Integrating CFD into the product development process - Industrial ...

Siemens PLM Software
Integrating CFD into the product
development process
www.siemens.com/nx
w h i t e p a p e r
Maximizing the business impact of technology requires far more than just buying the
right point functionality and handing it off to the analysis department. Digital
simulation needs to be at the core of every PLM business process because it enables
management to make faster, more informed decisions. Overall, increasing use of
digital simulation leads to better products that are more marketable, have better
performance and higher margins, all of which directly benefits the bottom line.
Siemens PLM Software

Integrating CFD into the product development process
Table of contents
Executive summary 1
Introduction 2
NX Flow for CFD 3
NX Flow technology 5
Integration with the product
development process 8
Summary 9

Executive summary
Computational fluid dynamics (CFD) allows engineers to numerically solve
very complex analyses that describe and predict the flow of air, gases or
liquids around or through a body or within an enclosed system. Analyzing the
flow patterns and pressure regulation of fluid within an automotive
transmission and the flow of air within an HVAC system are two typical
examples of CFD applications.
Traditionally, CFD has been considered a tedious and time-consuming
process performed exclusively by PhD specialists and, in many industries, not
part of a company’s mainstream product development strategy. More
recently, computer-aided engineering (CAE) has taken on a greater level of
importance within the product lifecycle management (PLM), and the
increasing use of CAE is being credited for the development of higher
quality, more marketable products. In turn, these innovative products feature
much increased performance and profit margins, all of which directly
benefits a company’s bottom line. As such, varying types of CAE analyses –
including CFD – are moving further upstream in the digital product
development process.
This paper describes the benefits of incorporating CFD as well as coupled
thermal/flow solutions into an integrated PLM environment and specifically
discusses the capabilities of NX ® Flow software – the new design-integrated
CFD solution from Siemens PLM Software.
1

Introduction
Various surveys and articles have recently been published on the topic
of computational fluid dynamics (CFD).The consensus is that
companies are facing multiple challenges in attempting to incorporate
CFD engineering analysis earlier and throughout the product
development process.The integration of CFD, like any other
engineering analysis technology (such as thermal or structural), relies
on the ability to quickly go from a concept design based on detailed
3D geometry to a discrete representation of the simulation model,
complete with associated physical boundary conditions; and ultimately
to the performance simulation results. In addition, the iterative nature
of the product development process calls for associativity between the
3D CAD part geometry as well as the assembly data and the
simulation model representation.
Regardless of the challenges, CFD can play a very beneficial role in
product development for specific applications that are dependent on
the physics of fluid flow. More specifically, CFD can help reduce or
eliminate costly prototypes. As a prediction solution, CFD analyses are
valuable in evaluating multiple ‘what if’ scenarios in an effort to obtain
an optimal design alternative. CFD analysis can help reduce design
cycle time without compromising quality by pinpointing fluid flow and
thermo-fluid related design flaws through digital simulation early in the
design process.
2

