Fluctuating lift and drag acting on a 5:1 rectangular cylinder in various turbulent flows

Yang Yang,Mingshui Li,Xiongwei Yang 한국풍공학회 2022 Wind and Structures, An International Journal (WAS Vol.34 No.1

In this paper, the fluctuating lift and drag forces on 5:1 rectangular cylinders with two different geometric scales in three turbulent flow-fields are investigated. The study is particularly focused on understanding the influence of the ratio of turbulence integral length scale to structure characteristic dimension (the length scale ratio). The results show that both fluctuating lift and drag forces are influenced by the length scale ratio. For the model with the larger length scale ratio, the corresponding fluctuating force coefficient is larger, while the spanwise correlation is weaker. However, the degree of influence of the length scale ratio on the two fluctuating forces are different. Compared to the fluctuating drag, the fluctuating lift is more sensitive to the variation of the length scale ratio. It is also found through spectral analysis that for the fluctuating lift, the change of length scale ratio mainly leads to the variation in the low frequency part of the loading, while the fluctuating drag generally follows the quasi-steady theory in the low frequency, and the slope of the drag spectrum at high frequencies changes with the length scale ratio. Then based on the experimental data, two empirical formulas considering the influence of length scale ratio are proposed for determining the lift and drag aerodynamic admittances of a 5:1 rectangular cylinder. Furthermore, a simple relationship is established to correlate the turbulence parameter with the fluctuating force coefficient, which could be used to predict the fluctuating force on a 5:1 rectangular cylinder under different parameter conditions.

Non-Gaussian features of dynamic wind loads on a long-span roof in boundary layer turbulences with different integral-scales

Xiongwei Yang,Qiang Zhou,Yongfu Lei,Yang Yang,Mingshui Li 한국풍공학회 2022 Wind and Structures, An International Journal (WAS Vol.34 No.5

To investigate the non-Gaussian properties of fluctuating wind pressures and the error margin of extreme wind loads on a long-span curved roof with matching and mismatching ratios of turbulence integral scales to depth ( / ), a series of synchronized pressure tests on the rigid model of the complex curved roof were conducted. The regions of Gaussian distribution and non-Gaussian distribution were identified by two criteria, which were based on the cumulative probabilities of higher-order statistical moments (skewness and kurtosis coefficients, Sk and Ku) and spatial correlation of fluctuating wind pressures, respectively. Then the characteristics of fluctuating wind-loads in the non-Gaussian region were analyzed in detail in order to understand the effects of turbulence integral-scale. Results showed that the fluctuating pressures with obvious negative-skewness appear in the area near the leading edge, which is categorized as the non-Gaussian region by both two identification criteria. Comparing with those in the wind field with matching / , the range of non-Gaussian region almost unchanged with a smaller / , while the non-Gaussian features become more evident, leading to higher values of Sk, Ku and peak factor. On contrary, the values of fluctuating pressures become lower in the wind field with a smaller / , eventually resulting in underestimation of extreme wind loads. Hence, the matching relationship of turbulence integral scale to depth should be carefully considered as estimating the extreme wind loads of long-span roof by wind tunnel tests.

Topology optimization for the design of perfect mode-converting anisotropic elastic metamaterials

Yang, Xiongwei,Kim, Yoon Young ELSEVIER (APPLIED SCIENCE) 2018 COMPOSITE STRUCTURES -BARKING THEN OXFORD- Vol.201 No.-

<P><B>Abstract</B></P> <P>This work is concerned with the topology optimization of anisotropic elastic metamaterials exhibiting perfect mode conversion, a newly discovered phenomenon that an incident longitudinal (transverse) mode is solely and maximally converted to a transmitted transverse (longitudinal) mode. The wave phenomenon occurs at a series of interference frequencies due to elaborate multimodal interferences, known as the perfect transmodal Fabry-Perot interferences. Because the metamaterial must satisfy unique anisotropic relations among its effective stiffness, design of its unit cell is difficult without a systematic strategy. Here, we propose a topology optimization method based on the effective material properties to design such artificial composites. The homogenization method is employed to evaluate the effective material properties and the anisotropy requirements are treated as a special form of constraints. Because there is no natural mass constraint, we propose to maximize the effective longitudinal-transverse coupling stiffness for stable convergence. The sensitivity analysis is performed analytically within the finite element framework to update the design variables. The validity and effectiveness of the developed method are verified by considering different lattice types and interference cases. Considering the wide potential applications of anisotropic metamaterials in industrial applications, the developed numerical method can be an important and critically useful design tool.</P>

