4.2 Numerical Methods and Software Engineering
- Finite Element Method in Unstructured Meshes (G. Chevalier, F. Ducros, B. Marquez)
- Meshing techniques (J. Bohbot, J.-Ch. Jouhaud, M. Montagnac, D. Darracq)
- Spalart-Allmaras (H. Pascal-Jenny)
- MAEVA (H. Pascal-Jenny)
- Development and implementation of a wake vortex model in
flight simulators (H. Moet, G. Grondin, D. Darracq)
- Software engineering (J. Bohbot, S. Champagneux, J.-Ch. Jouhaud,
Ph. Piras)
- Object Oriented CFD with elsA (M. Montagnac, G. Chevalier, S. Champagneux,
J.-Ch. Jouhaud)
- CFD Team Support (S. Champagneux, M. Labadens, F. Dabireau, V. Roche)
4.2.1 Finite Element Method in Unstructured Meshes (G. Chevalier, F. Ducros, B. Marquez)
The work on that field has followed three axis.
- first a work on the Domain Decomposition and preconditioning,
with firstly the implementation of the ILDU method in an official
version of AETHER, and secondly other improvements such as introduction of
explicit Runge-Kutta scheme, variables interpolation, LES
modeling. All these developments have been directly delivered to
Dassault Aviation in an official version of AETHER;
- second a work concerning the modeling relative to the LES of
complex unsteady flows. This work
has involved the development and validation of two
formulations of wall functions, validated on standard periodized
channel flow. CERFACS has also collaborated with Dassault Aviation through
the LESFOIL EC program, sharing expertise and data on the subject
([Ducros, 2000]);
- following the work on the preconditioning, a study has been
conducted on the influence of the node numbering on the robustness
and efficiency of the preconditioning. This was done on different
types of preconditioning.
4.2.2 Meshing techniques (J. Bohbot, J.-Ch. Jouhaud, M. Montagnac, D. Darracq)
Grid generation is a crucial problem for the computation of complex
aircraft configurations using a body fitted structured code.
Furthermore, due to the data management of structured grids, the local
refinements around the CAD (boundary layers, stagnation lines, wakes)
will be propagated through the whole grid even in zones where
gradients are expected to be low. This can lead to very large grids,
especially for complex geometries. Two approaches permit to
reduce this drawback: i) conservative patched grid, ii) automatic mesh refinement.
CERFACS is developing these techniques in order to help Airbus France in
reducing the turnaround time and in
enhancing the flow accuracy on a given grid size.
These two meshing techniques have been implemented and validated in NSMB.
Conservative Patched Grid (CPG) (M. Montagnac)
The purpose is to develop an efficient way to simplify the grid
generation and to deal with complex configurations using moderately
sized grids. The approach has been chosen to use NSMB on meshes
having no coincident interfaces. For this kind of meshes, blocks must
have common interfaces but do not need the same location of grid
nodes. This approach avoids mesh propagation from a block to
another block. The flexibility of this kind of mesh allows mesh
refinement and makes it
easier to cluster grid points.
A 3-D conservative patched grid algorithm using Jameson centered
scheme has been implemented in NSMB. The Multigrid acceleration
convergence technique has been extended when using patched grids
strategy. The parallel programming has been implemented to maintain
the CPU performance of the NSMB code. The efficiency of the method
has been demonstrated on complex geometry. The wake vortices behind a
civil aircraft (DLR-F11) have been calculated on ten processors using a
grid containing 3 millions nodes with non-coincident interfaces
(Fig.4.3).
These CPG techniques were also developped in elsA as a
product of the NSMB/elsA tightening.
Full totally non-coicident patched grid techniques for 3D
turbulent flows were implemented in the elsA code. Special
care was also taken to ensure the use of these techniques when
computing on parallel computers. An example is shown in the
Fig.4.4 for a 2D turbulent application : the RAE 2822 test case.
Adaptive Mesh Refinement (AMR) (J.-Ch. Jouhaud, M. Montagnac)
Another approach is to enrich the mesh with a hierarchical grid
structure, that is to say using an Adaptive Mesh Refinement Strategy
(AMR). It consists in splitting a cluster of grid cells in a
characteristics regions (shocks, boundary layers, wake vortices, ...)
and to automatically join these cells in new blocks. The activity on
AMR has increased over the period with a new algorithm to improve the method.
In particular, a parallel implementation has been realized. In the
next phase, extension
to viscous flows is planned.
The efficiency of the method has been demonstrated on a complex
geometry. The wake vortices behind a civil aircraft (F11) have been
calculated on ten processors
using a grid containing 1.5 millions nodes with two levels of refinement.
The most recent investigations were realized within the elsA tool.
Two major objectives were reached. Firstly, the extension of the AMR
method to turbulent transonic flows. Secondly, the addtion of local
time increment prolongations, as in standard multigrid strategies, in
order to increase the convergence rate. The RAE airfoil and the AS28
wing test cases have shown the efficiency of this new AMR extension
(Fig. 4.5).
Figure 4.3: Flow simulation behind a civil aircraft during the landing configuration. Application of the AMR technique.
