Mohammed R. Ahmed

Experimental Investigation of the Effect of Camber on the Aerodynamic Characteristics of Airfoils in Ground Effect

In the present paper, results from an experimental investigation of aerodynamic ground effect on two airfoils are presented. The flow characteristics over a symmetrical airfoil (NACA 0015) and a cambered airfoil (NACA 4415) were studied in a low speed wind tunnel. Experiments were carried out by varying the angle of attack from 0° to 10° and ground clearance from zero to one chord length. Pressure distribution on the surface of the airfoil was obtained with the help of pressure tappings. Mean velocity distributions were obtained over the surface of the airfoil. Profiles of mean velocity and turbulence intensity were obtained in the wake region at 0.5 and 1.0 chord length downstream of the trailing edge. It is found that pressure increases on the lower surface as the ground is approached. The flow accelerates over the airfoil, and a considerably higher mean velocity is observed near the suction peak location. For the symmetrical airfoil, the mean velocity over the surface was found to increase by nearly 30%, while for the cambered airfoil, an increase of nearly 60% was recorded for an angle of attack of 7.5°. The flow was found to separate almost near the trailing edge for angles of attack upto 10°, resulting in a thinner wake region and lower turbulence intensities for the symmetrical airfoil; while for the cambered airfoil, an early separation for an angle of attack of 10° was observed. Measurements in the wake region showed a defect in mean velocity profile at the corresponding values of ground clearance. For lower angles of attack, turbulence levels were higher in the wake region for the symmetrical airfoil, while for an angle of attack of 10°, very large defect in velocity was observed for the cambered airfoil model and the minimum velocity reduced to 20% of the freestream velocity.© 2003 ASME

A Regional University Perspective on Engineering Education of Multi-Cultural Freshman Students From South Pacific Countries

Of late, there is a growing need for quality engineers who have the ability to solve complex engineering problems with reasonable knowledge of ethics and economics. This has led many universities to pursue accreditation by professional engineering bodies. While the accreditation process installs a standardized system of quality teaching, it is important that the engineering entrants have a degree of understanding that allows implementation of quality teaching methods. This study looks at the performance of first year engineering students in a bid to identify major issues that students face in a Bachelor of Engineering program. The learning of students in the School of Engineering and Physics at the University of the South Pacific is influenced by interactions of at least 12 different cultures from the 12 member countries of the university. The study looks at how students perform across cultures in the first year mechanical engineering courses, mainly engineering mechanics and engineering graphics & design. The general trend over the last five years shows that while the student numbers in the program have been increasing, student performance in one course seems to be improving but declining in the other; the two courses differ considerable in contents, required skill sets, and assessment methodologies. The study also presents possible reasons for the varied performance by considering issues such as cultural and academic backgrounds, use of teaching tools and resources, and revisions to the course and program and looks at how multi-cultural engineering education can be improved. The number of female students taking up engineering as their major is also looked at and positive trends are seen with female participation increasing from 7.6% in 2008 to 13.9% in 2013.Copyright

Design and Performance Testing of a Ducted Savonius Turbine for Marine Current Energy Extraction

Marine current energy is a reliable and clean source of energy. Several marine current turbines have been developed over the years, most of the turbines perform well at velocities over 2 m/s and need to be installed at depths of 20–40 m. Placing an appropriately designed duct or shroud around the turbine significantly improves the turbine’s performance. Ducted Savonius turbines can operate at low depths, since large clearance is not required because turbulent flow has little effect on the performance of the Savonius rotor. Ducted Savonius turbine has simple components and can be easily fabricated in Pacific Island Countries (PIC) and other places that do not have advanced manufacturing industries. A ducted Savonius turbine was designed for a location in Fiji, to operate at a rated marine current speed of 1.15 m/s and cut in speed of 0.2 m/s. The model of ducted Savonius turbine, scaled down to 1/20, was fabricated and tested in a water stream with a velocity of 0.6 m/s and was validated with commercial Computational Fluid Dynamics (CFD) code ANSYS-CFX. Finally, a full scale numerical model was constructed to study the flow characteristics and compute the performance. The area ratio of the duct of 2.5:1 (inlet to turbine section) shows significant increase in kinetic energy and an improved turbine performance. The maximum efficiency of the turbine is around 50% at a tip speed ratio (TSR) of 3.5 and the maximum power produced is 10 kW at the rated speed of 1.15 m/s and 63.4 kW at a free-stream velocity of 2.15 m/s.© 2013 ASME

Optimization of Hydrofoils for Horizontal Axis Marine Current Turbines Using Genetic Algorithm