NX Flow for CFD
NX Flow offers a comprehensive suite of CFD capabilities for high
accuracy simulation of fluid flow. NX Flow provides a complete
framework for modeling and solving complex fluid flow problems by
virtue of being fully embedded in the NX environment. It supports fully
unstructured meshes, which can be generated using the NX finite
element modeling environment or a built-in meshing tool capable of
generating skin meshes for better resolution of near-wall effects and
convective heat transfer. It also supports fully disjoint meshes across
adjoining fluid volumes. Flow blockages can be modeled using 2D or
3D solid elements, with arbitrary porosity (including orthotropic),
surface roughness and convective attributes. NX Flow can be used in
tandem with the NX Thermal product for seamless modeling of fully
coupled forced and natural convection problems, including radiation
heat transfer. Flow boundary conditions are easily modeled, including
fan curves, time-varying inlet and outlet conditions, internal fans and
fluid recirculation loops. NX Flow offers a variety of turbulence models
for fluid flow modeling, including mixing length and k-ε models; secondorder
schemes are available for momentum, energy and turbulence.
The product enables effective simulation of both steady-state and
transient fluid flow problems, with effective handling of conjugate heat
transfer problems. NX Flow features an advanced and efficient
algebraic multigrid solver, ensuring a rapid and reliable solution.
A comprehensive set of post-processing tools is provided within the
NX environment.
As a result, NX Flow is a general-purpose CFD simulation package. Its
streamlined CFD modeling tools are fully integrated into the NX
digital product development environment; therefore, NX Flow
automatically recognizes NX CAD geometry and assembly changes by
adapting the fluid domain instantaneously. By operating within the NX
CAD environment, NX Flow is fully parametric.With NX Flow, the
CFD fluid domain needs to be defined only once.The local mesh seeds
on solid parts or surfaces and the boundary layer mesh need only to
be defined once if required.
As soon as the NX Flow solver is launched, the user can continue
working with NX (the user can continue to work on the next design
or simulation task while the CFD simulation results are being
generated). A window will inform the user of solution status and
completion.The user can also bring up the solver monitor form at any
point in time to check on status of the simulation solution and various
parameters such as convergence plots, minimum and maximum
pressure, velocity and temperature.
In a Teamcenter ® managed development environment, NX Flow’s
integration with the design process brings to bear flexible and powerful
tools for multiple positioning of parts and optimizing of shapes,
allowing the creation of multiple CFD results sets to increase your
product’s performance parameters.
NX Flow is a CFD solver that computes the solution to the Navier-
Stokes equations, which are nonlinear, coupled, partial differential
equations for the conservation of fluid mass, energy and momentum in
general and complex 3D geometry. It uses an element-based finite
volume method to discretize the solution fluid domain and an algebraic
multigrid method to solve the governing equations.What does that
mean? The NX Flow application breaks up the fluid volume into a large
number of well-defined smaller elements or blocks for which the
equations of physical behavior are well understood and can be solved
in the software.
NX Flow tackles industry-specific challenges such as:
• Industrial applications: design of pipes, valves and hydraulic
components
• Automotive: cabin climate control systems, powertrain or
automotive transmission, under hood cooling, exhaust systems
• Electronics: packaging and system-level analyses, air and liquid cooled
devices
• Medical devices: liquid cooling, viscous flows, etc.
• Military and defense applications: electronics enclosures to nozzle
designs
• Aircraft and avionics systems: cabin ventilation systems, cooling
systems, etc.
3

NX Flow for CFD (continued)
NX Flow solves practical engineering challenges such as:
• Computing pressure and velocity fields within and around complex
geometries
• Optimizing the placement of critical components
• Investigating both forced and natural convection cooling schemes
• Initial sizing of wing profiles, critical car component shapes and other
components or large assemblies
• Determining drag lift or other forces and pressures acting on
complex surfaces
• Designing of air comfort systems (HVAC cabin design)
• Predicting the operating point of a cooling fan
• Positioning and sizing of fans, inlets, vents and outlets
• Establishing spacing requirements between parts
• Optimizing the size and shape of heat sinks or designing the best
fluid pipe pattern throughout a complex assembly
4

