Experimental Study On Viv And Its Suppression Of Large Mass-Damping Cylinder

Circular cylinder is widely used in lots of engineering practices,such as the aerodynamic and hydrodynamic structures of construction engineering,mechanical engineering,and ocean engineering.When fluid flows past a bluff body such as cylinder,vortices generate from two shear layers with alternating counter-rotating vortex structures and lead to vortex-induced vibration(VIV)phenomenon.The lockin phenomenon may occar when the vortex shedding frequency is close to the natural frequency of cylinder.Cylinders may be destroyed when suffering continuous strong vibrations,which may bring serious influences to those structures,or even engineering and environmental accidents.Circular cylinder is the most frequently used structure in engineering practices.Therefore,the investigation on VIV and its suppression for circular cylinder is necessary.Till now,the VIV and its suppression for low mass-damping circular cylinder are widely studied,however,those for large mass-damping circular cylinder are scarce.The measurements of flow visualization and vibration have been established.Displacements at In-line and Cross-flow directions were measures using two laser sensors and the data acquisition is performend by a self-developed simultaneous measurement system.The smoke flow visualization technique were employed to help explain the differences of flow structures around bare cylinder and suppression devices.Present paper focuses on VIV for circular cylinder with large mass-damping parameter in air by wind tunnel test,and then investigates the response of cylinder with different kinds of devices such as fairing,splitter plate,helical strake as well as attached fur employed to suppress VIV.(1)The responses of cylinder with relative low mass-damping parameter(m*ζ=0.41)and flow visualization of fixed cylinder were experimental studied first.The trend of present results is in good agreement with that of previous results from classical experiments,which shows that the methods used in present paper is validation.Then we investigated VIV for cylinder with large mass-damping parameter(m*ζ=1.415)and found out that the amplitude response falls into the second peak,and the trajectory of bare cylinder is an incline ellipse-shape.(2)The influence on suppression efficiency of fairing shape angle was experimentally studied.The results show that fairings can suppress the amplitude response and narrow the lock-in region.Fairing with the shape angle of 60° is the optional configuration.More than 76% of the amplitude response can be suppressed by fairing of 60°.Power spectral density(PSD)can be suppressed by farings too,which proves that fairing can suppress VIV.(3)The influence on suppression efficiency of dimensionless splitter plate Length was experimentally studied.From the results of amplitude responses and flow visualization,it can be found those splitter plates with length of 0.25 D and 0.5D can reduce the amplitude responses and the width of lock-in region,but the suppression efficiency is limited,because their structures are close to the bare cylinder that the flow feild and amplitude response of them are simillar to that of bare cylinder.Splitter plates of 0.75 D,1.0D and 1.25 D can suppress the amplitude responses effectively by changing vortex shedding frequency and vortex structures,almost 78% and 67% amplitude responses in the CF and IL directions can be suppressed.The amplitude responses of all of the three splitter plates are close to each other.Splitter plates of 1.5D and 1.75 D can suppress the amplitude responses of bare cylinder in the test range,but these splitter plates can suffer galloping phenomenon when Ur reaches a certain value,which is too large to be measured by the test system.(4)The influence on sufferssion efficiency of helical strake geometric parameters were experimentally studied.From the results of flow visualization,it can be found that heilcal strake can suppress VIV effectively by varying the separation points of boundary layers and preventing the interaction between the shear layers as well as changing vortex shedding frequency and vortex structures.The amplitudes of straked cylinders are far smaller than those of bare cylinder and there is no remarkable peak in the amplitude responses.The "□"-type helical strakes of P/D=17.5D and P/D=5 have close suppression efficiency,which are slightly less than that of P/D=10.The suppression effectiveness of "D"-type strakes are better than that of "□"-type strakes.Yrms/D reduces as the height of helical strake decreases and the coverage of helical strake increases,but strake pitch is not an important factor on VIV suppression.Helical strakes of P/D=5,h/D=0.14("D"-type)is the most effective one,with which the VIV suppression efficiency on Yrms/D reaches up to over 85% compared with that of bare cylinder and the maximum suppression efficiency reaches up to 98% at Ur =20 in the peak region.Helical strake of "D"-type has a better suppression efficiency than that of "□"-type.PSD changes as the pitches and heights vary and the magnitude is far smaller than that of cylinder,there are more main peak frequencies in the magnitude of "D"-type helical strakes.It indicates that helical strake can effectively block the interaction between both sides of the separation layer and suppress vortex shedding.(5)The influence on suppression efficiency of dimensionless fur length was experimentally studied.From the results of flow visualization,it can be found that Fur swing freely with vortex and control vortices flexiblely.Fur can change the separation points and the quality of boundary latyers,avoid the interaction between two shear layers,as well as change the vortex shedding frequency and the vortex structures length.The amplitude responses are smaller than that of bare cylinder,and reduce as the length of fur increases.The amplitude response of fur at L/D=1.8 has been changed into a gentle curve in the test region,and the suppression efficiencies of fur with L/D=0.6~1.8 are close to each other.PSD is influenced by the length of fur,which is far smaller than that of bare cylinder.From the flow visualizations of circular cylinder with and without fur,Optimized selection of all parameters of suppression devices at present experimental conditions has been made from a comprehensive analysis:(1)Fairing of 60° is the optional one in the three farings;(2)Splitter plates ranged from 0.75 D to 1.25 D can suppress VIV effectively,which are the optional splitter plates;(3)Helical strake of "D"-type has a better suppression efficiency than that of "□"-type.Helical strake of P/D=5,h/D=0.14("D"-type)is the most effective one;(4)Fur with L/D=0.6~1.8 can suppress VIV effectively,the suppression efficiency of them are close to each other.All of them are the optional models.