Abstract
In the current chapter, some applications of tensor analysis to fluid dynamics are presented. Governing equations of fluid motion and energy are obtained and analyzed. We shall discuss about continuity equation, equation of motion, and mechanical energy transport equation and four forms of energy equation. Finally, we shall talk about the divergence from transfer equations of different parameters of motion. The tensor form of equations has advantages over the component form: these are, first, compact writing of equations and, second, independency from reference frames, etc. Moreover, it allows to obtain new forms of equations on the basis of governing ones easily.
Keywords
- stress tensor
- Navier–stokes equation
- energy
- continuity
- vorticity
- divergence form
1. Introduction
The mathematical model of moving fluid includes a set of equations, which are usually written as transport equations of main physical parameters—density, velocity, energy, etc. These equations are conservation laws in fluid flows. Traditionally the component form of the equations is usually used, but at the same time, the componentless form (Gibbs approach) could be applied to obtain and transform these equations. In this chapter several main conservation laws are discussed and represented in tensor form, which has many advantages against usually used component form, like simplicity and compactness, independence on reference frames, less errors in transformations, etc. Below we obtain and analyze continuity and momentum equations and vorticity and energy transport equations, and we discuss also about the divergent form of transport Eqs.
2. Continuity equation
Continuity equation is the mass conservation law for a fluid flow and is presented as a scalar equation, which connects density
where
Hereinafter, the Einstein summation convention is used by default.
It also can be written in two other equivalent forms [1, 2]:
In the case of incompressible fluid, we could obtain its simplified form:
Let us apply gradient operator to continuity equation (Eq. (1)):
As a result, we obtain vector equation:
which could be written in a more compact form:
or by a little bit different way:
These equations contain gradient of vector
For incompressible fluid the left part of this relation is equal to zero; therefore for rotation of velocity vector, we can write:
In the case of compressible fluid in accordance with Eq. (6), we have additional terms in the right part of the equation:
Continuity equation can be also written in tensor form:
The tensor
Finally, continuity equation can be written in form
Convective derivatives of density and pressure (and any another scalar quantitatives) also can be written in tensor form:
i.e., convective derivative is equal to trace of corresponding tensor.
In addition, for the divergence of the product of scalar and vector functions, we can obtain the following relation:
3. Equations of motion of fluid with constant and variable properties
The equation of a motion in terms of stress [4, 5] is
where
where
Then equation of motion of incompressible fluid (Navier–Stokes equation) at
Additional term
In case of compressible fluid with variable viscosity, the equation will contain a term with divergence
Let us introduce the denotation:
then we can write Eq. (16) as
Divergence of this tensor at
As a result, equation of motion of compressible fluid with variable viscosity has the form:
in Cartesian coordinates
If we represent fluid particle acceleration as the sum of local and convective terms, then (Eq. (19)) will take the form:
considering viscosity variability is especially important for turbulent flow modeling using the Boussinesq hypothesis with turbulent viscosity
Let us apply divergence operation to the Navier–Stokes equation for compressible fluid with variable viscosity. With this purpose we shall apply operation
If the fluid motion occurs in gravity force field, then there is potential
Function
As a result of applied divergence operation to Navier–Stokes equation at
In the case of incompressible fluid, we have
if also
Now we consider the general case of fluid motion, taking into account its compressibility.
The set of equations of motion of an incompressible fluid contains two—Navier–Stokes and continuity (one vector equation and one scalar equation) [2, 3]:
This set of two equations is closed: it contains two unknown quantities—velocity vector
In case of compressible flows at
where
where
Mendeleev-Clapeyron equation has the form
where
For liquids it usually supposes
Eqs. (1), (25), (26), and (27) are valid for laminar regime of motion. In case of turbulent regime in these equations, correlations will appear, caused by velocity, density, and temperature pulsations. For closure of the set of equations of turbulent motion, additional relations are required.
