Verifiable Outsourced Database Model: A Game-Theoretic Approach

In the verifiable database (VDB) model, a computationally weak client (database owner) delegates his database management to a database service provider on the cloud, which is considered untrusted third party, while users can query the data and verify the integrity of query results. Since the process can be computationally costly and has a limited support for sophisticated query types such as aggregated queries, we propose in this research a framework that helps bridge the gap between security and practicality. The proposed framework remodels the verifiable database problem using Stackelberg security game. In the new model, the database owner creates and uploads to the database service provider the database and its authentication structure (AS). Next, the game is played between the defender (verifier), who is a trusted party to the database owner and runs scheduled randomized verifications using Stackelberg mixed strategy, and the database service provider. The idea is to randomize the verification schedule in an optimized way that grants the optimal payoff for the verifier while making it extremely hard for the database service provider or any attacker to figure out which part of the database is being verified next. We have implemented and compared the proposed model performance with a uniform randomization model. Simulation results show that the proposed model outperforms the uniform randomization model. Furthermore, we have evaluated the efficiency of the proposed model against different cost metrics

EXPERIMENTAL INVESTIGATION OF A DOUBLE-STAGE SHROUDED HORIZONTAL AXIS WIND TURBINE

An experimental study is presented in this paper to check the ability of using a new power system, which utilizes the destroyed energy that is caused by the lens of a shrouded wind turbine. Instead of destroying the outer wind energy surrounding the wind turbine shroud by means of the lens, the lens will be replaced by a ring cascade that gives the duty of the lens in creating a low energy region behind the shroud to permit the flow with low energy to pass to the surrounding and in the same time can give excess power from the surrounding wind. A ring cascade working as second-stage wind turbine will be placed in the same location of the lens with its main dimensions. Experimental measurements are performed on three cases, namely a constant chord non twisted three blades wind turbine of 0.94 m outer diameter without shroud (bar wind turbine), shrouded wind turbine with lens, and shrouded wind turbine with second stage (ring cascade). The experimental results demonstrated that, the maximum power coefficient for the wind turbine in case of using a second stage is 0.4 compared to 0.34 for a flanged diffuser wind turbine. Furthermore, relatively uniform and wider operation range is obtained for different tip speed ratios

Improving performance of H-Type NACA 0021 Darrieus rotor using leading-edge stationary/rotating microcylinders: Numerical studies

In the current paper, a computational study has been performed to simulate a novel concept in which a fixed/ rotating microcylinder is installed upstream of the leading edge of the NACA 0021 airfoil, aiming to enhance the performance of the H-type Darrieus rotor. The idea behind implementing a fixed/rotating microcylinder is to enhance the lift to drag ratio due to increasing near-wall momentum through generated vortices transferring the momentum from the outer flow to the near-wall flow region of the airfoil. Parametric analyses for the microcylinder including its location, size, shape (circular, square and rhombus), and rotation were performed. The commercial Computational Fluid Dynamics (CFD) package ANSYS Fluent was used for solving the Unsteady Reynolds Averaged Navier-Stokes (URANS), and turbulence equations. The code has been based on Finite Volume Method (FVM) with the pressure-based solver developed for low-speed incompressible flows. The semiimplicit method for pressure-linked equation (SIMPLE) was used to solve the discretized equations. The URANS was used with adopting the SST k-& omega; turbulence model to find out the optimum rotor by utilizing the microcylinder model. An optimization methodology using Response Surface Optimization (RSM) based on Kriging method has been first performed to find the optimum size and location of a circular microcylinder. The optimization study indicated an optimum microcylinder diameter (d/C = 0.0085313), at an upstream chordwise distance and normal to it 0.070275 and 0.02303 of the chord length (C), respectively. The results showed also that a small static circular microcylinder of d/C = 0.009 and installed at 0.075C from the leading edge (MC5) is an efficient geometry at high regime of tip-speed ratios (TSR & GE; 2.2) rather than lower regime of TSRs. When these superior set of rotating (5 rad/s) microcylinder (MC5) parameters were adopted, significant enhancement of power coefficient (Cp) could be achieved and a considerable improvement of the maximum power coefficient (Cpmax) up to 120 % at TSR = 3 appeared. A physical analysis of the flow fields using the contours of vorticity, pressure and turbulence energy has been performed to illustrate the effect of rotating microcylinder for improving rotor performance. The vorticity contours showed the ability of rotating microcylinder for generating a strong vortex structure in its wake for enhancing turbulence production around the blades, which delayed the known strong stall of the blades by diminishing the separation bubble size. This generally improved the blades aerodynamic performance and enhancing the lift-to-drag ratio

Aerodynamic Analysis of a Wind-Turbine Rotor Affected by Pitch Unbalance

The aerodynamics of a rotor with pitch imbalance has been investigated experimentally and numerically in the present work. The comparison of mean velocity and turbulence intensity in the balanced and unbalanced cases indicated that a pitch imbalance modifies both the mean velocity and the turbulent activity; the latter is weakly increased by the imbalance. Spectral analysis indicated that the dynamics of the wake is also affected by the pitch imbalance since the tip vortices loose strength and disorganise more quickly than in the balanced case. The pitch imbalance has, however, a detrimental effect on the power coefficient and it affects the thrust coefficient as well. Only the blade affected by the imbalance shows significant modifications of the applied load, while the other blades operate with the same loading conditions.</jats:p

