Plenary Lecture

Unsteady Flow Simulation In Turbomachinery
A Numerical Challenge

Professor Francesco Martelli
Energetics Dept. "S. Stecco" - University of Florence
Firenze - Italy

Abstract: The prediction of unsteady flow field in turbine blades as well as in the turbomachinery stages is now an affordable item, and is required by the reduced margin for increasing efficiency, stability and life of propulsion components. The numerical tools are now capable to run within reasonable time 3D unsteady calculation for full stage, and the new techniques on the computation and parallel computer allow the improvements of results in terms of cost and accuracy. Despite this advantages many questions remain open and the physical modelling joint with the numerical improvements is still a challenge if it has to produce usable results ,compared with the experiments. On the other side the huge amount of data extracted from experiments require care and skillness to become usefull tools for design. The two activity interact and support each other in the attempt to improve design quality.
Aim of this paper is the report on some experience and the attempt to give some answer on that challenge, presenting results of an resent activity on modelling side compared with experiments as well.
A full-3D unstructured solver based on an upwind TVD finite volume scheme is developed and applied to the simulation of an unsteady turbine stage. Two different approaches are considered for the time accurate inviscid simulation of the unsteady stator/rotor interaction. The first consists of a classical explicit time accurate multi-step Runge-Kutta scheme. The second is based on a dual-time stepping strategy, which exploits the implicit time-marching Newton-Krylov method. In this case the linear solver of the implicit scheme consists of a preconditioned GMRES and ILU(0) incomplete factorisation. Both the explicit and implicit approaches are designed to run on parallel cluster of workstations. The development of the numerical strategy is discussed with particular concern on the validation of the unsteady model through a comparison against experiments, NISRE approach and a 3D steady stage computation.
The present work considers the application of the fully unstructured hybrid solver for internal viscous flows, as well. The multiblock version of the solver developed for turbine is considered, because of the highly improved performance as compared to the single domain version of the code. Moreover, the high numerical costs involved in 3D unsteady computations required the development of a new parallel single program multiple-data version of the numerical solver.
The results compare favourably with a set of time averaged and unsteady experimental data available for the turbine stage under investigation, which is representative of a wide class of aero-engines. This improved version of HybFlow is applied to the simulation of the BRITE HP turbine stage experimentally tested in the compression tube facility CT3 of the Von Karman Institute (Denos et al. 1999, 2000). Preliminary tests on viscous calculations show a good capability of the solver to manage complex flow conditions and geometry. Some example of calculation grids and results are reported in following figures.

In the final version of the papers results and comparision on rotor stator interaction will be reported in more details, and comments and suggestion on the pen question will be discussed.




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