CFD Basis For Ansys

Whats is CFD ?
Computational Fluid Dynamics (CFD) is the science of predicting fluid flow, heat and mass transfer, chemical reactions, and related phenomena by solving numerically the set of governing mathematical equations
More info can be found at wikipedia
How does CFD Help?
CFD helps a engineer in following ways:

• Helps in indept study of new design.
• Product development.
• Design Validation.
• Find Flaws in Design.

CFD analysis reduce total cost and time required for experimentation and data acquisition
CFD in ANSYS:
ANSYS CFD solvers are based on the finite volume method

- Domain is divided into finite set of control volume.
- General conservation (transport) equations for mass, momentum, energy, species, etc. are solved on this set of control volumes

How to analyze CFD problem :

Problem identification
What result needed

• Identify what result you are looking for (i.e Pressure drop , mass Flow rate etc.)
• Which physical model you will need for your analysis ?
• What assumption you will have to make ? 
• What easy assumption can you make (i.e Periodicity ,symmetry)
• How much accuracy is needed ?
• How quick do you need result ?

Identify the domain

• Isolate piece of system
• Beginning and end of computational domain

1.  Are boundary condition available at this domain  ?
2. Can it accomodate all the information
3. Can it be extended to point where reasonable data exists

• Can it be simplified or approximated as 2D or axis symmetric problem

Geometry

• You can extract fluid region from  solid part
• Make geometry from  scratch in design modeler
• You can simply  geometry by removing unnecessary features to simplify mesh , also you can use symmetry or periodicity to make model simple
• Sometimes model needs to  split so that boundary conditions or domain can be created

Mesh
A mesh divides a geometry into many elements . These are used by CFD solver to build control volume

• Mesh must resolve geometric feature of interest and also capture gradient of interest eg velocity , pressure,temp
• Find region of high gradient
• Sufficient computer memory should be available  for high no. of cells/node

More About Mesh will be posted in further blog

 Physics and Solver Settings

• Define material property i.e fluid , solid ,mixture
• Select appropriate physical models ie turbulence , combustion , multiphase etc
• Define boundary condition at all boundary zone
• Give initialvalueorpreviousolution
• Setup solver
• Setup convergence monitors

Compute solution

•  The discretised conservation equations are solved iteratively until convergence
• Convergence is reached when

1. Changes in solution from iteration to other are negligible
2. Overall property conservation is acheived
3. Data of interest have reached (eg drag,pressure drop) have reached steady value

• Accuracy of converged solution depends on 

1. Appropriateness & accuracy of physical model
2. quality of mesh and independance
3. Numerical errors

Examine result 

• Visualization tools can be used to study overall  flow pattern, find separation,find shock , shear layers etc
• Numerical Reporting tools can be used to calculate force , average heat transfer coefficient  , flux balances , surface and volume integral quantities

Update geometry 

• If physical model is  not appropriate

1.  Is the flow turbulent?
2. Is the flow unsteady?
3. Is the flow unsteady?
4. Are there compressibility effects ?
5. Are there 3D effects?

• If boundary condition are not correct
• If refine mesh gives more accurate result