Aeronautical Engineering

Spectrum sensing is a key function of cognitive radio to prevent the harmful interference with licensed users and identify the available spectrum for improving the spectrum's utilization. However, detection performance in practice is often compromised with multipath fading, shadowing and receiver uncertainty issues. To mitigate the impact of these issues, cooperative spectrum sensing has been shown to be an effective method to improve the detection performance by exploiting spatial diversity. While cooperative gain such as improved detection performance and relaxed sensitivity requirement can be obtained, cooperative sensing can incur cooperation overhead. The overhead refers to any extra sensing time, delay, energy, and operations devoted to cooperative sensing and any performance degradation caused by cooperative sensing. In this paper, the stateof-the-art survey of cooperative sensing is provided to address the issues of cooperation method, cooperative gain, and cooperation overhead. Specifically, the cooperation method is analyzed by the fundamental components called the elements of cooperative sensing, including cooperation models, sensing techniques, hypothesis testing, data fusion, control channel and reporting, user selection, and knowledge base. Moreover, the impacting factors of achievable cooperative gain and incurred cooperation overhead are presented. The factors under consideration include sensing time and delay, channel impairments, energy efficiency, cooperation efficiency, mobility, security, and wideband sensing issues. The open research challenges related to each issue in cooperative sensing are also discussed.

The unsteady flow past a reverse delta wing is simulated numerically to characterize the structure of the vortical flow and associated unsteady phenomena and quantify the impact of such structures on the aerodynamic performance. The most cost-effective grid-size/time-step combination is selected by performing a coupled sensitivity analysis. This numerical approach is successfully validated against well-established results for a delta wing. It is established that the flow past a reverse delta wing is always unsteady even at small angles of attack. The shear layer separating at the leading edge of the reverse delta wing rolls up into spanwise vortical structures that, as they are convected downstream, pair, realign, and reorganize, generating suction that contributes substantially to the lift produced by a reverse delta wing. The tip vortices confine the vortical structures to the leeward side and contribute to a less extent to lift. Power spectral density analysis shows that the unsteadiness of the lift coefficient is related to vortex shedding and reorganization of the vortical structures. Finally, it is confirmed that the lift-to-drag ratio of a delta wing and a reverse delta wing is, surprisingly, about the same.

Reverse delta wings have been used in Lippisch type wing-in-ground crafts since the 1960's of the previous century, due to their stabilizing pitching moment close to ground. Previous studies showed that a reverse delta wing behaves dierently from a regular delta wing, either in terms of aerodynamic performance or oweld structure. Up to this date only one study that includes a numerical simulation for the ow past a reverse delta wing have been published. In this thesis, we simulate numerically the unsteady ow past a reverse delta wing to understand better its aerodynamic performance, the structure of the oweld and the unsteady phenomena associated. The numerical simulations are performed using ANSYS Fluent commercial software. The turbulence model used is Delayed Detached-Eddy in conjunction with the kω-SST as a RANS model. We selected the grid size and time step by performing a time step and grid sensitivity analysis using the power spectral density of the time history of the lift coecient C L for the ow past a delta wing at angle of attack α = 20 • . Before simulating numerically the ow past a reverse delta wing, we perform unsteady numerical simulations for the ow past a delta wing at angles of attack ranging from 5 • to 20 • . We found that the lift of a reverse delta wing is unsteady even at angles of attack as low as 5 • , dierently from a delta wing. The power spectral density analysis shows that the time history of the lift coecient C L is related to the time history of vortex shedding. The numerical simulations showed that over the leeward side of the reverse delta wing, the shear layer separating at the leading edge rolls into spanwise vortical structures that are convected downstream. As the vortical structures are convected, they alter the pressure distribution over the leeward side of the reverse delta wing. The results also showed that the tip vortex of a reverse delta wing is formed entirely by ow coming from the lower side of the wing. When comparing the performance of the delta wing against the reverse delta wing, at angles of attack below 10 • , both wings generated the same lift and drag, while at higher angles of attack, the reverse delta wing generated less lift and had less drag than the delta wing, but the lift-to-drag ratio for both wings is almost the same. Acknowledgement I would like to thank my supervisor, professor Luca Cortelezzi, for his help, he dedicated to me most of his time and always provided advises and suggestions for the improvement of this thesis. I would also like to thank the Department of Aerospace Sciences and Technology at Politecnico di Milano for providing me the computational resources necessary to complete this work.

