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

TURBOMACHINERY
of each component, the flow is not necessarily solved with the same numerical method in the whole
system. For example, previous works indicate that LES is a very efficient method for predicting reactive
flows in a combustion chamber while RANS based approaches are more convenient for turbomachines.
Based on a literature review, the first step towards integrated simulations is to develop a framework to
exchange information between the combustion chamber (LES with the unstructured code AVBP) and the
high-pressure turbine (RANS with the structured code elsA). The coupling between AVBP and elsA has
been developed using the Open-PALM code coupling tool. The technique is based on the exchange of the
conservative variables from AVBP to elsA with a method similar to the Chimera approach while elsA sends
the pressure information to define the outlet non-reflective boundary condition of AVBP. This coupling
process is done at each iteration of the flow solver until convergence is achieved. It was validated in a
simple test case : a laminar Poiseuille channel flow. The technique needs now to be validated and tested
with more complex test cases.
3.1.4 Code coupling methods : application to elsA / elsA (F. Crevel, M. Montagnac)
For some specific applications the use of different time steps can be helpful to reduce the cost of an unsteady
flow simulation. For example, to describe a surge cycle in a gas turbine, it is mandatory to simulate both high
frequencies (generated by the blade passing frequency, o(10,000)Hz) and low frequencies (due to the surge
phenomenon, o(1)Hz). A simulation with a unique time step would need very large computing resources to
describe the low frequency phenomenon. In that context, a simulation that uses a large time step in parts
where only the description of low-frequency is important, coupled with a simulation that uses small time
steps (where high-frequencies must be computed) is an attractive approach. This method has been tested
by coupling two elsA simulations (considering two different time steps) with the Open-PALM coupling tool.
The coupling developed here is based on the exchange of conservative fields on an interface defined by
the user. The simulations are time accurate, which means that if N is the ratio between the time steps
used in the two simulations, the large-time-scale simulation does one time step while the small-time-step
simulation does N time steps. The small-time-step simulation sends to Open-PALM N fields while the
large-time-step simulation only sends one. Open-PALM allows to exchange fields after calculating a
moving average in order to smooth the solution over a coupling period. The application was tested on a
turbulent channel. The channel is split in two parts and a wake is injected in the upstream part. As shown
in Fig. 3.2, the wake is well transmitted in the downstream channel by the coupling method.
FIG. 3.2: Coupling of two elsA simulations with the code coupling tool Open-PALM. The fields, colored
with the density, show that the wake generated at the inlet of the upstream channel (up) is transmitted to the
downstream one (bottom).
148 Jan. 2010 – Dec. 2011