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Broad Front
CFD as a Tool to Support Development
New, innovative CAD and CAE systems have often helped what were previously exclusive high-end development technologies
to break through into the mainstream. This trend has been evident for some time in the shift of routine structural
mechanics calculations to the developer’s workstation.
Dr. Ivo Weinhold, Product Manager, NIKA GmbH
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Calculation of temperatures in an electronic device
Picture: NIKA GmbH
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It is interesting that the successful CAD and CAE systems in widespread industrial use are not reduced-function versions
of established high-end programs. They are always new products that combine innovative technologies and concepts with tried
and tested scientific and technical principles. As the core functions of the vast majority of products and methods are
physically based on or affected by fluid or thermodynamic processes in some way, simulation of these processes will follow
the same trend and increasingly establish itself a routine tool in most development departments. With programs like EFD.Lab
from NIKA, the first steps have already been taken.
Flow Is Difficult to Understand
It is obvious that, by their very nature, flow and heat transfer processes are much more difficult to understand and to model
than issues involving structural mechanics, for example. As a result, the corresponding simulation programs must provide
additional benefits to achieve widespread use. Essentially, this means that the specific knowledge held by calculation experts
has to be integrated into the software, ensuring that users can obtain usable results solely on the basis of their knowledge
of their own specific discipline. The specific knowledge we are dealing with relates to discretisation methods, grid generation
and numerical resolution of differential equation systems – things that have nothing to do with the engineer’s actual
technical tasks.
Of course, a simulation program cannot close gaps at an engineering level in terms of understanding of physical relationships,
appropriate delineation of tasks or evaluation of results. This is, and will remain, the responsibility of the engineer as
the user of the software. However, the reasons why flow simulations remain very complicated and time consuming are certainly
not to be found at this technical level. Rather, it is the conceptual inadequacies of many CFD programs that are still based
on 1980s technology and require extensive specialist knowledge from other disciplines to operate. As long as this basic
technology remains, improvements to the details, such as user interfaces tailored to specific tasks, reduction of functions
or formal links to CAD systems, are not much help in resolving the situation.
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Flow paths with speed represented in colour
Picture: NIKA GmbH
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Speed distribution on the walls
Picture: NIKA GmbH
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CFD – Keeping Pace with Design
The key lies in eliminating the true causes of the difficulties and allowing the engineer as a user to concentrate on his
technical and physical issues, which are often extremely complex in their own right. This is the only way to make flow simulations
efficient enough to deliver usable physical results in an appropriate timeframe. The acceptable timeframe is determined
directly by the frequency of design modifications in the development process. In other words, usable simulation results based
on one design version must be available sufficiently quickly to allow the findings to be incorporated into the next round of
modifications. This is the critical criterion for practical simulation alongside development in industry.
To actually enable flow simulations to be used as a development-based tool in practice, certain minimum technical requirements
must be met. These are the result of a simple analysis of the most important problems with traditional flow calculations and
represent the crucial “brake on productivity”.
In the age of digital engineering, the initial version of the geometry to be analysed normally exists as a complex 3D CAD data
record, which never includes the flow space necessary for grid generation as a separate geometry.
Handling the Geometry
EFD.Lab includes a powerful pre-processor, which provides import interfaces for original data from all major 3D CAD systems
and interfaces for universal standard formats. Users can also create their own components and assemblies using the full
functionality of a modern parametric volume modeller. One particularly attractive feature is that the initial version of
imported components and assemblies can also be directly modified and supplemented for subsequent alternative analyses and
then restored to its original format at the end and returned to the original system for direct processing.
EFD.Lab is based on geometry not on grids. This means that the definition of all necessary input data relates exclusively
to the “mechanical” 3D CAD geometry and not to a negative model of the flow space, which includes the calculation grid.
This concept is extremely productive in a multi-CAD environment or when working with data from different source systems.
It allows proper engineering methods to be used and makes modelling much easier to understand. EFD.Lab automatically
identifies the individual calculation areas for grid generation.
Grid Generation
In traditional CFD methods, grid generation is by some distance the most time consuming step. Automation of the process
brings major efficiency gains. It is very important that an automatic grid generator does not simply generate just any grid
at the push of a button, but takes into account the specific requirements of a flow calculation. EFD.Lab provides its own
automatic grid generator, which is based on NIKA’s Rectangular Adaptive Mesh (RAM) technology and works for even very
complex geometry with little or no additional input by the user. The result is a CFD grid that uses a numerically favourable
cell form and features adaptive local refinement to allow analyses that make efficient use of resources.
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Air flow in an air conditioned office
Picture: NIKA GmbH
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Convergence of Calculation
Even the fastest calculator is no use if an overnight calculation does not provide usable results due to a lack of
convergence and time consuming adjustment of the grid and the calculation settings are required. The numerical stability
of the calculator was therefore a high priority in the development of EFD.Lab. It is based on some fundamental technologies
developed by NIKA. Even under difficult conditions relating to the defined model, they provide a usable solution from the
first calculation, without the introduction of any measures that reduce quality, such as pure numerical diffusion. These
technologies include:
- The Rectangular Adaptive Mesh technology mentioned above
- Modified Wall Functions (MWF), which are based on an empirical approach developed by NIKA
- The LTTM automatic turbulence modelling system, which allows simulation of the physical flow characteristics
and their effects over the entire range of Reynolds numbers that is of interest from an engineering perspective
using a single model
- The ACC convergence control system.
Evaluation and Documentation of Results
The work involved in evaluating simulation results and the subsequent derivation, implementation and testing of
design modifications is often underestimated. Every simulation result must be subjected to a plausibility and accuracy
test by the engineer. EFD.Lab provides a variety of graphical and numerical evaluation functions closely related to
the original 3D geometry. This means that graphical representations are very clear and can often be easily understood
even by non-technical staff. A direct link to familiar Office applications for numerical evaluation in tables and
charts or to generate an automated results report enables presentation-ready documentation to be produced extremely
efficiently.
Integration into Development Processes
The trend towards using digital product data for the entire development and life cycle of a product includes
flow simulations. Regardless of their complex physical nature, the challenge for software developers is to realise
this in practice for professional users. One thing is clear: the physical issues faced by the user, the complexity
of the models and the requirements in terms of the efficiency of the calculation and the quality of the results do
not depend on the size of the company or the frequency of use. It is ultimately up to software manufacturers to
achieve what at first glance appears impossible: providing CFD software that is easier to use than high-end systems
yet has comparable features and the same quality of results at drastically reduced cost – allowing flow simulations
to be performed with sufficient technical and economic efficiency to support practical PLM integration.
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» Special edition from Digital Engineering Magazin 3/2003 (PDF, 2.047KB)
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