Experimental Investigation On The Flow Induced Vibration And Energy Extraction Of An Equilateral Triangle Prism

Flow induced vibration(FIV)is commonly encountered in engineering fields,such as oceanographic engineering,aeronautical engineering and long-spanned or high-rise buildings.The vortex induced vibration(VIV)due to the alternating shedding of vortices and the galloping caused by the periodic variations in the angle of attack of the flow are the most common FIV phenomena.FIV can cause destructive accidents.However,it can also bring benefits to mankind.Vortex Induced Vibration for Aquatic Clean Energy converter(VIVACE),which utilizes FIV of a cylinder to capture hydrokinetic energy,turns the destructive phenomena into a benefit.In order to improve the generating efficiency and the applicable velocity range of VIVACE,a series of FIV experiments for noncircular cylinders elastically mounted in a water channel are performed for the first time,and the FIV characteristics and the energy extraction performance of an equilateral triangle prism are emphatically discussed in the thesis.The main research contents and conclusions in this thesis are listed as follows:(1)A series of FIV experiments are performed to investigate the FIV characteristics of noncircular cylinders.The results show that the equilateral triangle prism has the strongest amplitude and the approximately constant frequency,and its response characteristics are beneficial to the power conversation in the environment where the flow velocity is larger and unsteady.(2)The results of FIV experiments of an equilateral triangle prism with different system stiffness indicate that the system stiffness changes the wake flow mode of the prism at the same reduced velocity thus affecting the amplitude and frequency responses.The amplitude ratio monotonically increases with the increase of the flow velocity,and the FIM of the prism can be divided into three primary regions: the vortex induced vibration(VIV)branch,the transition branch from VIV to galloping and the galloping branch.The transition branch is initiated accompanied with a steep increase in amplitude and a precipitous drop in frequency.The frequency presents “lock-in” phenomenon in the VIV upper branch and the galloping branch.Furthermore,the reduced velocity,amplitude and frequency ratios where the prism starts to oscillate decreases with the increase of the system stiffness.(3)The translation-rotation motion equation of the elastically mounted equilateral triangle cylinder is derived in this paper,and the constitution of the total vibration mass of the system and the relationship between the structural damping coefficient and the load resistance of a motor are expounded in the motion equation.Besides,the variable damping device is designed on the basis of the motion equation.(4)The results of FIV experiments of an equilateral triangle prism with different system damping ratios reveal that,as the system damping ratio increases,the amplitude ratio of the prism in all branches decreases,but the generated power and efficiency in the galloping branch increases.Besides,the galloping of the prism present,in order,the soft galloping,the hard galloping? and the hard galloping?with the increase in the system damping ratio.The optimal system damping ratio for generating electricity is the one that the prism can overcome to galloping.The maximum generated power and efficiency of the VIVACE with an equilateral triangle oscillator reach 53.56 W and 40.44%,respectively.And the maximum generated power and efficiency,which are achieved in the galloping branch,exceed those of the circular cylinder.(5)The effect of the Reynolds number on the FIV and the energy extraction of an equilateral triangle prism are investigated in the thesis.The results indicate that the higher Re number is beneficial for the prism to overcome larger damping and reache higher amplitude ratio thus improving the generated power and efficiency of VIVACE.In conclusion,the FIV of an equilateral triangle prism elastically mounted in a water channel presents the characteristics of the large amplitude response in an infinite range of flow velocities,the excellent vibration stationarity and the steady vibration frequencies.As a result,the VIVACE with an equilateral triangle oscillator has broader velocity range,higher generated power and efficiency and better power quality than those of the circular cylinder.The research in this thesis is of great importance for improving the use value of VIVACE.