Flutter performance of central-slotted plate at large angles of attack

Tang, Haojun,Li, Yongle,Chen, Xinzhong,Shum, K.M.,Liao, Haili Techno-Press 2017 Wind and Structures, An International Journal (WAS Vol.24 No.5

The flutter instability is one of the most important themes need to be carefully investigated in the design of long-span bridges. This study takes the central-slotted ideal thin flat plate as an object, and examines the characteristics of unsteady surface pressures of stationary and vibrating cross sections based on computational fluid dynamics (CFD) simulations. The flutter derivatives are extracted from the surface pressure distribution and the critical flutter wind speed of a long span suspension bridge is then calculated. The influences of angle of attack and the slot ratio on the flutter performance of central-slotted plate are investigated. The results show that the critical flutter wind speed reduces with increase in angle of attack. At lower angles of attack where the plate shows the characteristics of a streamlined cross-section, the existence of central slot can improve the critical flutter wind speed. On the other hand, at larger angles of attack, where the plate becomes a bluff body, the existence of central slot further reduces the flutter performance.

Flutter performance of central-slotted plate at large angles of attack

Haojun Tang,Yongle Li,Xinzhong Chen,K.M. Shum,Haili Liao 한국풍공학회 2017 Wind and Structures, An International Journal (WAS Vol.24 No.5

The flutter instability is one of the most important themes need to be carefully investigated in the design of long-span bridges. This study takes the central-slotted ideal thin flat plate as an object, and examines the characteristics of unsteady surface pressures of stationary and vibrating cross sections based on computational fluid dynamics (CFD) simulations. The flutter derivatives are extracted from the surface pressure distribution and the critical flutter wind speed of a long span suspension bridge is then calculated. The influences of angle of attack and the slot ratio on the flutter performance of central-slotted plate are investigated. The results show that the critical flutter wind speed reduces with increase in angle of attack. At lower angles of attack where the plate shows the characteristics of a streamlined cross-section, the existence of central slot can improve the critical flutter wind speed. On the other hand, at larger angles of attack, where the plate becomes a bluff body, the existence of central slot further reduces the flutter performance.

Flutter performance of box girders with different wind fairings at large angles of attack

Haojun Tang,Hang Zhang,Wei Mo,Yong-Le Li 한국풍공학회 2021 Wind and Structures, An International Journal (WAS Vol.32 No.5

The streamlined box is a common type of girders for long-span suspension bridges. Spanning deep canyons, long-span bridges are frequently attacked by strong winds with large angles of attack. In this situation, the flow field around the streamlined box changes significantly, leading to reduction of the flutter performance. The wind fairings have different effects on the flutter performance. Therefore, this study examines the flutter performance of box girders with different wind fairings at large angles of attack. Computational fluid dynamics (CFD) simulations were carried out to extract the flutter derivatives, and the critical flutter state of a long-span bridge was determined. Further comparisons of the wind fairings were investigated by a rapid method which is related to the input energy by the aerodynamic force. The results show that a reasonable type of wind fairings could improve the flutter performance of long-span bridges at large angles of attack. For the torsional flutter instability, the wind fairings weaken the adverse effect of the vortex attaching to the girder, and a sharper one could achieve a better result. According to the input energies on the girder with different wind fairings, the symmetrical wind fairings are more beneficial to the flutter performance.

Aerodynamic Instability of Stay Cables with Lighting Fixtures

Zhouquan Deng,Haojun Tang,Bo Hu,Yongle Li 대한토목학회 2021 KSCE JOURNAL OF CIVIL ENGINEERING Vol.25 No.7

In order to decorate the night view for cities, installing lighting fixtures on stay cables of existing cable-stayed bridges is an effective choice, but the aerodynamic characteristics of the stay cables will change significantly. This paper presents a study on the galloping stability of the stay cables equipped with lighting fixtures. The static aerodynamic coefficients were computed using computational fluid dynamics (CFD) simulations, and the reason for the negative slope of the lift coefficient was discussed based on the changes in pressure distributions and flow field characteristics. Wind tunnel tests were further carried out to verify the numerical results. The possible galloping instability intervals for the stay cables with lighting fixtures were calculated. The results show that the existence of lighting fixtures has significant effects on the aerodynamic performance of stay cables. The slope of the lift coefficient becomes negative within several intervals of angles of attack, which indicates the possible galloping instability. The sudden decrease in the lift coefficient is mainly related to the vortex which covers the concave between the lighting fixture and the stay cable and makes the separated flow cannot reattach.

