A growing interest has been devoted to the contra-rotating propellers （CRPs） due to their high propulsive efficiency, torque balance, low fuel consumption, low cavitations, low noise performance and low hull vibrati...A growing interest has been devoted to the contra-rotating propellers （CRPs） due to their high propulsive efficiency, torque balance, low fuel consumption, low cavitations, low noise performance and low hull vibration. Compared with the single-screw system, it is more difficult for the open water performance prediction because forward and aft propellers interact with each other and generate a more complicated flow field around the CRPs system. The current work focuses on the open water performance prediction of contra-rotating propellers by RANS and sliding mesh method considering the effect of computational time step size and turbulence model. The validation study has been performed on two sets of contra-rotating propellers developed by David W Taylor Naval Ship R ＆ D center. Compared with the experimental data, it shows that RANS with sliding mesh method and SST k-ω turbulence model has a good precision in the open water performance prediction of contra-rotating propellers, and small time step size can improve the level of accuracy for CRPs with the same blade number of forward and aft propellers, while a relatively large time step size is a better choice for CRPs with different blade numbers.展开更多
Forward modeling of elastic wave propagation in porous media has great importance for understanding and interpreting the influences of rock properties on characteristics of seismic wavefield. However,the finite-differ...Forward modeling of elastic wave propagation in porous media has great importance for understanding and interpreting the influences of rock properties on characteristics of seismic wavefield. However,the finite-difference forward-modeling method is usually implemented with global spatial grid-size and time-step; it consumes large amounts of computational cost when small-scaled oil/gas-bearing structures or large velocity-contrast exist underground. To overcome this handicap,combined with variable grid-size and time-step,this paper developed a staggered-grid finite-difference scheme for elastic wave modeling in porous media. Variable finite-difference coefficients and wavefield interpolation were used to realize the transition of wave propagation between regions of different grid-size. The accuracy and efficiency of the algorithm were shown by numerical examples. The proposed method is advanced with low computational cost in elastic wave simulation for heterogeneous oil/gas reservoirs.展开更多
基于GRAPES区域中尺度数值预报系统(GRAPES_MESO),针对700 h Pa、500 h Pa和200 h Pa的位势高度场H,温度场T,风场纬向分量U,经向分量V和地面降水场,在给定的模式物理过程下,分别考察了时间步长和空间分辨率对于模式预报效果的影响。研...基于GRAPES区域中尺度数值预报系统(GRAPES_MESO),针对700 h Pa、500 h Pa和200 h Pa的位势高度场H,温度场T,风场纬向分量U,经向分量V和地面降水场,在给定的模式物理过程下,分别考察了时间步长和空间分辨率对于模式预报效果的影响。研究结果表明,空间分辨率(0.3°×0.3°)相同时,各变量在不同层次的预报几乎都存在最优时间步长使得预报技巧最高,初步说明最优时间步长理论在复杂的偏微分方程组中的适用性。随后,将空间分辨率为0.3°×0.3°时最优时间步长(240 s)的预报结果与当前业务中(空间分辨率为0.15°×0.15°、时间步长为90 s)的预报结果进行比较,发现前者的变量H、T、U、V和地面降水场的预报技巧均高于后者,表明并不是空间分辨率越高预报效果越好。展开更多
提出了一种基于到达时间(time of arrival,TOA)和到达时间差(time difference of arrival,TDOA)的空中运动平台对目标高精度三维定位的无源定位方法。该方法使用3个辅站信号到空中运动平台的TOA以及辅站位置确定空中运动平台自身的位置...提出了一种基于到达时间(time of arrival,TOA)和到达时间差(time difference of arrival,TDOA)的空中运动平台对目标高精度三维定位的无源定位方法。该方法使用3个辅站信号到空中运动平台的TOA以及辅站位置确定空中运动平台自身的位置,然后依据目标散射回波到达各个辅站与空中运动平台的TDOA确定目标的位置。分析了三维TDOA目标定位模糊产生的原因,提出了一种无模糊的高精度TDOA目标位置求解算法。仿真结果表明,该算法比经典的TDOA定位算法精度高,而且不存在定位模糊,从而验证了该空中运动平台对目标进行无源定位方法的有效性以及正确性。展开更多
