Numerical Study on the Critical Frequency Response of Jet Engine Rotors for Blade-Off Conditions against Bird Strike

Abstract

Vibrations are usually induced in aero engines under their normal operating conditions. Therefore, it is necessary to predict the critical frequencies of the rotating components carefully. Blade deformation of a jet engine under its normal operating conditions due to fatigue or bird strike is a realistic possibility. This puts the deformed blade as one of the major safety concerns in commercially operating civil aviation. A bird strike introduces unbalanced forces and non-linearities into the engine rotor system. Such dynamic behavior is a primary cause of catastrophic failures. The introduction of unbalanced forces due to a deformed blade, as a result of a bird strike, can change the critical frequency behavior of engine rotor systems. Therefore, it is necessary to predict their critical frequencies and dynamic behavior carefully. The simplified approach of the one-dimensional and two-dimensional elements can be used to predict critical frequencies and critical mode shapes in many cases, but the use of three-dimensional elements is the best method to achieve the goals of a modal analysis. This research explores the effect of a bird strike on the critical frequencies of an engine rotor. The changes in critical mode shapes and critical frequencies as a result of a bird strike on an engine blade are studied in this research. Commercially available analysis software ANSYS version 18.2 is used in this study. In order to account for the material nonlinearities, a Johnson Cook material model is used for the fan blades and an isotropic–elastic–plastic–hydrodynamic material model is used for modeling the bird. The bird strike event is analyzed using Eularian and smoothed particle hydrodynamics (SPH) techniques. A difference of 0.1% is noted in the results of both techniques. In the modal analysis simulation of the engine rotor before and after the bird strike event, the critical failure modes remain same. However, a change in the critical frequencies of the modes is observed. An increase in the critical frequencies and excitation RPMs (revolution per minute) of each mode are observed. As the mode order is increased, the higher the rise in critical frequency and excitation RPMs. Also, a change in the whirl direction of the different modes is noted.

Dynamic Responses of the Aero-Engine Rotor System to Bird Strike on Fan Blades at Different Rotational Speeds

“…In the early simulations based on finite element analysis, the bird impact load was modelled as a uniform pressure vs. time at the impact area [10]. Afterwards, bird models based on Lagrangian, Eulerian, Arbitrary Lagrangian-Eulerian (ALE), and Smoothed Particles Hydrodynamics (SPH) formulations were developed and used in the bird strike simulations [11][12][13][14][15][16][17][18].…”

Modal Analysis and Rotor-Dynamics of an Interior Permanent Magnet Synchronous Motor: An Experimental and Theoretical Study

“…(2) the translational rotational modes (i.e., cylindrical rotational modes) are typical for the symmetric mechanical structures; and (3) the presence of torsional critical speeds and torsional vibration modes indicates the adequacy of the selected geometrical and material properties of the modeled structure [16,[18][19][20][21][22][23][24][25]. The output results of the rotational dynamic analysis in ANSYS Mechanical are provided as numerically computed values of eigenvalues f (Hz) (i.e., natural frequencies, also referred to as whirl frequencies) as a function of the rotational speed Ω (rpm) of the modeled rotating structure (i.e., f (Ω)).…”

“…(1) a strong dependency of the Coriolis effect, which affects the stiffness of the system, is reflected in the split (i.e., bifurcation) of the vibrational rotational modes; (2) the translational rotational modes (i.e., cylindrical rotational modes) are typical for the symmetric mechanical structures; and (3) the presence of torsional critical speeds and torsional vibration modes indicates the adequacy of the selected geometrical and material properties of the modeled structure [16,[18][19][20][21][22][23][24][25].…”

“…Such complete mechanical analysis should include both static modal analysis for vibration analysis of geometrical and material properties of the machine components and the rotor dynamics in order to investigate the vibration of the rotating parts. The modal analysis of the machines' stator has already been extensively reported for different types of electrical machines [5][6][7][8][9][10][11][12][13][14], while the rotor dynamic has mostly studied for the mechanical design of high speed machines constructed for special applications [1,2,4,[15][16][17][18][19][20][21][22][23][24][25]. To the best of the authors' knowledge, the complete mechanical analysis of the stator and rotor components of the same machine by the modal and rotor dynamic analysis has not been reported.…”

Transient Analysis and Safety Assessment of Turbofan Engine Structures during Bird Ingestion

“…Liebich [30] designed and developed a test rig that represented the rotor dynamic behavior of an aeroengine almost in a full size scale, which could study the unbalance response of the whole engine model. Badshah et al [31] established the rotor parts including the fan, compressor, and turbine, carried out the analysis of bird impact fan based on the SPH method, and obtained the impact response of the fan. Then, the modal analysis of the rotor parts with bird impact deformation was carried out at different rotational speeds, and it was found that the change of rotational speed had a greater impact on the modal response of the damaged engine rotor.…”