Analysis Of Sloshing Characteristics Of Two-layered Liquid In Tank

In recent years,more and more FPSOs,LNG,and other liquid carrying vessels have been used to store offshore oil and transport natural gas.In order to ensure the normal transportation and separation of crude oil on offshore engineering platforms,and to establish methods for studying the sloshing of damaged tanks,it is necessary to analyze the sloshing characteristics of stratified liquids in the tanks.Based on the theory of viscous Fluid mechanics,the RANS equation of SST k-w turbulence model is used to simulate the oil-water flow.The flow field is discretized using the finite volume discretization method,while the VOF method is used to capture the free liquid surface and oil-water interface,and the adaptive mesh method is used to capture interface motion.Using the STAR-CCM+software platform,numerical simulations were conducted on the oscillation of oil-water double-layer liquids.The main research content and conclusions of this article are as follows:(1)A two-dimensional numerical calculation model for double layer liquid sloshing in a liquid tank has been established.First,the natural frequency of Free surface sloshing and oil-water interface sloshing is obtained through the free attenuation numerical test of double-layer liquid sloshing,and the natural frequency of Free surface is greater than the natural frequency of the interface.Secondly,through calculation and comparison,the sloshing characteristics of single and double-layer liquids under different excitation conditions are analyzed: compared with single-layer liquids,the Free surface sloshing amplitude of double-layer liquids is smaller,and there is a certain phase lag,and the pressure impact amplitude is also reduced;The movement of oil-water interface of doublelayer liquid is slower than that of Free surface,and the upper liquid can inhibit the movement of the lower liquid.(2)According to the established numerical model of double-layer liquid sloshing,the sloshing characteristics of the tank under forced sway with different liquid loading rates were calculated;The sloshing frequency of the tank under external excitation includes the sloshing frequency of the tank and the sloshing natural frequency of the liquid itself.When far away from the resonance frequency,the spectrum analysis will have sum frequency and difference frequency.The contribution ratio of the natural frequency of the free liquid surface to the motion response of the two interfaces will increase with the increase of the liquid loading rate.When the external excitation frequency is small,high-order resonance may occur at the oil-water interface;In addition,as the liquid loading rate increases,the amplitude and pressure of free surface motion also increase,but the oscillation nonlinearity is stronger at low liquid loading rates.(3)The influence of two types of liquid depth ratio changes on the sloshing characteristics of double-layer liquids was studied:(1)The total height of the liquid is the same but the oil-water ratio is different,and the calculation includes free sloshing and liquid sloshing under forced lateral sway of the liquid tank.According to the calculation of free oscillation,at low liquid loading rate,the calculated natural frequency is greater than the theoretical value,while at high liquid loading rate,the calculated value is smaller than the theoretical value.Moreover,compared to the change in liquid depth ratio,the total liquid depth has a greater impact on the natural frequency of the two interfaces;The modal analysis of the oil-water interface reveals two natural frequencies of free liquid surface and oil-water interface oscillation.As the depth ratio increases,the motion response excited by the natural frequency of the oil-water interface gradually increases and the frequency doubling increases.Secondly,by analyzing the motion characteristics of the two interfaces under different external excitations,it was found that an increase in the oil-water ratio has no effect on the free surface motion.However,as the oil-water ratio increases,the wave height and pressure amplitude of the oil-water interface continue to decrease,but the peak amplitude changes differently.(2)The height of the lower layer of water is the same,but the height of the upper layer of oil is different.The influence of the height of the upper layer of liquid on the movement of the lower layer of liquid was calculated and analyzed.As the height of the upper layer of liquid increases,the inhibitory effect on the movement of the lower layer of liquid becomes more obvious,and the distribution of fluid velocity changes.