Investigation of a longfin inshore squid's swimming characteristics and an underwater locomotion during acceleration

In the present study, locomotion of a real longfin inshore squid (Doryteuthis pealeii) was numerically investigated. Geometry of a real squid was obtained using computed tomography (CT) images. In addition to a two-dimensional axisymmetric squid model, a modified squid model with no cavities around her head and an ellipse shaped model were generated with a fineness ratio (the ratio of body length to maximum body diameter) of 7.56. These numerical models were exposed to an acceleration with two different velocity programs. Added mass and basset forces on bodies were calculated during acceleration of the squid models starting from rest. Pressure and viscous drag forces were also calculated due to pressure variation along the squid models and friction on the surface of the models. The effect of a nozzle diameter on jet velocities and propulsive efficiency at all bodies were evaluated when time dependent velocity profiles (from 0 to 10 m/s in 0.5 and 1 s time durations) were set for the inlet of computational domain. The modified squid model required least thrust force during acceleration phase of time dependent velocity profile compared to the other models while the 0.02 m nozzle diameter provided largest propulsive efficiency for all models.     

Direct scantling assessment of propeller blades

Traditionally, propeller design has been focused on all activities necessary to obtain a propeller featuring a high efficiency, avoiding erosive cavitation for given operating conditions and having adequate structural strength. In recent years, more and more challenging requirements have been imposed, such as the reduction of radiated noise and pressures pulses, requiring more precise analyses and methods in the optimization of the propeller performance. On the other hand, the evaluation of the propeller strength still relies on simplified methods, which basically consider the blade as a cantilever beam subjected to characteristic static forces. Since the loads acting on a blade are variable in the blade revolution and in different operating conditions throughout the ship life, a procedure to account for the influence of fatigue phenomena is proposed. The fatigue assessment could reduce the safety factor in the propeller scantling rules and allow improving the quality of propeller design (e.g. obtaining higher efficiency, margin on cavitation phenomena, less noise).     

CFD investigations of OXYFLUX device,an innovative wave pump technology for artificial downwelling of surface water

No other environmental variable of such ecological importance to estuarine and coastal marine ecosystems around the world has changed so drastically, in such a short period of time, as dissolved oxygen in coastal waters. The prevalent methods for counteracting anoxic sea events are indirect measures which aim to cut-down anthropic loads introduced in river and marine environments. To date, no direct approaches, like artificial devices have been investigated except the WEBAP and OXYFLUX devices. The present paper adopts a numerical approach to the analysis of the pumped surface water as well as the analysis of the dynamic response of the OXYFLUX device. By means of a CFD-RANS code and through the application of overset grid method, the 1/16 OXYFLUX model’s dynamic response and pumping performance are evaluated. The appropriate grid is selected after an extensive sensitivity analysis carried out on 9 different grids. The CFD model is validated by comparing numerical and physical results of heave decay test, heave response, and surface water discharge under the action of regular waves. The extensive comparison with experimental results shows consistently accurate predictions. The main findings of the study show that nonlinear effects remarkable reduce the dynamic behaviour of the OXYFLUX and generate an unexpected second harmonic for pitch response intensifying the overtopping discharge also for small waves caused by the summer's low intensity winds.     

Numerical modelling of deep sea air-lift

Deep sea air-lifting of solid particles from depth of 1600 m is simulated with a mathematical model of the three-phase flow in an upward pipe. The computations are carried out for an axisymmetric domain in a transient way. Phase distributions, pressure and velocity profiles together with flow rates for all phases are presented and analysed. The influence of the pipe diameter on the air-lift efficiency was studied for air-lift pipes of different lengths and found to be significant. The lifting efficiency increases with the increase of the pipe diameter due to the reduction of the wall friction influence on the flow. In addition, the efficiency also increases with the increase of the solid particles volume fraction at the inlet. The presented numerical model can be utilized during various stages of the design of the air-lift pumps to help answer fundamental questions on the process, and during their operation to select optimal process parameters and to address possible problems.     

Study on deepwater conductor jet excavation mechanism in cohesive soil

Conductor casing jetting technique has been increasingly applied in deepwater drilling. The insight into the jetting excavation mechanisms is critical in guiding a successful conductor casing jetting operation. The real- time continuous jet excavation process is simulated with the volume of fluid (VOF) multiphase method of CFD (Computational Fluid Dynamics) ANSYS Fluent calculation software in the current study. The cohesive soil is modelled by using a kind of viscous fluid with Herschel-Bulkley model. In addition, a laboratory half round nozzle jet excavation test is designed for verification by comparison of the observed jet excavation profile with the numerical results. The sensitivity parameters affecting the conductor jetting excavation mechanism in cohesive soil are thus investigated. It is found that the application of Herschel-Bulkley (HB) model for cohesive soil and the VOF method of Fluent can provide a good simulation of jet excavation process. The maximum excavation depth can be determined by the undrained ultimate bearing capacity of the circular foundation with a bearing capacity factor of 6.7. The nozzle position, jet velocity and soil strength have significantly influence on the depth and width of the jet excavation profile in conductor oblique jet.     

