Optimal Arrangements of Cartridges In Carousel Type Mass Storage Systems

Optimal arrangements of cartridges and file partitioning schemes are examined in carousel type mass storage systems using Markov decision theory. It is shown that the Organ-Pipe Arrangement is optimal under different storage configurations for both the anticipatory as well as the non-anticipatory versions of the problem. When requests arrive as per an arbitrary renewal process this arrangement is also shown to minimize the mean queueing delay and the time spent in the system by the requests.Information Systems Working Papers Serie

Relation between molecular structure and ultrafast photoreactivity with application to molecular switches

Photoinduced ultrafast isomerizations are fundamental reactions in photochemistry and photobiology. This thesis aims for an understanding of the generic forces driving these reactions and a theoretical approach is set up, able to handle realistic systems, whose fast relaxation is mediated by conical intersections. The main focus is on the development of strategies for the prediction and accelerated optimization of conical intersections and their application to artificial compounds with promising physicochemical properties for technical applications as molecular switches. All calculations are based on advanced quantum chemical methods and mixed quantum-classical dynamics. In the first part of this thesis the two-electron two-orbital theory by Michl and Bonacic-Koutecky used in its original formulation to rationalize the structure of conical intersections in charged polyene systems is extended by including the interactions of the active pair of electrons with the remaining closed-shell electrons that are present in any realistic system. A set of conditions, called resonance and heterosymmetry conditions, for the formation of conical intersections in multielectronic systems are derived and verified by calculations on the basic units ethylene, cis-butadiene and 1,3-cyclohexadiene at various geometries and functionalizational patterns. The quantitative results help to understand the role of geometrical deformations and substituent effects for the formation of conical intersections and to derive rules of thumb for their qualitative prediction in arbitrary polyenes. An extension of the rules of thumb to conical intersection seams is formulated. The strategy pursued is to divide the molecular system into basic units and into functional groups. Each unit and its intersection space are treated independently, thereby reducing the dimensionality of the search space compared to the complete molecule. Subsequently, the interconnectivity of the intersection spaces of the different units is determined and an initial guess for the complete seam is constructed. This guess is then fed into a quantum chemistry package to finalize the optimization. The strategy is demonstrated for two multi-functionalized systems, hemithioindigo-hemistilbene and trifluoromethyl-pyrrolylfulgide. In the second part of this thesis state-of-the-art quantum chemical calculations and time-resolved transient and infrared spectroscopy are used to reconstruct the complex multi-channel isomerization mechanisms of hemithioindigo-hemistilbene and trifluoromethyl-indolylfulgide. Both the cis-trans isomerization in hemithioindigo-hemistilbene and the electrocyclic ring closure/opening in indolylfulgide are characterized by a charge transfer in the excited state. The ability of each system to stabilize this charge transfer is essential for the returning to the ground state. The relaxation to the ground state through extended regions of the seam is found to be the decisive step determining the reaction speed and the quantum yield. In the last part of this thesis mixed quantum-classical dynamics simulations at multi-configurational perturbation theory (MS-CASPT2) level, using Tully's fewest switches surface hopping approach, are performed to study the ultrafast photoreactivity of 1,3-cyclohexadiene in the gas-phase. For this purpose a numerical routine for the efficient calculation of non-adiabatic couplings at MS-CASPT2 level is presented. The major part of the excited molecules are found to circumvent the 1B2/2A1 conical intersection and reach the conical intersection seam between the excited state and the ground state instantaneuosly. Time constants for the evolution of the wavepacket on the bright 1B2-state, the relaxation into the 2A1-state and the return to the ground state are extracted. It is demonstrated that the accessibility of the conical intersection seam depends on its energetic and spatial relation to the minimum energy path, as well as on the momentum which is accumulated during relaxation on the excited state potential energy surface

Rheology and Collective Behavior in Living Tissue

Recent experiments and simulations have indicated that confluent epithelial layers, where there are no gaps or overlaps between the cells, can transition from a soft fluid-like state to a solid-like state, with dynamics that share many features with glass transitions. While a coherent picture has begun to form connecting the microscopic mechanisms that drive this transition with macroscopic observables, much less is known of its consequences in biological processes. Do tissues tune themselves to a fluid state in order to promote collective motion? Has evolution made use of the ability of tissues to tune themselves between fluid and solid states in programming the complex steps leading from the embryo to the organism? Here we describe our recent e↵orts to answer such questions using continuum and mesoscopic models. Employing the biophysical vertex model, active cells in confluent tissue are described as polygons with shape-based energies. Recent work has shown that this class of models yields a solid-liquid transition of tissue with evidence of glassy dynamics near the transition line. Here, we extend one such model to include the influence of cell division and cell death. With careful numerical studies, we refute a recent claim that the presence of such division and death will always fluidify the tissue. In the second part of the thesis, we develop a novel hydrodynamic model of confluent motile tissues that couples a structural order parameter for tissue rigidity to cell polarization. Using this continuum model we identify a new mechanism for pattern formation in confluent tissues via rigidity sensing that we name “morphotaxis”. We find that a single “morphotactic” parameter controls whether a tissue will remain homogeneous or will develop patterns such as asters and bands

A NUMERICAL APPROACH TO OHMIC LOSSES ASSESSMENT IN CONCENTRATING PHOTOVOLTAIC SYSTEMS

It is well known that concentrator solar cells operating under concentration experience a number of physical effects which affect their performances. In particular, ohmic losses can determine a noticeable performance worsening of concentrator solar cells. The goal of this dissertation is to develop a distributed electrical model of solar cell in order to simulate the operation of concentrator solar cells in a number of working conditions characteristic of Concentrating Photovoltaic (CPV) systems, such as uneven illumination profiles with arbitrary spectral distributions. To this end a mixed optical-electrical simulation tool has been developed in order to assess the performances of a typical concentrator solar cell in the case of illumination provided by different kinds of concentrators; in particular a Fresnel lens, a parabolic mirror and a freeform mirror have been considered and compared. At high concentration factors front contact grid pattern has a key role in extracting photogenerated charges, and hence it is another factor that can strongly affect the cell performances; for this reason the above mentioned distributed electrical model has also been applied to the assessment of ohmic losses impact on concentrator silicon solar cells performances in the presence of different kinds of front contact grid patterns. In particular, a comb-like geometry, a square-like geometry and a novel fractal autosimilar geometry have been simulated and compared. Another aspect investigated in this dissertation is the formation of voids in the solder joint region, during soldering process of concentrator solar cells to Metal Core Printed Circuit Boards (MC-PCB). Some commercially available silicon solar cells have been soldered in such a way that a great number of voids have arisen and their distribution has been revealed by X-ray inspection. Electrical and thermal behaviour of one of the cells has been assessed by a joint thermal-electrical simulation tool. In this thesis electrical, optical and thermal simulations have been performed by means of ORCAD PSPICE software, ZEMAX software and ADINA 8.7 software, respectively