NX Flow technology
Thermal couplings for joining disjoint solid or surface
meshes
Complex assemblies of parts with dissimilar mesh can be thermally
connected through thermal couplings. Creating these heat paths from
disjoint and dissimilar meshes is totally changing the paradigms for
CFD and thermo-fluid simulation modeling.The easy-to-use modeling
technique allows the engineer to create area-proportional
conductances between dissimilar element meshes where complicated
connections or CAD modifications had to be made.The conductances
are established at runtime where a variety of different conductance
types are created from the thermal coupling parameters defined by
the user.The couplings can even be defined as varying with different
model parameters such as temperatures or heat loads determined
during the solution.
Automatically joining disjoint fluid meshes
The ability to automatically join disjoint fluid meshing at runtime is a
powerful tool for reducing meshing effort and model sizes.The
technique is conceptually simple and consists of building and meshing
two or more subassemblies or versions of the same part of an
assembly so that they share one or more fluid mesh boundaries with
the remainder of the model.There is no need for nodes to match, but
mesh density on the matching faces should be similar.Within an NX
assembly, this method allows NX part models to be changed
instantaneously. A simple toggle called “Connect disjoint meshes” is
provided within the flow solver additional options form to enable
joining the disjoint fluid meshes at runtime.This meshing technique
embedded within the solver is extremely powerful and offers
significant time-savings when trying out different cooling heat sink
designs or optimizing the shape or CAD features of a specific part
within the assembly without having to remesh the entire model.
Viscous models
The first step before running any CFD analysis solution is to select a
viscous model for the flow analysis. NX Flow features a laminar flow
model, along with a variety of turbulent models. If the model is
turbulent, you can select the mixing length (algebraic), K-epsilon
(two-equation) or fixed turbulent viscosity models and then specify
the appropriate length scales options as needed. Choosing the
appropriate viscous model will dictate the accuracy of the fluid flow
results and is an important step to understand.The NX Flow help
page, “Defining the Viscous Model”, will guide the user with the selection
of laminar over turbulent models and determine which turbulent
viscous model to use and how to specify length scales options if
needed.
Modeling fluid buoyancy and natural convection
Modeling fluid buoyancy is essential for natural convection simulation.
Fluid density affects the total gravitational force on a volume of fluid,
The value of CFD at Cisco Systems
The Network-to-Users Business Unit of Cisco
Systems faces a continual challenge of heat
dissipation requirements affecting their product
development initiatives. Physical prototyping is
expensive and testing is time consuming.With an
integrated CFD solution, Cisco’s engineers have
complete control over the analysis models and
modifications can be done quickly and efficiently.
They know from experience that by using CFD
they can come within 10 percent accuracy of
physical test models; and early prototyping and
weeks of testing can be shortened significantly.
Final physical prototypes are built and tested only
at the end of the design cycle and serve as
verification rather than design purposes. Cisco
relies on CFD 100 percent for design purposes.
5

NX Flow technology (continued)
such that spatial variations in fluid density give rise to so-called
buoyancy forces. NX Flow allows solving for model fluid buoyancy for
natural convection or mixed forced-natural convection flow
simulations.The buoyancy model simulates the effects of gravity on the
fluid using the Boussinesq approximation.
Creating surfaces that automatically convect heat to
the fluid
The flow surfaces modeling technique enables the modeling of
convection on thermal model surfaces, to or from the fluid flow
domain, as well as surface drag on the fluid. NX Flow solves the
thermal (conduction) model and the fluid flow model individually.The
solution is thermally coupled (by convection) at the solid/fluid
interface defined by flow surfaces; therefore, at least one flow surface
is required for a coupled flow-thermal model. Use flow surfaces to
model geometry faces that constrain or direct the fluid flow (such as
PC boards, baffles, ducts, chambers, walls, external complex geometry,
etc.) with or without convection. During model simulation, heat paths
or conductances are automatically established from the surfaces and
obstructions to adjacent 3D fluid flow elements.The NX Flow solver
splits the fluid mesh at the embedded flow surface, automatically
creating a 3D obstruction to the fluid flow. Moreover, the built-in
meshing tools allow for generation of skin meshes (also called
boundary layer meshes) on surfaces for better resolution of near-wall
effects such as drag and convective heat transfer.The combination of
the flow surfaces modeling technique and skin meshing capability is
providing a complete set of tools to accurately model the near-wall
effects for multiple thermo-fluid interaction applications.
Auto-convect from all surfaces to the fluid
Auto-convect allows NX Flow users to avoid unnecessary repetition
of flow surface entities (and convecting flow blockages) that have
identical surface properties.When this option is selected, all surfaces
that are meshed having thermal solid material properties (2D or 3D)
and not already defined as flow surfaces (nor any other fluid boundary
conditions), are automatically treated as flow surfaces by the NX Flow
solver.Thus, these surfaces will convect to the fluid elements they
adjoin. Similarly, all volumes that are meshed with nonfluid 3D meshes
and not already defined as flow blockages will automatically model
convection from their surfaces.Thus, their surfaces will convect to the
fluid elements they adjoin.You can define one surface property to be
used for all these convecting surfaces or specify that the default
(smooth) surface property be used for all surfaces.
The value of CFD at Noranda Technology Centre
Noranda is a leading supplier of zinc for
galvanized steel products used to build homes,
automobiles and consumer goods. Noranda uses
CFD to quickly investigate various galvanizing bath
configurations as well as flow scenarios around the
snout area.The snout is a critical location where
the steel first comes into contact with the zinc
bath. CFD enables Noranda to see how the flow
develops near the strip surface. Modeling the
boundary layer is important for understanding
diffusion of aluminum from the zinc onto the steel
surface in order to control the reaction between
the iron and zinc.
6

