• Energy Research

To optimize wind farm layout and improve wind energy conversion, it is significant to accurately and efficiently model wind turbine wake. In this study, a novel dimensionality reduction method, i.e., delayed proper orthogonal decomposition (d-POD) is proposed and combined with long short-term memory (LSTM) network to predict the unsteady wind turbine wake velocity. Compared with standard POD, d-POD can extract coherence modes and mode coefficients that could highlight the main feature and smooth the noise, so that it achieves higher accuracy. Meanwhile, the deep network LSTM is employed to predict the temporal evolution of mode coefficients. Accordingly, the velocity distribution is reconstructed by the predicted mode coefficients and coherence mode. The proposed wake prediction surrogate model is validated by high-fidelity large eddy simulation data. Results show that d-POD-LSTM model can fast and precisely predict the unsteady wind turbine wake dynamics similarly as high-fidelity wake of LES for both near wake and far wake. More importantly, compared with conventional POD-LSTM model (i.e., delayed number d = 1), the performance of d-POD-LSTM model is superior. As delayed number increases from 1 to 16, the prediction error of d-POD coefficient reduces by 80%, and thus the wake prediction error decreases accordingly.

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.

Aerodynamic drag plays an important role in high-speed skiing. The wind-induced thrust or resistance of athletes, the sliding speed, and the work to overcome the aerodynamic drag are greatly affected by wind; therefore, reducing wind-induced drag is a focus of sport science. This paper proposes a method for evaluating the influence of wind on cross-country skiing performance, which is based on the athlete’s aerodynamic-drag-work relative to the environmental wind field and the establishment of a racetrack wind field model. Aiming at an athlete’s typical sport posture in the Yabuli Ski Field, the impact of field wind on the skier’s speed, the work done by the athlete to overcome aerodynamic drag, and the ratio of the field wind-induced work to the athlete’s total work are analyzed. Through the analysis of the athlete’s work to overcome aerodynamic drag and the wind resistance energy dissipation ratio in three training cases, it is shown that the field wind has a great influence on the athlete’s performance during sliding, which verified the effectiveness of the method. This method will provide coaches and sport researchers with accurate wind resistance energy dissipation data and provide a scientific basis for routine athletic training.

Abstract To suppress the vortex-induced vibration (VIV) of long-span bridges, an active flow control method, i.e. the external suction-blowing method (ESBM), is proposed in this study based on the efficiency of the three-dimensional spanwise-varying flow control method and the suction/blowing flow control method. The method can be achieved by cyclically arranging suction and blowing on the undersurface of the bridge. The effectiveness of the method for the vertical and torsional VIV of the bridge sectional model is verified through wind tunnel tests. The test results imply that the best perturbation position of the ESBM is the leading edge of the undersurface of the bridge. For dimensionless spanwise distances of 2–4 with single-hole flow coefficients of 2.27 × 10−3–3.03 × 10−3 and 1.45 × 10−3–1.94 × 10−3 during vertical and torsional VIV, respectively, the VIV can be completely suppressed. The results of the wake analysis show that this method can effectively suppress the scale of the spanwise vortex of the bridge, causing the VIV to disappear.

The flow through tandem square cylinders was investigated at a Reynolds number of 100 for oscillation amplitudes A = 0.1D to 0.7D and gaps L = 2.0D, 5.0D, and 6.0D, where D is the width of the cylinders. A moving reference frame method combined with the spectral/hp element method was employed to simulate the two-dimensional flow in the lock-in regime. Fluid forces, vorticity fields, power spectrum density, and pressure distribution were first investigated. Since surface pressure is directly connected with fluid forces, pressure and velocity field were synchronously analyzed by employing optimal dynamic mode decomposition. An underlying link between fluid forces and coherence modes was then uncovered. The results reveal that the move-induced forces and flow structures strongly depend on gaps and amplitudes in the lock-in regime. With respect to the dynamic mode decomposition analysis, odd-order modes contribute to lift forces, while even-order modes result in drag forces. The flow structures are dominated by at most three modes; as the amplitude increases, the high-order mode energy increases, coinciding with corresponding power spectrum density results of forces. Typical 2S, 2P, and C(2S) wakes were observed for various gaps and two representative amplitudes (A/D = 0 and 0.7), and their dominant modes show distinctive differences that lead to different local pressure shapes on the cylinders. It is the combined effects of local mode shape and global mode energy that account for the change in fluid forces for various gaps and two oscillating amplitudes.

202407 bcch ; Others ; National Natural Science Foundation of China ; Published ; 24 months ; Green (AAM)

The present wind tunnel study focuses on the effects of the steady-suction-based flow control method on the flutter performance of a 2DOF bridge deck section model. The suction applied to the bridge model was released from slots located at the girder bottom. The suction rates of all slots along the span were equal and constant. A series of test cases with different combinations of suction slot positions, suction intervals, and suction rates were studied in detail for the bridge deck model. The experimental results showed that the steady-suction-based flow control method could improve the flutter characteristics of the bridge deck with a maximal increase in the critical flutter speed of up to 10.5%. In addition, the flutter derivatives (FDs) of the bridge deck with or without control were compared to investigate the fundamental mechanisms of the steady-suction-based control method. According to the results, installing a suction control device helps to strengthen aerodynamic damping, which is the primary cause for enhanced flutter performance of bridge decks.