Numerical and Experimental Investigations on free surface characteristics of Sloshing Waves in externally Induced Tanks
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2011
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Abstract
A moving tank partially filled with liquid that undergoes periodic motions is known to exhibit sloshing. Fluid 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 free surface flow phenomenon is an important consideration in several engineering fields like safety assessment of power generation plants and for many other industrial purposes. Since early 1960s, the problem of liquid sloshing dynamics has been of major concern to aerospace, ocean and mechanical engineers. Civil engineers and seismologists have been studying liquid sloshing effects on large dams, oil tanks and elevated water towers under ground motion. The capture of liquid free surface flow properties such as displacement, velocity and pressure has always been a challenging work. Many experimental and numerical techniques are available to capture these properties. However, some of the techniques yield the best results. This thesis is concerned with capturing the dynamic properties of the liquid free surface in partially filled tanks. The liquid free surface elevation, free surface velocity and pressure distributions are captured by numerical and experimental investigations. The present investigation focuses on 2-D and 3-D rectangular domain with finite difference based Sigma-transformation (D -transformation) for regular and random excitations, and the results obtained are compared with benchmark data. Techniques such as SURF, Marker and Cell (MAC), Volume of Fluid (VOF) require complex computer programming to treat the time varying free surface boundary and the computational mesh needs to be updated at every time step. The D -transformation for treating the liquid free surface has gained wide spread popularity in recent years because of its simplicity and ease of implementation. This transformation is used to map the liquid domain onto a rectangle, such that the moving free surface in the physical plane becomes a fixed horizontal line in the computational mapped domain. Further, investigations of free surface velocity with the aid of flow visualisation technique in an externally induced sloshed tank subjected to surge motion is scarce. In the present investigation, the interface location method has been used for capturing the liquid free surface experimentally, during the external excitation of the tank under low frequency. The 2-D planar Particle Image Velocimetry (PIV) technique is used to calculate the U and V velocity of the free surface, while the interface location technique is used for vertical velocity of the interface (W). In the interface location method, the videos captured by the movie camera are then transferred to the computer and further image analysis is carried out. Intensity gradient at each point is calculated, where the location of maximum gradient is considered as the interface. The luminance contrast between the liquid and the air which indicates the location of the interface is determined by maximum intensity gradient method. The free surface velocity has been calculated by using the 2-D planar PIV technique. In this technique, the two images of each particle in the sheet are recorded in a short time interval. The images of marker particles in a fluid flow are used to measure instantaneous velocity fields. It is a technique which en...
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Supervisor: U. K. Saha
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MECHANICAL ENGINEERING