Thermal comfort indices are vital tools when assessing outdoor thermal comfort in hot and arid environments. Selecting a representative thermal comfort index for outdoor environments is challenging. This paper presents a comparative study of the suitability of seven different thermal comfort indices, namely PMV, discomfort index, cooling power index, Humidex, WBGT, SET, and UTCI in assessing outdoor thermal comfort. The thermal comfort indices were compared to the thermal sensation vote (TSV) obtained from a thermal comfort questionnaire of spectators seated in a semi-open air-conditioned stadium. Seated in six different zones, a total of 532 spectators participated in an online questionnaire. The results of the survey indicated high levels of climate acceptability, with small variations among the stadium zones and between genders. Almost 40% of the spectators reported feeling ‘‘cool’‘, while 28% of the spectators were feeling ‘‘slightly cool’’ and 21% reported a ‘‘neutral’’ thermal perception. Hence, CFD simulations were used to predict the values of the seven thermal comfort indices. The thermal comfort indices' values, obtained from the CFD simulations, were compared to their counterparts obtained from the questionnaire. The WBGT index showed good agreement to the actual questionnaire data with an average difference of 8.8%. The other six indices yielded an average range of difference of (15%–46%). The WBGT index deemed the most suitable to assess outdoor thermal comfort for hot and arid regions, followed by the UTCI and the SET indices, with average differences of 14% and 15%, respectively. The CPI index deemed not suitable for hot and arid regions compared to other indices.

Evaluation of the thermal comfort is essential for complex ventilation systems design. Assessment of thermal indices requires representative velocity and pressure fields' values. When simulating the air flow in large facilities such as stadium, the effect of crowds' geometrical features needs to be captured. Using porous models approximations to simulate the aerodynamic effect of detailed spectators' geometry reduces the required mesh size and associated processing time. This paper investigates the use of different porous media models approximations for capturing the effect of large crowds inside complex building systems, such as stadiums. Their efficiency of capturing the effect of spectators on the air flow were compared to the simulation of the exact spectators' geometry. The exact spectators' geometrical model was of a stadium tiers section with 28 spectators. Using a wind tunnel, the exact spectator's model results were validated against a 1:10 scaled physical model. The experiments included PIV and hot-wire velocity measurements. The results of the pressure drop were used to obtain the coefficients needed to utilize the porous models. Compared to the exact spectators' case, the three-dimensional porous volume model approximation yielded an average absolute error of 24.5% in velocity, while the two-dimensional porous jump model yielded results with an average error of 1.5%. In comparison to the exact model cooling load, the results yielded a difference of 6% for the 2D porous jump and 6.5% for the 3D porous volume. Nevertheless, both models yielded more representative results than the case of simulation of empty bleachers.

Windbreaks such as vegetation barriers, fences, and buildings' perforated facades are used in various environmental and wind mitigation applications. Numerical simulation of the airflow through windbreaks is challenging as modeling of the exact geometry needs large computational power. The computational cost can be reduced by modeling the windbreaks using the porous media model. The computational fluid dynamics (CFD) model uses empirical coefficients that need to be determined experimentally. This research develops empirical formulae to determine the needed viscous and inertial loss coefficients as a function of porosity for perforated windbreaks. Particle image velocimetry (PIV) experiments and pressure measurements were performed in a wind tunnel for scaled windbreaks models of porosity ratios ranging from 0.25 to 0.6. CFD simulations were carried out for the exact geometry and for the approximated numerical model. The CFD results of both models were compared against the experimental results. Hence, a correlation between the windbreaks porosity and the needed coefficients to utilize the CFD porous media model was obtained. Compared to exact geometry CFD results, the average velocity and pressure drop values obtained from the porous media model yielded an average error of 10.5% and 12%, respectively.

This work aims to study and simulate the flow field around light a/c using numerical solution based on solving Reynolds Averaged Navier-Stokes equations coupled with K-ω SST turbulence model. Solution strategy began by analyzing wing alone then analyzing the whole a/c configuration. This strategy allows studying the wing flow field carefully and analyzing the vortices which occur over the wing and capture their effective regions at critical design conditions like stall condition. These regions were captured by plotting pressure contours and flow pathlines. The aerodynamic behavior of this aircraft was investigated at different angles of attack and side slip angles. The x, y and z forces and moments were calculated at flight speed of 50m/s and at sea level conditions. Lift and drag curves for different angles of attack were plotted. The maximum lift coefficient for this a/c was 1.67 which occurred at angle of attack 16 o , the maximum lift to drag ratio (L/D) found to be 14 which occurred at α=3 o , and the zero lift drag coefficient was 0.0342. Also the yawing moment coefficient was plotted for different side slip angles as well as rolling moment. The longitudinal stability derivatives with respect to angle of attack, speed variation (u), pitch rate (q) and time rate of change of angle of attack were calculated using obtained CFD results. Regarding lateral stability only side slip derivatives were calculated.

