Browsing by Subject "vibration"
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Item Backward Precessional Whip and Whirl for a Two Point Rubbing Contact Model of a Rigid Rotor Supported by an Elastically Supported Rigid Stator(2011-10-21) Kumar, Dhruv DileepThe present work investigates the phenomena of whip and whirl for a rigid rotor contacting at two bearing locations. The idea originated from an anemometer consisting of a rotor with an elastically supported stator undergoing the phenomena of dry friction whip and whirl at the two bushing contacts. To analyze the behavior, a mathematical model similar to the anemometer is developed and analyzed assuming two possible solutions, Mode1 (normal reaction forces in phase at two contacts) and Mode 2 (normal reaction forces out of phase at two contacts). Analytical solutions are only possible for the models with same RCl (Radius to Clearance ratio) at the two rub locations. A simulation model is constructed using the Texas A&M University (TAMU) Turbomachinery Laboratory rotordynamics software suite XLTRC? comprised of Timoshenko beam finite elements to form multiple degrees of freedom rotor and stator models. The nonlinear connections at the rub surface are modeled using Hunt and Crossley?s contact model with coulomb friction. Dry friction simulations are performed for three separate models depending on the rotor?s mass disk location with respect to the contact locations. The three models used have (1) Disk at center location (2) Disk at 3/4 location (3) Disk at overhang location. The adequacy of the analytical solution is investigated using the above simulations. Also, cases are explored where the general assumed solution would not solve the mathematical model, e.g. different RCl ratios at the two contacts. Simulations are performed for increasing as well as decreasing running speeds. There is partial agreement between simulation predictions and the analytical solutions for the cases with the mass center at centered and at 3/4 location. First, whirl-to-whip transitions occur at near the combine rotor-stator bounce frequency for both disk at center and disk at 3/4 location. The case with overhang mass disk predicts the two contacts to whip and at different frequencies simultaneously. Neither of the analytical solutions predicts a case where precession occurs at two different frequencies at the two contact points. Predictions for models with different RCl on the Backward Precessional (BP) graph imitate whirling. The BP graph predicts increasing BP frequency with increasing rotor speeds which is a characteristic of whirling, whereas investigation of individual contact velocities suggest that they are slipping at all conditions, one of them slipping more than the other netting a whirling like motion. For the overhang model with different RCl, apart from whipping at different frequency the two contacts also whirl at different frequencies corresponding to the RCl at the respective contacts. Simulations for decreasing rotor speed predict jump down from whirl- to-whip different BP frequency as compared to the jump up from whip-to-whirl for the speed up.Item Courtship acoustics and mating in Cotesia, a genus of parasitoid wasps(2009-05-15) Joyce, Andrea LeeCotesia are parasitoid wasps (Hymenoptera: Braconidae) that are used for biological control of pest moths (Lepidoptera: Noctuidae, Pyralidae) that damage agricultural crops. This dissertation investigated courtship acoustics and mating, and their relevance to biological control, in members of the Cotesia flavipes species complex, and a noncomplex member, Cotesia marginiventris. The first study investigated whether courtship acoustics were species specific for two members of the Cotesia flavipes complex, C. flavipes and C. sesamiae, and for C. marginiventris. During courtship, male Cotesia fan their wings and produce low amplitude sounds and substrate vibrations. The airborne and substrate components of courtship were similar within a species. However, the courtship acoustics of each species was distinct. The duration and frequency of several courtship acoustic components distinguished each species, while some components did not differ among species. The second study investigated mating success and transmission of courtship vibrations on natural and artificial rearing substrates for Cotesia marginiventris. Mating success was measured on plastic, glass, corn and bean leaves, and chiffon fabric. Mating success was lowest on plastic and glass, intermediate on corn and bean leaves, and highest on chiffon. Substrate influenced transmission of courtship vibrations. Durations of courtship vibrations were longer on corn, bean and chiffon than on plastic. Frequency modulation occurred on corn, bean and chiffon, and amplitude was greatest on chiffon. The mating success of normal and dealated males was higher on chiffon than on glass, suggesting that courtship communication relied in part on substrate vibrations. The third study examined female and male mate choice in a solitary and a gregarious species, C. marginiventris and C. flavipes, respectively. Females of the solitary species, C. marginiventris, mated more frequently with large than small males, and this did not appear to be the result of male competition. Male choice for female size was not apparent in C. marginiventris. Females of the gregarious parasitoid, C. flavipes, mated with large or small males with similar frequencies, and male-male competition was not observed. In the male choice experiment, C. flavipes males attempted copulation and mated more with smaller females, and smaller females accepted males more than large females.Item Discussion of Induction Motor Effect on Rotordynamics(2015-05-13) Han, Xu 1986-In this dissertation, the influence of using induction motor in machinery train on rotordynamics is discussed. Two areas are considered ? the use of variable frequency drives (VFDs), which control and drive the induction motor; and the unstable forces due motor eccentricity, which is resulted from motor rotor lateral motion. VFDs ? The dissertation documents fatigue related mechanical failures in VFD motor machinery due to mechanical vibrations excited by drive torque harmonics which are created by PWM switching. Present effort models the coupled system with full electrical system including DC bus, inverter, motor, and an industrial mechanical system including flexible couplings, gearboxes and multiple inertias. The approach extends failure prediction beyond simple occurrence of resonance, to fatigue life evaluation based on Rain-flow algorithm, which is suitable for both steady state and transient startup mechanical response. The use of multilevel inverters is demonstrated having the possibility actually exacerbate resonance and fatigue failure. The model is also compared to an industrial test case, which provides good agreement. Motor eccentricity ? In this dissertation, a MEC modeling method is proposed to calculate both the radial and tangential motor eccentric force. The proposed model is also coupled with the motor electric circuit model to provide capability of transient simulation. FEM (Ansys Maxwell) is used to verify the proposed