An isolated slit was placed in a single box girder to obtain passive leading-edge suction and trailing-edge jet flow to control the unsteady aerodynamic force and modify the flow structure.The Great Belt East Bridge w...An isolated slit was placed in a single box girder to obtain passive leading-edge suction and trailing-edge jet flow to control the unsteady aerodynamic force and modify the flow structure.The Great Belt East Bridge was used as a physical model at a geometric scale of 1:125.Wind tunnel experiments were conducted at an incoming airflow speed of 10 m/s,and the Reynolds number was calculated as 2.3×104 using the test model height and wind speed.The surface pressure distribution was measured,and the aerodynamic force acting on the test model with and without the isolated slit was calculated by integrating the pressure result.It was found that the control using an isolated slit can dramatically decrease the fluctuating surface pressure distribution and aerodynamic force.An analysis on the power spectral density of the lift force revealed that the isolated slit accelerated vortex shedding.Moreover,high-speed particle image velocimetry was used to investigate the wake flow structure behind the test model.A vortex separated from the upper surface was pushed to a lower location and the wake flow structure was modified by the isolated slit.A proper orthogonal decomposition(POD)of the flow field showed that the first two POD modes in the controlled case contributed less energy than those in the uncontrolled case,indicating that more energy was transferred to higher modes,and small-scale vortices had more energy.A secondary instability structure was found in the wake flow for a nondimensional jet momentum coefficient J of 0.0667.展开更多
Wind tunnel tests were carried out to investigate the aerodynamic interference between a triple-box girder and trains,involving static aerodynamic forces and vortex-induced vibrations(VIVs).Static and dynamic sectiona...Wind tunnel tests were carried out to investigate the aerodynamic interference between a triple-box girder and trains,involving static aerodynamic forces and vortex-induced vibrations(VIVs).Static and dynamic sectional models of the girder and trains were employed for aerodynamic force measurement and VIV test,respectively.Results indicate that the aerodynamic interference effect on static aerodynamic forces of both the girder and trains is remarkable.When a single train exists,the horizontal position of the train has a small effect on aerodynamic coefficients of the girder.When two trains meet on the girder,the drag coefficient of the girder is significantly reduced compared with that of without train or with a single train;besides,during the whole meeting process,aerodynamic forces of the leeward train first drop and then increase suddenly.The fluctuation of aerodynamic force could cause redundant vibration of the train,which is unfavorable for safety and comfort.A train on the girder could worsen the girder VIV performance:a new vertical VIV appears in the triple-box girder when a train is on the girder,and the torsional VIV amplitude increases significantly when the train is on the windward side.展开更多
For the 64 most basic ways to construct a hash function H:{0,1} → {0,1}n from a block cipher E:{0,1}n × {0,1}n → {0,1}n, Black et al.provided a formal and quantitative treatment of the 64 constructions, and pro...For the 64 most basic ways to construct a hash function H:{0,1} → {0,1}n from a block cipher E:{0,1}n × {0,1}n → {0,1}n, Black et al.provided a formal and quantitative treatment of the 64 constructions, and proved that 20 schemes are collision resistant.This paper improves the upper and lower bounds and make contrast with a hash constructed from a random oracle.These 20 schemes have only one kind of collision resistance upper and lower bounds.In addition, we present new advantages for finding second preimages.展开更多
基金Projects(51978222,51722805,U2106222) supported by the National Natural Science Foundation of ChinaProject(HIT.BRETIV 201803) supported by the Fundamental Research Funds for the Central Universities,China。
文摘An isolated slit was placed in a single box girder to obtain passive leading-edge suction and trailing-edge jet flow to control the unsteady aerodynamic force and modify the flow structure.The Great Belt East Bridge was used as a physical model at a geometric scale of 1:125.Wind tunnel experiments were conducted at an incoming airflow speed of 10 m/s,and the Reynolds number was calculated as 2.3×104 using the test model height and wind speed.The surface pressure distribution was measured,and the aerodynamic force acting on the test model with and without the isolated slit was calculated by integrating the pressure result.It was found that the control using an isolated slit can dramatically decrease the fluctuating surface pressure distribution and aerodynamic force.An analysis on the power spectral density of the lift force revealed that the isolated slit accelerated vortex shedding.Moreover,high-speed particle image velocimetry was used to investigate the wake flow structure behind the test model.A vortex separated from the upper surface was pushed to a lower location and the wake flow structure was modified by the isolated slit.A proper orthogonal decomposition(POD)of the flow field showed that the first two POD modes in the controlled case contributed less energy than those in the uncontrolled case,indicating that more energy was transferred to higher modes,and small-scale vortices had more energy.A secondary instability structure was found in the wake flow for a nondimensional jet momentum coefficient J of 0.0667.
基金Project(52025082) supported by the National Natural Science Foundation for Distinguished Young Scholars of ChinaProject(CX20190288) supported by Hunan Provincial Innovation Foundation for Postgraduate,China。
文摘Wind tunnel tests were carried out to investigate the aerodynamic interference between a triple-box girder and trains,involving static aerodynamic forces and vortex-induced vibrations(VIVs).Static and dynamic sectional models of the girder and trains were employed for aerodynamic force measurement and VIV test,respectively.Results indicate that the aerodynamic interference effect on static aerodynamic forces of both the girder and trains is remarkable.When a single train exists,the horizontal position of the train has a small effect on aerodynamic coefficients of the girder.When two trains meet on the girder,the drag coefficient of the girder is significantly reduced compared with that of without train or with a single train;besides,during the whole meeting process,aerodynamic forces of the leeward train first drop and then increase suddenly.The fluctuation of aerodynamic force could cause redundant vibration of the train,which is unfavorable for safety and comfort.A train on the girder could worsen the girder VIV performance:a new vertical VIV appears in the triple-box girder when a train is on the girder,and the torsional VIV amplitude increases significantly when the train is on the windward side.
基金the National Natural Science Foundation of China (No. 60573028)
文摘For the 64 most basic ways to construct a hash function H:{0,1} → {0,1}n from a block cipher E:{0,1}n × {0,1}n → {0,1}n, Black et al.provided a formal and quantitative treatment of the 64 constructions, and proved that 20 schemes are collision resistant.This paper improves the upper and lower bounds and make contrast with a hash constructed from a random oracle.These 20 schemes have only one kind of collision resistance upper and lower bounds.In addition, we present new advantages for finding second preimages.