Experimental study on hydrodynamic performance of SWATH vessels in calm water and in head waves

An extensive experimental investigation on four SWATH hull forms has been conducted in calm water and in regular waves at University of Naples Federico II. Calm water tests have been analyzed in the range of Froude number Fr_T from 0.1 to 0.6. For all four SWATH configurations at the speed, corresponding to Fr_T 0.32, the behaviour in regular waves has been tested. The results of heave, pitch and vertical accelerations are presented in nondimensional form as RAO. For the “most promising” SWATH #4 configuration, a set of stabilizing fins have been designed and an active stabilization system has been developed. The developed SWATH#5 has been tested in calm water on three displacements in the range of Fr_T from 0.1 to 0.65. The dynamic wetted surface has been identified and the residual resistance coefficient C_R as well as R_T/Δ are reported. Seakeeping tests have been performed in regular head sea and in head and following irregular sea at Fr_T = 0.50. The conditions for the occurrence of dynamic longitudinal instabilities have been identified. The results allows to comment the effect of slenderness of struts and SWATH’s immersed bodies on resistance and seakeeping and concerns the applicability of SWATH concept to small craft.     

CFD modelling of a small–scale fixed multi–chamber OWC device

Wave Energy Converters (WECs) have excellent potential as a source of renewable energy that is yet to be commercially realised. Recent attention has focused on the installation of Oscillating Water Column (OWC) devices as a part of harbor walls to provide advantages of cost–sharing structures and proximity of power generation facilities to existing infrastructure. In this paper, an incompressible three–dimensional CFD model is constructed to simulate a fixed Multi–Chamber OWC (MC–OWC) device. The CFD model is validated; the simulation results are found to be in good agreement with experimental results obtained from a scale physical model tested in a wave tank. The validated CFD model is then used for a benchmark study of 96 numerical tests. These investigate the effects of the PTO damping caused by the power take–off (PTO) system on device performance. The performance is assessed for a range of regular wave heights and periods. The results demonstrate that a PTO system with an intermediate damping can be used for all chambers in the MC–OWC device for most wave period ranges, except for the long wave periods. These require a higher PTO damping. An increased incident wave height reduces the device capture width ratio, but there is a noticeable improvement for long wave periods.     

基于CFD的单个建筑物风场模拟参数选择

运用流体力学软件FloEFD对沽源单个建筑物周围的风场进行数值模拟。通过不断改变模型中计算参数的设置进行一系列模拟试验,对比模拟试验结果,并与观测资料进行比较,分析不同计算参数对模拟结果的影响,并获得适用于该模型的最佳参数。主要研究的计算参数包括计算域高度,初始网格等级,局部初始网格等级和不同平均风速剖面形式。结果表明:计算域高度从3倍建筑物高度开始,空腔区的长度、漩涡中心位置以及再发展区的边界位置基本保持稳定。随着初始网格等级的增加,空腔区的长度、再发展区的边界位置及计算时间逐渐增大。局部初始网格等级对模拟结果影响不显著。以两种不同平均风速剖面形式进行模拟,迎风漩涡长度不同,背风面影响不大。与观测资料比较显示,最优参数组合为:计算域高度为3倍建筑物高度,初始网格等级为4、局部初始网格等级为4、平均风速剖面形式为指数律。     

Development and application of simulation technique for hydrokinetic hammer

The computer simulation is an important method for hydrokinetic hammer design. Various kinds of simulation measures with their technical characters and applications being taken during the computer aided design are enumerated. Computer simulation supports plenty of valuable references to the designer. Each type of simulation process is used to explore the exact aspect of the performance of hydrokinetic hammer and each type of simulation method has its own excellences and deficiencies. Thus the integrative simulation methods based on modern computational technology are brought forward to obtain the perfect capability of the whole product. Along with the development of computer hardware and software, various kinds of platforms have been provided to different simulation methods that can be carried out with distinct working flows. The jet flow element is the core part of the hydrokinetic hammer. We can build the ideal simulation model of it by means of CFD (computational fluid dynamics) technology. On the other hand, to set up the digital model of piston and hammer, the best way is to build the virtual prototype using automatic dynamic analysis of mechanical system. As a result of the argumentation, we think the technique of Virtual Prototype and CFD are the prime way to process the combined computer simulation for hydrokinetic hammer.