Cfd Basics

© 2009 ANSYS, Inc. All rights reserved. 1 ANSYS, Inc. Proprietary© 2009 ANSYS, Inc. All rights reserved. 1 ANSYS, Inc. Proprietary

© 2009 ANSYS, Inc. All rights reserved. 2 ANSYS, Inc. Proprietary

Outline

• CFD Basics• Application of CFD in Hydro Power Plants• Piping system design challenges• Case studies• More Turbo Machinery solutions by ANSYS• Conclusion• Questions and Discussion


What is CFD?

• Computational Fluid Dynamics (CFD) is the numerical study of how things flow.

• CFD is used as a tool by many engineers (mechanical, chemical, etc.) across a broad range of industries.

• CFD can provide detailed information about what is happening in a process where fluid is in motion.


Uses of CFD

• Architectural• Environmental

Process/Food Industry

Automotive/MechanicalAerospace

Safety/HealthPower Generation

Bio-medical

Sports


The Building Block

• The most basic building block in CFD is the conservation volume

Conservation volume: what goes in + what is made/destroyed = what goes out

what goes in what goes out

what ismade/destroyed

conservation volume


Assembling the Blocks

• Simple algebraic equations describe how the conservation volumes are connected.

• These simple equations conserve mass, momentum and energy.

The conservation volumes are assembled together to fill space.


Conservation Volumes Assembled Around A Car

• Once the conservation volumes are assembled, a flow field can be calculated.


The Final Solution

• The final flow field may look like this.


CFD Basic Concepts

• The Control Volume• Conservation Laws• Governing Equations of CFD• Linear and Non-Linear Partial Differential

Equations• Boundary Conditions• Discrete Methods


Steps in a CFD Simulation

• Step I: Pre-processing step - define the problem– Define the geometry– Define the domain(s)– Define boundary and initial conditions– Define the mesh– Define solver parameters

• Step II: Solution step– Solve the governing equations

• Step III: Post-processing step– Analyze results


Step I: Pre-processing• Define the geometry

– Domain in which the governing equations will be solved and solution obtained

– Create a B-rep Solid• Define the fluid domain.

– Create fluid regions (fluid, solid, conducting solid, porous media)

– Assign fluid properties (viscosity, thermal conductivity, specific heat, etc…)

– Select the physical models (turbulence models, compressibility, buoyancy, two-phase flow, etc…)


Step I: Pre-processing

• Grid Generation– Process of generating finite volumes or cells– Tetrahedral/Pyramidal/Prismatic/Hexahedral cells created– Surface mesh and volume mesh make up the grid


Step I: Pre-processing

• Define the boundary conditions– Needed on all external surfaces of geometry


Step II: Solving• Solve the governing

equations– Set the flow solver options– Iteratively solve the

governing equations– Obtain convergence


Step III: Post-processing• Qualitative

– Graphical format– Pressure, Temperature distribution– Velocity field


Step III: Post-processing• Quantitative

– Quantitative post-processing is the reduction of data to produce performance coefficients

– Simplifies comparison of designs– Application dependent– Examples:

• 1. Pressure drop in a duct• 2. Lift, drag coefficients of airfoil• 3. Head, Efficiency etc. in pump• 4. Torque, Efficiency etc. in a torque converter• 5. Average NOx emission at a furnace exit

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Cfd Basics