Figure 4.4: elsA/Patched grid technique applied to the RAE2822 test case
Figure 4.5: elsA/AMR technique applied to the AS28G airfoil
4.2.3 Spalart-Allmaras (H. Pascal-Jenny)
In the framework of 'elsA-NSMB' convergence activities, the
update of the one equation turbulence model functionnalities for a
production-level use has been addressed during year 2001. This topic
follows the work previously realized for the algebraic turbulence
model of Baldwin-Lomax. The LU-SSOR implicit algorithm extension to
the well-known Spalart-Allmaras (SA) turbulence model has been
transfered from the NSMB code to the elsA software. Specific
treatments (unknown's positivity monitoring, parallelism, ...) have
also been implemented leading to a robust and efficient solver for the
computation of turbulent flows over full aircraft configurations. The
extension of the multigrid algorithm to the SA model is in progress
and will be finalized during the beginning of year 2002. These
activities are part of a contract with Airbus France with the goal of
providing the same level of efficiency with both softwares in order to
switch from one to the other in a transparent way inside the design
process.
4.2.4 MAEVA (H. Pascal-Jenny)
During year 2001, CERFACS has seen a growing involvement in the
developpement of the elsA software through the MAEVA project
(Modelisations Aerothermiques des Ecoulements en Ventilation Avion)
between Airbus France and ONERA. Indeed, CERFACS has been in
charge of implementing a new turbulence model
dedicated to strongly thermaly coupled flows (impinging jets for
instance), namely the (k, e, v'2, f) model of Durbin.
This model is described by 3 additionals transport equations (k,
e, v'2) and another one for f which is elliptic. An
explicit version of the model has been implemented in the
elsA software this year and validations are in progress.
Extensions are scheduled for the year 2002.
4.2.5 Development and implementation of a wake vortex model in
flight simulators (H. Moet, G. Grondin, D. Darracq)
In the framework of the S-Wake program, the CERFACS is involved in the
safety aspects of
aircraft wake vortices.
An engineering wake vortex behaviour model (called VORTEX)
developed with the purpose to do probabilistic investigations has been
improved by integrating several decay models which govern the decay of
circulation due to turbulence, stratification, viscosity and
crosswind. The effect of crosswind on wake vortices has been
investigated in order to incorporate the effect properly in the VORTEX
model. A new decay model accounting for aircraft parameters and
atmospheric turbulence characteristics has been developed at
CERFACS
[Möet, 2001].
An analytical model has been developed
which is integrated in industrial (Airbus Deustschland) and research
(TU-Berlin, NLR, TU-Braunschweig) flight simulator environments to
perform parametric studies for wake vortex encounters. These
parametric studies (including variation of vortex parameters) have the
purpose to obtain the safety hazard for an encounter, to obtain
handling criteria and to determine the influence of cockpit motion on
piloted simulations. Furthermore, a numerical model is developed which
uses a more realistic vortex velocity distribution is used for the
same purposes as the analytical model and to simulate a more realistic
wake vortex encounter and to investigate the effects of 'low-vortex'-
designs of future very large transport aircraft.
4.2.6 Software engineering (J. Bohbot, S. Champagneux, J.-Ch. Jouhaud,
Ph. Piras)
Dealing with the CFD code NSMB, the CERFACS is more than ever considered
as the official provider as much from the Airbus France point of view
as the one of EADS Launch Vehicles . Integration, management and validation
processes have consequently been improved in order to face this
responsibility in a reliable way (control versioning with both CVS
(integration) and SCCS (development), testing over a wide range a
representative test cases). Transferring, optimizing and benchmarking
the CFD code NSMB on various parallel platforms have also been
carried out and continued, motivated by both Airbus France as well as
EADS Launch Vehicles for the renewal of their own computing facilities. Algorithm
performance improvements and various optimizations have been realized
leading to an increased efficiency of the code both on vector and RISC
architectures. Finally, the harmonization between computational tools
coming from CERFACS and Airbus France has been continued with the
delivery and the use of new software (JEDM, GAME, LAMA3D) so that
developers at CERFACS can work within an industrial environment in order
to reduce the cost of the know how transfer when a development is
completed.
4.2.7 Object Oriented CFD with elsA (M. Montagnac, G. Chevalier, S. Champagneux,
J.-Ch. Jouhaud)
In the year 2000 the collaboration with ONERA has started on the
object-oriented code named elsA ensemble logiciel de
simulation Aérodynamique. The elsA software comes along
with procedures to enhance productivity in a multi-user and
multi-platform environment: validation database, unitary test cases,
cvs management tools, software quality program. This work takes fully
part of the framework wanted by Airbus France for the harmonization
between NSMB and elsA.
The CERFACS elsA team was in charge of the conception and the
development of the implicit time-integration LU-SSOR method as well as
of the Baldwin-Lomax algebraic turbulence model with generalized
distances. It has also contributed to write the specification and the
user guide manuals. Some validation and unitary tests have been
integrated in the corresponding databases too.
All developments have been tested on usual cases as 2D-3D multi-blocks
sequential or parallel square nozzle, flat plates, NACA and RAE
profiles. These developments were integrated in the official version
of elsA.
The code has been installed on FUJITSU VPP700 and VPP5000 vector
machines to run the S3CH configuration (see Fig.4.6) for
Airbus France.
Figure 4.6: Cp distribution around the S3CH (wing+pylon+nacelle) configuration
4.2.8 CFD Team Support (S. Champagneux, M. Labadens, F. Dabireau, V. Roche)
This year, the CFD Team was involved in improving its communication by
redesigning its website
[Roche, 2000]
(http://www.cerfacs.fr/cfd). The most important work
consisted in updating information about the team that now fit its
current structure. A wide modernization has been conducted to make it
more attractive, simpler to navigate and easier to maintain. A
specific effort has been made to rationalize the management of the CFD
Team bibliography that now consists of more than 600 references from
year 1986 to 2001 from which some are available on-line.
|
|
