Computational Fluid Dynamics

CFD 6: Conservation of Chemical Species

The transport is quantified by the vector field Ji (x, t ) of the flux of a species i , which denotes the direction and the rate of the mass flux of the species per unit area at the point x. In the same manner as in the derivation, we can find that the rate of change by diffusion of the mass content of species i in a fluid element of unit volume is ∇ · Ji .
The concentration of species can be expressed in terms of the mass fraction mi (x, t ), which is the ratio of the mass of species i to the total mass of the mixture in the same small volume. Another possibility is to use the concentration of species Ci = mi * ρ defined as the mass of species i per unit volume. The conservation law is

CFD 5 : Conservation of Mass

It expresses the law of conservation of mass. In a flow with density ρ(x, t ) and velocity V(x, t ),

we consider a fluid element of volume δV. Since, according to the definition, its mass δm = ρδV must remain constant:

CFD 4 : Continuity Equation

Let us consider the two-dimensional situation. The element has the sizes dx and L, volume δV = Ldx.

The velocity field is purely one-dimensional, but x-dependent with u = u(x).

During the time interval dt , the right-hand side boundary moves together with fluid molecules by the distance u(x + dx)dt.

The corresponding increase of volume is Ldtu(x + dx). At the same time, the volume decreases by Ldtu(x) due to the motion of the left-hand side boundary. The time rate of volume change per unit volume is

CFD 3 : Intro to Governing Equations

From the physical viewpoint, the equations describing fluid flows and heat and mass transfer are simply versions of the conservation laws of classical physics, namely:
• Conservation of chemical species (law of conservation of mass)
• Conservation of momentum (Newton’s second law of motion)
• Conservation of energy (first law of thermodynamics)
In some scenarios, additional equations are needed to account for other phenomena, such as, for example, entropy transport (the second law of thermodynamics) or electromagnetic fields.

CFD 2 : Why CFD ?

Processes of a fluid flow such as motion, rotation, and deformation of small fluid particles can be taken into account.
Of course, the opportunities come at a high cost, most important one in the form of dramatically increased complexity of the governing equations.