Computational Fluid Dynamics, known today as CFD, is defined as the set of
methodologies that enable the computer to provide us with a numerical simulation of
fluid flows.
We use the word ‘simulation’ to indicate that we use the computer to solve numerically
the laws that govern the movement of fluids, in or around a material system,
where its geometry is also modeled on the computer. Hence, the whole system is
transformed into a ‘virtual’ environment or virtual product. This can be opposed to
an experimental investigation, characterized by a material model or prototype of the
system, such as an aircraft or car model in a wind tunnel, or when measuring the flow
properties in a prototype of an engine.
This terminology is also referring to the fact that we can visualize the whole system
and its behavior, through computer visualization tools, with amazing levels of realism,
as you certainly have experienced through the powerful computer games and/or movie
animations, that provide a fascinating level of high-fidelity rendering. Hence the
complete system, such as a car, an airplane, a block of buildings, etc. can be ‘seen’
on a computer, before any part is ever constructed.
I.1 THE POSITION OF CFD IN THEWORLD OF VIRTUAL PROTOTYPING
To situate the role and importance of CFD in our contemporary technologicalworld, it
might be of interest to take you downthe road to the globalworld of Computer-Assisted
Engineering or CAE. CAE refers to the ensemble of simulation tools that support
the work of the engineer between the initial design phase and the final definition of
the manufacturing process. The industrial production process is indeed subjected to
an accelerated evolution toward the computerization of the whole production cycle,
using various software tools.
The most important of them are: Computer-Assisted Design (CAD), Computer-
Assisted Engineering (CAE) and Computer-Assisted Manufacturing (CAM) software.
The CAD/CAE/CAM software systems form the basis for the different phases
of the virtual prototyping environment as shown in Figure I.1.1.
This chart presents the different components of a computer-oriented environment,
as used in industry to create, or modify toward better properties, a product. This
product can be a single component such as a cooling jacket in a car engine, formed
by a certain number of circular curved pipes, down to a complete car. In all cases the
succession of steps and the related software tools are used in very much similar ways,
the difference being the degree of complexity to which these tools are applied.
The Definition Phase
The first step in the creation of the product is the definition phase, which covers
the specification and geometrical definition. It is based on CAD software, which
allows creating and defining the geometry of the system, in all its details. Typically,
large industries can employ up to thousands of designers, working full time on CAD
software. Their day-to-day task is to build the geometrical model on the computer
screen, in interaction with the engineers of the simulation and analysis departments.
This CAD definition of the geometry is the required and unavoidable input to the
CFD simulation task.
Figure I.1.2 shows several examples of CAD definitions of different models, for
which we will see later results of CFD simulations. These examples cover a very wide
range of applications, industrial, environmental and bio-medical
Figure I.1.2a, is connected to environmental studies of wind effects around a block
of buildings, with the main objective to improve the wind comfort of the people
walking close to the main buildings. To analyze the problem we will have to look at
the wind distribution at around 1.5m above the ground and try to keep these wind
velocities below a range of 0.5–1.0 m/s. Figure I.1.2b shows a CAD definition of an
aircraft, in order to set up a CFD study of the flow around it.
Figure I.1.2c is a multistage axial compressor, one of the components of a gas
turbine engine. The objective here is to calculate the 3D flow in all the blade rows,
rotors and stators of this 3.5 stage compressor, simultaneously in order to predict the
performance, identify flow regions generating higher losses and subsequently modify
the blading in order to reduce or minimize these loss regions.
Figure I.1.2d, fromVan Ertbruggen et al. (2005), is a section of several branches of
the lung and the CFD analysis has as objective to determine the airflow configuration
during inspiration and to determine the path of inhaled aerosols, typical of medical
sprays, in function of the size of the particles. It is of considerable importance for
the medical and pharmaceutical sector to make sure that the inhaled medication will
penetrate deep enough in the lungs as to provide the maximal healing effect. Finally,
Figure I.1.2e and f show, respectively, the complex liquid hydrogen pump of theVULCAIN
engine of the European launcher ARIANE 5 and an industrial valve system,
also used on the engines of the ARIANE 5 launcher. A CFD analysis is applied in
both cases to improve the operating characteristics of these components and define
appropriate geometrical changes.
I.1.2 The Simulation and Analysis Phase
The next phase is the simulation and analysis phase, which applies software tools
to calculate, on the computer, the physical behavior of the system. This is called
virtual prototyping. This phase is based on CAE software (eventually supported by
experimental tests at a later stage), with several sub-branches related to the different
physical effects that have to be modeled and simulated during the design process. The
most important of these are:
• Computational Solid Mechanics (CSM): The software tools able to evaluate
the mechanical stresses, deformations, vibrations of the solid parts of a system,
including fatigue and eventually life estimations. Generally, CSM software will
also contain modules for the thermal analysis of materials, including heat conduction,
thermal stresses and thermal dilation effects. Advanced software tools
also exist for simulation of complex phenomena, such as crash, largely used in
the automotive sector and allowing considerable savings, when compared with
the cost of real crash experiments of cars being driven into walls.
• Computational Fluid Dynamics (CFD): It forms the subject of this book, and
as already mentioned designates the software tools that allow the analysis of
the fluid flow, including the thermal heat transfer and heat conduction effects
in the fluid and through the solid boundaries of the flow domain. For instance,
in the case of an aircraft engine, CFD software will be used to analyze the flow
in the multistage combination of rotating and fixed blade rows of the compressor
and turbine; predict their performance; analyze the combustor behavior, analyze
the thermal parts to optimize the cooling passages, cavities, labyrinths, seals and
similar sub-components. A growing number of sub-components are currently
being investigated with CFD tools; while the ultimate objective is to be able to
simulate the complete engine, from compressor entry to nozzle exit. An example
of a complex simulation of a cooled gas turbine blade is shown in Figure I.1.3.
In this simulation, the external flow around the cooled turbine interacts with
the cooling flow ejected from the internal cooling passages. You can observe
the very complex three-dimensional flow, which is affected by the secondary
vortices, connected to the presence of the end-walls and by the tip clearance
flow at the upper blade end.
• Other simulation areas related to specialized physical phenomena are also currently
applied and/or in development, such as Computational Aero-Acoustics
(CAA) and Computational electromagnetics (CEM). They play an important
role when effects such as reduction of noise or electromagnetic interferences
and signatures are important design objectives.
I.1.3 The Manufacturing Cycle Phase
In the last stage of the process, once the analysis has been considered satisfactory and
the design objectives reached, the manufacturing cycle can start. This phase will
attempt to simulate the fabrication process and verify if the shapes obtained from the
previous phases can be manufactured within acceptable tolerances. This is based on
the use of CAM software. This area is in strong development, as a growing number
of processes are being simulated on computer, such as Forging, Stamping, Molding,
Welding, for which appropriate software tools can indeed be found.
With the exploding growth of the computer hardware performance, both in terms
of memory and speed, industrial manufacturers expect to simulate, in the near future,
a growing number of design and fabrication processes on computer, prior to any prototype
construction. This concept of virtual product associated to virtual prototyping
is a major component of the technological progress, and it has already a considerable
impact in all areas of industry. This impact is prone to grow further and to become a
key-driving factor to all aspects of industrial analysis and design.