Parallel ribbon vortex: A phenomenological flow feature in an atmospheric boundary layer near sloped terrain

Large eddy simulation with the explicit fifth-order weighted compact nonlinear schemes is performed to investigate the aero-optical distortion caused by a transonic flow over a cylinder at Reynolds numbers of Re = 1 × 105, Re = 2 × 105, and Re = 4 × 105. Proper orthogonal decomposition identifies two dominant modes: the antisymmetric “shifting” mode and the symmetric “breathing” mode, both peaking at the non-dimensionalized Strouhal number StD = 0.18. Optical distortion refers to the phenomenon where a beam passes through a non-uniform and fluctuating flow field, resulting in defocus, jitter, and significant energy reduction. Optical calculations for 12 cases demonstrate that local shock waves and turbulent wakes notably exacerbate optical distortion. In terms of temporal results, the peak frequency of the beam aligns with StD = 0.18 when passing through the cylinder shockwave and falls within StD = 0.16–0.42 as it crosses the shear layer. Significant fluctuations are observed in the turbulent wake and local shock wave, with frequency peaks ranging from StD = 0.12–0.72. Additionally, streamwise flow structures are found to primarily impact optical distortion. Comparative analysis across the three Reynolds numbers indicates that optical distortion is insensitive to Reynolds number variations within the same order of magnitude. The employed grid sufficiently resolves key flow structures impacting beam transmission.

Numerical investigation of optical distortions by turbulent wake and shock wave in the transonic flow

Tan,

Xu,

Yin

2024

Insight on the surface fluctuating pressures considering distorted turbulence around a rectangular bluff body

Su,

Li,

Yang

et al. 2024

The distortion of turbulence approaching a bluff body plays a dominant role in determining the unsteadiness of the fluctuating surface pressures. A three-dimensional (3D) spectral approach is proposed to decouple the effects of the distorted turbulence into the blocking effect, distortion effect, and 3D effect. Wind tunnel tests are conducted to investigate the unsteady behavior of the windward fluctuating surface pressures on a rectangular bluff body, considering the effect of the ratio of the turbulence scale to the structural feature size λ = Lu/D. The results show that the fluctuating pressure on the surface of a rectangular cylinder is mainly affected by the low-frequency blocking effect, the high-frequency turbulent distortion effect, and the full-frequency turbulence 3D effect. The low-frequency blocking effect is related to the surface spatial position, but is less affected by λ; the high-frequency distortion effect does not depend on the spatial position of the pressure but is affected by λ; the 3D effect is affected by both the spatial position of the pressure and λ. When λ approaches infinity, the distortion and 3D effects can be ignored, and the quasi-steady theory is valid for the fluctuating pressure. Finally, one-wavenumber and 2D fluctuating pressure generalized spectrum models considering the unsteady effects and the spatial position of the surface pressure are proposed, and their accuracy is effectively verified by the test results.

Numerical study of wind characteristics at a long-span bridge site in mountain valley

Jing,

Li,

et al. 2024

Wind loads play an important role in the design and construction of long-span bridges, especially for that built in mountain valley, and wind characteristics in mountain terrain are still unclear. To further understand the wind characteristics at a bridge site in a mountain valley, large eddy simulations were employed to comprehensively study the wind characteristics at the bridge site in an L-shaped canyon. Different wind directions consisting of 0°, 45°, 90°, 135°, 180°, 225°, 270°, and 315° were considered, and the wind characteristics on the main girder and towers, as well as the flow field, were comprehensively investigated. The results indicate that the mean and fluctuating wind characteristics are significantly different from those specified in the relevant specifications, and it is inappropriate to apply the specifications directly to mountainous terrain. When wind flows along a channel, it leads to a large wind speed amplification factor, strong non-Gaussianity, and large kurtosis and skewness. The steeper the slope of the mountain, the stronger the non-Gaussianity of the fluctuating wind. The coherence function remains nearly unchanged when the distance between the two points is constant, regardless of the location. The wind characteristics at the main girder and towers are so different that they should be considered separately and taken into account in engineering practice. This study provides a reference for the wind-resistant design of bridges and structures built on mountainous terrains.