Abstract
Wing downwash study and estimation of downwash effect on the tail plane is important task during aircraft design process, although a lot of papers and works has been done, but the experimental work is the most important, the progress in CFD simulation has reached to the point it is ale to reduce the number of run in the wind tunnel. In this work CFD has been utilized to calculate the downwash angle and downwash gradient with respect to angle of attack over a high aspect ratio of MALE UAV. The results of the simulation shall be used in the estimation and calculation of longitudinal static stability analysis of the UAV.

Keywords: wing downwash, CFD, static longitudinal Stability, MALE UAV.

Sudan, in specific, confronts a critical crises relating to fossil fuel resources, and mostly concerning the production of electricity energy from fired thermal power plants. One of these Garri "2" combined cycle power plants produce about 180 MW. In the light of the second law of thermodynamics, exergy analysis is considered a powerful and effective tool to investigate the optimization of engineering devices. In order to evaluate exergetic efficiency and exergy destruction of each part, exergy analysis has been carried out for Garri "2;" while in order to achieve it, exergy balance and entropy generation were analyzed. Conclusively, the results proved that combustion chambers are the main sources of exergy destruction due to their high irreversibility. These results in detail illustrate the following percentages: 63% from the total exergy destruction, followed by 13.6% of gas turbines, 6.4% steam turbines, 6.3% heat recovery steam generators (HRSGs), 4.7% stacks (exhaust gases), 3.8% compressors and 2.3% cooling systems. Moreover, the results affirmed that thermal and exergetic efficiencies for the entire plant are (38%, 49%) respectively.

The determination of an accurate value of a moment of inertia plays a vital role in designing decent mechanical parts as general such as a flight control system for aircraft. This why an accurate value for moment of inertia is needed. A test rig was designed and made form cardboard and then developed for wood. Experiments were carried out, to get to the final shape and product for the test rig, and it was manufactured with dimensions of 2.7x2.5 meters and with a holder dimension of 60x60 cm that bears up to 10 kg. Moreover, an electronic circuit has been used to measure the period of oscillations. The moment of inertia of a rectangular block of wood and a UAV is found. The moment of inertia was measured analytically and experimentally for the block of wood, and by comparing the values it founds that the error ratio from 1%to5% in X axis and-6% to 7% in Y axis. For the UAV the moment of inertias error ratio in X axis from 2% to 5% and in Y axis from 11% to 14%. It is noticed that the greater the additional weight, the greater the accuracy of the device.

Under the scenario of an underlay cognitive radio network, we propose in this paper two adaptive schemes using switched transmit diversity and adaptive modulation in order to increase the spectral efficiency of the secondary link and maintain a desired performance for the primary link. The proposed switching efficient scheme (SES) and bandwidth efficient scheme (BES) use the scan and wait combining technique (SWC) where a transmission occurs only when a branch with an acceptable performance is found, otherwise data is buffered. In these schemes, the modulation constellation size and the used transmit branch are determined to minimize the average number of switched branches and to achieve the highest spectral efficiency given the fading channel conditions, the required error rate performance, and a peak interference constraint to the primary receiver (PR). For delay-sensitive applications, we also propose two variations of the SES and BES schemes using power control (SES-PC and BES-PC) where the secondary transmitter (ST) starts sending data using a nominal power level which is selected in order to minimize the average delay introduced by the SWC technique. We demonstrate through numerical examples that the BES scheme increases the capacity of the secondary link when compared to the SES scheme. This spectral efficiency improvement comes at the expense of an increased average number of switched branches and thus an increased average delay. We also show that the SES-PC and the BES-PC schemes minimize the average delay while satisfying the same spectral efficiency as the SES and BES schemes, respectively.

Drones are increasingly vital in numerous fields, such as commerce, delivery services, and military operations. Therefore, it is essential to develop advanced systems for detecting and recognizing drones to ensure the safety and security of airspace. This paper aimed to develop a robust solution for detecting and recognizing drones and birds in airspace by combining a radar system and a visual imaging system, and contributed to this effort by demonstrating the potential of combining the two systems for drone detection and recognition. The results showed that this approach was highly effective, with a high overall precision and accuracy of 88.82% and 71.43%, respectively, and the high F1 score of 76.27% indicates that the proposed combination approach has great effectiveness in the performance. The outcome of this study has significant practical implications for developing more advanced and effective drone and bird detection systems. The proposed algorithm is benchmarked with other r...