BOUNDARY CONDITIONS, RESIDUALS, AND TURBULENCE Flashcards
What is SET AT THE INFLOW BOUNDARY CONDITION? (3)
- Initial Velocity (mass flow)
- Temperature
- Density
What is SET AT THE OUTFLOW BOUNDARY CONDITION? (2)
- Mass Flow
- Pressure
What is the MOST ROBUST INLET-OUTLET BOUNDARY CONDITION COMBINATION?
INLET: Velocity
OUTLET: Static Pressure
Where should an OUTLET BE PLACED?
Downstream at a distance 10x the height of the last object
What WALL CONDITIONS SHOULD BE CONSIDERED? (4)
- No slip condition (most common)
- Free slip condition
- Rotating/moving walls
- Wall roughness
Why are SYMMETRY PLANES USEFUL IN CFD?
Can significantly reduce the amount of computation required
* No flow of mass/heat across the symmetry plane
What are RESIDUALS?
A measure of convergence, the closer to zero the more converged the solution
Typically set to R≈110^(-5) or 110^(-6)
What does it mean if the RESIDUALS ARE INCREASING?
The solution is diverging
What is used to FORCE A SOLUTION TO CONVERGE?
Under-relaxation factor implemented, this limits the amount a variable can change during a solution step
Under-relaxation factor, α typically set to 0.4-1.0
What are MONITOR POINTS?
Probe points to take specific readings at points of interest - equivalent to a physical sensor
What is the STRUCTURE OF TURBULENCE?
- Represented by velocity fluctuations around a mean (3D)
- Comprised of eddies of varying sizes
What is meant by VORTEX STRETCHING?
- One end of an eddy moves into a faster-flowing region
- Stretching occurs as one end moves faster than the other, cross-sectional area decreases
- Angular velocity increases to conserve angular momentum
Why does VORTEX STRETCHING CAUSE AN INCREASE IN TEMPERATURE?
The smallest eddies’ energy is converted into heat energy due to friction from the fluid (viscosity)
