Forward and Backward Mode Excitation of Flexible Rotor Supported by Tilting Pad Bearings: Numerical and Experimental Investigation

2014 ◽  
Author(s):  
Weimin Wang ◽  
Qihang Li ◽  
Jinji Gao ◽  
Timothy Dimond ◽  
Paul Allaire
Keyword(s):  
Frequency Response ◽  
Number Field ◽  
Response Function ◽  
Frequency Response Function ◽  
Operating Conditions ◽  
Modal Parameters ◽  
Polynomial Method ◽  
Centrifugal Compressors ◽  
Simulation Results ◽  
Sine Sweep

Understanding rotor modal excitations is crucial for high performance centrifugal compressors and other rotating machines. Assuring low vibration levels of such machines at operating conditions before delivery is important both for original equipment manufacturers (OEMs) and end users. In this paper, transient simulations of a full scale test rig rotor subject to sine sweep excitations are performed to investigate the forward and backward rotor whirling response. The applied sine sweep excitations are circular forward, circular backward, and elliptical forward, respectively. The effects of excitation force amplitude are also investigated to determine the minimum force required to accurately identify the rotor system modal parameters. The transient simulation results are then used to investigate a forward and backward mode system identification method for rotating machinery stability based on sine-sweep excitations. Both simulations and experimental testing on a full size rotor with an electromagnetic actuator were performed to verify and validate the method. The traditional Multiple Input Multiple Output (MIMO) Frequency Response Function (FRF) is transformed into a directional Frequency Response Function (dFRF) form. This transformation recasts the real number field into complex number field via a transformation matrix. This transformation separates the MIMO FRFs into forward and backward components, which improves the accuracy of the identified results. This method is used to identify the first forward bending modal parameters to estimate rotor stability. The rational polynomial method is used to fit and identify both the dFRFs. Excellent correlation was obtained between simulation results and the identification experiments. The results of this paper provide new insights for avoidance of rotor instability in centrifugal compressors.

scholarly journals Numerical and Experimental Stability Investigation of a Flexible Rotor on Two Different Tilting Pad Bearing Configurations

2014 ◽  
Vol 2014 ◽  
pp. 1-11 ◽  
Author(s):  
Weimin Wang ◽  
Qihang Li ◽  
Feng He ◽  
Paul Allaire
Keyword(s):  
Frequency Response ◽  
Response Function ◽  
Frequency Response Function ◽  
High Performance ◽  
System Stability ◽  
Polynomial Method ◽  
Inlet Temperature ◽  
Centrifugal Compressors ◽  
Rational Polynomial ◽  
Rotor Bearing

Rotordynamic stability is crucial for high performance centrifugal compressors. In this paper, the weighted instrumental variable (WIV) based system identification method for rotating machinery stability is investigated based on a sine sweep forward excitation with an electromagnetic actuator. The traditional multiple input multiple output (MIMO) frequency response function (FRF) is transformed into a directional frequency response function (dFRF). The rational polynomial method (RPM) combined with WIV is developed to identify the rotor’s first forward mode parameters. This new approach is called the COMDYN method. Experimental work using the COMDYN method is carried out under different rotating speeds, oil inlet temperatures, and pressure conditions. Two sets of bearings with preloads 0.1 and 0.3 are investigated. A numerical rotor-bearing model is also built. The numerical results correlate reasonably well with the experimental results. The investigation results indicate that the new method satisfies the desired features of rotating machine stability identification. Furthermore, the system log decrement was improved somewhat with the increase of oil inlet temperature. The increase of oil supply pressure affects the rotor-bearing system stability very slightly. The results of this paper provide new and useful insights for potentially avoiding instability faults in centrifugal compressors.

Change of modal parameters due to crack growth in a tripod tower platform

1993 ◽  
Vol 20 (5) ◽  
pp. 801-813 ◽  
Author(s):  
Yin Chen ◽  
A. S. J. Swamidas
Keyword(s):  
Finite Element ◽  
Frequency Response ◽  
Response Function ◽  
Frequency Response Function ◽  
Experimental Results ◽  
Modal Testing ◽  
Modal Parameters ◽  
Response Functions ◽  
Frequency Response Functions ◽  
Strain Frequency

Strain gauges, along with an accelerometer and a linear variable displacement transducer, were used in the modal testing to detect a crack in a tripod tower platform structure model. The experimental results showed that the frequency response function of the strain gauge located near the crack had the most sensitivity to cracking. It was observed that the amplitude of the strain frequency response function at resonant points had large changes (around 60% when the crack became a through-thickness crack) when the crack grew in size. By monitoring the change of modal parameters, especially the amplitude of the strain frequency response function near the critical area, it would be very easy to detect the damage that occurs in offshore structures. A numerical computation of the frequency response functions using finite element method was also performed and compared with the experimental results. A good consistency between these two sets of results has been found. All the calculations required for the experimental modal parameters and the finite element analysis were carried out using the computer program SDRC-IDEAS. Key words: modal testing, cracking, strain–displacement–acceleration frequency response functions, frequency–damping–amplitude changes.

Modelling the Response of Force Transducers Under Sine-Sweep Calibration

2015 ◽  
Author(s):  
Nicholas Vlajic ◽  
Ako Chijioke
Keyword(s):  
Frequency Response ◽  
Response Function ◽  
Output Voltage ◽  
Frequency Response Function ◽  
Use Of Force ◽  
Applied Force ◽  
Dynamic Calibration ◽  
Force Transducers ◽  
Sine Sweep

In the dynamic calibration of force transducers using swept-sine excitation, the sensitivity (the output voltage divided by the applied force) of the transducer can start to decrease (or roll-off) at higher frequencies. It has been proposed that this roll-off originates from the finite stiffness and dissipation of the transducer. In other words, the roll-off is caused by a mechanical frequency response of the transducer, and the sensitivity is proportional to this frequency response function via a constant. The focus of this study is the origin of the observed roll-off in sensitivity. The findings of this study have application to the dynamic calibration and use of force transducers.

Modal Testing and Parameter Identification of Active Magnetic Bearing System

1995 ◽  
Author(s):  
Chong-Won Lee ◽  
Young-Ho Ha ◽  
Cheol-Soon Kim ◽  
Chee-Young Joh
Keyword(s):  
Parameter Identification ◽  
Frequency Response ◽  
Response Function ◽  
Frequency Response Function ◽  
Magnetic Bearing ◽  
Experimental Results ◽  
Active Magnetic Bearing ◽  
Modal Testing ◽  
Modal Parameters ◽  
Bearing System

Abstract Complex modal testing is employed for parameter identification of a four-axis active magnetic bearing system. In the test, magnetic bearings are used as exciters while the system is in operation. The experimental results show that the directional frequency response function, which is properly defined in the complex domain, is a powerful tool for identification of bearing as well as modal parameters.

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