Mesh Generation Flashcards
What is mesh generation?
Dividing physical space into a large number of geometrical elements
What are the three kinds of meshes?
Surface
Volume
Surface-Volume
What are seven different types of Grids/Meshes?
- Structured Grid
- Overset Grid
- Hybrid Grid
- Unstructured Mesh
- Body-fitted Grid
- Mixed Grid
- Cartesian Grid
What are properties of structured grids?
- Each grid point connected with two neighbors
- Easy access of flow variables
- Difficult to generate for complex bodies
- Longer grid generation time
- Each grid point is uniquely defined
- Grid elements are quadrilaterals in 2D and hexahedral in 3D in
What are three basic grid topologies in 2D?
- C-grid
- H-grid
- O-grid
What are four methods for creating structured grids?
- Algebraic Grid Generation
- Conformal Transformation
- Elliptical Grid Generation
- Hyperbolic grid generation
What are the three steps of grid generation?
- Definition of surface
- Surface grid creation
- Volume grid creation
What are some important grid requirements?
- No holes between grid cells
- Grid cells do not overlap
- No abrupt changes in grid cell volume or stretching ratio
- Elements should be as regular as possible
- No large kinks in grid line
What are the properties of unstructured meshes?
- No particular ordering to grid cells
- Neighboring points cannot be directly identified, a connectivity matrix is needed
- Triangles / quadrilaterals in 2D and tetrahedral/ hexahedral/ prism / pyramid elements in 3D
- Can be created much more quickly
- Difficult to apply to turbulence models
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What is the C- Grid?
- Grid clustering in far field
- High resolved regions in wake
What is H-grid?
- Employed in turbomachinery
- Grid clustering in far field
- Uses points least efficiently
What is O-grid?
- Poor grid quality at a sharp trailing edge
- Uses points most efficient
What are the main principles for creating structured grids?
- Constant coordinate system in the mapped surface of the body
- Governing equations are solved in the transformed cartesian domain
- Results are mapped back to the physical domain
- Governing equations also have to be transformed
What is Algebraic grid generation?
- Most basic way
- Direct description of the coordinate transformation between physical and computational domain
- Generates Structured grid
What is Conformal Transformation?
- Relates an easily constructed mesh over a simple geometry to a mesh around a general shape
- Angles are preserved in conformal transformation, so an orthogonal grid around a simple geometry will remain orthogonal around the general shape
- Restricted to two-dimensional cases
- Generated structured grid
What is elliptical grid generation?
- Elliptic PDE is solved to create grid
- Grid is solution of PDE so it has the shape of a flow field
- Grid quality can be controlled
- Create initial grid and solve Laplace equation on initial grid
- Generates structured grid
What is hyperbolic grid generation?
- Hyperbolic grids can be grown away from initial data surface with no need for multiple solutions to be obtained
- Do not require boundaries enclosing the area where the grid is created
- can often be created faster and with a higher level of control
- Generates structured grid
What are three basic approaches for generating unstructured meshes?
- Delaunay triangulation
- Advancing front methods
- Octree methods
What is Delaunay Triangulation?
- For a set of points, obtain the best set of triangles
- Generates unstructured grid
What is Octree / Quadtree Concept?
- Region surrounding the surface of interest is divided into a collection of rectangles followed by a division of rectangles into triangles
What is the difference between super and subsonic flow in terms of information propagation?
- Subsonic flow propagates information in all directions
- Supersonic flow only propagates information downstream within Mach cone
What are important structured cell grid properties?
- Jacobian
- Cell Shape
- Cell orthogonality at a surface boundary
- Cell streaching
What are important unstructured cell properties?
- Flow alignment and boundary layer gradients
- Cell Planeness
- Cell skew and smoothness
- Cell isotropy and spacing
What is the Jacobian?
- It defines the transformation from physical space to computational space
How does cell shape affect solution accuracy for structured cell grid?
Highly skewed cells can lead to inaccuracy
How does cell orthogonality at surface affect the solution accuracy for structured cell grid?
- Structured grid flow solvers give most accurate results when the grid cells are orthogonal
How does cell stretching affect solution accuracy for structured cell grid?
- The more we stretch the grid, form one cell to the next, the more the truncation error.
- Any adjacent cells should not have lengths that differ by more than 20 percent to avoid error
How does cell planarness affect solution accuracy for unstructured cell grids?
- Non planar faces introduce errors for unstructured cell grids
- All quadrilateral faces should be as plane as possible
How does cell skew and smoothness affect solution accuracy for unstructured cell grids?
- The face centroid should be located centrally with respect to the solution points at the neighboring cell centroids to avoid error
How many regions does the boundary layer have and what are they called?
- Inner and outer
What are the three sublayers in the inner layer of the boundary layer?
- Viscous
- Transitional
- Fully turbulent
What is the formula for y+ calculation for first estimation?
- Compute Reynolds number
- Estimate skin friction
- Compute wall sheer stress
- Compute friction velocity
- Compute wall distance
What is the rule for viscous sublayer in mesh generation?
It starts at the wall and is continuous to y+ = 10
What are some grid manipulation methods?
- Adaptation
- Fluid Structure Interaction (deformat.)
- Chimera Methods
- Sliding Interface
What is the purpose of grid adaptation and what are its three steps?
- To make sure that the grid is fine enough in high gradient areas.
- Identify high gradient regions
- Refine mesh in high grad. areas
- Coarsen unimportant areas
What are some indicators to identify high gradient regions in mesh?
- Gradient based indicator
- Residual based indicator
- Reconstruction based indicator
What are some methods for refining mesh in high gradient regions?
- Local remeshing
- Delaunay triangulation
- Edge-face swapping
- Octree
What is Fluid-Structure interaction and what are its steps?
- It is useful for modeling of a rigid body with aerodynamic forces.
- Use forces on structure model
- Transfer Model deformation to CFD solver
- Calculate simulation with deformed model
What are three approaches to Fluid-Structure Interaction?
- Monolithic Approach
- One way coupling
- Two way coupling
What is the monolithic approach for fluid structure interaction?
- One global system of equations to describe fluid, structure and interaction
- Simultaneously solve for entire aeroelastic system
What is the one way coupling approach for fluid structure interaction?
- CFD solver gives aerodynamic solution for a defined time span
- Transfer solution to structural solver
- Computation of structural response
- No feedback loop to CFD Solver!
What is the two way coupling approach for fluid structure interaction?
- Partition into fluid and structural domain
- Separate solution and mutual interaction of solvers
- Further divided into explicit and implicit approach
- Explicit approach solves once per time step and has first order accuracy
- Implicit approach has converged solution in physical time step
What is overset grid generation and why is it used?
Used in cases where geometry cannot be represented using a single contiguous grid.
- Use blocks of overlapping structured grids
- Exchange boundary layer information via interpolation
- Chimera steps
What are chimera steps?
- Grid generation
- Hole cutting
- Determination of interpolation weights
What is sliding interface and when is it used?
- Used in cases where there is movement of part relative to boundaries of interest
- Move boundaries and adjust mesh
- Nodes move rigidly in a given dynamic mesh zone
- Multiple cell zones connected through non conformal interfaces
What are properties of sliding interface?
- Can be partially overlapping
- Solution is inherently unsteady
What is grid sensitivity study and when is it used?
- Used when you want to prove that the simulation provides accurate results and is not dependent on the grid
- Select parameters
- Refine mesh until solution is independent of grid
- Assure grid resolves features in best way
