Theoretical analysis and experimental research on non-cavitation noise modulation mechanism of underwater counter-rotation propeller

The underwater counter-rotation propeller non-cavitation noise has an obvious mod- ulation characteristic which is due to the interaction of flow and blade. A modulation mecha- nism is presented in this paper. A sound pressure spectrum model is presented to describe its non-cavitation noise with application of generalized acoustic analogy method, the modulation mechanism is expressed with the improvement of sound pressure model. The power spectrum and modulation spectrum are presented by numerical simulation. Theoretical analysis and nu- merical simulation results are verified by the cavitation tunnel experiment. The modulation model of counter-rotation propeller is beneficial to the prediction modulation characteristics and identification of underwater high-speed vehicles.

Prediction of line-spectrum noise induced by high speed vehicle counter-rotation propellers in water

Line-Spectrum noise of counter-rotation propellers has constructed the main part of the radiated noise of high speed vehicles in water. The line-spectrum noise of the counter-rotation propellers is due to the interaction between fore or aft propeller and wake of the vehicle,and the interaction between fore and aft propeller. Based on a combination of the lifting surface theory and acoustic method, the prediction of line-spectrum noise is presented in this paper.Theoretical calculation method, characteristics and numerical prediction of the line-spectrum noise are detailed too. The effect of different wake and different distance between fore and aft propeller on the propeller noise is also studied by numerical method. The agreement of predicted results compared with existing experimental data is quite satisfactory.

Coupling of RANS and BEM Solvers for Simulation of Propeller Behind the Hull in Full Appended Model

To design a propeller for ship power plant,the interaction between ship hull and propeller must be taken into account.The main concern is to apply the wake effect of ship stern on the propeller performance.In this paper,a coupled BEM(Boundary Element Method)/RANS(Renolds-Averaged Navier−Stokes)solver is used to simulate propeller behind the hull in the self-propulsion test.The motivation of this work is to develop a practical tool to design marine propulsion system without suffering long computational time.An unsteady boundary element method which is also known as panel method is chosen to estimate the propeller forces.Propeller wakes are treated using a time marching wake alignment method.Also,a RANS code coupled with VoF equation is developed to consider the ship motions and wake field effects in the problem.A coupling algorithm is developed to interchange ship wake field to the potential flow solver and propeller thrust to the RANS code.Based on the difference between hull resistance and the propeller thrust,a PI controller is developed to compute the propeller RPM in every time step.Verification of the solver is carried out using the towing tank test report of a 50 m oceanography research vessel.Wake factor and trust deduction coefficient are estimated numerically.Also,the wake rollup pattern of the propeller in open water is compared with the propeller in real wake field.

Counter-Rotating Type Horizontal-Axis Bidirectional Propellers for Tidal Stream Power Unit

Tidal stream power units with horizontal-axis propellers are one of promising technologies for generating the renewable green energy. The ebb and flow require that the power unit must operate in bidirectional tidal streams. Hence a tidal stream power unit with counter-rotating type horizontal-axis bidirectional propellers is proposed in this paper. The blades with fully-symmetrical hydrofoils were optimized numerically. The output and flow conditions predicted by the computational fluid dynamics simulations are compared with the results of the wind tunnel experiments at the higher tip speed ratios, which are of expected usual operating conditions of this unit. The numerical and experimental results show good agreements. It is also confirmed that the flow discharged from the counter-rotating type propellers has no swirling component, though the single propeller generates the unacceptable swirling component.

A Numerical Prediction of Tip Vortices from Tandem Propellers in the Counter-Rotating Type Tidal Stream Power Unit

Ocean energy has a potential of providing a large amount of renewable energy around the world. One of the forms of ocean energy, tidal stream power is widely recognized as the continuous, predictable and eco-friendly ocean energy source. Unique tandem propellers that can counter-rotate have been designed to generate electric power effectively from a tidal stream. This type of power unit has several advantages compare to the conventional unit with a single propeller. At the design of the tidal stream power unit, it is important to investigate the structure of the tip vortex tubes shedding to predict the load of the propeller. In this research, we investigated the tip vortex shedding using the CFD method for the conventional single propeller and counter-rotating type tandem propellers and estimated the performance efficiency using RANS (Reynolds Averaged Navier-Stokes) model and we confirmed the limitation of RANS model on the calculation of the tip vortex stretching.

