Influences of a Varying Sill at the Seabed on Two-Dimensional Radiation of Linear Water Waves by a Rectangular Buoy

The presence of a sill alters the traditional flat seabed conditions, imposing significant effects on hydrodynamic characteristics of the wave–body interaction problem. Eigenfunction matching method (EMM), which is a typical analytical method, is employed to precisely evaluate these effects on two-dimensional radiation of linear water waves by a rectangular buoy. The agreement with the published data and numerical results by the boundary element method (BEM) indicates the correctness of the present EMM. Four aspects involving varying height of the sill, varying width of the sill, varying water depth of the right semi-infinite subdomain, and varying relative positions between the buoy and the sill are considered. Analytical results show that among all the hydrodynamic coefficients, the added mass coefficient of heave is affected most by the varying sill.

Study on Added Mass Coefficient and Coupling Effect of Concentric Tube Bundles

The added mass coefficient is an important parameter when calculating the flow induced vibration (FIV) in heat exchangers. The calculation method of the added mass coefficient for tube bundles in triangular and square arrays had been presented in relevant standards which like GB/T 151. Presently, concentric arrays of tube bundles are applied extensively in the heat exchangers at nuclear power plants. However, the method for calculating the added mass coefficient of tube bundles in concentric arrays has not been given in these standards. In this paper, in order to calculate the added mass coefficient of a central flexible tube surrounded by rigid tubes in concentric arrays, the fluid-structure interaction models of tube bundles has been established by ANSYS CFX. The influence of the circumferential and radial distance of the tube bundles on the added mass coefficient has been studied by calculating the natural frequency. The results are expected to provide the calculation reference for added mass coefficient of the tube bundles in concentric arrays. The coupling vibration of tube bundles in concentric arrays has been studied by the other model which is made up of all the flexible tubes, and the coupling vibration of the tube bundles is simulated by the rigid wall motion of ANSYS CFX. According to simulation results, there exists a band of amplitude peaks in the coupling vibration of the tube bundles. It is shown that as the circumferential and radial distance increases, the band of amplitude peaks becomes narrow.

Hydrodynamic Design of a Morphic Autonomous Underwater Vehicle Using Neural Networks

With the advent of Artificial Intelligence and new manufacturing techniques, Autonomous Underwater Vehicles (AUVs) have started to prevail over their manned version in terms of cost efficiency when it comes to accomplish tasks in ocean exploration, offshore platform and ship maintenance or other military missions. As progress has been made over the past years, autonomy remains a topical issue for the untethered AUVs. Drawing its inspiration from nature, this paper aims at minimizing the energy consumed by the device on a specific mission by allowing its shape, parameterized with Bezier curves, to morph throughout time. The framework is restricted to one dimensional trajectories only. A first step consisting of finding the optimal velocity and shapes added mass coefficient in surge as functions of time for a given mission is presented. Then a way of determining the succession of shapes the AUV must take so that it has the right added mass coefficient at any time is proved and used. This last part is made computationally affordable by using a Neural Network instead of a Boundary Element Method to evaluate the hydrodynamic coefficient in surge of the shape. Outliers detection and elimination are being performed on the training dataset to increase the predictive model reliability and robustness.

Flight Performance Analysis of Hybrid Airship Considering Added Mass Effects

In this paper, an analysis is applied to a hybrid airship considering the added mass. First, based on the dynamic mesh technology, a computational fluid dynamics (CFD) method is employed to obtain the added mass coefficient matrix. Through a validation process using the 6:1 prolate spheroid, the 6 × 6 added mass matrix of hybrid airship is obtained. After a dynamic modeling, the equations of motion with added mass are developed. Through the linearization based on small perturbation, the linearized longitude model is used to simulate the dynamic response of a trim condition. The take-off and landing performance has been analyzed and affected by the added mass. The result shows an obvious vertical destabilizing trend on the hybrid airship dynamics due to the added mass and the inertial effect has little influence on the vehicle during the take-off and landing.

