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This study investigates the flutter response of a rectangular cylinder model with an aspect ratio of 5 at the Reynolds number Re = 100 via direct numerical simulation. The effects of two key parameters, i.e., the moment of inertia and reduced flow velocity, on the aerodynamic performance and dynamic responses of the cylinder in the state of torsional flutter are investigated. To reveal the flutter mechanism, the high-order dynamic mode decomposition (HODMD) analysis is conducted to decompose the flow field. The results show that both an increase in the moment of inertia and a higher reduced flow velocity lead to a larger torsional amplitude and a corresponding decrease in torque. At the same time, the primary frequency decreases and the size of the shedding vortex gradually enlarges. The vortices shed from the leading edge and the trailing edge of the model form a 2P wake pattern. The leading-edge vortex is significantly larger than the trailing-edge vortex in terms of strength and size. The leading edge plays a dominant role and only contributes to the odd-order HODMD modes while the even-order modes are deemed inconsequential. As the moment of inertia increases, the total energy of the higher-order modes increases, which has the same results as the power spectral density of torque, reflecting increased nonlinearity and complexity of the system. Similarly, increasing the reduced flow velocity at the same moment of inertia has similar excitation effects. © 2023 Author(s).

Understanding and modeling the flow field and force development over time for flow past twin tandem cylinders can promote insight into underlying physical laws and efficient engineering design. In this study, a new surrogate model, based on a convolutional neural network and higher-order dynamic mode decomposition (CNN-HODMD), is proposed to predict the unsteady fluid force time history specifically for twin tandem cylinders. Sampling data are selected from a two-dimensional direct numerical simulation flow solution over twin tandem cylinders at different aspect ratios (AR = 0.3-4), gap spacing (L{*} = 1-8), and Re = 150. To promote insight into underlying physical mechanisms and better understand the surrogate model, the HODMD analysis is further employed to decompose the flow field at selected typical flow regimes. Results indicate that CNN-HODMD performs well in discovering a suitable low-dimensional linear representation for nonlinear dynamic systems via dimensionality augment and reduction technique. Therefore, the CNN-HODMD surrogate model can efficiently predict the time history of lift force at various AR and L{*} within 5% error. Moreover, fluid forces, vorticity field, and power spectrum density of twin cylinders are investigated to explore the physical properties. It was found three flow regimes (i.e., overshoot, reattachment, and coshedding) and two wake vortex patterns (i.e., 2S and P). It was found the lift force of the upstream cylinder for AR < 1 is more sensitive to the gap increment, while the result is reversed for the downstream cylinder. It was found that the fluctuating component of the wake of cylinders decreases with increasing AR at L{*} = 1. Moreover, flow transition was observed at L{*} = 4.

In recent years, an increasing number of projects have been developed in complex mountainous areas. The wind environment in mountainous areas, extremely complex due to the undulating terrain and diverse landscapes, is a key factor threatening the structural safety of buildings and their appurtenances in mountainous areas. Therefore, it is important to study the wind environment in complex terrain to clarify the wind resistance of structures in mountainous areas. Computational fluid dynamics (CFD) approaches are commonly used to examine wind fields in complex terrain; however, due to the limited range of terrain considered, direct modeling using terrain elevation data can result in truncated elevation differences, affecting the accuracy of numerical simulations. To address the problem of truncated elevation differences at terrain boundaries, the parameters of the wind tunnel contraction curve are optimized regarding the wind tunnel contraction section design principle. Moreover, several transition curves are analyzed and evaluated by numerical simulation methods, and a transition curve applicable to the terrain boundary transition form is proposed. The proposed terrain transition curves are applied to model the terrain of complex mountainous ski resort areas to be used in CFD numerical simulations. Furthermore, the accuracy of the numerical simulation is verified through a comparison with the field-measured data. Results indicate that the proposed method can accurately and effectively reflect the wind environment characteristics of a ski resort area. The proposed terrain transition curve provides a theoretical basis and case support for designing the terrain model boundary transition section, which can be used as a reference for wind tunnel and numerical simulation studies in complex mountainous areas.

Two-dimensional flow past two tandem near-ground plates with inclination angles of 25° at the Reynolds number of 150 is numerically simulated via the high-order spectral element method. Plate-to-ground gap is varied from G = 0.2L to 1.6L with intervals of 0.2L at two representative inter-plate spacings (i.e., X = 2.5L and 6L). The ground effect on the fluid force, power spectral density, asymmetric gap flow, and wake structure of plates is systematically evaluated. Then, the high-order optimal mode decomposition (HOOMD) method is proposed to synchronously analyze the velocity and pressure fields. The results show that the fluid force and flow structure are closely dependent on G. The presence of the ground inhibits vortex shedding when G < 0.6L; as the gap increases from 0.6 L to 1.4 L, the fluctuating forces are continuously enhanced until the ground effect basically disappears at G > 1.4L. The ground effect exacerbates the asymmetry of the vortex structure near the upper and lower parts of the inclined plates, consequently changing the fluid force. The downstream plate is more sensitive to the ground effect because of impingement from the upward-biased jet flow generated in the narrow gap between the upstream plate and ground. The HOOMD method well captures the spatial morphology and temporal evolution features of different dominant modes at the transition or vortex shedding flow regime. Mode analysis affords a correspondence between the coherent vortex structure and fluid force of plates. Furthermore, the ground effect can simultaneously change the global mode energy and local pressure mode shape, subsequently influencing the fluid force. However, the global mode energy plays the determinant role in the variation of the fluid force of plates with the plate-to-ground distance herein. © 2023 Author(s).