4. Vorticity vector and its associated tensor
Vorticity
in component form
where
Vorticity vector
And vice versa, vector
In Eq. (30) spin tensor
Let us prove expression Eq. (30):
The same for Eq. (31):
Let us descry components of this second-rank tensor: when
values for all
The matrix of components of this tensor is
It can be seen that it is a matrix of components of the antisymmetric tensor
It is easy to see also that
5. Vorticity transport equation
Rotation of convective acceleration of a fluid particle could be written as
Evidence of this equation can be performed by writing convective acceleration according to the formula:
Gradient of vorticity is the pseudo tensor of rank 2:
Trace of this tensor is
Let us assume that the fluid is incompressible,
now we apply curl operation (
We could rewrite the second term of the left part in this form:
but
As a result, we can obtain transport equation of vortices in an incompressible viscous fluid, which is named as the generalized Helmholtz equation:
or in more compact form:
It is necessary to note that in the case of compressible fluid and at
This transport equation can be written in another form, considering the equality
If we apply divergence operation to Eq. (36), then for incompressible fluid we obtain
because
On the other hand,
and, finally, we have
For the second power of vorticity, we can write
and now, if we scalar multiply transport equation of vortices by
then we obtain scalar transport equation of
For incompressible fluid
As we already know
Let us write one more equation:
Therefore,
This equation also can be written in the next form:
One more interesting relation is
but as we already mentioned,
The vector product of gradients of scalar functions gives us a vector; in terms of rotation of a vector function, we can write
Really, the left part of this equation is a vector:
and the right part is.
Therefore, Eq. (44) is valid.
The vector product of gradients of scalar functions also can be written as
Here we have in component form:
It is easy to see that this is equal to expression for
6. Mechanical energy equation
Mechanical energy balance equation can be obtained as a scalar product of each member of Eq. (12) on velocity vector
Transformations of the left part lead us to the following results:
It is easy to see that in sum the left part is the material derivative of kinetic energy of a fluid particle—quantity
Usually stress tensor is defined as the sum
Now we can represent equation of mechanical energy balance (Eq. (46)) considering
The first member of the right part of Eq. (46) is power of stresses
It is easy to be proven if we rewrite this expression in component form in Cartesian coordinates. In this case the left part of Eq. (49) is
The first term of the right part of Eq. (49) in component form is
The second term of the right part of Eq. (49) in component form is
Finally Eq. (49) in component form is
Due to the symmetry of the stress tensor
We could simplify the last term of the right part of Eq. (46) if we introduce potential
After substituting the above expressions into Eq. (46), we obtain the equation for mechanical energy of a fluid flow:
In the left part of this equation, we observe the total mechanical energy of a fluid flow as the sum of kinetic and potential energy of the flow [6]. Often the right part of Eq. (51) is written in another form, where stress tensor is written as the sum
The first member is
The third member is
When we substitute these terms in Eq. (52) and then in Eq. (51), the equation for mechanical energy of a fluid flow will take the form:
As a result, we could conclude that the rate of change of total mechanical energy of a flow is equal to the sum of the powers of the pressure forces and viscous friction.
Navier–Stokes equation for a steady flow of viscous incompressible fluid is
The Laplacian of velocity in the right part can be written in the form:
It can be obtained by consideration of operation
The member with the basis vector
The same can be written for the members with basis vectors
And therefore formula Eq. (55) is valid.
