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:
- Forumulate the Flow Problem
- Model the Geometry and Flow Domain
- Establish the Boundary and Initial Conditions
- Generate the Grid
- Establish the Simulation Strategy
- Establish the Input Parameters and Files
- Perform the Simulation
- Monitor the Simulation for Completion
- Post-process the Simulation to get the Results
- Make Comparisons of the Results
- Repeat the Process to Examine Sensitivities
In further detail, these steps include:
Formulate the Flow Problem
The first step of the analysis process is to formulate
the flow problem by seeking answers to the following questions:
- what is the objective of the analysis?
- what is the easiest way to obtain those objective?
- what geometry should be included?
- what are the freestream and/or operating conditions?
- what dimensionality of the spatial model is required? (1D, quasi-1D, 2D, axisymmetric, 3D)
- what should the flow domain look like?
- what temporal modeling is appropriate? (steady or unsteady)
- what is the nature of the viscous flow? (inviscid, laminar, turbulent)
- how should the gas be modeled?
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.
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
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.
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.
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.
Perform the Simulation
The simulation is performed with various possible with options for
interactive or batch processing and distributed processing.
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.
Post-Process the Simulation to get the Results
Post-Processing involves extracting the desired flow properties
(thrust, lift, drag, etc...) from the computed flowfield.
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.
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:
- flow conditions
- initial conditions
- marching strategy
- grid topology and density
- turbulence model
- chemistry model
- flux model
- artificial viscosity
- boundary conditions
- computer system
Further information can be found on the pages entitled
Verification Assessment and
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