How Does a CFD Code Work

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How Does a CFD Code Work
Department of Mechanical Engineering
MEHB463
CFD Chap. 1 Introduction to CFD

• A . Pre-Processor
• Geometry definition or computational domain
• Generate mesh / grid or subdivisions
• Definitions of fluid properties
• Definition of physical or chemical phenomena to
  be modeled
• Specifications of boundary and initial
  conditions

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How Does a CFD Code Work .
Department of Mechanical Engineering
MEHB463
CFD Chap. 1 Introduction to CFD

• B. SOLVER
• This is the heart of the code
• Involved applications of the discretized equation
  to each of the CV
• Basically 3 Steps are Involved -
• Approximation of the unknown flow variables
• Discretization Substitution of approximation
  into governing flow equations and subsequent
  manipulations
• Solution of Algebraic Equation For Each Node
• 3 Methodologies are used Finite Difference,
  Finite Volume, Finite Element
• Most Commercial Code use FV e.g. FLUENT, CFX,
  PHOENICS

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How Does a CFD Code Work .
Department of Mechanical Engineering
MEHB463
CFD Chap. 1 Introduction to CFD

• B. SOLVER cont
• Finite Volume Method
• Basically 3 Steps are Involved -
• Formal Integration of the Governing Equation For
  All CVs
• Discretization substitution of FD-type equation
  of the Convection, Diffusion and Sources Terms
• Solution of Algebraic Equation For Each Node eg
  by TDMA, Stone Algorithm, Conjugate Gradient
• FVM Mimics the underlying physical conservation
  principles.
• General Conservation Law
• Rate of change of ? in the CV Convection
  Flux of ? into CV Diffusion Flux of ? into
  the CV Rate of Source Creation of ? in the CV

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For example, for general fluids motion the PDE
is called the Navier Stokes equations
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How Does a CFD Code Work .
Department of Mechanical Engineering
MEHB463
CFD Chap. 1 Introduction to CFD
Small control volume where equations are applied
Structured H-mesh
inlet
periodic
Unstructured adaptive mesh
exit
exit
inlet
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How Does a CFD Code Work .
Department of Mechanical Engineering
MEHB463
CFD Chap. 1 Introduction to CFD

• Post-Processor
• Graphical Display of the Output Results
• Eg. Contours, Line Plot, Vector Plot, Surface
  Plot, Particle Track

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Introduction to CFD analysis using GAMBIT FLUENT
GAMBIT
FLUENT
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Test case using GAMBIT FLUENT

• Problem Identification and Preprocessing
• Define your modeling goals. ?
• 2. Identify the domain you will model ?
• 3. Design and create the grid ?
• Solver Execution
• 4. Set up the numerical model.
• 5. Compute and monitor the solution.
• Post-Processing
• 6. Examine the results.
• 7. Consider revisions to the model.

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Pre-Requisites of CFD-User
Department of Mechanical Engineering
MEHB463
CFD Chap. 1 Introduction to CFD

• Aware that the underlying physics are very
  complex and results generated is as good as the
  physics embedded and as worst as its operator
• The knowledge required are -
• Understanding of the Physics of the flow problem
  Flow equations physical behaviour, model
  embedded, approximation made, fundamental physics
  of flow e.g. steady-unsteady, compressibility,
  significant factors influencing the flow
• Understanding of Numerical Algorithm Basic
  concepts of Numerical Method such as convergence,
  consistency stability Basics of discretization
  scheme such as conservativeness
  transportiveness, numerical viscosity, false
  convection Effect of grid size, orthogonolity
  etc.

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Scope of This Course
Department of Mechanical Engineering
MEHB463
CFD Chap. 1 Introduction to CFD

• The Objective is to Provide all the Necessary
  Pre-Requisites of Good CFD User
• This is Achieved by -
• Understanding Basic Governing Equations, Their
  Physical Meanings Possible Approximations
• Understanding the Finite Volume Methods and
  Associated Numerical Discretization Methods as
  Applied to Fluid Flow Conservation Equations
• Understanding the concept of turbulence and its
  importance in CFD. Different types of turbulence
  models

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• End of lecture

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MEHB463
Department of Mechanical Engineering
CFD Chap. 2 Fluid Flow Governing Equations
Summary of Governing Equations. In
conservation or divergence form
Continuity
x-mom
y-mom
z-mom

• Energy

Equation of state - Thermal eqn
of state p ?RT Caloric eqn of
state e CvT
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