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Search Results: 1 - 10 of 223787 matches for " R Bassily "
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Customized toric intraocular lens implantation for correction of extreme corneal astigmatism due to corneal scarring
R Bassily, J Luck
Clinical Optometry , 2010, DOI: http://dx.doi.org/10.2147/OPTO.S9305
Abstract: stomized toric intraocular lens implantation for correction of extreme corneal astigmatism due to corneal scarring Case report (3583) Total Article Views Authors: R Bassily, J Luck Published Date March 2010 Volume 2010:2 Pages 21 - 24 DOI: http://dx.doi.org/10.2147/OPTO.S9305 R Bassily, J Luck Ophthalmology Department, Royal United Hospital, Combe Park, Bath, UK Abstract: A 76-year-old woman presented with decreased visual function due to cataract formation. Twenty-five years prior she developed right sided corneal ulceration that left her with 10.8 diopters (D) of irregular astigmatism at 71.8° (steep axis). Her uncorrected visual acuity was 6/24 and could only ever wear a balanced lens due to the high cylindrical error. Cataract surgery was planned with a custom designed toric intraocular lens (IOL) with +16.0 D sphere inserted via a wound at the steep axis of corneal astigmatism. Postoperative refraction was -0.75/+1.50 × 177° with a visual acuity of 6/9 that has remained unchanged at six-week follow-up with no IOL rotation. This case demonstrates the value of high power toric IOLs for the correction of pathological corneal astigmatism.
Customized toric intraocular lens implantation for correction of extreme corneal astigmatism due to corneal scarring
R Bassily,J Luck
Clinical Optometry , 2010,
Abstract: R Bassily, J LuckOphthalmology Department, Royal United Hospital, Combe Park, Bath, UKAbstract: A 76-year-old woman presented with decreased visual function due to cataract formation. Twenty-five years prior she developed right sided corneal ulceration that left her with 10.8 diopters (D) of irregular astigmatism at 71.8° (steep axis). Her uncorrected visual acuity was 6/24 and could only ever wear a balanced lens due to the high cylindrical error. Cataract surgery was planned with a custom designed toric intraocular lens (IOL) with +16.0 D sphere inserted via a wound at the steep axis of corneal astigmatism. Postoperative refraction was -0.75/+1.50 × 177° with a visual acuity of 6/9 that has remained unchanged at six-week follow-up with no IOL rotation. This case demonstrates the value of high power toric IOLs for the correction of pathological corneal astigmatism.Keywords: intraocular lens, corneal ulceration, visual acuity, scarring
Ergodic Secret Alignment
Raef Bassily,Sennur Ulukus
Mathematics , 2010,
Abstract: In this paper, we introduce two new achievable schemes for the fading multiple access wiretap channel (MAC-WT). In the model that we consider, we assume that perfect knowledge of the state of all channels is available at all the nodes in a causal fashion. Our schemes use this knowledge together with the time varying nature of the channel model to align the interference from different users at the eavesdropper perfectly in a one-dimensional space while creating a higher dimensionality space for the interfering signals at the legitimate receiver hence allowing for better chance of recovery. While we achieve this alignment through signal scaling at the transmitters in our first scheme (scaling based alignment (SBA)), we let nature provide this alignment through the ergodicity of the channel coefficients in the second scheme (ergodic secret alignment (ESA)). For each scheme, we obtain the resulting achievable secrecy rate region. We show that the secrecy rates achieved by both schemes scale with SNR as 1/2log(SNR). Hence, we show the sub-optimality of the i.i.d. Gaussian signaling based schemes with and without cooperative jamming by showing that the secrecy rates achieved using i.i.d. Gaussian signaling with cooperative jamming do not scale with SNR. In addition, we introduce an improved version of our ESA scheme where we incorporate cooperative jamming to achieve higher secrecy rates. Moreover, we derive the necessary optimality conditions for the power control policy that maximizes the secrecy sum rate achievable by our ESA scheme when used solely and with cooperative jamming.
