Fanny M. Besem

Vortex-Induced Vibration and Frequency Lock-In of an Airfoil at High Angles of Attack

Vortex-induced vibration is a fluid instability where vortices due to secondary flows exert a periodic unsteady force on the elastic structure. Under certain circumstances, the shedding frequency can lock into the structure natural frequency and lead to limit cycle oscillations. These vibrations may cause material fatigue and are a common source of structural failure. This work uses a frequency domain, harmonic balance (HB) computational fluid dynamics (CFD) code to predict the natural shedding frequency and lock-in region of an airfoil at very high angles of attack. The numerical results are then successfully compared to experimental data from wind tunnel testings.

Forced Response Sensitivity of a Mistuned Rotor From an Embedded Compressor Stage

The frequency mistuning that occurs due to manufacturing variations and wear and tear of the blades has been shown to significantly affect the flutter and forced response behavior of a blade row. While tuned computational fluid dynamics (CFD) analyses are now an integral part of the design process, designers need a fast method to evaluate the localized high blade responses due to mistuning. In this research, steady and unsteady analyses are conducted on the second-stage rotor of an axial compressor, excited at the first torsion vibratory mode. A deterministic mistuning analysis is conducted using the numerical modal forces and the individual blade frequencies obtained experimentally by tip timing data. The mistuned blade responses are compared in the physical and traveling wave coordinates to the experimental data. The individual and combined impacts of frequency, aerodynamic, and forcing function perturbations on the predictions are assessed, highlighting the need to study mistuned systems probabilistically.

Developing a Reduced-Order Model of Nonsynchronous Vibration in Turbomachinery Using Proper-Orthogonal Decomposition Methods

The paper develops a reduced-order model of non-synchronous vibration (NSV) using proper orthogonal decomposition (POD) methods. The approach was successfully developed and implemented, requiring between two and six POD modes to accurately predict CFD solutions that are experiencing non-synchronous vibration. This POD method was first developed and demonstrated for a transversely-moving, two-dimensional cylinder in cross-flow. Later, the method was used for the prediction of CFD solutions for a two-dimensional compressor blade.This research is the first to offer a proper orthogonal decomposition approach to the reduced-order modeling of non-synchronous vibration in turbomachinery. Modeling non-synchronous vibration is especially challenging because NSV is caused by complicated, unsteady flow dynamics; this initial study helps researchers understand the causes of NSV, and aids in the future development of predictive tools for aeromechanical design engineers.Copyright

Developing a Reduced-Order Model of Non-Synchronous Vibration in Turbomachinery Using Proper Orthogonal Decomposition Methods

The paper develops a reduced-order model of non-synchronous vibration (NSV) using proper orthogonal decomposition (POD) methods. The approach was successfully developed and implemented, requiring between two and six POD modes to accurately predict CFD solutions that are experiencing non-synchronous vibration. This POD method was first developed and demonstrated for a transversely-moving, two-dimensional cylinder in cross-flow. Later, the method was used for the prediction of CFD solutions for a two-dimensional compressor blade.This research is the first to offer a proper orthogonal decomposition approach to the reduced-order modeling of non-synchronous vibration in turbomachinery. Modeling non-synchronous vibration is especially challenging because NSV is caused by complicated, unsteady flow dynamics; this initial study helps researchers understand the causes of NSV, and aids in the future development of predictive tools for aeromechanical design engineers.Copyright

Student Perceptions on the New Teaching Methods for Engineers: The Influence of Podcasting on Education

This paper presents the perceptions of engineering students who followed podcasted courses during their higher education. Podcasting is widely used in remote education, but it also benefits on-campus students because it supports flexible and personalized teaching. The first three generations of students from an international Master’s degree program participated in a survey to give their impressions about the use of podcasting in their program. This preliminary survey targeted three aspects of podcasting: the format, the effect on learning experience, and the effect on student isolation. Results showed a majority of students considered recorded lectures a very helpful tool to support traditional on-campus lectures. The students appreciated the opportunity to pause and re-watch the videos to learn at their own pace. However, few students would consider a purely remote education; they felt less engaged in their education because of the lack of direct contact with teachers and peers. This highlights the importance of the social networking that happens on-site. In conclusion, the main advantage of podcasting is to compensate for the lack of individually adapted teaching in higher education. However, it will not completely replace traditional lectures without the development of both new tools to facilitate the professor-student interactions, and of teaching techniques to keep students engaged in their studies.Copyright

Influence of the Tip Clearance on a Compressor Blade Aerodynamic Damping

The flutter CFD calculations conducted in industry generally use a single-row, single-passage approach with a minimum of detailing to reduce the computational time as much as possible. The question arises of what level of detailing is necessary for accurate aerodamping predictions. In this study, the influence of tip clearance size on the steady flowfield and aerodynamic damping is assessed for a high-pressure compressor second-stage rotor. The steady analyses are conducted on an isolated blade passage by imposing the experimental boundary conditions at the inlet and outlet of the passage. Two operating points are considered: 74 and 100% corrected speed. It is observed that, at the same pressure ratio, the mass flow decreases with a larger tip gap due to the tip clearance flow rolling into a vortex on the blade suction side. Then, unsteady analyses are conducted by enforcing the first torsion mode shape at the corresponding natural frequency. For all interblade phase angles, the same trend is observed: th...