磁共振成像PROPELLER数据网格化中的采样密度补偿新算法

PROPELLER磁共振数据的重建是一个典型的K空间非笛卡尔采样数据的重建问题.由于现有网格化重建算法中的密度补偿需要计算每个采样点的密度补偿系数,须对非笛卡尔分布的数据进行卷积运算,给定N采样点,该卷积运算需要N×N/2次距离运算,由于PROPELLER采集的数据量N很大,计算耗时非常长.本文提出PRO-PELLER数据网格化重建中的密度补偿新算法,通过基于网格化分量全为1的向量来计算在均匀网格点上的采样密度分布值进而加以补偿,使得算法复杂度大大下降.实验表明,本文算法比现有算法的运行时间缩短400多倍,而重建质量与原有算法基本相同.

低张MRCP结合Propeller LAVA动态增强扫描在壶腹周围病变中的应用价值研究

目的:探讨低张胰胆管水成像(magnetic resonance cholangiopancreatography,MRCP)结合Pro-pellerLAVA多期动态增强扫描在壶腹周围病变综合诊断中的临床应用价值。方法:对34例壶腹周围病变患者在常规MR扫描基础上行低张MRCP及十二指肠低张后PropellerLAVA多期动态增强扫描,对原始图像进行3DMIP及MPR重建后处理,观察病变的间接及直接征象并与临床随访及病理结果对照。结果:34例中31例梗阻性质诊断明确,1例胰头癌漏诊,2例炎症误诊为肿瘤,定性诊断准确率为91.2%。30例行PropellerLAVA增强扫描,直接征象显示率达93.3%,肿瘤浸润范围诊断符合率为80.8%。26例肿瘤性病变中壶腹部癌5例,胆总管远端癌11例、胰头癌9例、壶腹周围十二指肠腺癌1例,定位准确率分别为88.5%(23/26)、84.6%(22/26)、88.5%(23/26),十二指肠腺癌误诊为乳头癌。另外直观显示了4例病变局限、直径在2.0cm以内的早期壶腹周围癌。结论:低张MRCP结合PropellerLAVA动态增强扫描能够全面、直观显示壶腹部病变的间接及直接征象,在明确肿瘤性病变的范围及周围组织、血管浸润方面具有重要意义,对壶腹部较小或隐匿病变亦具有较高的应用价值。

磁共振成像PROPELLER采样数据重建中的运动估计新算法

PROPELLER(推进器)采样技术能够利用K空间中心重叠采样区域的数据来估计采集过程中受检查者的运动进而加以补偿,对运动伪影的消除效果非常显著。然而,由于其重建时的运动估计是基于最大化频域空间上相关系数的配准算法,该算法为了实现旋转估计与平移估计的分离,在进行旋转估计时,仅仅采用K空间数据的模,在数据量有限的情况下造成估计精度较低,在重建图像上表现为模糊及星条状伪影。本研究基于最大化图像空间上的互信息提出一种PROPELLER采样数据的运动估计新算法,首先由每个K空间带进行傅立叶逆变换后取模重建出系列临时图像,对这些图像进行模糊增强后以互信息作为相似性测度迭代搜索最优的运动参数。实验证明,该方法能显著提高PROPELLER采样数据重建中运动估计与补偿的精度,从而更好地消除伪影,特别是用于有运动时T1加权头部成像时。