Causal Erasure Channels
Raef Bassily,Adam Smith
Mathematics , 2014,
Abstract: We consider the communication problem over binary causal adversarial erasure channels. Such a channel maps $n$ input bits to $n$ output symbols in $\{0,1,\wedge\}$, where $\wedge$ denotes erasure. The channel is causal if, for every $i$, the channel adversarially decides whether to erase the $i$th bit of its input based on inputs $1,...,i$, before it observes bits $i+1$ to $n$. Such a channel is $p$-bounded if it can erase at most a $p$ fraction of the input bits over the whole transmission duration. Causal channels provide a natural model for channels that obey basic physical restrictions but are otherwise unpredictable or highly variable. For a given erasure rate $p$, our goal is to understand the optimal rate (the capacity) at which a randomized (stochastic) encoder/decoder can transmit reliably across all causal $p$-bounded erasure channels. In this paper, we introduce the causal erasure model and provide new upper bounds and lower bounds on the achievable rate. Our bounds separate the achievable rate in the causal erasures setting from the rates achievable in two related models: random erasure channels (strictly weaker) and fully adversarial erasure channels (strictly stronger). Specifically, we show: - A strict separation between random and causal erasures for all constant erasure rates $p\in(0,1)$. - A strict separation between causal and fully adversarial erasures for $p\in(0,\phi)$ where $\phi \approx 0.348$. - For $p\in[\phi,1/2)$, we show codes for causal erasures that have higher rate than the best known constructions for fully adversarial channels. Our results contrast with existing results on correcting causal bit-flip errors (as opposed to erasures) [Dey et. al 2008, 2009], [Haviv-Langberg 2011]. For the separations we provide, the analogous separations for bit-flip models are either not known at all or much weaker.
Local, Private, Efficient Protocols for Succinct Histograms
Raef Bassily,Adam Smith
Computer Science , 2015, DOI: 10.1145/2746539.2746632
Abstract: We give efficient protocols and matching accuracy lower bounds for frequency estimation in the local model for differential privacy. In this model, individual users randomize their data themselves, sending differentially private reports to an untrusted server that aggregates them. We study protocols that produce a succinct histogram representation of the data. A succinct histogram is a list of the most frequent items in the data (often called "heavy hitters") along with estimates of their frequencies; the frequency of all other items is implicitly estimated as 0. If there are $n$ users whose items come from a universe of size $d$, our protocols run in time polynomial in $n$ and $\log(d)$. With high probability, they estimate the accuracy of every item up to error $O\left(\sqrt{\log(d)/(\epsilon^2n)}\right)$ where $\epsilon$ is the privacy parameter. Moreover, we show that this much error is necessary, regardless of computational efficiency, and even for the simple setting where only one item appears with significant frequency in the data set. Previous protocols (Mishra and Sandler, 2006; Hsu, Khanna and Roth, 2012) for this task either ran in time $\Omega(d)$ or had much worse error (about $\sqrt[6]{\log(d)/(\epsilon^2n)}$), and the only known lower bound on error was $\Omega(1/\sqrt{n})$. We also adapt a result of McGregor et al (2010) to the local setting. In a model with public coins, we show that each user need only send 1 bit to the server. For all known local protocols (including ours), the transformation preserves computational efficiency.
Experimental Study of Filling Ratio Effect on the Thermal Performance in a Multi-Heat Pipe with Graphene Oxide/Water Nanofluids  [PDF]
Mohamed Salem, Tarek A. Meakhail, Magdy A. Bassily, Shuichi Torii
World Journal of Nano Science and Engineering (WJNSE) , 2016, DOI: 10.4236/wjnse.2016.64014
Abstract: This experimental study is performed to investigate heat transfer performance of a multi-heat pipe cooling device in the condition of different filling ratios (40%, 60%, 80% and 100%) and different constant heat fluxes (10 - 30 W). Here, pure water (distilled water) and graphene oxide (GO)/water nanofluids are employed respectively as working fluid. GO/water nanofluids were synthesized by the modified Hummers method with 0.05%, 0.10%, 0.15%, and 0.20% volume concentrations. Multi-heat pipe is fabricated from copper; the heating and cooling sections are the same size and both are connected by four circular parallel tubes. Temperature fields and thermal resistance are measured for different filling ratio, heat fluxes and volume concentrations. The results indicated that the thermal performance of heat pipe increased with increasing the concentration of GO nanoparticles in the base fluid, while the maximum heat transfer enhancement was observed at 0.20% volume concentration. GO/water nanofluids showed lower thermal resistance compared to pure water; the optimal thermal resistance was obtained at 100% filling charge ratio with 0.20% volume concentration. Studies were also demonstrated that heat transfer coefficient of the heat pipe significantly increases with increasing the input heat flux and GO nanoparticles concentration.