Windproof ability of aerodynamic measures to improve the wind environment above a truss girder

Zewen Wang,Haojun Tang,Yongle Li,Junjie Guo,Zhanhui Liu 한국풍공학회 2021 Wind and Structures, An International Journal (WAS Vol.32 No.5

Aerodynamic measures have been widely used for improving the flutter stability of long-span bridges, and this paper focuses their windproof ability to improve the wind environment for vehicles. The whole wind environment around a long-span bridge located in high altitude mountainous areas is first studied. The local wind environment above the deck is then focused by two perspectives. One is the windproof effects of aerodynamic measures, and the other is whether the bridge with aerodynamic measures meets the requirement of flutter stability after installing extra wind barriers in the future. Furthermore, the effects of different wind barriers are analyzed. Results show that aerodynamic measures exert potential effects on the local wind environment, as the vertical stabilizer obviously reduces wind velocities behind it while the closed central slot has limited effects. The suggested aerodynamic measures have the ability to offset the adverse effect of the wind barrier on the flutter stability of the bridge. Behind the wind barrier, wind velocities decrease in general, but in some places incoming flow has to pass through the deck with higher velocities due to the increase in blockage ratio. Further comparison shows that the wind barrier with four bars is optimal.

Wake effects of an upstream bridge on aerodynamic characteristics of a downstream bridge

Zhenhua Chen,Zhenyun Lin,Haojun Tang,Yongle Li,Bin Wang 한국풍공학회 2019 Wind and Structures, An International Journal (WAS Vol.29 No.6

To study the wake influence of an upstream bridge on the wind-resistance performance of a downstream bridge, two adjacent long-span cable-stayed bridges are taken as examples. Based on wind tunnel tests, the static aerodynamic coefficients and the dynamic response of the downstream bridge are measured in the wake of the upstream one. Considering different horizontal and vertical distances, the flutter derivatives of the downstream bridge at different angles of attack are extracted by Computational Fluid Dynamics (CFD) simulations and discussed, and the change in critical flutter state is further studied. The results show that a train passing through the downstream bridge could significantly increase the lift coefficient of the bridge which has the same direction with the gravity of the train, leading to possible vertical deformation and vibration. In the wake of the upstream bridge, the change in lift coefficient of the downstream bridge is reduced, but the dynamic response seems to be strong. The effect of aerodynamic interference on flutter stability is related to the horizontal and vertical distances between the two adjacent bridges as well as the attack angle of incoming flow. At large angles of attack, the aerodynamic condition around the downstream girder which may drive the bridge to torsional flutter instability is weakened by the wake of the upstream bridge, and the critical flutter wind speed increases at this situation.

Effects of Secondary Elements on Vortex-Induced Vibration of a Streamlined Box Girder

Xingyu Chen,Fang Qiu,Haojun Tang,Yongle Li,Xinyu Xu 대한토목학회 2021 KSCE JOURNAL OF CIVIL ENGINEERING Vol.25 No.1

Box girders are aerodynamically favorable, and the paper presents a study on the vortex-induced vibration (VIV) of this girder type at large angles of attack. Taking a box girder as an example, the form of the guardrails and the position of the maintenance tracks were improved to increase the VIV performance. The aerodynamic mechanism was further investigated according to the change in flow field around the girder. Results showed that the VIV performance of the box girder at positive angles of attack is worse. Improving secondary elements is an effective way to increase the VIV performance. Adding four rectangle bars above the original guardrails could achieve the target by preventing the generation of the vortex and blocking its movement. Moving the maintenance tracks inwards with a reasonable distance could also achieve the target as the flow passes through the girder more smoothly. With the combination of the optimal form of the guardrails and the optimal position of the maintenance tracks, the VIV performance of the bridge is higher. The countermeasures provide reference for the inhibition of the VIV of box girders.

