obtained:
formula_73
Substitution of these inverted relations in the Euler momentum equations yields:
formula_74
and by dividing for the first coefficient:
formula_75
Now defining the Froude number:
formula_76
the Euler number:
formula_77
and the coefficient of skin-friction or the one usually referred as 'drag' co-efficient in the field of aerodynamics:
formula_78
by passing respectively to the conservative variables, i.e. the momentum density and the force density:
formula_79
the equations are finally expressed (now omitting the indexes):
 2 \nabla \cdot \boldsymbol \tau + \frac 1 {\mathrm{Fr}} \mathbf g
&lt;/math&gt;
Cauchy equations in the Froude limit (corresponding to negligible external field) are named free Cauchy equations:
 2 \nabla \cdot \boldsymbol \tau
&lt;/math&gt;
and can be eventually conservation equations. The limit of high Froude numbers (low external field) is thus notable for such equations and is studied with perturbation theory.
Finally in convective form the equations are:
 \mathbf f 
&lt;/math&gt;
3D explicit convective forms.
Cartesian 3D coordinates.
For asymmetric stress tensors, equations in general take the following forms:
formula_80
Cylindrical 3D coordinates.
Below, we write the main equation in pressure-tau form assuming that the stress tensor is symmetrical (formula_81):
formula_82