# 8.3.1 Generalization of Mathematical Approach for Derivations

- Page ID
- 743

In this section a general approach for the derivations for conservation of any quantity e.g. scalar, vector or tensor, are presented. Suppose that the property \(\phi\) is under a study which is a function of the time and location as \(\phi(x,y,z,t)\). The total amount of quantity that exist in arbitrary system is

\[

\label{dif:eq:math:phiG}

\Phi = \int_{sys} \phi\,\rho\,dV \tag{61}

\]

Where \(\Phi\) is the total quantity of the system which has a volume \(V\) and a surface area of \(A\) which is a function of time. A change with time is

\[

\label{dif:eq:math:DphiDt1}

\dfrac{D\Phi}{Dt} = \dfrac{D}{Dt} \int_{sys} \phi\,\rho\,dV \tag{62}

\]

Using RTT to change the system to a control volume (see equation (??)) yields

\[

\label{dif:eq:math:DphiDt}

\dfrac{D}{Dt} \int_{sys} \phi\,\rho\,dV =

\dfrac{d}{dt} \int_{cv} \phi\,\rho\,dV + \int_{A} \rho\,\phi\,\pmb{U}\cdot dA \tag{63}

\]

The last term on the RHS can be converted using the divergence theorem (see the appendix) from a surface integral into a volume integral (alternatively, the volume integral can be changed to the surface integral) as

\[

\label{dif:math:divergenceTheorem}

\int_{A} \rho\,\phi\,\pmb{U}\cdot dA = \int_{V} \nabla\cdot\left(\rho\,\phi\,\pmb{U} \right) dV \tag{64}

\]

Substituting equation (64) into equation yields

\[

\label{dif:eq:RTTextended1}

\dfrac{D}{Dt} \int_{sys} \phi\,\rho\,dV = \dfrac{d}{dt} \int_{cv} \phi\,\rho\,dV +

\int_{cv} \nabla\cdot\left(\rho\,\phi\,\pmb{U} \right) dV \tag{65}

\]

Since the volume of the control volume remains independent of the time, the derivative can enter into the integral and thus combining the two integrals on the RHS results in

\[

\label{dif:eq:RTTextended}

\dfrac{D}{Dt} \int_{sys} \phi\,\rho\,dV = \int_{cv} \left( \dfrac{d \left(\phi\,\rho\right)}{dt} +

\nabla\cdot\left(\rho\,\phi\,\pmb{U} \right) \right) dV \tag{66}

\]

The definition of equation (61) LHS can be changed to simply the derivative of \(\Phi\). The integral is carried over arbitrary system. For an infinitesimal control volume the change is

\[

\label{dif:eq:math:infinitesimalChange}

\dfrac{D\,\Phi}{Dt} \cong \left( \dfrac{d \left(\phi\,\rho\right)}{dt} +

\nabla\cdot\left(\rho\,\phi\,\pmb{U} \right) \right) \overbrace{dx\,dy\,dz}^{dV} \tag{67}

\]

### Contributors

Dr. Genick Bar-Meir. Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.2 or later or Potto license.