CERFACS CERFACS Scientific Activity Report Jan. 2010 – Dec. 2011

3 Turbomachinery
The turbomachine group is the most recent component of the CFD team and gathers around 12 people. The
main effort of the group has been put on the development and the application of methods to simulate
unsteady flows in compressor and turbine components. Recent breakthroughs include the successful
implementation of the harmonic balance method in elsA and the application of LES (both with elsA
and AVBP) to address complex flow phenomena (such as shock/boundary layer interaction and laminarturbulent
transition) in turbomachine configurations. Another promising work is the code coupling activity :
coupling methods are developed in cooperation within the CFD team (COMB and AAM groups) and with
the GLOBC team (which develops the code coupling tool Open-PALM). The target is to prepare techniques
for the 2020 challenge ”COUGAR” (for which the objective is to perform an unsteady flow simulation in
a whole gas turbine). The work of the turbomachinery group also led to industrial applications, supported
by SAFRAN, Airbus and EDF. The group is involved in European Projects (e.g. COPA-GT, a Marie-Curie
program, and FACTOR). The cooperation with research partners (VKI, LMFA, ONERA) has also been
strengthened through the co-direction of Phd students and the definition of common test cases (CREATE,
LS89, etc.).
3.1 Numerical methods and software
The development of numerical methods for turbomachinery focuses on three main topics : the reduction
of the computational cost for unsteady flows, the quantification of the solution to unknown/uncertain
parameters and the code coupling activity.
3.1.1 Spectral methods : development of the Harmonic Balance Technique
(T. Guedeney)
Some flows are controlled by a strong periodic forced activity : for turbomachinery flows for example,
developing a method which includes this information allows computations that are more efficient than
classical unsteady CFD approaches (Dual Time Stepping, etc.). The Time Spectral Method (TSM) has been
implemented in the code elsA following this philosophy. Thanks to Fourier spectral analysis, the unsteady
Navier-Stokes equations can be read as 2N + 1 steady problems coupled by a source term (with N the
number of harmonic of a frequency given by the user). The phase-lagged functionality [7] has been also
implemented in the spectral method to reduce the computational domain to only one single blade passage
per blade row.
The TSM is able to deal with only one frequency and its harmonics (for example the blade passing
frequency) and is therefore limited to a single stage computation. To extend the method to multistage
turbomachines, it is mandatory to take into account several frequencies which are not necessarily multiples
of a base frequency. This new method is called the Harmonic Balance Technique (HBT). To implement
the HBT, all features developped for TSM were adapted to a multifrequential formulation. However, while
the position of the time instants with TSM was simply equally distributed along the flow period, the same
approach with HBT led to serious conditioning problems, especially for multifrequential test cases. To
circumvent this issue, different methods such as the APFT algorithm (which stands for Almost Periodic
Fourier Transform) or genetic algorithms have been implemented. The development of the HBT is now
completed and the validation step has already started [CFD42].
146 Jan. 2010 – Dec. 2011