Call number: LD2668 .T4 1962 B39

Call number: LD2668 .T4 1979 S94 / Master of Science

published_or_final_version / Environmental Management / Master / Master of Science in Environmental Management

published_or_final_version / abstract / Mechanical Engineering / Master / Master of Philosophy

In the modern enterprises it is common that support and maintenance of IT environments are outsourced to third parties. In this setting, unencrypted confidential data may pose a problem since administrators maintaining the outsourced system can access confidential information if stored unencrypted. This thesis work, performed at ELITS, presents a solution to this problem; a design of a collaborative storage system where all files at rest (i.e. stored on disk) and in transit remain encrypted is proposed. The design uses a hybrid encryption scheme to protect the encryption keys used. The keys can safely be stored in a centralized database as well as sent to the clients without risk of unauthorized parties gaining access to the stored data. The design was also implemented as a proof of concept in order to establish that it was possible to realize.

This work evaluates the performance of Peer-to-Peer storage systems in structured Peer-to-Peer (P2P) networks under the impacts of a continuous process of nodes joining and leaving the network (Churn). Based on the Distributed Hash Tables (DHT), the peer-to-peer systems provide the means to store data among a large and dynamic set of participating host nodes. We consider the fact that existing solutions do not tolerate a high Churn rate or are not really scalable in terms of number of stored data blocks. The considered performance metrics include number of data blocks lost, bandwidth consumption, latencies and distance of matched lookups. We have selected Pastry, Chord and Kademlia to evaluate the e ect of inopportune connections/disconnections in Peer-to-Peer storage systems, because these selected P2P networks possess distinctive characteristics. Chord is one of the rst structured P2P networks that implements Distributed Hash Tables (DHTs). Similar to Chord, Pastry is based on a ring structure, with the identi er space forming the ring. However, Pastry uses a di erent algorithm than Chord to select the overlay neighbors of a peer. Kademlia is a more recent structured P2P network, with the XOR mechanism for improving distance calculation. DHT deployments are characterized by Churn. But if the frequency of Churn is too high, data blocks can be lost and lookup mechanism begin to incur delays. In architectures that employ DHTs, the choice of algorithm for data replication and maintenance can have a signi cant impact on the performance and reliability. PAST is a persistent Peer-to-Peer storage utility, which replicates complete les on multiple nodes, and uses Pastry for message routing and content location. The hypothesis is that by enhancing the Churn tolerance through building a really e cient replication and maintenance mechanisms, it will: i) Operate better than a peer-to-peer storage system such as PAST especially in replica placement strategy with a fewer data transfers. ii) Resolve le lookups with a match that is closer to the source peer, thus con- serving bandwidth. Our research will involve a series of simulation studies using two network simulators OverSim and OMNeT++. The main results are: Our approach achieves a higher data availability in presence of Churn, than the original PAST replication strategy; For a Churn occuring every minute our strategy loses two times less blocks than PAST; Our replication strategy induces an average of twice less block transfers than PAST.

Thermal energy storage (TES) systems play a crucial part in the success of concentrated solar power as a reliable thermal energy source. The economics and operational effectiveness of TES systems are the subjects of continuous research for improvement, in order to lower the localized cost of energy (LCOE). This study investigates the use of spherical tanks and their role in sensible heat storage in liquids. In the two tank system, typical cylindrical tanks were replaced by spherical tanks of the same volume and subjected to heat loss, stress analysis, and complete tank cost evaluation. The comparison revealed that replacing cylindrical tanks by spherical tanks in two tank molten salt storage systems could result in a 30% reduction in heat loss from the wall, with a comparable reduction in total cost. For a one tank system (or thermocline system), a parametric computational fluid dynamic (CFD) study was performed in order to obtain fluid flow parameters that govern the formation and maintenance of a thermocline in a spherical tank. The parametric study involved the following dimensionless numbers: Re (500-7500), Ar (0.5-10), Fr (0.5-3), and Ri (1-100). The results showed that within the examined range of flow characteristics, the inlet Fr number is the most influential parameter in spherical tank thermocline formation and maintenance, and the largest tank thermal efficiency in a spherical tank is achieved at Fr = 0.5. Experimental results were obtained to validate the CFD model used in the parametric study. For the flow parameters within the current model, the use of an eddy viscosity turbulence model with variable turbulence intensity delivered the best agreement with experimental results. Overall, the experimental study using a spherical one tank setup validated the results of the CFD model with acceptable accuracy.

Collection of the author's previous publications. Includes bibliographies.

