Capturing the Motion of the Free Surface of a Fluid Stored within a Floating Structure

Abstract

Large floating structures, such as liquefied natural gas (LNG) ships, are subject to both internal and external fluid forces. The internal fluid forces may also be detrimental to a vessel’s stability and cause excessive loading regimes when sloshing occurs. Whilst it is relatively easy to measure the motion of external free surface with conventional measurement techniques, the sloshing of the internal free surface is more difficult to capture. The location of the internal free surface is normally extrapolated from measuring the pressure acting on the internal walls of the vessel. In order to understand better the loading mechanisms of sloshing internal fluids, a method of capturing the transient inner free surface motion with negligible affect on the response of the fluid or structure is required. In this paper two methods will be demonstrated for this purpose. The first approach uses resistive wave gauges made of copper tape to quantify the water run-up height on the walls of the structure. The second approach extends the conventional use of optical motion tracking to report the position of randomly distributed free floating markers on the internal water surface. The methods simultaneously report the position of the internal free surface with good agreement under static conditions, with absolute variation in the measured water level of around 4 mm. This new combined approach provides a map of the free surface elevation under transient conditions. The experimental error is shown to be acceptable (low mm-range), proving that these experimental techniques are robust free surface tracking methods in a range of situations.

Comparison of a Floating Cylinder with Solid and Water Ballast

“…Wave gauges (WG) made of copper tape are applied on the inside wall. This instrumentation is introduced by Gabl et al [59] and allows the measurement of internal run up at the walls. In the presented investigation, the movement of the internal water surface is not specifically monitored, hence the main goal is the comparison of the body motions between the ballast variations, namely water or solid, for a wide range of wave conditions.…”

“…At the beginning of each test day, all probes are calibrated to maintain the high accuracy of the measurements. The accuracy of a resistive wave gauge is typically in the range of 1 mm [59,64,65].…”

Experimental Data of a Floating Cylinder in a Wave Tank: Comparison Solid and Water Ballast

“…All edges were sharp, and the mooring is described and investigated in Gabl et al [13]. The cylinder incorporates copper tape wave gauges on the wall and floating balls measured by the motion capturing system for additional measurement of the inside free surface [17].…”

“…The wave gauges were calibrated at the beginning of each measurement day covering changes of the water surface of ±10 cm. The accuracy of this measurement system is typically in the order of 1 mm [17,22,23]. Table 3.…”

Versatile image-based measurements of granular flows and water wave propagation in experiments of tsunamis generated by landslides

“…Computer vision has received a boost in the last 30 years mainly because the recent inexpensive and technically improved digital cameras are attractive for low-cost measuring systems (Jain et al 1995;Shapiro and Stockman 2001). Image-based measuring systems are suitable for laboratories of hydraulics and have been used in several experimental practices (Erikson and Hanson 2005;Yao and Wu 2005;Iglesias et al 2009;Wang et al 2012;Gabl et al 2018). In order to study impulsive water wave generated by landslides, a specific application of image-based measurements has been set up in the Fluvial Morphdodynamics I laboratory of the Sediment Transport Research Group (GITS) in the Technical University of Catalonia (Bregoli et al 2017).…”