| By Nicole Diana, FLUENT Product Manager
View the pdf of this article
The fall release of FLUENT 6.2 promises many exciting new features and
capabilities. It focuses on areas such as dynamic mesh, multiphase flows,
acoustics, reacting flows, rotating equipment, and heat transfer and phase
change. Application areas include automotive in-cylinder flows, positive displacement
pumps, store separation and missile launch, bubble column reactors,
fluidized bed reactors, flow-induced noise prediction, turbomachinery,
fuel cells, underhood flows, and automotive climate control.
Velocity vectors are shown for a 2D rotary engine simulation, solved using the
dynamic mesh model
Dynamic mesh
While the dynamic mesh model has served as the foundation for automotive
in-cylinder applications, it has been widely used for other industrial
applications as well, ranging from chemical process to biomedical. Dynamic
hanging node adaption, new in FLUENT 6.2, allows for more accurate solutions
with lower overall cell counts. The remeshing and layering capabilities
have been expanded, and include the use of symmetry boundary conditions
with local re-meshing, and a more flexible “2.5-D” remeshing technique
that is particularly useful for simulating certain types of pumps. A new built-in
six degrees of freedom solver is also available for applications with
unconstrained motion, including store separation, missile launch, and tank
sloshing. Parallel scalability has also been improved.
Multiphase flows
A major new improvement for multiphase systems is the ability to include
species transport and homogeneous reactions with all multiphase models,
including the Eulerian and VOF models. For heterogeneous reactions, the
reaction rates must be specified in a user-defined function (UDF); however,
the stoichiometry is easily specified in the graphical user interface (GUI).
The Reynolds stress turbulence model can now be used in conjunction with
the Eulerian model for the prediction of highly swirling multiphase mixtures.
Several enhancements have been added to the granular model to produce more realistic results in certain circumstances. The granular model
is now available with the mixture model, providing an economical alternative
to the full Eulerian model when appropriate.
Several improvements have been made to the VOF model. A new variable
time-stepping scheme provides improved robustness and efficiency
for cases where the velocity changes significantly with time, as with tank
sloshing and filling. An interface-capture scheme provides more efficient
convergence for free surface problems in which the final steady state solution
is more important than intermediate solutions, as is common in certain
ship hydrodynamics applications. An open channel boundary
condition and support for inviscid flows further improves FLUENT’s capabilities
for marine applications.
A new wall film model has been added to FLUENT’s suite of spray models.
It is particularly important for in-cylinder combustion simulations. New
tracking schemes, automated tracking scheme selection, and error-controlled
time-step adaption result in more accurate and efficient particle tracking
with the discrete phase model.
Fischer-Tropsch synthesis in a bubble column, used to produce liquid hydrocarbons
from carbon monoxide and hydrogen, is modeled using the Eulerian multiphase
model with chemical reactions; the liquid volume fraction (top) and bubble velocity
vector field, colored by the dissolution rate of H2 (bottom) are shown
Prominent noise sources on a sedan,
computed using the acoustics
capabilities in FLUENT
Acoustics
FLUENT 6.2 provides an unparalleled suite of acoustics capabilities. The
Ffowcs-Williams Hawkings (FW-H) model that was introduced in FLUENT
6.1 has been extended to handle rotating surfaces, so that fan noise can
be modeled. When used in conjunction with rotating reference frames, fan
noise predictions can be obtained based on a steady-state simulation, providing
an efficient and powerful tool. The FW-H model is now also available
with the coupled solver. A new set of broadband source models allows
acoustic sources based on the results of steady-state simulations to be estimated.
These models are practical tools for evaluating design modifications.
Reacting flows
FLUENT 6.2 includes models for diesel autoignition and gasoline spark ignition.
(See the article on page 23.) A new stiff chemistry algorithm for the
segregated solver is available for laminar flows. When combined with ISAT,
laminar finite rate chemistry problems can be solved more quickly and efficiently.
Several enhancements have also been added to the NOx model. Surface
chemistry mechanisms can be imported in the Surface CHEMKIN format.