PROPELLER技术在改善固定义齿脑梗塞患者图像伪影中的意义

目的探讨磁共振螺旋桨技术(periodically rotated overlapping parallel lines enhanced reconstruction,PROPELLER)在改善固定义齿脑梗塞患者MR图像伪影中的作用。方法对2013年1月至2015年1月来我科行MRI检查装有固定义齿的患者81例,分别采用PROPELLER DWI技术和常规FSE DWI序列进行头颅MRI扫描,对采集到的MRI图像进行观察。结果PROPELLER DWI检出脑梗塞病灶78例,未检出病灶3例分别是额叶1例,颞叶2例;FSE DWI检出脑梗塞病人20例,未检出病灶61例,额叶、颞叶、小脑及脑干出现伪影分别为19例、39例、1例、2例。结论 PROPELLER技术能有效消除磁敏感伪影,改善固定义齿脑梗塞患者的MR图像质量,增加脑梗塞病灶的检出率,对改善MR图像质量有非常好的作用。

磁共振Propeller技术消除伪影的临床应用

目的:探讨Propeller技术在临床的应用价值。方法:日常工作中随机对19例磁共振头颅检查中出现躁动或有固定金属异物患者,应用Propeller技术进行成像,与常规T2FRFSE序列、DWI序列进行对比扫描。其中因躁动产生运动伪影进行PropellerT2加权成像11例,因金属异物产生伪影进行PropellerDWI成像8例,同时进行PropellerT2WI、DWI成像5例。结果:19例应用Propeller技术进行T2WI、DWI成像消除因运动、金属引起的伪影均获得满意图像。结论:应用PropellerT2WI、DWI成像技术明显消除患者因运动或金属异物造成的伪影,可生成高分辨无伪影具有临床诊断意义的理想图像。

金属结扎丝对颅脑MR影像的影响与Propeller技术的临床应用

目的探讨金属结扎丝对头部MRI的影响及Propeller技术在消除头部金属伪影中的应用价值。方法对10例上颌前牙因外伤松动、脱位的患者,用金属结扎丝结扎固定后分别用FSE序列T1加权像,普通EPI扩散加权成像序列(EPIDWI)、Propeller扩散加权成像序列(PropellerDWI)成像并进行对比。结果10例患者在FSE序列T1加权像矢状面MRI中均有重度的磁敏感伪影;普通EPIDWI序列颅脑解剖组织处有明显的磁敏感伪影;PropellerDWI技术能明显减少磁敏感伪影生成满意的影像。结论金属结扎丝对颌面部及颅脑MR成像有较大的影响;PropellerDWI成像技术能明显消除磁敏感伪影获得具有临床诊断价值的理想影像。

基于Voronoi图的磁共振PROPELLER数据网格化算法

PROPELLER数据采集成像算法是磁共振成像中的一项新技术,由于其数据的网格化直接影响该成像算法的效果,因此提出一种基于Voronoi图的网格化算法.该算法对采样数据集进行网格分组查找,快速消除位置相同点;加入边缘闭包后进行Voronoi网格化,计算Voronoi网格面积并将其作为网格化的密度补偿权函数,提出基于网格分组的快速网格化算法并成像.实验表明,该算法运行速度快,成像清晰,图像对比度和细节较好,信噪比得到有效提高.

PROPELLER磁共振成像数据重建中的仿射运动校正新算法

PROPELLER(Periodically Rotated Overlapping ParallEL Lines with Enhanced Reconstruction)磁共振成像方法对刚性运动伪影的消除效果非常显著,已经在头部磁共振成像中获得了成功应用.但是刚性运动一般仅存在于头部成像中,人体其它部位成像往往伴随着不同程度的软组织拉伸变形.对于这种软组织变形必须基于非刚性运动模型才能准确地进行描述并加以校正.本文将PROPELLER采样中的每个k-空间条经过傅立叶逆变换重建得到临时图像,通过基于仿射运动模型的图像配准算法获得非刚性运动信息,然后根据仿射变换的频域性质,对PROPELLER采样中的每个k-空间条进行校正,最后经网格化重建得到最终图像.仿真实验与真实数据实验表明,相对于现有的PROPELLER重建算法,本文所提算法对于刚性运动与仿射运动造成的伪影均具有很好的校正效果.

