Designing and developing new aerospace propulsion systems is time-consuming and expensive. Computational simulation is a promising means for alleviating this, due to the flexibility it provides for rapid and relatively inexpensive evaluation of alternative designs, and because it can be used to integrate multidisciplinary analysis earlier in the design process. In this paper a numerical method based on CFD computations for predicting the performances given by turbine engines during transients is presented. I. INTRODUCTION Designing and developing new aerospace propulsion systems is time-consuming and expensive. Computational simulation is a promising means for alleviating this, due to the flexibility it provides for rapid and relatively inexpensive evaluation of alternative designs, and because it can be used to integrate multidisciplinary analysis earlier in the design process. However, integrating advanced computational simulation analysis methods as CFD into computational simulation software system is a challenge. A prerequisite for the successful implementation of such a program is the development of an eective simulation framework for the representation of engine components, like compressor, turbines, nozzles, etc. To address these issue has been developed a numerical model based on CFD computation for predicting the performances given by a gas turbine engine during transients. From a physical point of view we may distinguish two kinds of transients, according to the considered reference time. The low frequency transients are mainly related to the relatively large inertia of the rotors. The range in time of these phenomena is of the order of magnitude of some seconds: for instance, the time needed to reach the maximum thrust from idling. The high frequency transient are strictly related to phenomena such as the pressure wave propagation through the engine: the range in time amounts to some hundredth or tenths of second. The formulation of these two problems is dierent. For the low frequency transients, the method given in Refs. 1, 2 may be followed, on the base of the lumped volume gas dynamics and on the hypothesis that the performances of each whole element of engine ( a multistage compressor, for example) are the ones given by the steady-state map. The peculiar details of the unsteady flow inside the element are lost, in this analysis. The eect of the unsteadiness as regard the gasdynamics, are taken into account as mass and energy in control volumes, associated with each component of the engine. The flow properties vary in time, but are kept constant as some average value through the volume. This formulation is applied by the analytical techniques as well by the numerical ones, in cases confined to low frequency phenomena. In the case of high frequency transients, the interest is directed to the prediction of the pressure waves propagation, in the meaning usually accepted in unsteady gasdynamics. In this case it is required the looking inside the single component of the engine and the formulation of simplified models which may be treated easily by the numerical techniques, typical of unsteady aerodynamics. And, what is more important, the assumed models should be realizable in describing the actual process occurring in the component (compressor, turbine, etc.). In the final paper will be show some results of computations about typical fast transients, which may take place in a gas turbine engine, as an impulsive change in the inlet flow conditions. The methodology used is based on a numerical technique developed for CFD (Ref. 3) computations and on a classical concepts, widely used in turbomachinery studies, as regards the modeling of the complicated actual machine.