As fossil fuels near depletion and their detrimental side effects become prominent on ecosystems, the world is searching for renewable sources of energy. Tidal energy is an emerging and promising renewable energy resource. Tidal turbines can extract energy from the flowing water in a similar way as wind turbines extract energy from the wind. The upside with tidal turbines is that the density of water is approximately 800 times greater than that of air and a tidal turbine harnessing the same amount of power as a wind turbine can be considerably smaller in size. At the heart of the horizontal axis marine current turbines are carefully designed hydrofoil sections. While there is a growing need to have hydrofoils that provide good hydrodynamic and structural performances, the hydrofoils also have to avoid cavitation for safe operation. This study uses a genetic algorithm optimization code to develop hydrofoils which have the desired qualities mentioned above. The hydrofoil problem is parameterized using a composite Bezier curve with two Bezier segments and 11 control points. Appropriate curvature conditions are implemented and geometric constraints are enforced to maintain the hydrofoil thickness between 16 to 18%. XFOIL is used as the flow solver in this study. The hydrofoils are optimized at Reynolds number of 2 million and for angles between 4 to 10 degrees. The best foil from the results, named USPT4 is tested for performance with the CFD code ANSYS CFX. The CFX results are validated with experimental results in a wind tunnel at the same Reynolds number. The hydrofoil’s performance is also compared with a commonly used NACA foil.Copyright

Performance Improvement of a Wind Turbine Blade Designed for Low Wind Speeds With a Passive Trailing Edge Flap

The flow characteristics and the lift and drag behavior of a newly designed thick trailing-edged airfoil that was provided with fixed trailing edge flaps (Gurney flaps) of 1% to 5% height right at the back of the airfoil were studied at different low Reynolds numbers (Re) and angles of attack for possible applications in wind turbines suitable for the wind speeds of 4–6 m/s that are common in the Pacific Island Countries. A thick trailing-edged blade section, AF300, that was designed and tested in a recent work for small horizontal axis wind turbines to improve the turbine’s startup and performance at low wind speeds was chosen for this study. Experiments were performed on the AF300 airfoil in a wind tunnel at different Re, flap heights and angles of attack. Pressure distributions were obtained across the surface of the airfoil and the lift and drag forces were measured for different cases. It was found that the flap considerably improves the suction on the upper surface of the airfoil resulting in a high lift coefficient. For some of the angles, in the case of 3 mm and 4 mm flaps, the peak Cp values on the suction surface were significantly higher compared to those without the flap. However, at angles of attack of 12° and above, this unusually high Cp on the upper surface close to the leading edge caused flow separation for some cases as the flow could not withstand the strong adverse pressure gradient. The CFX results matched most of the experimental results without flaps, except that the suction peak was lower numerically. The difference was higher for the case with flaps. It is clear from the results that trailing-edge flaps can be used to improve the performance of small wind turbines designed for low wind speeds.Copyright

Design of a Horizontal Axis Wind Turbine for Fiji

The demand and cost of electricity has increased for Pacific Island Countries (PICs). The electricity from main grid does not reach rural areas and outer islands of Fiji. They burn fuel for electricity and daily lighting. Therefore, there is a need to look for alternative energy sources. Wind turbine technology has developed over the past years and is suitable for generating electricity by tapping wind energy. However, turbines designed to operate at higher wind speed do not perform well in Fiji, because Fiji’s average wind velocity is around 5–6 m/s. A 10 m, 3-bladed horizontal axis wind turbine is designed to operate at low wind speed, cut in speed of 3 m/s, cut off speed of 10 m/s and rated wind speed of 6 m/s. The blade sections were designed for different locations along the blade. The airfoil at the tip (AF0914) a has maximum thickness of 14% and maximum camber of 9%; the thickness varies linearly to the root, at the root the airfoil (AF0920) has a maximum thickness of 20% and maximum camber of 9%. The aerodynamic characteristics of airfoil AF0914 were obtained using Xfoil and were validated by experimentation, at turbulence intensities (Tu) of 1% and 3%, and a Reynolds number (Re) of 200,000. The aerodynamic characteristics of other airfoils were also obtained at operating Re at the turbulence intensities of 1% and 3%. These airfoils have good characteristics at low wind speed, and were used to design the 10 m diameter 3-bladed HAWT for Fiji. The turbine has a linear chord distribution for easy manufacturing purpose. Twist distribution was optimized using Blade Element Momentum (BEM) theory, and theoretical power and turbine performance were obtained using BEM theory. At the rated wind speed of 6 m/s and a TSR of 6.5, the theoretical efficiency of the rotor is around 46% and maximum power is 4.4 kW. The turbine has good performance at lower wind speeds and is suitable for Fiji’s conditions.Copyright

Turbulent mixing enhancement with a 10° chute mixer

Purpose – Turbulent mixing of two co‐axial jets having a low annular to core area ratio is enhanced by employing a chute mixer, directing part of the annular stream at 10° towards the core region. Aims to present results from measurements of time‐averaged and fluctuating components of velocity under cold flow conditions.Design/methodology/approach – Experiments were conducted at a bypass ratio of 0.47 which is a typical value for low bypass turbofan engines. Contours of time‐averaged velocity and streamwise and transverse turbulence intensities were obtained by making detailed measurements close to the chutes. Distributions of time‐averaged velocity and turbulence intensity were obtained at different axial locations downstream of the chute mixer. Total and static pressure measurements were also performed.Findings – The high velocity annular stream was found to quickly diffuse after entering through the chutes and mix with the core stream due to high turbulence generation. A strong transverse turbulence co...