Quiz 4 Flashcards
What is Philip’s rule number 1?
No grid, no solution
What two main types of grids exist?
Structured
Unstructured
What kind of grid does FDS use?
Structured with the possibility of multiple blocks
Does FDS have a conforming or non-conforming mesh?
Non-conforming, the mesh does not conform to the geometry, the geometry has to conform to the mesh
What are the main characteristics of an unstructured grid?
o No inherent order o Cannot be mapped to a Cartesian grid o We need to define ▪ The nodes and elements ▪ The connectivity graph o Lack of inherent structure provides a VERY flexible approach to grid generation
What are some signs of poor mesh quality?
o Large jumps in mesh size
o Twisted cells
o Cells with one or more very short edges
What is the difference between an analytical solution and a numerical solution?
o Analytical Solution provides closed from expressions and variation of the dependent variables continuously through the entire domain.
o Numerical solution provides solution at discrete points in the domain - grid points – and the governing equations replaced with systems of algebraic equations
What is more numerically accurate, a first order scheme or a second order scheme?
Second order scheme (at higher computational cost)
What are some key characteristics of an explicit and implicit scheme?
o Both require a set of algebraic equations.
o Implicit
▪ Unknowns coupled in space
▪ Numerically expensive
▪ Stability not limited by choice of time step
o Explicit - Equations not coupled in space
o Numerically inexpensive
o Stability limited by choice of time step
Theoretically, if bound by the CFL stability criteria, what happens to the time step when the grid size is reduced by a factor of 2? And how does it affect the overall simulation time?
Time step half as small (half the distance within in a cell but the same velocity), 8 times the cells so a total of 16 times longer computation time.
