PhD Oral Exam - Iman Hazrati Ashtiani
When studying for a doctoral degree (PhD), candidates submit a thesis that provides a critical review of the current state of knowledge of the thesis subject as well as the student’s own contributions to the subject. The distinguishing criterion of doctoral graduate research is a significant and original contribution to knowledge.
Once accepted, the candidate presents the thesis orally. This oral exam is open to the public.
The adverse effects of liquid cargo slosh on dynamic responses and safety performance of partially filled road vehicles are well known. The general-purpose railway tank car may also encounter partial fill conditions due to variations in density of the liquid cargo and track load limit. The additional slosh forces and moments from the partially filled state of the tank may further affect the wheel-rail forces, dynamic response, and safety of the rail vehicles. The coupled sloshing cargo and vehicle system dynamics have been investigated in only a few studies because of complexity and high computation demand. The objective of this study is to investigate the influence of liquid cargo sloshing on the dynamic performance of railway tank cars. Other than detail modelling, the topics of lateral dynamics, curving performance and switch passing responses of partially filled railway tank cars through a co-simulation approach have been addressed in this dissertation.
The nonlinear damping characteristics of friction wedges in the secondary suspension of a freight wagon are investigated considering non-smooth unilateral contact, multi-axis motions, slip-stick conditions, and geometry of the wedges. The parameters of the contact pairs within the suspension were identified to achieve smooth and efficient numerical solutions while ensuring adequate accuracy. The friction wedge model was integrated into the multibody dynamic model of a three-piece bogie to study the effects of wedge properties on hunting characteristics. The resulting 114-degrees-of-freedom wagon model incorporated constraints due to side bearings, axle boxes, and the centre plates, while the wheel-rail contact forces were obtained using the FastSim algorithm. The simulation results were obtained to study hunting characteristics of the wagon in terms of critical speed and the predominant oscillation frequency. The study also examined the effects of wedge friction and geometry on lateral stability characteristics of the freight car. The results showed subcritical Hopf bifurcation of dynamic responses of the wagon. The parametric study showed an increase in the wedge angle, friction coefficient, and springs free length to yield higher critical speed. The validated dynamic model of the wagon is further used to investigate the effects of liquid sloshing on hunting speed of partly filled tank car. An analytical liquid slosh model is used to capture dynamic response of the liquid cargo in a horizontal cylindrical tank using up to five fundamental modes in the roll plane under lateral as well as yaw motions of the tank car. The liquid slosh model is co-simulated with the comprehensive nonlinear model of a railway tank car to evaluate the lateral dynamic response of the tank car. The results suggest that fill levels and the corresponding slosh forces can adversely affect the lateral stability performance and yield lower critical hunting speed of railway tank car.
The influence of liquid cargo sloshing on the dynamic performance of railway tanker in a typical curve negotiation is further examined using the proposed coupled co-simulation model. The performance measures include car roll angle, unloading of the wheelset and derailment quotient. The results clearly demonstrate that partial state of the tank car and resulting slosh could lead to a significantly larger dynamic response of the system and may result in separation of wheel and rail contact at lower forward speed in comparison to the rigid cargo assumption. Dynamic simulations of partially filled railway tank cars without fluid slosh consideration will thus lead to underestimation of overturning critical conditions on the curving manoeuvres. Above performance measures in a switch-passing manoeuvre is finally examined for different fill ratios and switch geometries. The results obtained for the coupled vehicle and liquid slosh model clearly showed strong interactions between the switch-induced transient liquid slosh and vehicle dynamics for the partial-fill ratio of 60% and less. The effect of fluid slosh on the car body roll angle and wheelset unloading ratio was observed to diminish with fill ratio above 90%. Neglecting the contributions due to dynamic slosh force and roll moment may lead to overestimation of the critical speed in switch-passing manoeuvres if the car is partially filled.