MOMENTUM AND PRACTICAL CFD

Question 1

What is a BODY FORCE, GIVE TWO EXAMPLES

Answer 1

A force that acts directly on the entire mass of the fluid element
* Gravity
* Coriolis

Question 2

What is a SURFACE FORCE, GIVE TWO EXAMPLES

Answer 2

A force that acts directly on the surface of the fluid element
* Pressure
* Normal/Shear

Question 3

What are the TWO FUNDAMENTAL TYPES OF STRESS?

Answer 3

Direct Stress
* Compression/Extension
* Normal
* Pressure

Shear Stress
* Shear
* Along a surface
* Viscous Forces

Question 4

Where should MESH ELEMENTS BE CONCENTRATED? (3)

Answer 4

• Areas of high flow gradients (boundary layer >10 nodes within boundary layer)
• Areas with significant changes in geometry
• Ideal grids are isotropic and finely spaced

Question 5

What should be AVOIDED WHEN MAKING A MESH? (4)

Answer 5

• Highly skewed/warped elements: <0.9
• Elements with high aspect ratios (Ratio of side lengths) <400
• High expansion rates (transition): <1.5
• Sudden changes in mesh density

Question 6

What is the DIFFERENCE BETWEEN AN ERROR AND AN UNCERTAINTAINTY?

Answer 6

An error is a recognizable deficiency in a model not caused by a lack of knowledge whereas uncertainty is due to a lack of knowledge

Question 7

What are some INPUT UNCERTAINTIES? (3)

Answer 7

• Domain Geometry;
  - Discrepancy between CAD design intent and manufactured part (manufacturing tolerances)
  - Effect of roughness
  - Geometry represented by a finite number of straight edges and arcs ( limited accuracy
• Boundary Conditions;
  - Difficult to get flow variables such as velocity, and temperature with high accuracy
  - Choice of type and location of open boundaries can be challenging
• Fluid Properties;
  - Assumption of constant fluid properties is often an approximation

Question 8

What is meant by VERIFICATION?

Answer 8

Determining that a model implementation accurately represents the developer’s conceptual description of the model and the solution to the model. “Solving the equations right”

Question 9

What is meant by VALIDATION?

Answer 9

The process of determining the degree to which a model accurately represents the real world. “Solving the right equations”