Estimación de la Eficiencia de una Hélice Operando en Régimen Compresible Subsónico

La información experimental disponible para hélices no es útil cuando el número de Mach de la punta de la pala es mayor a . Con el fin de verificar esta aseveración, se propuso un caso de estudio para una hélice Navy 5868-9 con perfil aerodinámico Clark-Y de 4 palas operando a un número de Mach de avance de 0.59 y un número de Mach en la punta de la pala de 0.95. La eficiencia experimental de la hélice (obtenida de gráficas) se compara con la eficiencia obtenida empleando la teoría combinada y con la eficiencia obtenida al corregir la teoría combinada por efectos de compresibilidad con la metodología propuesta en este trabajo. Se concluye que la información experimental disponible para hélices no es adecuada cuando el número de Mach en la punta de la pala es mayor que el Mach crítico, siendo más conveniente el resultado teórico corregido por compresibilidad. The experimental information available for propellers is not useful when the Mach number of the tip of the blade is greater than 0.3. In order to verify this assertion, a case study was proposed for a Navy propeller 5868-9 with a 4-blade Clark-Y airfoil section operating at an advance Mach number of 0.59 and a Mach number at the tip of the blade of 0.95. The experimental efficiency of the propeller (obtained from graphs) is compared with the efficiency obtained using the combined theory and with the efficiency obtained by correcting the combined theory for compressibility effects with the methodology proposed in this paper. It is concluded that the experimental information available for propellers is not suitable when the Mach number at the tip of the blade is greater than the critical Mach, being more convenient the theoretical result corrected by compressibility.

Aerodynamic Analysis and Design of High-Performance Sails

High-performance sails, such as the ones used on the America Cup boats, require sails whose aerodynamic characteristics approach those of rigid wings, yet permit a reduction in sail area in high wind and sea conditions. To this end, two-cloth sails are coming into use. These sails are constructed out of an articulated forebody that is a truncated ellipse, the aft of which has sail tracks, or rollers, along the edges to accommodate the twin sails. As the sails on either side need to be of the same length, due to the requirement to sail on different tacks, the two cloth sections need to be of equal length. The requirement then is to have their clews separated and able to slide over each other. More importantly, the transition between the rigid mast section and sails needs to be as aerodynamically smooth as possible in order to reduce drag and hence maximize the lift to drag ratio of the airfoil section that is made up of the mast and twin sails. A computational analysis using ANSYS CFX is presented in this chapter which shows that the aerodynamic characteristics of this type of two-cloth sail are almost as good as those of two-element rigid wing sections. Optimum sail trim configurations are analyzed in order to maximize the thrust production. Applications may soon extend beyond competitive sailing purposes for use on sailing ships equipped with hydrokinetic turbines to produce hydrogen via electrolysis (energy ships). Additionally, high performance sails can be used onboard cargo ships to reduce overall fuel consumption.

Effect of Gurney flap on the aerodynamic behavior of an airfoil in mutational ground effect

In this research, the effect of Gurney flap on the aerodynamic behavior of an airfoil in mutational ground effect is investigated. To perform this, lift and drag coefficients of NACA 4412 airfoil section in the presence and absence of Gurney flap in mutational ground effect are evaluated and compared. To provide a better illustration of the effect of Gurney flap on the aerodynamic behavior of the airfoil section in the mutational ground effect, results are obtained during the takeoff and landing processes. Validation of the used numerical model is also performed by comparison of the obtained results with those of other works and reasonable agreements were seen. Results show that inclusion of Gurney flap to the airfoil leads to higher variations of lift and drag coefficients during takeoff and landing process. During the takeoff process, the flapped airfoil results in a higher lift decrement and drag increment although an increase in distance of airfoil from the ground or angle of attack causes the lift decrement of the flapped airfoil to get close to that of clean airfoil. It is shown that during takeoff process, downwash is generated around the airfoil as the airfoil leaves the step and as the airfoil gets away from the step, the generated downwash decreases. During the landing process, at each distance of airfoil form the ground and angle of attack, the lift decrement and drag increment of the flapped airfoil are significant compared to that of the clean airfoil. Results exhibit that during landing process, upwash is generated around the airfoil as the airfoil reaches the step and further airfoil move on the step leads to the decrease in the upwash. The decrement in lift coefficient and also the increment in drag coefficient during landing process are more remarkable than those in takeoff process.