Another useful new feature is the ability to create custom material databases.
This allows material properties and reaction mechanisms that have
been specified in one case to be used again in a different case. PrePDF has
been completely integrated into FLUENT 6.2. In addition to improved usability,
the integration has also resulted in reduced computation times.
Rotating equipment
A new sliding mesh model is available in FLUENT 6.2 that addresses the
robustness issues associated with earlier versions of the model. The improvements
also apply to non-conformal interfaces in general. Of particular interest
to turbomachinery users is automated full multigrid initialization. This
feature allows an initial solution field to be computed so that a higher Courant
number can be used with the coupled implicit solver, resulting in faster overall
convergence. Non-reflecting boundary conditions are now supported
for unsteady flows.
Heat transfer and phase change
A new solar load model has been implemented in FLUENT 6.2. The solar
load model allows the radiant energy transport between the sun and a wall
surface to be solved, based on the intensity and position of the sun. This
model is important for automotive and building climate control applications.
For the discrete ordinates (DO) model, a second order discretization option
is now available for obtaining more accurate results, particularly for cases
with high optical thickness. A UDF can be used to customize the absorption
coefficient for each band with the gray-banded DO model. Several improvements
have also been made to the surface-to-surface radiation model, including
the ability to specify the number of faces per surface cluster for different
boundaries, allowing fine clustering to be used only where it is needed.
The result is a more accurate solution without the memory penalty associated
with using fine clustering everywhere.
A wet steam model has been implemented in FLUENT 6.2. Two-phase
flow in steam turbine applications is one area that will benefit. Cases with
shell conduction no longer require encapsulation in parallel; this significantly
improves calculation efficiency. Several enhancements have also been made
to both the cavitation and heat exchanger models.
Solver improvements
Several enhancements to the numerics in FLUENT 6.2 result in improved
solver accuracy and efficiency. For the segregated solver, an improved Rhie-Chow interpolation technique has been implemented that can significantly
improve accuracy on tetrahedral meshes. In addition, a new, locally 3rd
order discretization scheme has been implemented for both the segregated
and coupled solvers.
Time-dependent flows
FLUENT 6.2 contains some significant improvements in non-iterative transient
solution methods. A non-iterative PISO solution scheme is now available,
and an implicit fractional step method has been implemented for
incompressible and mildly compressible flows. Both methods result in significant
speed-ups compared with the iterative transient solution method.
Neither, however, can be used with dynamic mesh, sliding mesh, reacting,
or Eulerian multiphase flows.
Some improvements have also been made to the coupled solver that
result in better accuracy along with a speed-up in the time required per
iteration. Additional modifications were made for unsteady cases that result
in improved accuracy and convergence for the coupled solver when implicit
time-stepping is used. In particular, the coupled implicit solver now captures
pressure wave propagation with accuracy similar to the coupled explicit
solver. Further, the number of iterations required for these problems is significantly
reduced.
Turbulence
In the area of turbulence modeling, there is an effort underway to improve
the large eddy simulation (LES) capability. FLUENT 6.2 includes several new
sub-grid scale viscosity models, including two dynamic models that improve
the quality of LES predictions. To help accelerate
convergence, an LES simulation can be launched
from a steady-state RANS solution using turbulence
synthesization. Two new methods for specifying
the stochastic velocity boundary condition at inlets,
more rigorous than previous offerings, are now available.
The usability of the detached eddy simulation
(DES) model has also been enhanced.
Solids distribution in an unbaffled stirred tank containing
four LIGHTNIN A310 impellers, solved using the Eulerian
granular multiphase and Reynolds stress models
And more…
FLUENT 6.2 also includes add-on modules for
proton-exchange membrane (PEM) fuel cells and
solid oxide fuel cells (SOFC). The modules will be
included on the FLUENT 6.2 CD, but a special license
will be required to activate them.
There are several other enhancements in
FLUENT 6.2, including substantial improvements
to the speed of reading cases files into the parallel
version of FLUENT, new particle animation capabilities,
the ability to display multiple monitors in
a single graphics window, and more.
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