Numerical study on the wave pattern characteristic of air layer in cavity and the effects of multiple influence factors

Air-layer drag reduction (ALDR) technology for ship energy saving is getting more and more attention in recent years because of the outstanding drag reduction effect. In order to promote practical application, it is necessary to fully understand the two phase flow characteristics of the air layer. Recent experimental studies have shown that the surface of the air layer presents wave pattern, which has an important influence on its damage risk. However, it is difficult to measure the wave pattern quantificationally due to the interference of equipment. The main goal of the present paper is to investigate the wave pattern characteristic of air layer in cavity using numerical simulation method. On this basis, the effect of flow and geometric influence factors are discussed to understand the key control conditions. A computational fluid dynamics (CFD) numerical method based on Reynolds averaged Navier-Stokes (RANS) equations and volume of fluid (VOF) interface capturing method is established, and has been successfully applied in the simulation of air layer wave pattern. Both 2-D and 3-D simulations are carried out, aiming at analyzing air-water interface flow and vortex flow directly. Based on the simulation results, several important conclusions about the mechanism of air layer wave pattern can be obtained. Firstly, it is found to be an inherent characteristic that the wave height of the upstream air layer is higher than that of the downstream. The extremely high wave peak is easy to contact with the flat plate, leading to the breakup of air layer and a “central blank area” phenomenon. With the help of flow analysis, it is found that this characteristic is mainly caused by the strong counterclockwise vortex behind the bow wedge block. Secondly, the air layer stability is reduced with the increase of water flow velocity by affecting the wave height. There is a saturation point of air flow rate to reach maximum thickness of air layer. Thirdly, cavity configuration has obvious influence on air layer stability by influencing vortex flow field. The increase of cavity depth and width can aggravate the unsteady and nonlinear characteristics of air layer. Finally, comprehensive design criteria are concluded from the view of geometrical configuration and flow conditions. A cavity with the moderate depth and width can avoid the upstream damage of air layer. Longitudinal position of air nozzles should be set within the low pressure zone behind the wedge block for stable air layer formation.

船舶螺旋桨模型表面流动形态分析

流动形态观测是流体力学实验测量的一项重要内容,其可用于水动力作用机理、流场特性和特征分析,以及流动数值模拟的准确性验证。该文对船舶螺旋桨模型表面流动形态,及其与水动力的联系进行了概括。通过油流显示,螺旋桨模型叶片表面主要包含三种流动形态:径向为主的流动、周向为主的流动以及两者之间的过渡流动。这三种流动形态与叶片表面的流动状态相关,径向为主流动处于层流区,周向为主的流动处于湍流区,过渡流动为转捩区或流动分离区。螺旋桨的水动力与叶片表面流动状态相关,在螺旋桨设计中要高精度预报水动力,必须使用能够同时模拟层流和湍流的数值方法。

不同工况下无轴泵喷推进器间隙流动损耗特性数值分析

为研究无轴泵喷推进器间隙流动特性以及流动损耗机理,采用雷诺平均(RANS)方程对不同工况下无轴泵喷推进器周围流场进行数值模拟。该文首先进行了推进器水动力性能计算,通过与试验结果的比较,评估了湍流模型和网格数量对结果的影响。在此基础上,进一步采用柱坐标系下涡量输运方程和熵增理论,分析了不同工况下推进器间隙流动特征和能量损耗机理。结果表明,计算结果与试验吻合程度较高;进一步由涡量输运方程分析可得方程右侧其影响涡量输运的各项多集中于间隙及间隙进出口区域,其中黏性耗散项对间隙区涡量变化起到主要作用,加剧流动不稳定性;由熵增结果可知,间隙区域中由黏性耗散引起的功率损失占据主导地位,由湍流耗散产生的损失可以忽略不计;同时,间隙流出口区由于流体流动方向与主流方向垂直,二者相互碰撞的同时受到叶轮抽吸作用向下游运动,导致该区域流动较为复杂,流动损耗较大;且随着进速系数提高,由轮缘所引发的流动损耗对推进器水动力性能有一定影响。

Study of tip vortex cavitation inception and vortex singing

Tip vortex cavitation(TVC)is an important cavitation phenomenon in marine propeller.The formation and evolution of tip vortex cavitation are hot topics consistently both in engineering application and mechanism research.In this paper some recent studies on tip vortex cavitation inception and the noise of tip vortex cavitation evolution are presented.The effects of both flow field and water qualities on tip vortex cavitation inception are considered by experiments and numerical simulations.The results show that besides the average minimum pressure in the vortex core the turbulence fluctuation and water qualities including air content and nuclei distribution have great influence on tip vortex cavitation inception.Based on the idea of first nucleus cavitating in tip vortex core new prediction formula for tip vortex cavitation inception is proposed.The synchronous technique of high speed video observation and noise measurement are adopted to study the development of tip vortex cavitation.S-type total noise characteristics are obtained when cavitation number from low to high.Vortex singing is found in the case where the tip vortex cavitation just before leaves the tip region.The excited mechanism of vortex singing is proposed by analyzing the wave propagation on the interface of vortex cavity.

Experiment on surface wake of internal waves generated by underwater vehicle in stratified fluids

The surface flow field of internal waves generated by the underwater vehicle is very weak.In order to study the characteristics of the surface wake of internal waves,a surface particle image velocimetry(PIV)technique which can be used to measure the flow field in the order of mm/s is developed.Breakthrough is made with respect to the key technique measuring the micro-velocity flow field of internal waves on the water surface in stratified fluids.The wake generated by SUBOFF model is measured in stratified fluid tank,and the surface flow field of internal waves is successfully measured for the first time.The experimental results are compared with predicted results by the classical Tuck’s internal wave theory.The results show the characteristics of the surface wake signature of internal waves and the variation of the angle between internal wave beams and the surface flow velocity of internal wave with the towing speed of the model are in good agreement.It provides support for further research on wake signature remote sensing of internal waves in laboratory.