Differentially Private Empirical Risk Minimization: Efficient Algorithms and Tight Error Bounds
Raef Bassily,Adam Smith,Abhradeep Thakurta
Computer Science , 2014,
Abstract: In this paper, we initiate a systematic investigation of differentially private algorithms for convex empirical risk minimization. Various instantiations of this problem have been studied before. We provide new algorithms and matching lower bounds for private ERM assuming only that each data point's contribution to the loss function is Lipschitz bounded and that the domain of optimization is bounded. We provide a separate set of algorithms and matching lower bounds for the setting in which the loss functions are known to also be strongly convex. Our algorithms run in polynomial time, and in some cases even match the optimal non-private running time (as measured by oracle complexity). We give separate algorithms (and lower bounds) for $(\epsilon,0)$- and $(\epsilon,\delta)$-differential privacy; perhaps surprisingly, the techniques used for designing optimal algorithms in the two cases are completely different. Our lower bounds apply even to very simple, smooth function families, such as linear and quadratic functions. This implies that algorithms from previous work can be used to obtain optimal error rates, under the additional assumption that the contributions of each data point to the loss function is smooth. We show that simple approaches to smoothing arbitrary loss functions (in order to apply previous techniques) do not yield optimal error rates. In particular, optimal algorithms were not previously known for problems such as training support vector machines and the high-dimensional median.
Performance Investigation of the Solar Chimney Power Plants Heater Case Study in Aswan, Egypt  [PDF]
Magdy Bassily Hanna, Tarek Abdel-Malak Mekhail, Omar Mohamed Dahab, Mohamed Fathy Cidek Esmail, Ahmed Rekaby Abdel-Rahman
Journal of Power and Energy Engineering (JPEE) , 2016, DOI: 10.4236/jpee.2016.410003
Abstract: Solar collector is a thermal device that uses the heated air in the power generation and many engineering applications. The purpose of the present work is to study the performance and temperature distribution for the solar collector which uses heated air in solar chimney power generation that it consist of three parts, a turbine-generator unit which is used in the generation of electric energy, and cylindrical chimney is fixed vertically and finally a solar collector under the climatic conditions of Egypt-Aswan is studied. This site is specified as the hottest site because the nearest of this location from the Tropic of cancer. Experiments are performed in ten summer days of May and June 2015 with different solar radiations and clarity of the sky. Hourly values of global solar radiation and some meteorological data (temperature, pressure, velocity, etc.) for measuring days are obtained by measuring devices. Inlet and outlet temperatures of air from a solar collector and velocity at junction region. In this work, attempt has been made to present the effect of environmental factors such as ambient temperature, the clarity of the sky and solar radiation on the performance of solar collector. The temperature of the base and the cover of the solar collector, the variation of solar radiation, solar collector efficiency, heat transfer coefficient, the velocity at the junction region between the chimney base, the outlet of the solar collector and temperature distribution along the air heater are discussed. A prediction for the results of the solar collector were performed by using developed theoretical model was made by this study which is based on the previous works. The numerical study has used a commercial code CFX, ANSYS 16.1 to simulate the flow through the collector. The study show that the outlet air temperatures from the solar collector and the velocity at the junction are depending on the climate condition such as ambient temperature and solar radiation, the differences in air temperature at the solar collector ranging between 8° - 24°. It is concluded that the theoretical model is basically valid for the system under study, and theCFD simulation can be used conveniently to predict the performance of the system, the comparison between them and experimental result shows a good agreement.