Asymptotic theory of bimodal quarter-wave impedance matching for full mode-converting transmission

Yang, Xiongwei,Kim, Yoon Young American Physical Society 2018 Physical review. B Vol.98 No.14

Surface Modification Method of Polyacrylonitrile (PAN) Fibers by L-cysteine Coupling Protein

Zixuan Yang,Yao Yao,Yujiao Huang,Wu Chen,Xiongwei Dong 한국섬유공학회 2019 Fibers and polymers Vol.20 No.12

This study reports an innovative environmental friendly polyacrylonitrile (PAN) fiber grafting modificationmethod, which is using L-cysteine as a coupling agent to link the hydrolyzed PAN fibers with protein macromolecules bycovalent bonds. After hydrolysis of PAN fibers, protein grafting was carried out after cysteine treatment. This method couldimprove the grafting efficiency of grafted polyacrylonitrile fiber without producing toxic by-products to human body andenvironment. The structure and surface appearance of the grafted fibers were characterized by FTIR, SEM, XRD and XPS. Itwas proved that the protein was successfully grafted onto the surface of acrylic fibers. The grafted fibers also showedimproved mechanical properties and good hygroscopicity.

What Drives China’s 2015 Stock Market Surges and Turmoil?

Jui-Jung Tsai,Yang-Chao Wang,Xiongwei Li 한국증권학회 2019 Asia-Pacific Journal of Financial Studies Vol.48 No.3

Using high-frequency data, we investigate China’s stock market surges and turmoil from 2014 to 2015 by employing GARCH family models to evaluate the effects of government policies, economic factors, and related announcements. The results indicate that China’s stock market has primarily been driven by government policies rather than economic factors. During the period, the commentaries of the People’s Daily further amplified the direction of market movements in the market surges and turmoil. In addition to individual policies and announcements, we explore the structural changes in volatility in different market stages and provide explanations for the distinct features of each stage.

Spatial correlation of aerodynamic forces on 5:1 rectangular cylinder in different VIV stages

Yongfu Lei,Yanguo Sun,Tianyi Zhang,Xiongwei Yang,Ming-shui Li 한국풍공학회 2022 Wind and Structures, An International Journal (WAS Vol.34 No.1

To better understand the vortex-induced vibration (VIV) characteristics of a 5:1 rectangular cylinder, the distribution of aerodynamic force and the non-dimensional power spectral density (PSD) of fluctuating pressure on the side surface were studied in different VIV development stages, and their differences in the stationary state and vibration stages were analyzed. The spanwise and streamwise correlations of surface pressures were studied, and the flow field structure partitions on the side surface were defined based on the streamwise correlation analysis. The results show that the variation tendencies of mean and root mean square (RMS) pressure coefficients are similar in different VIV development stages. The RMS values during amplitude growth are larger than those at peak amplitude, and the smallest RMS values are observed in the stationary state. The spanwise correlation coefficients of aerodynamic lifts increase with increase of the peak amplitude. However, for the lock-in region, the maximum spanwise correlation coefficient for aerodynamic lifts occurs in the VIV rising stage rather than in the peak amplitude stage, probably due to the interaction of vortex shedding force (VSF) and self-excited force (SEF). The streamwise correlation results show that the demarcation point positions between the recirculation region and the main vortex region remain almost constant in different VIV development stages, and the reattachment points gradually move to the tailing edge with increasing amplitude. This study provides a reference to estimate the demarcation point and reattachment point positions through streamwise correlation and phase angle analysis from wind tunnel tests.