NX Flow technology (continued)
Automatic runtime meshing
The fluid domain can simply be picked as a general volume surrounding
all other parts without having to actually mesh the fluid volume. All that
needs to be meshed are the internal solid (nonfluid) components (using
either 2D mesh on surfaces or 3D mesh on solid components within
the fluid volume).The picking of the fluid volume is done during the
application of boundary conditions such as the inlet and outlet
conditions.The fluid mesh is then created at runtime using advanced
meshing technology. The NX meshing tools can obviously be leveraged
to mesh the fluid volume prior to runtime as well.The combination of
both options provides true fluid domain meshing flexibility and an
automatic and instantaneous way for the skilled engineer to investigate
different fluid flow scenarios by moving parts around or modifying
some features within the selected fluid domain without having to
remesh the fluid volume unnecessarily.This automatic fluid domain
meshing capability is introduced specifically for NX Flow and is a great
CFD model preparation time saver for iterative designs and CFD
scenario investigations.
Interdisciplinary interaction simulations
The coupling of thermal and structural models for conducting a
structural thermo-elastic analysis can be greatly simplified by virtue of
having the thermal and structural models sharing the same NX
environment tools and user interface, a common design database and
the same advanced simulation toolset.The coupling of thermal and fluid
flow models occurs automatically when NX Flow is used in conjunction
with NX Thermal (Siemens PLM Software’s solution for thermal
analysis). A temperature mapping algorithm can be used to map steadystate
or transient NX Flow temperature results onto another finite
element model with a different mesh.This technology allows the
transfer of thermal model results data onto an independent structural
model of similar geometry for thermal stress and distortion analysis.
Similarly, fluid pressures and forces can be mapped onto the structural
model for quasi-static or transient stress and distortion analysis.
7

Integration with the product development process
It is a common situation that companies use multiple CAE systems
from different software vendors for simulating different disciplines; a
simple example is structural and fluid simulation applications. One of
the most common (but often forgotten or taken for granted) linking of
different applications is the comparison of physical test results against
the theoretical analysis assumptions.
Sometimes these different disciplines can be treated independently, but
more often they are linked in some way.This can be achieved by using
built-in multi-physics applications, linking of solvers using message
passing interface (MPI) techniques and often customers create their
own techniques to transfer data between applications. However, in
many cases the link between applications or solvers is not so tightly
defined.
All of these situations are part of the product development lifecycle
and are considered to be PLM issues. Using NX Flow and other CAE
applications that are tightly coupled within the PLM context and NX
digital product development environment will ultimately yield
significant time and cost savings as well as innovative products.
8

Summary
The traditional “experts only” domain of CFD and coupled
flow/thermal technology is making its way into the mainstream of
digital product development.Advances in CAD integration, automated
geometry abstraction and meshing algorithms and high-performance
solvers that take advantage of the rapid advances in desktop computer
power will soon lead to a much higher utilization of flow and multiphysics
performance simulations earlier in the product design process.
This increased adoption rate for CFD tools over the next 5-10 years
will be similar to the significant advancements experienced in
mainstream structural analysis tools such as Nastran over the past
decade.
In addition, new knowledge-enabled “process-centric” user
environments for non-expert design engineers that leverage the
expertise and best practices of CAE/CFD domain experts will lead to
a much broader utilization of digital performance simulation in the
context of PLM.The business impact will be substantial to those
companies that leverage these new CAE tools as a strategic
competitive advantage in new product development.
9

About Siemens PLM Software
Siemens PLM Software, a business unit of
Siemens Industry Sector, is a leading global
provider of product lifecycle management (PLM)
software and services with 4.6 million licensed
seats and 51,000 customers worldwide.
Headquartered in Plano,Texas, Siemens PLM
Software’s open enterprise solutions enable a
world where organizations and their partners
collaborate through Global Innovation Networks to
deliver world-class products and services. For more
information on Siemens PLM Software products
and services, visit www.siemens.com/plm.
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