Experimental and Numerical Investigation of the Solar Chimney Power Plant’s Turbine  [PDF]
Magdy Bassily Hanna, Tarek Abdel-Malak Mekhail, Omar Mohamed Dahab, Mohamed Fathy Cidek Esmail, Ahmed Rekaby Abdel-Rahman
Open Journal of Fluid Dynamics (OJFD) , 2016, DOI: 10.4236/ojfd.2016.64025
Abstract: The solar chimney power plant is a relatively new electricity generation concept, based on renewable energy, combining the greenhouse effect with the chimney suction. The solar chimney powerplant consists of three parts, the solar collector, the chimney and the turbine generator unit, of which the study was focused on the later part. To evaluate the turbine performance inside the solar chimney powerplant, experimental system was constructed in Aswan, Egypt that has a metrological site (23°58'N and 32°47'E) occurs. The system was constructed to evaluate the performance of the solar chimney turbine and power generation characteristic in the hottest site where Aswan is located at the nearest of the Tropic of Cancer at the summer season. Velocity, electric power generation and the turbine efficiency are studying in this work. The numerical analyses were performed by using a commercial code CFX, ANSYS 16.1 to simulate the flow through the turbine and overall system. The study shows that the range of power generated (1.2 W - 4.4 W). It can be estimated, according to the results, the variation trend in pressure drops with the turbine rotation speed increase with small differences when the turbine rotation speed surpasses 1800 rpm with average efficiency of 57%. It is concluded that the theoretical model is basically valid for the system under study, and the CFD simulation can be used conveniently to predict the performance of the system, the comparison between them and experimental result shows a good agreement.
An Investigation on the Effect of Endwall Movement on the Tip Clearance Loss Using Annular Turbine Cascade
Hesham M. El-Batsh,Magdy Bassily Hanna
International Journal of Rotating Machinery , 2011, DOI: 10.1155/2011/489150
Abstract: The aerodynamic losses in gas turbines are mainly caused by profile loss secondary flow, and tip leakage loss. This study focuses on tip leakage flow of high-pressure turbine stages. An annular turbine cascade was constructed with fixed blades on the casing, and the distance between blade tip and the hub was considered as tip clearance gap. The effect of endwall movement on loss mechanism was investigated by using experimental and numerical techniques. The measurements were obtained while the hub was fixed but the numerical calculations were carried out for both stationary and moving cascades. Upstream and downstream flows were measured by using a calibrated five-hole pressure probe. The steady incompressible turbulent flow was obtained by solving Reynolds averaged Navier-Stokes equations and by employing shear stress transport (SST) k-ω turbulence model. The total pressure loss coefficient obtained from the numerical technique was compared with the experimental measurements, and the comparison showed good agreement. Tip clearance vortices were observed in the tip clearance gap. It was found through this study that end-wall movement reduces tip leakage loss through the cascade. 1. Introduction The applications of gas turbine engines have growing importance in the fields of electric power generation as well as marine and aerospace applications. Gas turbine engines have large powers, and, therefore, increasing the efficiency of gas turbines reduces fuel consumption and increases the economic operation of these machines. In order to increase the efficiency, detailed and fundamental comprehension of loss mechanisms is necessary. In turbines, clearance gaps are necessary to avoid the contact between rotating blades and the fixed casing. Tip leakage flow is induced due to pressure difference between blade pressure and suction surfaces. The leakage flow interacts with the passage vortex and produces aerodynamic losses. Although tip clearance in low-pressure turbines is usually about 1% of the blade height, tip clearance loss increases the total flow losses in a turbine and contributes to the efficiency degradation of gas turbines. In high-pressure stages, tip clearance gap is typically about 6% of the blade height due to the reduction of the blade length [1]. In this case, tip clearance loss has a larger influence on the overall stage loss. In previous studies, experimental and numerical techniques were used to investigate the flow and loss mechanisms in turbine cascades. While experimental measurements give clear qualitative indications on loss levels,
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