Reducing the clearances between rotating and fixed parts is an important factor in increasing the performances of turbomachines. The physical counterpart however is an evolution in possible rotor-stator contacts capable of causing unstable dynamic behavior. A proper prediction of the rotor-stator contact occurrences and associated induced phenomena, has therefore become of a great interest for aero-engine mechanical engineers. Most numerical simulations involving rotor-stator contact can be divided into two types of physical behavior. The first focuses on contact induced blade/casing interactions, in only taking into account the blades and casing flexibility. The second type of behavior takes into account the shaft dynamic while neglecting blade flexibility. Future designs of aircraft engines will however raise the need to combine these two types of models. Since, the structural components are more flexible, the dynamic coupling between engine modules is increased. This paper proposes a study based on a structure representative of the whole aircraft engine, including the contacts that may arise between the fan-blade tips and fan casing. We have introduced a fully-coupled phenomenological model with flexible blades, shaft and casing. Furthermore, this model includes an elastic link between shaft and casing to simulate the fan frame behavior. We begin by explaining the linear results, which highlight the dynamic couplings between these various model components. During a second step, this paper presents the nonlinear results obtained by introducing a contact law. These results demonstrate the influence of the whole engine dynamic on contact-related behavior with special focus on the system dynamic stability.

No abstract

The improvement of efficiency in the design of turbomachines requires a reliable prediction of the vibrating behavior of the whole structure. The simulation of blades vibrations is decisive and this is usually based on elaborated finite element model restricted to the bladed-disk. However the blades dynamic behavior can be strongly affected by interactions with other parts of the engine. Global dynamic studies that consider these other parts are required but usually come with a high numerical cost. In the case of a bi-rotor architecture, two coaxial rotors with different rotating speed can be coupled with a bearing system. The mechanical coupling between the shafts generates energy exchange that alters the dynamic behavior of the blades. The equations of motion of the whole structure that take into account the coupling contain periodic time-dependent coefficients due to the difference of rotational speed between both rotors. Equations of this kind, with variable coefficients, are typically difficult to solve. This study presents a preprocessing method to guarantee the elimination of time-dependent coefficients in the bi-rotor equations of motion. This method is tested with a simplified finite element model of two bladed-disks coupled with linear stiffnesses. We obtain accurate results when comparing frequency analysis of preprocessed equations with time-integration resolution of the initial set of equations. The developed methodology also offers a substantial time saving.

This paper deals with fluid-structure interactions (FSI), involving a blade profile, submitted to different sources of excitations, as if it were included in a real engine. Two forces of excitation will be considered on the NACA 64A010 airfoil, described in : an external force, due to a forced rotation motion of the blade, and an aerodynamic force, induced by fluid flow around the structure. By using the Harmonic Balance Method, the airfoil’s motion equation becomes an algebraic problem. Then, this system is solved for each frequency of a chosen range. Therefore, the fluid effect on the translation motion of the profile is studied. To compute the time periodic aerodynamic field, the Time Spectral Method, implemented in the Onera’s elsA solver, is used for a fast and efficient resolution. This method relies on a time-integration scheme that turns the resolution of the turbulent Navier-Stokes problem into the resolution of several coupled steady state problems computed at different instants of the time period of the movement. The Theodorsen approach with several hypothesis exposed in allows an analytic estimation of the unsteady lift effort. The two approaches are compared for an imposed motion. In order to predict the dynamic behavior of the system, a fully coupled numerical methodology is developed. For each frequency and at each iteration, TSM supplies the flow field which is used by HBM as a nonlinear excitation on the structure to computate a periodic response and conversely, HBM supplies the new deformed mesh used by TSM to compute the flow field. This strategy has the advantage that all computations take place in the spectral domain, allowing thus to find the steady-state behavior of the fluid and the structure without computing any transient state. The analysis provides the Frequency Forced Response. Some frequencies in the range corresponding to a contribution change between structure and fluid damping are precisely highlighted.