model. Parametric study is performed on the motor radial and tangential eccentric forces. Also a Jeffcott rotor model is used to study the influence of the motor eccentric force on mechanical stability. A stability criteria of the bearing damping is calculated. The motor radial and tangential eccentric forces are all curved fitted to catch their nonlinearity, which are used in time domain simulation. Nonlinear motions are observed, including limit cycle and jumping phenomena. The results of this dissertation show that both the use of VFDs and the motor lateral motion (motor eccentricity) can cause severe mechanical vibration problem in a rotating machinery train. Both of these two problems need to be carefully concerned in design stage.Item Effects of Induced Acoustic Vibrations on Droplet Shedding on Hybrid Micro-structured Surfaces(2014-04-28) Lai, Chen-LingArtificial hydrophobic and superhydrophobic surfaces have been studied in the last ten years in an effort to understand the effects of structured micro- and nano- scale features on droplet motion and self-cleaning mechanisms. Among these structured surfaces, hybrid surfaces consisting of a combination of hydrophilic and hydrophobic materials have been designed, fabricated and characterized to understand how surface properties and morphology affect enhanced droplet growth rates and droplet shedding during condensation. However, use of hybrid surfaces in condensation leads to a strong pinning effect that takes place between the condensing droplets and the hydrophobic-hydrophillic edge, leading to a significant contact angle hysteresis effect. In an effort to circumvent the pinning effect, a vibration-induced droplet shedding method has been explored to overcome contact angle hysteresis and facilitate droplet shedding at lower rolling angles. To understand the effects of hybrid surface morphology and vibration modes on droplet removal from surfaces used for condensation, this research study focuses on the effects of acoustic-induced vibrations on droplet sliding at different inclined angles on hybrid surfaces. A hydrophilic surface (silicon surface) has been used as the baseline in the study to be able to uncover the effects of vibration on pinned droplets. Firstly, the relationship between sliding angles and droplet volume was investigated experimentally for hybrid surfaces with different spacings. Then, the effects of natural resonance frequencies of droplets with different volumes on different surfaces were also studied using a resonance model and a customized experimental setup. Acoustic-induced vibrations were then applied to the surfaces to understand the effects of single or multiple resonance frequencies on droplet sliding angles. Droplet vibration and roll-off processes were experimentally characterized using a high speed imaging system. An acoustic sensor was also used to measure the induced frequencies and amplitudes. Experimental results to date show that hybrid surfaces with larger spacing leads to lower droplet sliding angles. Furthermore, droplets under the influence of acoustic waves depict different contour morphologies when vibrating at different resonance frequencies. Moreover, droplet sliding angles can be reduced through vibration when the proper combination of droplet size and surface morphology is prescribed. Future studies will consider the use of acoustic waves in actual condensers.Item The influence of internal friction on rotordynamic instability(Texas A&M University, 2004-09-30) Srinivasan, AnandInternal friction has been known to be a cause of whirl instability in built-up rotors since the early 1900's. This internal damping tends to make the rotor whirl at shaft speeds greater than a critical speed, the whirl speed usually being equal to the critical speed. Over the years of research, though models have been developed to explain instabilities due to internal friction, its complex and unpredictable nature has made it extremely difficult to come up with a set of equations or rules that can be used to predict instabilities accurate enough for design. This thesis deals with suggesting improved methods for predicting the effects of shrink fits on threshold speeds of instability. A supporting objective is to quantify the internal friction in the system by measurements. Experimental methods of determining the internal damping with non-rotating tests are investigated, and the results are correlated with appropriate mathematical models for the system. Rotating experiments were carried out and suggest that subsynchronous vibration in rotating machinery can have numerous sources or causes. Also, subsynchronous whirl due to internal friction is not a highly repeatable phenomenon.Item The use of maximum rate of dissipation criterion to model beams with internal dissipation(Texas A&M University, 2004-09-30) Ko, Min SeokThis thesis deals with a systematic procedure for the derivation of exact expression for the frequency equation of composite beams undergoing forced vibration with damping. The governing differential equations of motion of the composite beam are derived analytically for bending and shear deformation. The basic equations of Timoshenko beam theory and assumption of maximum rate of dissipation are employed. The principle involved is that of vibration energy dissipation due to damping as a result of deformation of materials in sandwich beam. The boundary conditions for displacements and forces for the cantilever beam are imposed and the frequency equation is obtained. The expressions for the amplitude of displacements are also derived in explicit analytical form. Numerical results of the displacement amplitude in cantilever sandwich beam varying with damping coefficient are evaluated.Item Validation of computer-generated results with experimental data obtained for torsional vibration of synchronous motor-driven turbomachinery(Texas A&M University, 2004-09-30) Ganatra, Nirmal KirtikumarTorsional vibration is an oscillatory angular twisting motion in the rotating members of a system. It can be deemed quite dangerous in that it cannot be detected as easily as other forms of vibration, and hence, subsequent failures that it leads to are often abrupt and may cause direct breakage of the shafts of the drive train. The need for sufficient analysis during the design stage of a rotating machine is, thus, well justified in order to avoid expensive modifications during later stages of the manufacturing process. In 1998, a project was initiated by the Turbomachinery Research Consortium (TRC) at Texas A&M University, College Station, TX, to develop a suite of computer codes to model torsional vibration of large drive trains. The author had the privilege of developing some modules in Visual Basic for Applications (VBA-Excel) for this suite of torsional vibration analysis codes, now collectively called XLTRC-Torsion. This treatise parleys the theory behind torsional vibration analysis using both the Transfer Matrix approach and the Finite Element approach, and in particular, validates the results generated by XLTRC-Torsion based on those approaches using experimental data available from tests on a 66,000 HP Air Compressor.