An Experimental and Computational Investigation of a Pulsed Air-Jet Excitation System on a Rotating Bladed Disk

Nonsynchronous vibrations are a difficult problem to address for turbomachines due to the complex nature of the forcing. Such vibrations can be caused by vortex shedding, flow instabilities, stall cells, or flutter. Testing a design with such excitations can be difficult in practice due to the required forcing. This work demonstrates an experimental excitation method using pulsed air jet excitation to create nonsynchronous vibrations in engine hardware rotating at nominal design speeds. Experimental runs were conducted to excite a number of engine orders (EOs). Blade tip timing was used to measure the blade response without interfering with the blade dynamics. The bladed disk was held at a constant rotational speed while the air jets were pulsed at a sweeping frequency to simulate rotating forcing. Computational models of the physical system were constructed using parametric reduced order models that incorporate the effects of rotational speed and small mistuning. The computational model was used in simulations that mimic the experiment; the forcing was swept across the blades while being pulsed. This results in a system response that cannot be captured using traditional harmonic analyses. The computational and experimental datasets were compared through mistuning values, amplitudes, and the nodal diameter (ND) content in the system response.

Dynamic Analysis of a Flexible Rotor Supported on Two Turbulent Model Journal Bearings with Micropolar Fluid Lubrication

This paper presents the complex dynamic analysis of a flexible rotor–bearing system supported by two turbulent micropolar fluid film journal bearings under nonlinear suspension. The Modified Reynolds equation based on the assumptions of turbulent flow and the micropolar parameters has been considered. The system considers Short bearing approximation to simplify the numerical computations. The pressure distribution thus obtained is used to find out the resulting forces about the journal center in the radial and tangential directions. The Non-dimensional dynamic equations are derived considering appropriate non dimensional parameters and solved using MATLAB for a wide range of non-dimensional speed ratios. Plots of the journal center trajectories and rotor center trajectories are obtained. The results show that the system undergoes undesirable nonsynchronous vibrations due to bearing center displacement. Micropolar fluid is found to stabilize the system even when the flow of the system becomes turbulent. The study presented enhances the understanding of the nonlinear dynamics of turbulent journal bearings with respect to dimensionless parameters.

Effect of Tip Clearance on the Prediction of Nonsynchronous Vibrations in Axial Compressors

This work investigates the effect of tip clearance size and operating temperature on the predictions of the critical rotor speed at which nonsynchronous vibrations (NSV) can be encountered in a turbine engine axial flow compressor. It has been proposed that the tangential tip clearance flow, observed at high blade loading near stall, can act as an impinging resonant jet on the upcoming blades and could be the underlying physics behind NSV. A model, in the form of an equation to predict the critical blade tip speed at which NSV can occur, was proposed based on the Jet-Core Feedback Theory and was experimentally verified by Thomassin et al. (2008, “Experimental Demonstration to the Tip Clearance Flow Resonance Behind Compressor NSV,” Proceedings of GT2008: ASME Turbo Expo Power for Land, Sea and Air, Berlin, Germany, Jun. 9–13, Paper No. GT2008-50303). In the equation, a factor k that was called the “tip instability convection coefficient” was measured experimentally and found to be influenced by the tip clearance size and operating temperature. This factor has a significant impact on the accuracy of the NSV predictions obtained using the proposed model. This paper propose a numerical experiment to determine the effect of tip clearance size and temperature on k, in order to improve the critical NSV tip speed predictions using the proposed model. A review of the NSV model is presented along with the relevant background theory on the subject. Two different blade geometries are simulated to provide a generic approach to the study. The leakage flow velocity is calculated to estimate k and a correlation is proposed to model the behavior of the k parameter as a function of the tip clearance size. The latter was found to significantly improve the critical NSV speed predictions. The effect of operating temperature on k is also discussed. Finally, the variation of k with the aerodynamic loading is assessed and compared with available data in the literature to strengthen the generic nature of the results.

Noise Characteristics Corresponding to High-Level Rotor Blades Nonsynchronous Vibrations in an Axial Compressor

Nonsynchronous vibrations (NSVs) with high amplitude levels in the first rotor blades of a multi-stage axial compressor have been observed. The excitation is aerodynamically caused and associated with a unsteady flow field, including sound field. In order to investigate the characteristics of sound field in the axial compressor, the noise inner compressor casing are measured simultaneously with the vibration of the rotor blades on a high pressure compressor component rig testing. The results show that noise with specific frequency structures appear in the axial compressor under a pre-arranged structure adjustment and the specific operating conditions, and the noise spectrum characteristics are analyzed detailedly. Some influence factors such as rotating speed and corrected mass flow rate on noise characteristics are discussed emphatically. The results presented in this paper can be a reference for further understand of the characteristics of unsteady flow field and the effects of the high intensity sound waves on the rotor blades.