1. https://www.grc.nasa.gov/WWW/wind/valid/tutorial/process.
html
CFD Analysis Process
The general process for performing a CFD analysis is outlined below so as to provide
a reference for understanding the various aspects of a CFD simulation. The process
includes:
1. Forumulate the Flow Problem
2. Model the Geometry and Flow Domain
3. Establish the Boundary and Initial Conditions
4. Generate the Grid
5. Establish the Simulation Strategy
6. Establish the Input Parameters and Files
7. Perform the Simulation
8. Monitor the Simulation for Completion
9. Post-process the Simulation to get the Results
10. Make Comparisons of the Results
11. Repeat the Process to Examine Sensitivities
12. Document
In further detail, these steps include:
1. Formulate the Flow Problem
The first step of the analysis process is to formulate the flow problem by
seeking answers to the following questions:
o what is the objective of the analysis?
o what is the easiest way to obtain those objective?
o what geometry should be included?
o what are the freestream and/or operating conditions?
o what dimensionality of the spatial model is required? (1D, quasi-1D, 2D,
axisymmetric, 3D)
o what should the flow domain look like?
o what temporal modeling is appropriate? (steady or unsteady)
o what is the nature of the viscous flow? (inviscid, laminar, turbulent)
o how should the gas be modeled?
2. Model the Geometry and Flow Domain
The body about which flow is to be analyzed requires modeling. This generally
involves modeling the geometry with a CAD software package.
Approximations of the geometry and simplifications may be required to allow
an analysis with reasonable effort. Concurrently, decisions are made as to the
extent of the finite flow domain in which the flow is to be simulated. Portions
of the boundary of the flow domain conicide with the surfaces of the body
geometry. Other surfaces are free boundaries over which flow enters or leaves.
The geometry and flow domain are modeled in such a manner as to provide
input for the grid generation. Thus, the modeling often takes into account the
structure and topology of the grid generation.
3. Establish the Boundary and Initial Conditions
Since a finite flow domain is specified, physical conditions are required on the
boundaries of the flow domain. The simulation generally starts from an initial
solution and uses an iterative method to reach a final flow field solution.
4. Generate the Grid
The flow domain is discretized into a grid. The grid generation involves
defining the structure and topology and then generating a grid on that topology.
Currently all cases involve multi-block, structured grids; however, the grid
blocks may be abbuting, contiguous, non-contiguous, and overlapping. The
grid should exhibit some minimal grid quality as defined by measures of
orthogonality (especially at the boundaries), relative grid spacing (15% to 20%
stretching is considered a maximum value), grid skewness, etc... Further the
maximum spacings should be consistent with the desired resolution of
important features. The resolution of boundary layers requires the grid to be
clustered in the direction normal to the surface with the spacing of the first grid
point off the wall to be well within the laminar sublayer of the boundary layer.
For turbulent flows, the first point off the wall should exhibit a y+ value of less
than 1.0.
5. Establish the Simulation Strategy
The strategy for performing the simulation involves determining such things as
the use of space-marching or time-marching, the choice of turbulence or
chemistry model, and the choice of algorithms.
6. Establish the Input Parameters and Files
A CFD codes generally requires that an input data file be created listing the
values of the input parameters consisted with the desired strategy. Further the a
grid file containing the grid and boundary condition information is generally
required. The files for the grid and initial flow solution need to be generated.
7. Perform the Simulation
The simulation is performed with various possible with options for interactive
or batch processing and distributed processing.
8. Monitor the Simulation for Completion
As the simulation proceeds, the solution is monitored to determine if a
"converged" solution has been obtained, which is iterative convergence. Further
discussion can be found on the page entitled Examining Iterative Convergence.
9. Post-Process the Simulation to get the Results
Post-Processing involves extracting the desired flow properties (thrust, lift,
drag, etc...) from the computed flowfield.
10. Make Comparisons of the Results
The computed flow properties are then compared to results from analytic,
computational, or experimental studies to establish the validity of the computed
results.
11. Repeat the Process to Examine Sensitivities
The sensitivity of the computed results should be examined to understand the
possible differences in the accuracy of results and / or performance of the
computation with respect to such things as:
o dimensionality
o flow conditions
o initial conditions
o marching strategy
o algorithms
o grid topology and density
o turbulence model
o chemistry model
o flux model
o artificial viscosity
o boundary conditions
o computer system
Further information can be found on the pages entitled Verification
Assessment andValidation Assessment.
12. Document
Documenting the findings of an analysis involves describing each of these steps
in the process.
Last Updated: Thursday, 17-Jul-2008 07:25:21 EDT
Responsible NASA Official/Curator: John W. Slater
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