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  • [ March 08, 2019]

    Building Toward the Digital Twin for Smart Logistics Operation

    | In this talk, we will introduce some new initiatives in Singapore in transforming the industry into the next generation industry, in particular in the logistics and maritime sectors. We will also present some of our latest work in developing a simulation optimization framework for digital twin for various systems, such as warehouse system and port system. This framework O2DES has the capability in incorporating optimization in the simulation, and optimizing the system parameters based on simulation output.
  • [ March 08, 2019]

    AGV Routing and Control for Transshipment Container Terminal

    | In this presentation, we develop a framework for routing and controlling the Automated Guided Vehicles (AGVs). As the AGV routing and control are done in real time, the framework requires the solutions to be generated very fast. To address this computational challenge, the solution framework consists of two stages. The first stage determines the rough routes of the AGVs. The second stage determine the detail routings and schedules of the AGVs. As solving the detail routings and schedules require high computing resource, we decompose the port layout into submodules where each of them can be solved almost independently. The output of the first stage (rough routes) becomes an input to the second stage which solve the detail routes and schedules for real time control. Whenever an AGV completes a task, the first stage will be called to revise the rough routes of the AGVs which includes the new task which is assigned to the AGV that has completed the previous task. The new rough routes will become input for second stage to solve the detail routes and schedules. We show this framework is promising and is able to generate the detail routes and schedules very fast.
  • [ Janurary 07, 2019]

    Turbulent/non-turbulent interfaces and their relation to turbulent entrainment

    | In many relevant flow cases, such as in jets or turbulent boundary layers, turbulent regions of the flow are bounded by non-turbulent fluid. The interface between such regions, the so-called turbulent/non- turbulent interface (TNTI) is known to remain sharp and well-defined even after long times. Here, we investigate how processes at the TNTI are related to turbulent entrainment, i.e. the incorporation of previously non-turbulent fluid into the turbulent flow. We show that turbulent entrainment is a viscous, small-scale process that is amplified by interface contortions across a range of scales. We also elucidate how these processes change in the presence of a stable stratification across the TNTI.
  • [ December 18, 2018]

    A Global Perspective on Energy Challenges and Mechanics

    | Based in Aberdeen, the “Energy Capital of Europe", from our recent research collaborations with the energy industries, and the experience chairing the 1st-9th International Symposium on Energy, this talk explores issues related to energy challenges and mechanics from a global perspective. The aim is to get a complete view of energy-related problems. The following topics are covered:
    - Life on earth currently faces a threat on a genuinely global scale.
    - Asset life extension.
    - The DESERTEC idea was initially developed in Germany for power production at a large scale, making use of solar energy in the deserts of North Africa with the option to transport a significant fraction of power to Europe.
    - Fusion energy is among the most environmentally friendly sources of energy.
  • [ December 18, 2018]

    Highly efficient differentiation of human iPSCs into functional endothelial cells using ETV2 mRNA

    | Current protocols to differentiate human induced pluripotent stem cells (h-iPSCs) into endothelial cells (h-iECs) lack reliability. Here we describe a method for rapid, consistent and highly efficient generation of h-iECs from h-iPSCs. The protocol entails delivery of modified mRNA encoding the transcription factor ETV2 at the intermediate mesodermal stage of differentiation. This approach reproducibly differentiated thirteen diverse h-iPSC lines into h-iECs with exceedingly high efficiency. In contrast, differentiation with the standard protocol, which relies on endogenous ETV2, was inefficient and notably inconsistent. Our method generated h-iECs that were functionally competent in many respects, including ability to form perfused vascular networks in vivo. Importantly, timely activation of ETV2 was critical and bypassing the mesodermal stage produced putative h-iECs with reduced expansion potential and that lacked ability to form functional vessels. Our protocol could have broad application in regenerative medicine and reliably provide an unlimited number of autologous h-iECs for vascular therapies.
  • [ December 14, 2018]

    Data-Driven Control of a Production System by Using Marking-Dependent Threshold Policy

    | This study is motivated by the need of developing effective data-driven control methods for production systems. Our aim is answering the following research questions: How can we control production by using partial information (markings) obtained from a production system?; How can the control parameters be determined by using the real-time data? How well does the data-driven control policy perform?; and How can the markings be selected for a given system?
    In order to analyze the performance of a production system controlled with partial information, we introduce the Marked Markov Arrival Process (MMAP) framework to model a system that generates different signals, referred as markings, based on the system status. We then propose a marking-dependent threshold policy to control the system. An analytical method that is based on a matrix analytical approach is developed to analyze a production/inventory system with MMAP information and demand arrivals and a partially observable production time process modeled as a MMAP. A mathematical programming formulation is used to determine the optimal thresholds of the control policy based on the matrix geometric model of the system. We then present a joint simulation and optimization (JSO) approach to determine the parameters of the threshold policy by using the shop-floor data collected from the system.
    We test these methods on two production systems.
    We show that using a marking-dependent control policy together with a JSO approach that determines the policy parameters works effectively as a data-driven control method for manufacturing.
  • [ December 05, 2018]

