Browsing by Author "Vistamehr, Arian"
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Item Analysis of Automotive Turbocharger Nonlinear Response Including Bifurcations(2010-10-12) Vistamehr, ArianAutomotive turbochargers (TCs) increase internal combustion engine power and efficiency in passenger and commercial vehicles. TC rotors are usually supported on floating ring bearings (FRBs) or semi-floating ring bearings (SFRBs), both of which are inexpensive to manufacture. However, fluid film bearings are highly nonlinear components of TC units and contribute to the complex behavior (i.e. bifurcations and frequency jumps between a first whirl frequency and a second whirl frequency) of the entire rotor-bearing system (RBS). The frequency jump phenomenon concerns the TC manufacturing industry due to increased levels of noise generation. This thesis presents progress on assessing the effects of some bearing parameters and operating conditions on the RBS dynamic forced performance and the frequency jump phenomenon. A fluid film bearing model is integrated into a finite element rotordynamics computational model for numerical prediction of the TC linear and nonlinear (time transient) forced response. Since automotive TCs operate with variable rotational speed, predictions are conducted with shaft acceleration/deceleration. Over most of its operating speed range, TC rotor nonlinear response predictions display two subsynchronous whirl frequencies w1 and w 2 representing a conical mode and a cylindrical bending mode, respectively. At low shaft speeds w1 is present up to a shaft speed (Omega bifurcation), where there is a frequency jump from w1 to w 2. The second whirl frequency may persist up to the highest shaft speeds (depending on operating conditions). Results show during rotor deceleration the Omega bifurcation may be different from the one during rotor acceleration (hysteresis). Predictions show the following factors delay the Omega bifurcation: increasing oil supply pressure, decreasing oil supply temperature, and increasing shaft acceleration. Also, rotor imbalance distribution greatly affects Omega bifurcation and the shaft amplitude of total motion. Overall, this study shows the sensitivity of bifurcations and frequency jump phenomenon in TC nonlinear response due to various bearing parameters and operating conditions. Further analysis is required to generalize these findings and to assess the effect of other bearing parameters (i.e. clearances, outer film length, ring rotation, etc.) on this phenomenon. In addition further validation of the predictions against test data is required for refinement of the predictive tool.Item Understanding dynamic balance during walking using whole-body angular momentum(2014-08) Vistamehr, Arian; Neptune, Richard R.Maintaining dynamic balance during walking is a major challenge in many patient populations including older adults and post-stroke hemiparetic subjects. To maintain dynamic balance, whole-body angular-momentum has to be regulated through proper foot placement and generation of the ground-reaction-forces. Thus, the overall goal of this research was to understand the mechanisms and adaptations used to maintain dynamic balance during walking by analyzing whole-body angular-momentum, foot placement and ground-reaction-forces in older adults and post-stroke subjects. The analysis of healthy older adults showed that they regulated their frontal-plane angular-momentum poorly compared to the younger adults. This was mainly related to the increased step width, which when combined with the dominant vertical ground-reaction-force, created a higher destabilizing external moment during single-leg stance. The results also suggested that exercise programs targeting appropriate foot placement and lower extremity muscle strengthening, particularly of the ankle plantarflexors and hip abductors, may enhance balance control in older adults. During post-stroke hemiparetic walking, ankle-foot-orthosis and locomotor therapy are used in an effort to improve the overall mobility. However, the analyses of healthy subjects walking with and without a solid ankle-foot-orthosis showed that they can restrict ankle plantarflexor output and limit the successful regulation of angular-momentum and generation of forward propulsion. Thus, the prescription of solid ankle-foot-orthosis should be carefully considered. The analysis of hemiparetic subjects walking pre- and post-therapy showed that locomotor training did not improve dynamic balance. However, for those subjects who achieved a clinically meaningful improvement in their self-selected walking speed, their change in speed was correlated with improved dynamic balance. Also, improved balance was associated with narrower mediolateral paretic foot placement, longer anterior nonparetic steps, higher braking ground-reaction-force peaks and impulses, higher (lower) propulsive ground-reaction-force peaks and impulses from the paretic (nonparetic) leg, and higher vertical ground-reaction-force impulses from both legs during the late stance. Further, simulation analyses of hemiparetic walking highlighted the importance of ankle plantarflexors, knee extensors and hip abductors in maintaining balance and revealed the existence of compensatory mechanisms due to the paretic leg muscle weakness. Collectively, these studies showed the importance of ankle plantarflexors and hip abductors in maintaining dynamic balance.