Non-uniform wind environment in mountainous terrain and aerostatic stability of a bridge

Xingyu Chen,Junjie Guo,Haojun Tang,Yongle Li,Lei Wang 한국풍공학회 2020 Wind and Structures, An International Journal (WAS Vol.30 No.6

The existence of a dam has potential effects on the surrounding wind environment especially when it is located in mountainous areas. In this situation, the long-span bridge over the reservoir can easily be exposed to non-uniform incoming flows, affecting its wind-resistance performance. This paper presents a study on the aerostatic stability of such a bridge. Wind tunnel tests were first carried out to investigate the wind environment above a mountainous reservoir. The results show that the angle of attack and the wind speed along the bridge axis show obvious non-uniform characteristics, which is related to the inflow direction. When winds come from the south where the river is winding, the angle of attack varies along the span direction significantly. The finite element model for the bridge was established using ANSYS software, and effects of non-uniform wind loads on the aerostatic stability were computed. Non-uniform angle of attack and wind speed are unfavorable to the aerostatic stability of the bridge, especially the former. When the combined action of non-uniform angle of attack and wind speed is considered, the critical wind speed of aerostatic instability is further reduced. Moreover, the aerostatic stability of the bridge is closely related to the dam height.

Blockage effects on aerodynamics and flutter performance of a streamlined box girder

Yong-Le Li,Junjie Guo,Xingyu Chen,Haojun Tang,Jingyu Zhang 한국풍공학회 2020 Wind and Structures, An International Journal (WAS Vol.30 No.1

Wind tunnel test is one of the most important means to study the flutter performance of bridges, but there are blockage effects in flutter test due to the size limitation of the wind tunnel. On the other hand, the size of computational domain can be defined by users in the numerical simulation. This paper presents a study on blockage effects of a simplified box girder by computation fluid dynamics (CFD) simulation, the blockage effects on the aerodynamic characteristics and flutter performance of a long-span suspension bridge are studied. The results show that the aerodynamic coefficients and the absolute value of mean pressure coefficient increase with the increase of the blockage ratio. And the aerodynamic coefficients can be corrected by the mean wind speed in the plane of leading edge of model. At each angle of attack, the critical flutter wind speed decreases as the blockage ratio increases, but the difference is that bending-torsion coupled flutter and torsional flutter occur at lower and larger angles of attack respectively. Finally, the correction formula of critical wind speed at 0o angle of attack is given, which can provide reference for wind resistance design of streamlined box girders in practical engineering.

Antibacterial and Antioxidant Composite Fiber Prepared from Polyurethane and Polyacrylonitrile Containing Tea Polyphenols

Ya Wen,Ruifang Zhao,Xueqian Yin,Yidong Shi,Haojun Fan,Yi Zhou,Lin Tang 한국섬유공학회 2020 Fibers and polymers Vol.21 No.1

Antibacterial and antioxidant properties exert an indispensable effect on biomedical and packaging materialswhich are mainly made of fiber-based fabrics. The current research illustrates a novel type of composite fiber which wasprepared from thermoplastic polyurethane (PU), polyacrylonitrile (PAN) and tea polyphenols (TP) through wet-spinningtechnology. The PU-PAN mass ratio was optimized upon the mechanical property; TP was incorporated into the compositefiber with different contents. The obtained fibers were fully characterized in terms of mechanical, thermal, morphologicalproperties as well as surface wettability. Additionally, in vitro TP release, antioxidant and antibacterial activity of suchcomposite fibers before and after washing were under investigation systematically and orderly. The findings showed that TPloadedcomposite fibers with an average diameter around 120 μm performed a sustained TP release behavior and desirablecytocompatibility. According to the capacity on scavenging 2, 2-diphenyl-1-picrylhydrazyl (DPPH) free radicals, theobservation of scavenging ratio reaching to 97 % proved the excellent antioxidant performance. Furthermore, the idealantibacterial activity against Gram-negative E. coli and Gram-positive S. aureus was also demonstrated. Collectively, thisstudy provides a facile strategy to design and prepare a novel type of composite fibers and fabrics, which may be put intoapplication as wound dressing and tissue conduit.