One more useful expression based on Eq. (55) is
Using Eqs. (34) and (55) and considering the mass force in field of gravity with the help of potential
where
In incompressible fluid
The gradient of total mechanical energy of a fluid particle
It is possible to obtain the divergence form of Eq. (54) for incompressible fluid considering
Then Eq. (54) will take the form:
Using concepts of identity tensor
The last term of Eq. (59) can be considered in a more simple form due to relation for incompressible fluid
7. Energy equation for moving fluid
The first law of thermodynamics connects internal energy, heat, and work. In the case of moving fluid, it can be written as follows:
where
The physical meaning of this equation is that the rate of change of internal energy per unit volume is equal to rate of energy supply due to heat conduction, due to dissipation of mechanical energy of the flow, and due to heat from external or internal sources. Since stress tensor
This is the energy equation in terms of transfer of specific internal energy
Vector
where
Fourier’s law of thermal conductivity can also be written in terms of enthalpy, which for an ideal gas is related to temperature by the formula
where
In Cartesian coordinates Eq. (62) can be written as follows:
The terms
where
Now we could write the dissipative term
Thus, in component form Rayleigh function
or in usual notations
This function can also be written in the componentless form:
For perfect gases [6, 7] internal energy is connected with temperature by the relation
The energy equation (Eq. (62)) can be also written in terms of enthalpy
Here we also used continuity equation (Eq. (2)). Finally, we can obtain energy equation in the form of enthalpy transport as
This is the second form of energy equation for the perfect gas in which
One more form of the energy equation can be written if we introduce stagnation enthalpy
Here we used the relation:
which is easy to be proven if we write it down in the component form considering symmetry of stress tensor
As a result, the dissipative term in Eq. (73) can be written as follows:
where
In usual axis designations, the first term is
while second term is
Finally function
The fourth form of the energy equation can be written in terms of entropy
Hence we have
and then if we substitute quantity
All forms of the equation energy (in terms of internal energy, enthalpy, stagnation enthalpy, and entropy) are equivalent.
Equation for temperature field of an arbitrary gas in the form of equation of transport of temperature
The subscripts in derivatives here fix the parameters, with the constancy of which the derivatives are calculated. From these formulas the expressions for derivatives can be obtained:
If we substitute them into Eq. (62) and Eq. (71), we obtain two forms of the equation energy in terms of temperature transport.
In heat transfer problems, boundary conditions are specified in three different kinds—the first, second, and third kind:
The boundary conditions of the first kind consist in setting the temperature on the surface of the body.
The boundary conditions of the second kind are setting of the distribution of the heat flux density q on the surface of the body.
The boundary conditions of the third kind consist in setting the temperature of the flow over the surface of the body and the heat transfer conditions on its surface.
8. Divergence form of transport equations
Material derivative of any physical quantity Θ multiplied by density
This directly follows from the continuity equation [Eq. (2)].
Let us consider in detail the following cases for three ranks of a certain physical quantity Θ.
8.1 Quantity Θ is a scalar
Let us assume that quantity Θ is temperature
We could prove this equality if we write the left and right parts in component form. For the left part, we have
For the right part, we have
Since the expression in parentheses is zero (due to continuity equation), the equality of the left and right parts is obvious.
8.2 Quantity Θ is a vector
Let us assume that quantity Θ is velocity
Here in the last term, we see tensor
We could prove this equality if we write the left and right parts in the component form and use continuity equation.
For the left part, we have
The first term of the right part is
The second term of the right part is
The right part as a whole is
Therefore, the expression (Eq. (83)) is valid in case Θ is a vector.
8.3 Quantity Θ is a tensor
Let us assume that Θ is a tensor, for instance, stress tensor
The left part of this equation in the component form can be written as follows:
The right part is
Therefore, the expression (Eq. (83)) is also valid in case Θ is a tensor.
It is necessary to note the derivative
There are different forms of deviatoric stress rate for an arbitrary second-rank tensor
Jaumann G.:
Rivlin R.:
Truesdell C.:
Oldroyd J., Sedov L.I., etc. [8, 10, 11]
At present, the question of which derivative is more appropriate to use when constructing rheological equations is unclear. The most common is the rotational derivative by Gustav Jaumann. The corresponding material derivative in the form by Jaumann, for an arbitrary tensor of the second rank, has the form
Material derivative in the form by Rivlin is written as follows:
It is easy to see that Rivlin’s derivative differs from Jaumann’s one by the additional term
Rotational derivative of a symmetric tensor is also a symmetric tensor. As an example, let us consider the rotational derivative of strain-rate tensor and spin tensor:
As a result, we have obtained the symmetrical second-rank tensor.
9. Conclusions
In this chapter, some applications of tensor calculus in fluid dynamics and heat transfer are presented. Typical transformations of equations and governing relations are discussed. Main conservation equations are given and analyzed. The governing equations of fluid motion and energy were obtained.
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