Basic Concepts

Question 1

The approximation of a continuously-varying quantity in terms of values at a finite number of points is called discretization.

How does the affect flow field?

Answer 1

The flow field is discretized; i.e. field variables (𝜌, 𝑢, 𝑣, 𝑤, 𝑝, …) are approximated by their values at a finite number of nodes.

Question 2

The approximation of a continuously-varying quantity in terms of values at a finite number of points is called discretization.

How does the affect equations of motion?

Answer 2

The equations of motion are discretised:
derivatives → algebraic approximations
(continuous) (discrete)

Question 3

The approximation of a continuously-varying quantity in terms of values at a finite number of points is called discretization.

What is the significance of algebraic equations?

Answer 3

The resulting system of algebraic equations is solved to give values at the nodes.

Question 4

The main stages in a CFD simulation are:

Answer 4

Pre-processing:
– formulate problem (geometry, equations, boundary conditions);
– computational mesh (set of control volumes).

Solving:
– discretise the governing equations;
– solve the resulting algebraic equations

Post-processing:
– analyse (calculate derived quantities: forces, flow rates, … );
– visualise (graphs and plots)

Question 5

The equations of fluid flow are based on fundamental physical conservation principles:

Answer 5

• mass: change of mass = 0
• momentum: change of momentum = force × time
• energy: change of energy = work + heat

In fluid flow these are usually expressed as rate equations; i.e. rate of change = …

Additional equations may apply for non-homogeneous fluids (e.g. particle load, dissolved chemicals, multiple species, …)

Question 6

Conservation principles may be expressed mathematically as either:

Answer 6

• integral (control-volume) equations;
• differential equations