As the computer industry grows, so will the requirements for data storage. Magnetic memory has been the most stable method in terms of capacity and recording/reading speed. However, we have reached the point where a substantial increase in the capacity cannot be produced without increasing the size of the system. When compact discs (CDs) were introduced in the 1980�s they revolutionized the concept of data storage. While the initial force behind compact discs could easily be said to be the music industry, once recordable and rewritable discs became available they quickly found more use in the computer industry as backup devices. Since their inception, the capacity requirements have far exceeded what is available on a compact disc, and they are now following the same path as magnetic memories. Following this trend, it could be assumed that digital versatile discs or digital video discs (DVDs) have a limited lifetime as a storage medium. In fact it has been noted (Higuchi et al., 1999) that the maximum capacity of digital video discs will be reached in 3 � 5 years. The question then is, what comes next? The efficiency of conventional optical data storage is extremely poor. For an optically thick recording medium, both CDs and DVDs use less than 0.01% of the total volume to store the information. Three-dimensional bit optical data storage endeavors to increase the efficiency by recording information in a volume that is greater than 90% of the total volume. The concept of three-dimensional bit optical data storage was first proposed by Parthenopoulos and Rentzepis in 1989, where they demonstrated that capacities far exceeding that of compact discs could be achieved. Three-dimensional bit optical data storage relies on creating a highly localised chemical or physical change within a recording medium, such that further layers can be recorded without causing interference. Ideally the chemical/physical change in the material should be reversible to enable erasable/rewritable data storage. In order to create a highly localised effect nonlinear excitation can be used; whereby the excitation is limited to a small region around the focal spot. Depending on the material and recording method there are several techniques for reading the information such as transmission imaging or reflection confocal microscopy. However, all the recording and reading methods require focusing to a deep position within a recording medium, such focusing encounters spherical aberration as a result of the difference in the refractive indices between the immersion and recording media. This thesis has concentrated on several areas to understand and develop the concept of three-dimensional bit optical data storage. The photorefractive effect in crystals has been studied for many years and is now widely used in optoelectronic devices. The use of photorefractive polymers is a relatively new and exciting development in optical data storage. Until now they have been used solely in the area of holographic data storage. The research in this thesis was conducted using photorefractive materials that were fabricated in two polymer matrices, poly(N-vinylcarbazole) (PVK) and poly(Methyl Methacrylate) (PMMA). The recording samples also consisted of the following compounds in various proportions, 2,5-dimethyl-4-(p-nirtophenylazo)anisole (DMNPAA), 2,4,7-trinitro-9-fluorenone (TNF) and N-ethylcarbazole (ECZ). In this project two-photon excitation was used as the recording mechanism to achieve erasable/rewritable data storage in a photorefractive polymer. As a result of two-photon excitation, the quadratic dependence of excitation on the incident intensity produces an excitation volume that is confined to the focal region in both the transverse and axial directions. Therefore, focusing the laser beam above or below its previous position provides a method by which layers of information can be recorded in the depth direction of a material, without causing interference from neighbouring layers. The feasibility of two-photon excitation in photorefractive polymers is demonstrated in this thesis. The quadratic relationship between excitation and incident light in two-photon excitation requires high photon density to ensure efficient excitation. The use of ultra-short pulsed lasers, while effective, is not a practical solution for an optical data storage system. This thesis demonstrates the ability to produce three-dimensional erasable/rewritable data storage in a photorefractive polymer using continuous wave illumination. Using this technology it has been possible to achieve a density of 88 Gbits/cm3, which corresponds to a capacity of 670 Gbytes on a compact disc sized recording medium. This is an increase of 1000 times the capacity of a CD and 130 times the capacity of current DVDs. While erasable optical data storage is an exciting prospect there are problems associated with the deterioration of the information. For long term information storage a permanent recording process would be more practical. It is demonstrated that there is a point after which further increases in the recording power result in the formation of a micro-cavity. While two-photon excitation is the recording method for erasable data storage, the increase in power results in an increase in ultra-violet absorption such that multi-photon excitation may occur. This thesis demonstrates the ability to record multi-layered arrays of micro-cavities. The change in refractive index associated with an erasable bit is less than 1%. As a result only phase sensitive reading methods (transmission imaging or differential interference contrast (DIC) microscopy) can be used to image a recorded bit. Both transmission and DIC imaging systems have poor axial resolution and therefore limit the density of the recording system, as well as being large optical systems. The introduction of a split or quadrant detector reduces the size of the optical reading system and is demonstrated to be sensitive enough to detect the phase changes of a recorded bit. However, the change in refractive index across a micro-cavity is large enough that reflection confocal microscopy can be used to detect a bit. It is demonstrated in this thesis that multi-layered micro-cavity arrays can be read using reflection confocal microscopy. Focusing of light to deep positions within an optical thick recording medium has the effect of increasing spherical aberration resulting from the refractive index mismatching between the immersion and recording media. The work in this thesis illustrates the effect of spherical aberration on the performance of both the recording and reading systems. The work conducted in this thesis shows the ability to record multi-layered erasable/rewritable information in a photorefractive polymer using pulsed and continuous wave two-photon excitation. It has also been demonstrated that through multi-photon excitation multi-layered micro-cavity arrays can be fabricated. It has also been illustrated that while spherical aberration deteriorates the performance of the recording and reading systems it is possible to achieve a density of greater than 88 Gbits/cm3.

Thesis (M. Phil.)--University of Hong Kong, 2007. / Title proper from title frame. Also available in printed format.