A numerical investigation is conducted on the flow past a transversely oscillating square cylinder at various amplitudes with a Reynolds number of 200 in the “lock-on” region. Floquet stability analysis was then applied to study the three-dimensional instability of its wake. The results show that mean recirculating region sizes decrease and force coefficients increase as the oscillating amplitude increases. The dynamic mode decomposition analysis reveals that although oscillating amplitude changes the topology of high order harmonic modes, the primary modes share similar features, which result in the classical “2S” mode at various amplitudes. The distinct high-order-harmonic modes lead to the difference of the flow pattern at various amplitudes. For the three-dimensional instability, the oscillating amplitudes can be divided into three stages, I (0 ≤ A/D < 0. 3), II (0.3 ≤ A/D < 0.4), and III (0.4 ≤ A/D < 0.7). In region I, the dominant Floquet mode is Mode-A corresponding to a long wavelength λ_z/D ≈ 3.49. As the oscillating amplitude increases to region III, the wavelength of the dominant mode abruptly changes to a much shorter wavelength λ_z/D ≈1.14 (a new mode called Mode-SA). Mode-SA shows similar features to Mode-A, except that this mode dissipates rapidly in the wake. Region II is the transitional region.

Crack assessment is an essential process in bridge detection. In general, most non-contact crack detection techniques are not suitable for widespread use. The reason for this is that they all need to position the ruler at the inspection site in advance or calibrate the camera unit pixel size at a certain distance in a very intricate process. However, the object distance method in this paper can complete the calculation using only the crack image and the working distance, which are provided by an acquisition system equipped with a camera and laser range finder. First, the object distance method and the scale method are compared by calculating the crack width, and the results show that the object distance method is the more accurate method. Then, a double edge pixel statistical method is proposed to calculate the crack length, which solves the problem of redundant and missing pixels. In addition, the conventional mosaic algorithm is improved to realize an image mosaic for the more efficient splicing of crack images. Finally, a series of laboratory tests were conducted to verify the proposed approach. The experiments showed that the precision of crack length extraction can reach 92%, and the improved algorithm stitching precision can reach 98%.

A straight and an arc-shaped curtain are combined to enhance the aerodynamic performance of the Savonius wind rotor. The straight-arc curtain is placed in front of the Savonius wind rotor to reduce the negative moment on the convex blade and increase the positive moment on the concave blade. The static and dynamic performances of the Savonius wind rotor with and without the curtain are investigated based on the computational fluid dynamics method. The results show that the static torque is higher with the curtain than without it at the same angle-of-attack. The maximum mean power coefficient with the curtain is increased by about 50% compared with conventional Savonius wind rotor. Additionally, the flow field around the rotor with the straight-arc curtain is presented, and the flow control mechanics of the straight-arc curtain are discussed.

The tree sway frequency is an important part of the dynamic properties of trees. In order to obtain trees sway frequency in wind, a method of tracking and measuring the sway frequency of leafless deciduous trees by adaptive tracking window based on MOSSE was proposed. Firstly, an adaptive tracking window is constructed for the observed target. Secondly, the tracking method based on Minimum Output Sum Of Squared Error Filter (MOSSE) is used to track tree sway. Thirdly, Fast Fourier transform was used to analyze the horizontal sway velocity of the target area on the trees, and the sway frequency was determined. Finally, comparing the power spectral densities (PSDs) of the x axis acceleration measured by the accelerometer and PSDs of the x axis velocity measured by the video, the fundamental sway frequency measured by the accelerometer is equal to the fundamental sway frequency measured by video. The results show that the video-based method can be used successfully for measuring the sway frequency of leafless deciduous trees.