The unsteady characteristics and mechanisms of the front stagnation point oscillation for the flow around a square cylinder are investigated with the two-dimensional time-resolved particle image velocimetry technique in the range of 1420 ≤ Re ≤ 8600. The front stagnation point oscillates with the frequency equal to the wake vortex-shedding frequency. The Reynolds number has no effect on the amplitude and the dimensionless frequency of the front stagnation point. It is revealed that the front stagnation point moves from the centerline to one side when the wake vortex from the opposite side is forming and moving toward the centerline. This growing wake vortex stays attached to the rear surface of the square cylinder. After the wake vortex is detached from the rear surface, the front stagnation moves from the maximum displacement back toward the centerline. Furthermore, the quantitative correspondence between the location of front stagnation point and the wake pattern is revealed via time-lag correlation. The movement of the front stagnation point and the variation of typical proper orthogonal decomposition coefficient of the wake are nearly opposite in phase.
As the rapid development of equipment for aerospace, national defense, biological engineering and precision machinery, traditional milling is gradually difficult to satisfy the growing demand for accuracy and machining surface quality. It is an effective method to introduce ultrasonic vibration into milling processing. Ultrasonic vibration assisted milling(UVAM) has been widely studied recent years for its extensive adaptability on materials, good cutting effect, high efficiency, long cutting life, et al. And it is an emerging trend to obtain biomimetic surface texture and certain surface performance by UVAM. In this paper, the characteristics of squamous surface and ultrasonic vibration assisted milling surface are analyzed and discussed. Ultrasonic vibration assisted milling surface is formed by tool tip path and influenced by vibration frequency and amplitude, cutting tool diameter, feed speed, spindle speed, spindle rotation accuracy, etc. The squamous surface feature is extracted based on image processing and compared with ultrasonic vibration assisted milling surfaces. The results show that the feature curves of squamous surface can be regarded as the same type of function curve with tool tip trajectories. The spindle precision only plays a role in surface features distribution uniformity during UVAM. Squamous surface shall be obtained by UVAM with certain vibration and machining parameters. It is concluded that UVAM is suitable for machining squamous surface, which is helpful to guide practical UVAM for machining biomimetic surface.
Gust response has consistently been a concern in engineering. Critical theories have been proposed in the past to predict the unsteady lift response of an airfoil experiencing vertical gusts by Atassi, and longitudinal gusts by Greenberg. However, their applicability for an airfoil with non-zero angles of attack still needs clarification. Thus, force measurements are conducted to examine these theories’ validity and quasi-steady corrections are applied to compensate potential disparities between the idealised and real flow conditions. Velocity measurements are performed to scrutinise the effect of gusts on the flow around the airfoil, and subsequently to reveal the underlying mechanism governing the airfoil's response to gust-induced perturbations. In the study, two pitching vanes are arranged upstream to generate periodic vertical and longitudinal gusts, whereas a downstream airfoil with angles of attack of 0–12° is subjected to two gust types. It is found that Greenberg's theory demonstrates superior predictive capability in pre-stall regimes, with the potential for its effectiveness to be expanded to post-stall regimes through theoretical refinements. In contrast, Atassi's theory exhibits significant deviations from experimental outcomes across the measured angles of attack. Nevertheless, a modified version of the theory aligns better with experimental results at small angles of attack, whereas substantial discrepancies persist as the angle of attack increases. In the pre-stall regime, the aerodynamic response of the airfoil to vertical gusts displays a linear correlation with the flow angle near the leading edge. In the post-stall regime, the vertical gust induces dynamic stall of the airfoil. The flow angle has an essential effect on the lift coefficient but it alone is inadequate to dictate the trend of the lift coefficient. The vorticity statistics show that negative vortex circulation strongly correlates with the lift coefficient. Thus, further correction of the theory or a new vortex model can be expected to predict the lift variation.
The statistical characteristics and flow structures for turbulent flow over smooth, convergent and divergent riblets flat plate with zero pressure gradient are investigated with two-dimensional time-resolved particle image velocimetry (TRPIV). It is shown in the wall-normal planes of the convergent and divergent riblets flat plate that, compared to the smooth flat plate, the local boundary layer thickness, wall friction velocity, turbulent fluctuation and Reynolds stress are evidently reduced over the divergent surface. Furthermore, the effect of convergent riblets flat plate on turbulent boundary layer flow is different from the divergent one, which causes the near wall fluid move away and results in an increment of about 43% for turbulent boundary layer thickness. Meanwhile, the large scale coherent structures are more likely to be formed for flow over convergent riblets surface, this is not benefit for drag reduction. Besides, the population of the prograde vortices reaches a maximum value in the log region of turbulent boundary layer, and which appears much closer to the divergent riblets surface than the convergent one. The ejection and sweep induced by the prograde vortices make a great contribution to the mean shear in turbulent boundary layer, and it is the decrease of the prograde vortices which results in the reduction of the wall friction for the diverqent riblets surface.
To investigate the flow mechanism of micro-air-vehicle's layout, hydrogen bubble visualization on a series of models with different flexibility and sweep angle whose ratio of span to chord length is 0.5 was carried out in water tunnel.As the results shown,the rule of flow structure to obey at low Reynolds number is listed as follows: the flow around flexible wing follows the rule which is Ω vortex-one pair of leading vortex-one pair of leading and dual vortex-one pair of leading vortex,dual-vortex and tri-vortex-one pair of leading and dual-vortex-one pair of leading vortex as the sweep angle increases,the flow structures over rigid wing are almost the same as flexible one but without tri-vortex structure.
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