2.2 Planning Your CFD Analysis

For each of the problem-solving steps, there are some questions that you need to consider:

• Defining the Modeling Goals
  • What results are you looking for, and how will they be used?
    • What are your modeling options?

    • What physical models will need to be included in your analysis?

    • What simplifying assumptions do you have to make?

    • What simplifying assumptions can you make?

    • Do you require a unique modeling capability?
      • Could you utilize user-defined functions (written in C)?

  • What degree of accuracy is required?

  • How quickly do you need the results?

  • How will you isolate a piece of the complete physical system?

  • Where will the computational domain begin and end?
    • Do you have boundary condition information at these boundaries?

    • Can the boundary condition types accommodate that information?

    • Can you extend the domain to a point where reasonable data exists?

  • Can it be simplified or approximated as a 2D or axisymmetric problem?

• Creating Your Model Geometry and Mesh

  ANSYS FLUENT uses unstructured meshes in order to reduce the amount of time you spend generating meshes, to simplify the geometry modeling and mesh generation process, to allow modeling of more complex geometries than you can handle with conventional, multi-block structured meshes, and to let you adapt the mesh to resolve the flow-field features. ANSYS FLUENT can also use body-fitted, block-structured meshes (e.g., those used by FLUENT 4 and many other CFD solvers). ANSYS FLUENT is capable of handling triangular and quadrilateral elements (or a combination of the two) in 2D, and tetrahedral, hexahedral, pyramid, wedge, and polyhedral elements (or a combination of these) in 3D. This flexibility allows you to pick mesh topologies that are best suited for your particular application, as described in the User's Guide.

  You can adapt all types of meshes (except for polyhedral) in ANSYS FLUENT in order to resolve large gradients in the flow field, but you must always generate the initial mesh (whatever the element types used) outside of the solver, or one of the CAD systems for which mesh import filters exist.

  The following questions should be considered when you are generating a mesh:

  • Can you benefit from other ANSYS, Inc. products such as CFX, ANSYS Icepak, or ANSYS Airpak?

  • Can you use a quad/hex mesh or should you use a tri/tet mesh or a hybrid mesh?
    • How complex is the geometry and flow?

    • Will you need a non-conformal interface?

  • What degree of mesh resolution is required in each region of the domain?
    • Is the resolution sufficient for the geometry?

    • Can you predict regions with high gradients?

    • Will you use adaption to add resolution?

  • Do you have sufficient computer memory?
    • How many cells are required?

    • How many models will be used?

• Setting Up the Solver and Physical Models

  For a given problem, you will need to:

  • Import and check the mesh.

  • Select the numerical solver (e.g., density based, pressure based, unsteady, etc.).

  • Select appropriate physical models.
    • Turbulence, combustion, multiphase, etc.

  • Define material properties.
    • Fluid

    • Solid

    • Mixture

  • Prescribe operating conditions.

  • Prescribe boundary conditions at all boundary zones.

  • Provide an initial solution.

  • Set up solver controls.

  • Set up convergence monitors.

  • Initialize the flow field.

• Computing and Monitoring Your Solution
  • The discretized conservation equations are solved iteratively.
    • A number of iterations are usually required to reach a converged solution.

  • Convergence is reached when:
    • Changes in solution variables from one iteration to the next are negligible.
      • Residuals provide a mechanism to help monitor this trend.

    • Overall property conservation is achieved.

  • The accuracy of a converged solution is dependent upon:
    • Appropriateness and accuracy of physical models.

    • Mesh resolution and independence.

    • Problem setup.

• Examining and Saving Your Results

  Examine the results to review the solution and extract useful data.

  • Visualization tools can be used to answer such questions as:
    • What is the overall flow pattern?

    • Is there separation?

    • Where do shocks, shear layers, etc. form?

    • Are key flow features being resolved?

  • Numerical reporting tools can be used to calculate the following quantitative results:
    • Forces and moments

    • Average heat transfer coefficients

    • Surface and volume integrated quantities

    • Flux balances

• Revising Your Model

  Once your solution is converged, the following questions should be considered when you are analyzing the solution:

  • Are physical models appropriate?
    • Is flow turbulent?

    • Is flow unsteady?

    • Are there compressibility effects?

    • Are there 3D effects?

  • Are boundary conditions correct?
    • Is the computational domain large enough?

    • Are boundary conditions appropriate?

    • Are boundary values reasonable?

  • Is the mesh adequate?
    • Can the mesh be adapted to improve results?

    • Does the solution change significantly with adaption, or is the solution mesh independent?

    • Does boundary resolution need to be improved?