Propeller技术消除头部磁共振伪影的应用

目的:探讨Propeller技术对减少头部运动伪影及磁敏感伪影的应用价值。方法:对53例磁共振头颅检查时躁动或难以自控的患者行常规FRFSE T2WI和Propeller T2WI轴位扫描;对31例常规扩散加权图像(DWI)磁敏感伪影明显者进行Propeller DWI扫描。观察并对照分析FRFSE T2WI和Propeller T2WI以及DWI与Propeller DWI的图像质量状况。结果:53例在T2WI存在明显运动伪影的患者,在行Propeller T2WI扫描后明显减少或消除了运动伪影,图像质量级别显著升高[(2.88±0.32,(1.11±0.30),P=0.00)]。31例在DWI有显著磁敏感伪影的患者,采用Pro-peller DWI后,明显消除磁敏感伪影,图像质量明显提高[(2.84±0.35,(1.06±0.17),P=0.00)]。结论:应用Propeller技术能够明显减少或消除T2WI及DWI上运动伪影及磁敏感伪影,显著改善图像质量。

PROPELLER技术在头颅MR检查中的应用进展

磁共振检查技术,以其特有的无辐射、高分辨率成为目前最常用的检查手段之一。磁共振检查在获得良好的组织对比度的同时,也是存在伪影最多的检查方法,它的主要伪影包括设备伪影、运动伪影及磁敏感伪影。尤其是在高场强磁共振检查中,运动伪影和磁敏感伪影更为突出。如何避免因运动伪影、固定义齿产生的严重磁敏感伪影,以及呼吸运动等非刚性运动伪影是广大学者广泛关注与研究的问题。

基于模糊增强的磁共振成像PROPELLER采样数据优质重建算法

目的提出一种基于模糊增强的重建算法,以改进PROPELLER采样数据中现有重建算法的不足。方法由每个K空间带补零重建出临时图像,然后提出模糊增强算法,增强后在图像域进行运动参数估计,进行运动补偿后由网格化重建算法重建出结果图像。结果该算法对于运动参数估计的精度更高,对刚性运动伪影的消除效果明显优于现有算法。结论该算法可以显著提高PROPELLER采样数据重建图像质量。

Propeller LAVA增强扫描在壶腹周围癌诊断中的应用研究

目的:研究Propeller LAVA(Liver Accqusition with Volume Acceleration)肝脏三维容积快速多期动态增强扫描对壶腹周围癌的应用价值。材料与方法:应用Propeller LAVA对21例疑壶腹周围癌的病人进行十二指肠低张后增强及延时扫描,应用工作站对所得数据行三维处理,并结合其MRCP和FIESTA(Fast Imaging EmployingSteady-state Accquisition采用稳态采集的快速图像)采集的图像,以3级评分法,判断Propeller LAVA增强扫描对壶腹周围癌直接征象及其临床分型。结果:21例行Propeller LAVA增强扫描者有20例显示肿瘤的直接征象及累及范围,MRCP及FIESTA序列分别有0例和2例显示肿瘤的直接征象及累及范围,三者差异具有统计学意义。(P<0.05)Propeller LAVA增强扫描根据不同类型可表现为肿瘤强化及延迟强化。结论:Propeller LAVA增强扫描能显示壶腹周围癌的直接征象及累及范围,在壶腹周围癌的鉴别诊断上明显优于MRCP和FIESTA序列。

磁共振Propeller技术消除患者因运动及磁敏感效应造成伪影的作用(英文)

