摘要：Very limited attention has already been paid to the velocity behavior in the wake region in unstead...Very limited attention has already been paid to the velocity behavior in the wake region in unsteady aerodynamic problems. A series of tests has been performed on a flapping airfoil in a subsonic wind tunnel to study the wake structure for different sets of mean angle of attack, plunging amplitude and reduced frequency. In this study, the velocity profiles in the wake for various oscillation parameters have been measured using a wide shoulder rake, especially designed for the present experiments. The airfoil under consideration was a critical section of a 660 k W wind turbine.The results show that for a flapping airfoil the wake structure can be of drag producing type, thrust producing or neutral, depending on the mean angle of attack, oscillation amplitude and reduced frequency. In a thrust producing wake, a high-momentum high-velocity jet flow is formed in the core region of the wake instead of the conventional low-momentum flow. As a result, the drag force normally experienced by the body due to the momentum deficit would be replaced by a thrust force.According to the results, the momentum loss in the wake decreases as the reduced frequency increases. The thrust producing wake pattern for the flapping airfoil has been observed for sufficiently low angles of attack in the absence of the viscous effects. This phenomenon has also been observed for either high oscillation amplitudes or high reduced frequencies. According to the results, for different reduced frequencies and plunging amplitudes, such that the product of them be a constant, the velocity profiles exhibit similar behavior and coalesce on each other. This similarity parameter works excellently at small angles of attack. However, at near stall boundaries, the similarity is not as evident as before.显示全部
摘要：A new vortex sheet model was proposed for simulating aircraft wake vortex evolution.Rather than beg...A new vortex sheet model was proposed for simulating aircraft wake vortex evolution.Rather than beginning with a pair of counter-rotating cylindrical vortices as in the traditional models, a lift-drag method is used to initialize a vortex sheet so that the roll-up phase is taken into account. The results of this model report a better approximation to a real situation when compared to the measurement data. The roll-up induced structures are proved to influence the far-field decay.On one hand, they lead to an early decay in the diffusion phase. On the other hand, the growth of linear instability such as elliptical instability is suppressed, resulting in a slower decay in the rapid decay phase. This work provides a simple and practicable model for simulating wake vortex evolution, which combines the roll-up process and the far-field phase in simulation. It is also proved that the roll-up phase should not be ignored when simulating the far-field evolution of an aircraft wake vortex pair, which indicates the necessity of this new model.显示全部
