Proper orthogonal decomposition, dynamic mode decomposition, wavelet and cross wavelet analysis of a sloshing flow

Internal hydrodynamics and its coupling with structural dynamics are non-negligible processes in the design phase of aerospace systems. An improved understanding of the nature of this coupling would allow for greater flexibility in modeling and design of such systems, and could lead eventually to the development of suitable active and/or passive control strategies for enhanced performances. In this manuscript we apply a number of data analysis techniques: proper orthogonal decomposition, dynamic mode decomposition and wavelet transform and their combination to time-resolved images of a liquid sloshing within an enclosure. We use these techniques to identify fluid-dynamic modes in space and time and to verify their coupling with the structural dynamics of vibrating structures. In particular we consider the transient case of a water tank mounted on a free oscillating cantilever. As the acceleration amplitude decays, we observe and quantify the transition from incoherent flow to standing waves. Our results show that the content of the images is very informative and can be used for quantitative analysis. As the main outcome, the hydrodynamic modes are identified using POD and DMD, and related to known features of sloshing flow, such as the frequency of the first symmetric free surface mode. Additionally we perform a comparison of wavelet transforms of POD time coefficients and measured acceleration signals at the tank base. Viewing the latter as the input and the former as the output of the fluid-dynamic system, we are able to correlate the enhanced damping of the cantilever oscillation to the different regimes of the hydrodynamic field

Sloshing in a closed domain under unidirectional excitation

1145-1153Sloshing is a phenomenon where a partially filled tank is exerted into various environmental sea conditions, such as wave and wind. Sloshing in a tank of liquefied natural gas carrier can lead to structural damage of tank structures and motion instability of the carrier. Thus, sloshing analysis needs to be conducted beforehand to minimize the risk of damages. This paper presents experimental and numerical study on sloshing phenomenon in a prismatic membrane tank model under unidirectional excitation with 30% water filling condition. A regular wave motion stimulated by the linear actuator was applied to the model tank and recorded by a video camera. Meanwhile, OpenFoam software was used to simulate the sloshing numerically in a volume of fluid method based on Navier-Stokes theorem. The sloshing patterns and free surface elevation in the prismatic membrane model tank, with the same input amplitude and frequency, were investigated for both cases. Both experimental and simulation results showed reasonable agreement on the sloshing profile, while the internal free surface elevation in the closed domain indicated a deviation with maximum absolute error of 4.9 cm

A STUDY ON THE TWO-ROW EFFECT IN THE SLOSHING PHENOMENON

In this study, changes in fluid impact loads inside a tank were examined according to a two-row tank arrangement in an LNG-FPSO (Liquefied Natural Gas-Floating Production Storage Offloading) vessel. The motion RAO (Response Amplitude Operator) of the LNG-FPSO, coupled with the sloshing phenomenon inside the tank, was calculated by using HydroStar by Bureau Veritas. The motion simulation in the tank was conducted under filling ratios of 30%H, 60%H, and 80%H. The RAO in each condition was calculated according to the one-row and the two-row tank arrangement. The motion response spectrum using the calculated RAO and the JONSWAP (Joint North Sea Wave Analysis Project) spectrum were computed by implementing irregular motion according to each filling ratio and tank arrangement. The sloshing phenomenon inside the tank was implemented by using a 6-DOF (Degree Of Freedom) sloshing motion platform; impact pressure on the walls of the tank was measured with pressure sensors installed inside the tank. The sloshing experiment was conducted under the three filling ratios in the one-row and the two-row tank arrangement and impact loads were compared under each filling ratio according to the one-row and the two-row tank arrangement

Fluid technology (selected components, devices, and systems): A compilation

Developments in fluid technology and hydraulic equipment are presented. The subjects considered are: (1) the use of fluids in the operation of switches, amplifiers, and servo devices, (2) devices and data for laboratory use in the study of fluid dynamics, and (3) the use of fluids as controls and certain methods of controlling fluids

Soft metal blanket with optional anti-sloshing conceptual designs to improve pressure control for floating and land-based liquefied natural gas tanks

 A conceptual design for an additional in-tank system in liquefied natural gas (LNG) tanks (in offshore or land-based plants) is proposed for efficient control of tank pressure. This system involves simple supplementary components to standard boil-off handling systems. The design concept builds upon the recently recognised duality in tank pressure behaviour in large LNG tanks. Such behaviour is exploited to promote conditions where tank pressure naturally trends to lower levels and limits the significant and abrupt pressure increases that would otherwise occur from time to time during routine operations. The concept involves a soft floating metal blanket, which involves simple low-cost components, requires no additional power to run, is easily retrofitted to and removed from existing tanks. The construction modifications for tanks required are minor and could be beneficial to both land-based and offshore plants. In offshore plants this system is suitable for sheltered locations where LNG cargo sloshing is not an issue. The design concept can though be modified as a more complex and connected structure (an anti-sloshing floating soft metal blanket) to provide combined anti-sloshing and pressure-control capabilities for offshore applications. Both concepts provide their greatest potential benefits to offshore floating storage and regasification units and floating storage units with tanks constrained by tank strength design limits, typically those converted from LNG carriers. Additionally, the solutions presented have direct relevance to shore-based LNG tanks due to their simpler geometry and sloshing-free status.Cited as: Kulitsa, M., Wood, D.A. Soft metal blanket with optional anti-sloshing conceptual designs to improve pressure control for floating and land-based liquefied natural gas tanks. Advances in Geo-Energy Research, 2019, 3(4): 424-447, doi: 10.26804/ager.2019.04.0