CFD Flashcards
Computational Fluid Dynamics
Fluid (gas and liquid) flows are governed by partial differential equations which
represent conservation laws for the mass, momentum, and energy.
Computational Fluid Dynamics (CFD) is the art of replacing such PDE systems
by a set of algebraic equations which can be solved using digital computers.
Fluid flow
What is fluid flow?
Fluid flows encountered in everyday life include
• meteorological phenomena (rain, wind, hurricanes, floods, fires)
• environmental hazards (air pollution, transport of contaminants)
• heating, ventilation and air conditioning of buildings, cars etc.
• combustion in automobile engines and other propulsion systems
• interaction of various objects with the surrounding air/water
• complex flows in furnaces, heat exchangers, chemical reactors etc.
• processes in human body (blood flow, breathing, drinking . . . )
• and so on and so forth
CFD
What is CFD?
Computational Fluid Dynamics (CFD) provides a qualitative (and sometimes even quantitative) prediction of fluid flows by means of
• mathematical modeling (partial differential equations)
• numerical methods (discretization and solution techniques)
• software tools (solvers, pre- and postprocessing utilities)
CFD enables scientists and engineers to perform ‘numerical experiments’
(i.e. computer simulations) in a ‘virtual flow laboratory’ real experiment CFD simulation
Why use CFD ?
Why use CFD?
Numerical simulations of fluid flow (will) enable
• architects to design comfortable and safe living environments
• designers of vehicles to improve the aerodynamic characteristics
• chemical engineers to maximize the yield from their equipment
• petroleum engineers to devise optimal oil recovery strategies
• surgeons to cure arterial diseases (computational hemodynamics)
• meteorologists to forecast the weather and warn of natural disasters
• safety experts to reduce health risks from radiation and other hazards
• military organizations to develop weapons and estimate the damage
• CFD practitioners to make big bucks by selling colorful pictures :-)
Examples of CFD applications
Examples of CFD applications
Aerodynamic shape design
CFD simulations by L ̈ohner et al.
Smoke plume from an oil fire in Baghdad CFD simulation by Patnaik et al.
Experiments vs. Simulations
CFD gives an insight into flow patterns that are difficult, expensive or impossible
to study using traditional (experimental) techniques
Experiment : Quantitative description of flow Quantitative prediction of flow phenomena using measurements phenomena using CFD software • for one quantity at a time • at a limited number of points and time instants • for a laboratory-scale model • for a limited range of problems and operating conditions
CFD
- for all desired quantities
- with high resolution in space and time
- for the actual flow domain
- for virtually any problem and realistic operating conditions
Error sources: measurement errors, Error sources: modeling, discretization
flow disturbances by the probes, iteration, implementation
Experiments vs. Simulations
As a rule, CFD does not replace the measurements completely but the amount
of experimentation and the overall cost can be significantly reduced.
Experiments versus simulations
Experiments Simulations • expensive • slow • sequential • single-purpose
- cheap(er)
- fast(er)
- parallel
- multiple-purpose
Equipment and personnel
are difficult to transport
CFD software is portable,
easy to use and modify
The results of a CFD simulation are never 100% reliable because
• the input data may involve too much guessing or imprecision
• the mathematical model of the problem at hand may be inadequate
• the accuracy of the results is limited by the available computing power
Fluid characteristics
Fluid characteristics
Macroscopic properties
ρ density μ viscosity p pressure T temperature v velocity
Classification of fluid flows
Classification of fluid flows
viscous inviscid compressible incompressible steady unsteady laminar turbulent single-phase multiphase
The reliability of CFD
The reliability of CFD simulations is greater
• for laminar/slow flows than for turbulent/fast ones
• for single-phase flows than for multi-phase flows
• for chemically inert systems than for reactive flows
How CFF makes predictions ?
How does CFD make predictions?
CFD uses a computer to solve the mathematical equations for the problem
at hand. The main components of a CFD design cycle are as follows:
• the human being (analyst) who states the problem to be solved
• scientific knowledge (models, methods) expressed mathematically
• the computer code (software) which embodies this knowledge and
provides detailed instructions (algorithms) for
• the computer hardware which performs the actual calculations
• the human being who inspects and interprets the simulation results
CFD is a highly interdisciplinary research area which lies at the interface of
physics, applied mathematics, and computer science
CFD analysis process
CFD analysis process
- Problem statement information about the flow
- Mathematical model IBVP = PDE + IC + BC
- Mesh generation nodes/cells, time instants
- Space discretization coupled ODE/DAE systems
- # Time discretization algebraic system Axb
- Iterative solver discrete function values
- CFD software implementation, debugging
- Simulation run parameters, stopping criteria
- Postprocessing visualization, analysis of data
- Verification model validation / adjustment