    Emulsion and foam stability: one drop and bubble at a time

    | The stability of emulsions and foams are affected by a number of physical processes. As thin liquid films surrounding bubbles and drops drain, film rupture can occur, leading to coalescence and to coarsening. A common approach to appreciating stability in these systems is to create a bulk emulsion or foam and to follow the rate of creaming (for an emulsion) or collapse and drainage (for a foam). In this lecture, an alternative approach is described whereby individual bubbles and drops are monitored as they approach a fluid/fluid interface until coalescence ensues. These measurements utilize a newly developed instrument, the Dynamic Fluid Film Interferometer (DFI) where draining film thicknesses can be measured in space and time as well as the pressure drop (Laplace pressure) across the interface. Application of the DFI to study two problems is presented: (1) coalescence of oil/water emulsions in the presence of asphaltenes and (2) antifoaming of lubricating oils.
  • [ December 03, 2018]

    Screening and Tomographic Reconstruction of Defects by Structural Guided Ultrasonic Waves

    | Detecting and quantifying damages in large structures is of growing interest in various industries. Conventional ultrasonic methods are tedious and expensive, especially for inaccessible areas. Ultrasonic structural guided wave offers an attractive alternative for the rapid inspection of defects from a remote position in a large structure. In this talk, two interesting applications of ultrasonic guide waves will be discussed: defect screening in structural features such as weld, stiffeners and bends, as well as defect characterization using guided wave tomographic approach. A numerical forward model is used to predict the scattering of guided wave through defects, and an iterative inverse model is developed to reconstruct the defect profile. The imaging algorithm allows higher order diffraction and scattering to be considered in its numerical solver, thus can provide accurate inversion results. Despite two guided wave applications, recent progress in the research of acoustic metamaterial and laser ultrasound will also be shared in the talk.
  • [ December 03, 2018]

    Airway smooth muscle cells migration and orientation on 3D surfaces: taking side

    | Airway smooth muscle cells (ASMCs) are known to migrate and assemble into helical band structures around the tubular airway wall, with the helical angle variable in states from normal to disease such as asthma. However, the mechanism underlying such morphogenesis of the ASMCs remains largely unknown.
    Using concave/convex tubular/folding substrate, we found that on the surface of the micropatterned tubular substrate, the migration and orientation of the ASMCs were closely dependent not only on the tube’s radius, but also on which side of the tube either the concave or convex side. Furthermore, we demonstrate that the migration and orientation behaviors of the ASMCS were dependent on the contractile state of the cells, primarily on the actin-myosin interaction. These behaviours were not observed with other pulmonary cell types such as FBs and EPCs.
    Our findings indicate that under in vitro culture condition the ASMCs could quickly determine the microenvironment of the tubular/folding micropatterned curved surface, and change their behaviors accordingly, which ultimately led the cells to self assemble into a morphology most fit for their survival and function. This phenomenon may have important implications to better understand the mechanisms of tissue morphogenesis of tubular organs such as the pulmonary airways in both health and diseased.
  • [ November 28, 2018]

    Control of “Valley” Properties in 2D Materials by Magnetism

    | Exploiting the “valley” degree of freedom to store and manipulate information is an emerging direction of condensed matter physics, and provides a novel paradigm for future electronics. Valley is the local extremum in the electronic band structure. Transition metal dichalcogenides (TMDC), such as MoS2, WS2 and WSe2, are semiconductor analogy of graphite with atomic layers bonded together by Van der Waals interactions. A monolayer TMDC with broken inversion symmetry possesses two degenerate valleys that can be selectively excited by circularly polarized light. Breaking the valley degeneracy allows convenient control of valley degree of freedom. This can be done by applying an external magnetic field to Zeeman split the band edge states. We demonstrate that the valley properties can be controlled by magnetism. We show that valley splitting can be enhanced by more than an order of magnitude, utilizing the interfacial exchange field from a ferromagnetic substrate. We further show that transition metal doped TMDs demonstrate ferromagnetism with their magnetization tunable by light. These approaches open up new avenue for valley control forvalleytronics applications.
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