Performance of Baffles In A Sway Excited Sloshing Rectangular Tank

Abstract

The liquid in partially filled tanks tends to slosh when subjected to external disturbances. The safety of liquid transportation system necessitates the liquid sloshing problem with great practical importance. The knowledge of liquid sloshing frequency and hydrodynamic force on the wall is essential for seismic design of liquid storage tanks. Liquid motion in partially filled tanks may cause large structural loads if the period of tank motion is close to the natural period of fluid inside the tank. The large liquid movement in tanks creates highly localized impact pressure on tank walls and also displays a violent disturbance in the fluid. Hence, this highly nonlinear nature of the problem is the greatest interference in solving such a problem analytically and even computationally. Ship structures are likely subjected to impact pressure actions arising from sloshing, slamming, and green seas while in service. The accelerations arising from the motions of a ship in a sea way produce sloshing loads, that is, inertial reactions, on partially filled liquid cargo tank structures of the ship. Motions of liquid cargo vessels such as oil tankers often produce severe sloshing loads. Sloshing implies movement of a free liquid surface inside a vessel. A shake-table experiment for various depths of water is investigated to examine the effect of sloshing. The shake table is designed and devised to measure the sloshing force and the concept is tried for patentability as well. An experimental program is performed to study the phenomena of liquid sloshing and to assess the sloshing oscillation expected on the side walls in a partially filled rectangular tank. The sloshing tank of model scale 1:43 (Nasar et al. 2008) is fitted into the horizontal shake table. In order to examine the sloshing effects, a liquid fill level with an aspect ratio (hs/l, where hs is static liquid depth, l is tank length) of 0.163, 0.325 and 0.488 is considered which corresponds to 25% and 50% and 75% liquid fill levels. In view of suppressing sloshing oscillation and baffle wall configurations which is made up of acrylic sheets materials with different porosities of 4.4%, 6.8% and 9.2% and by placing baffle at a distance of L/2 conditions and L/3 & 2L/3 conditions in a rectangular tank are studied. The movement of fluid in a rectangular tank has been studied using another set of experimental approach with mild steel plates materials and different baffle configurations were adopted for analysing the sloshing oscillation, natural frequencies and variation in wave deflection. The adopted iporosities in the present study are 15%, 20% and 25 %. Porous screen is placed inside the tank at L/2 location and L/3 & 2L/3 locations and study is extended for single and double porous screen for better energy absorption. The parametric studies were carried out to show the liquid sloshing effects in terms of slosh frequencies, maximum free surface elevation and hydrodynamic forces acting on the tank walls. Capacitance wave probes have been placed at tank ends to record the free surface water elevation. At distinct locations along the length of the tank, time histories of sloshing oscillations () are recorded. The behaviour of sloshing oscillation is observed for the excitation. The frequencies of excitation ranges from 0.4566 Hz to 1.9757 Hz which covers up to fifth mode sloshing frequencies and the amplitude of 8mm is adopted. Sloshing force is captured by load cells and it is also analysed. Linear Variable Displacement Transducers (LVDT) are used to measure the displacement of the shake table. In the present study single and dual porous screen under the action of wave signals were analysed to understand the wave control performance due to porosity parameters. The results of maximum free surface elevation (max), root mean square surface elevation (rms), Sloshing Dynamics, sloshing forces (F’max, F’av and Fs), energy dissipation (Ew) and spectral moments (mo) are presented here. The suppression of resonant sloshing motion by porous baffle is analysed. A higher sloshing oscillation is observed for the aspect ratio (hs/l) of 0.325 than compared with other two fill levels. The tank with a water depth of hs/l= 0.325 and porous baffle of 15% porosity shows attenuation in sloshing force of about 32.78% whereas the 20% and 25% porosities of baffles show attenuation of 39.74% and 38.86% respectively in comparison with no baffle condition at L/3 & 2L/3 locations. The sloshing of liquid in containers is a ubiquitous phenomenon that represents one of the most fundamental fluid-structure interaction problems. By virtue, the liquid in a partially filled tank tends to slosh when subjected to external disturbances and is a vicious resonant fluid motion in a moving tank. In view of this difficulty, the sloshing effects are analysed by employing Computational Fluid Dynamics (CFD) and is validated with experimental tests. The fill levels of the sloshing tanks are varied for three different fill levels of aspects ratios (hs/l - 0.163, 0.325 and 0.488). Further, a comparative study is performed with porous and no porous presence of baffle iiconditions. The results from CFD correlate well with the experimental measurements on the sloshing effect. The numerical study shows a good correlation with the experiment results and the deviation of numerical simulation in comparison with experimental findings is found to be 9.24%. Results presented in this study can be used in the ship tanks in view of dynamic stability, reduction of sloshing energy and avoidance of roof impact.