背景:运动伪影及磁敏感伪影严重影响MR成像质量,Propeller技术采用填充模式,以辐射状的叶片用螺旋的方式采集数据,在很大程度上解决了运动伪影及磁敏感伪影的问题。目的:探讨Propeller技术在头部伪影消除的应用价值。设计、时间及地点:对比观察实验,于2006-07/2007-04在解放军成都军区昆明总医院完成。参试者:21例磁共振头颅日常检查中出现躁动或不能自控患者,5例固定义齿致磁敏感伪影明显者。方法:对21例磁共振头颅日常检查躁动或不能自控患者,常规T2FRFSE序列运动伪影明显,用Propeller技术进行T2WI扫描并进行对比;5例常规扩散加权图像序列磁敏感伪影明显者进行Propeller 扩散加权图像扫描并进行对比。主要观察指标:Propeller技术应用后运动伪影与磁敏感伪影的改变情况。结果:21例应用Propeller技术进行T2WI扫描明显减少或消除了运动伪影,5例Propeller扩散加权图像明显消除磁敏感效应引起的伪影,均生成较满意图像。结论:应用Propeller T2WI及扩散加权图像成像技术可以明显减少或消除患者因运动及磁敏感效应造成的伪影,显著提高图像质量,获得具有临床诊断价值的理想图像。

Scale effects on non-cavitation hydrodynamics and noise of highly-skewed propeller in wake flow

Regarding the scale effects on propeller＇s noncavitation hydrodynamics and hydroacoustics, three similar 7bladed highly-skewed propellers in the wake flow are addressed with diameters of 250, 500 and 1 000 mm, respectively. The discrete line-spectrum noise and its standardized spectrum level scaling law, together with the total sound pressure level are analyzed. The non-cavitation noise predictions are completed by both the frequency domain method and the time domain method. As a fluctuated noise source, the time-dependent fluctuated pressure and normal velocity distribution on propeller blades are obtained by the unsteady Reynolds-averaged Navier-Stokes （ URANS ） simulation. Results show that the pressure coefficient distribution of three propellers on the 0.7R section is nearly superposed under the same advance ratio. The periodic thrust fluctuation of three propellers can exactly reflect the tonal components of the axial passing frequency （APF） and the blade passing frequency （BPF）, and the fluctuation enhancement from the small to the middle propeller at the BPF is greater than that from the middle to the big one. By the two noise prediction methods, the increment of the total sound pressure level from the small to the big propeller differs by 2.49 dB. Following the standardized scaling law, the spectrum curves of the middle and big propellers are nearly the same while significantly differing from the small one. The increment of both the line-spectrum level and the total sound pressure increases with the increase in diameter. It is suggested that the model scale of the propeller should be as large as possible in engineering to reduce the prediction error of the empirical scalin~ law and weaken the scale effects.

T1 FLAIR PROPELLER序列在3.0T磁共振颅脑增强成像上对抑制各种伪影的应用

目的:探讨3.0T磁共振上利用T1 FLAIR PROPELLER序列在颅脑增强成像时对消除各种血管搏动伪影及头部运动伪影和金属伪影等的应用价值。方法:对108例患者行常规头部平扫和增强检查,应用PROPELLER技术进行钆造影剂注射后的T1 FLAIR三平面扫描,与常规T1 FLAIR三平面序列进行对比,并进行图像伪影评价。结果:108例患者常规增强T1 FLAIR序列均有不同程度的血管搏动伪影,25例患者有明显的运动伪影,22例患者有金属伪影(固定义齿或手术缝合钉、钛板等),这些伪影严重影响图像质量,有部分导致诊断的假阳性判断。应用PROPELLER技术后的T1 FLAIR序列,对比发现,108例均明显消除了血管搏动伪影,106例没有明显运动伪影,22病例金属伪影均有所减轻,获得了有诊断价值的图像。两种序列图像质量有显著差异(P<0.01)。结论:应用PROPELLER技术的T1 FLAIR序列,能最大限度地消除头部增强扫描中的血管搏动伪影以及降低运动伪影、金属伪影,可提供更有临床诊断价值的理想图像。