Test to Determine Hydrodynamic Interaction Between Drilling Riser, Drill String and Mud

Drilling riser damping can have a significant effect on the dynamic response of the drilling system, especially the fatigue response of the wellhead and conductor system. The presence of any drilling riser damping helps to diminish the transfer of riser motions into the wellhead system, which can improve any wellhead fatigue issues. One of the little studied contributors to the system damping is the hydrodynamic damping effect of the interaction between the drill pipe and drilling fluid inside the riser. As part of the ongoing Structural Well Integrity Joint Industry Project (JIP), finite element analysis (FEA) with a wide range of drag and inertia coefficients is conducted to simulate the mud drag and inertia on the drill string and the riser. These sensitivity studies demonstrate that the mud drag and inertia on the drill string could be one of the key driving factors in riser system damping. To verify the FEA work, the Structural Well Integrity JIP conducted a laboratory test to determine the drag coefficient and added mass coefficient of drill pipe inside a marine drilling riser with and without flowing mud and water in the riser annulus. In this paper, the test setup and the test matrix are introduced, and the methodology for determining drag coefficient and added mass coefficient are explained. The test results are also presented and compared with published test data for open water.

Nonlinear Vertical Vibration of Tension Leg Platforms with Homotopy Analysis Method

One of the useful methods for offshore oil exploration in the deep regions is the use of tension leg platforms (TLP). The effective mass fluctuating of the structure which due to its vibration can be noted as one of the important issues about these platforms. With this description, dynamic analysis of these structures will play a significant role in their design. Differential equations of motion of such systems are nonlinear and providing a useful method for its analysis is very important. Also, the amount of added mass coefficient has a direct effect on the level of nonlinearity of partial differential equation of these systems. In this study, Homotopy analysis method has been used for closed form solution of the governing differential equation. Linear springs have been used for modeling the stiffness of this system and the effects of torsion, bending and damping of water have been ignored. In the study of obtained results, the effect of added mass coefficient has been investigated. The results show that increasing of this coefficient decreases the bottom amplitude of fluctuations and the system frequency. The obtained results from this method are in good agreement with the published results on the valid articles.

Wall Proximity and Curvature Effect on Added Mass Forces in Two-Phase Cross-Flow

This paper studies the effect of wall proximity and wall curvature on the added mass coefficient of a spherical bubble. Results are based on a semi-analytical method. This information is essential to completely characterize finely dispersed bubbly flows in two-phase cross flow. In such flows small spherical gas bubbles are present in a continuous liquid phase close to a cylinder. This paper uses solid harmonics to solve 3D potential flow around a bubble and a wall. A new technique is developed to calculate the flow potential around a sphere and a cylinder using solid harmonics. Several configurations were calculated: one bubble close to an infinite wall, one bubble close to a cylinder and one bubble close to a spherical wall. Our results are compared with previous studies. As expected added mass forces increase in the vicinity of the wall and for lower curvature. The main purpose of this work is to understand the effect of wall curvature and proximity on added mass. These results are suitable for further use, particularly as added mass models for multiphase flow averaged equations.

Void Fraction Effect on Added Mass in Bubbly Flow

This paper proposes a relation for the added mass coefficient of spherical bubbles depending on void fraction based on results obtained by a semi-analytical method. This information is essential to completely characterize finely dispersed bubbly flows, where small spherical gas bubbles are present in a continuous liquid phase. Most of the closure relations for Euler-Euler or Euler-Lagrange models are obtained from experiments involving single bubbles. Their applicability to systems with high void fraction is therefore questionable. This paper uses solid harmonics to solve 3D potential flow around bubbles. Several configurations were calculated for different numbers of particles and spatial arrangements. Our results are compared with previous studies. Depending on the model proposed by previous authors, added mass forces could increase or decrease with the void fraction. This paper solves these discrepancies. The main purpose of this work is to develop simple formulas fitting our semi-analytical results. These simple formulas are suitable for further use, particularly as added mass models for multiphase flow averaged equations.