Experimental and Numerical Study of the Wind Tunnel Blockage Effects on the Behaviour of a Horizontal Axis Wind Turbine

none4siThis study presents an experimental and numerical investigation of the effects of tunnel blockage, defined as the ratio of the free portion of a wind tunnel cross-section with and without the rotor of a small-size (diameter 2Â&nbsp;m) horizontal-axis turbine (HAWT). Experimental measurements were performed on three rotors with different number of blades, namely three, five and six, and different tip speed ratios, in the closed-loop closed-test chamber wind tunnel of the University of Perugia. Numerical wind tunnel simulations were performed at the University of Genoa through a steady-RANS method with SST k-ω turbulence model by means of the CFD solver OpenFOAM. The wind turbine was represented numerically by the actuator disc method. Simulations have been compared to experimental results for validation.mixedEltayesh A.; Burlando M.; Castellani F.; Becchetti M.Eltayesh, A.; Burlando, M.; Castellani, F.; Becchetti, M

Aerodynamic Analysis of a Wind-Turbine Rotor Affected by Pitch Unbalance

The aerodynamics of a rotor with pitch imbalance has been investigated experimentally and numerically in the present work. The comparison of mean velocity and turbulence intensity in the balanced and unbalanced cases indicated that a pitch imbalance modifies both the mean velocity and the turbulent activity; the latter is weakly increased by the imbalance. Spectral analysis indicated that the dynamics of the wake is also affected by the pitch imbalance since the tip vortices loose strength and disorganise more quickly than in the balanced case. The pitch imbalance has, however, a detrimental effect on the power coefficient and it affects the thrust coefficient as well. Only the blade affected by the imbalance shows significant modifications of the applied load, while the other blades operate with the same loading conditions

Experimental and numerical investigation for PV cooling by forced convection

A photovoltaic (PV) cell is very sensitive to temperature changes where decreasing temperature plays the main role in the increase of PV electrical efficiency and output power. Therefore, researchers used different techniques for cooling PV modules to avoid immoderate heating PV modules and to decrease the panel temperature, resulting in raising power output, energy efficiency, performance, and life of the panel. In the present work, PV panels are cooled by forced convection. Cooling airflow characteristics and panel temperature distribution are examined using computational fluid dynamics (CFD). In order to evaluate CFD predictions, experimental measurements are obtained in a sunny day on 23rd September 2021 for the period from 10 AM to 4 PM. The experimental measurements are performed on three different arrangements of PV panels. The uncooled panel was considered as the reference case. Two different cooling methods were examined: PV panels with forced air-cooling using a lower duct and supplying air using the blower, and PV panels with forced air-cooling using small fans arranged symmetrically on the backside of the PV panels. The temperatures determined by the CFD calculations are compared to the experimentally measured temperatures and it was found to be in good agreement. The results showed that cooling PV using small backside fans can enhance the performance and achieve a maximum total increase of 2.1% in PV panel efficiency with 7.9% saving energy. Using the blower cooling technique achieves a maximum total increase of 1.34% in PV panel efficiency with 4.2% saving energy

Experimental and Numerical Investigation of the Effect of Blades Number on the Dynamic Response of a Small Horizontal-Axis Wind Turbine

The new energy scenario is rising a strong interest towards the distributed production from renewable sources; among them, wind seems to be one of the most interesting and at the same time critical due to the challenges of the conversion technology. Operation of small wind energy conversion systems is generally very complex due the extremely variable regime of operation and the high turbulent wind, so that a deep knowledge of the dynamic response of such devices is crucial not only for optimising their performances but also to make them comfortable to be used in residential areas. For these reasons, the present work is focused on analysing the effect in changing the number of blades on a small horizontal axis wind turbine through an experimental campaign in the wind tunnel. The turbine dynamics has been characterised running some “wind ramp” tests and analysing the rotor capability in following the wind as well the vibrations transmitted by the turbine during the operation. Results demonstrate that a higher number of blades, despite a small decrease of performance, can make the machine more efficient in operating at low wind regimes. At the same time, a higher number of blades can make the rotor efficient even at lower speed of rotation, thus limiting the risks of having high magnitude vibrations

Aerodynamic Analysis of a Wind-Turbine Rotor Affected by Pitch Unbalance

The aerodynamics of a rotor with pitch imbalance has been investigated experimentally and numerically in the present work. The comparison of mean velocity and turbulence intensity in the balanced and unbalanced cases indicated that a pitch imbalance modifies both the mean velocity and the turbulent activity; the latter is weakly increased by the imbalance. Spectral analysis indicated that the dynamics of the wake is also affected by the pitch imbalance since the tip vortices loose strength and disorganise more quickly than in the balanced case. The pitch imbalance has, however, a detrimental effect on the power coefficient and it affects the thrust coefficient as well. Only the blade affected by the imbalance shows significant modifications of the applied load, while the other blades operate with the same loading conditions