"雪龙"号极地科考船是推动我国极地科学考察事业发展的重要工具,"雪龙"号在数十次的极地科考过程中累积了大量的航迹数据,其中蕴含的巨大价值亟须挖掘。针对科考船的航迹分段是将科考船移动轨迹分为停留与行驶两部..."雪龙"号极地科考船是推动我国极地科学考察事业发展的重要工具,"雪龙"号在数十次的极地科考过程中累积了大量的航迹数据,其中蕴含的巨大价值亟须挖掘。针对科考船的航迹分段是将科考船移动轨迹分为停留与行驶两部分,合理的分段方法可以分离出信息更丰富的航迹段,有利于航迹知识提取。然而,由于原始航迹信息密度分布不均等原因,现有的航迹分段方法往往会造成分段过多等问题,结果并不理想。本文针对该问题,提出了一种针对科考航迹整体的时域差分(Time Domain Difference,TDD)分段方法。本方法基于时间域对航速进行差分处理,有效降低了因为航速波动频繁对分段结果的影响。同时,考虑到该方法的差分步长在航迹处理过程中的不明确性,本文将差分后航迹的路程损失和航速波动幅值进行归一化处理,提出了航迹差分时间步长的动态确定方法,并以速率阈值对航迹进行分段。最后本文以第29次南极科考航迹数据为例,将本方法与经典的具有噪声的基于密度的聚类方法DBSCAN(Density-Based Spatial Clustering of Applications with Noise)进行了比较,实验结果表明本文提出的方法可有效降低航迹分段时分段过多的问题,在分段准确性和时间效率等方面结果更优。展开更多
基金supported by the National Natural Science Foundation of China(Grant No.51079157)
文摘A growing interest has been devoted to the contra-rotating propellers （CRPs） due to their high propulsive efficiency, torque balance, low fuel consumption, low cavitations, low noise performance and low hull vibration. Compared with the single-screw system, it is more difficult for the open water performance prediction because forward and aft propellers interact with each other and generate a more complicated flow field around the CRPs system. The current work focuses on the open water performance prediction of contra-rotating propellers by RANS and sliding mesh method considering the effect of computational time step size and turbulence model. The validation study has been performed on two sets of contra-rotating propellers developed by David W Taylor Naval Ship R ＆ D center. Compared with the experimental data, it shows that RANS with sliding mesh method and SST k-ω turbulence model has a good precision in the open water performance prediction of contra-rotating propellers, and small time step size can improve the level of accuracy for CRPs with the same blade number of forward and aft propellers, while a relatively large time step size is a better choice for CRPs with different blade numbers.
基金supported by the National Basic Research Program of China (No. 2013CB228604)the National Science and Technology Major Project (No. 2011ZX05030-004-002,2011ZX05019-003)the National Natural Science Foundation (No. 41004050)
文摘Forward modeling of elastic wave propagation in porous media has great importance for understanding and interpreting the influences of rock properties on characteristics of seismic wavefield. However,the finite-difference forward-modeling method is usually implemented with global spatial grid-size and time-step; it consumes large amounts of computational cost when small-scaled oil/gas-bearing structures or large velocity-contrast exist underground. To overcome this handicap,combined with variable grid-size and time-step,this paper developed a staggered-grid finite-difference scheme for elastic wave modeling in porous media. Variable finite-difference coefficients and wavefield interpolation were used to realize the transition of wave propagation between regions of different grid-size. The accuracy and efficiency of the algorithm were shown by numerical examples. The proposed method is advanced with low computational cost in elastic wave simulation for heterogeneous oil/gas reservoirs.
文摘基于GRAPES区域中尺度数值预报系统(GRAPES_MESO),针对700 h Pa、500 h Pa和200 h Pa的位势高度场H,温度场T,风场纬向分量U,经向分量V和地面降水场,在给定的模式物理过程下,分别考察了时间步长和空间分辨率对于模式预报效果的影响。研究结果表明,空间分辨率(0.3°×0.3°)相同时,各变量在不同层次的预报几乎都存在最优时间步长使得预报技巧最高,初步说明最优时间步长理论在复杂的偏微分方程组中的适用性。随后,将空间分辨率为0.3°×0.3°时最优时间步长(240 s)的预报结果与当前业务中(空间分辨率为0.15°×0.15°、时间步长为90 s)的预报结果进行比较,发现前者的变量H、T、U、V和地面降水场的预报技巧均高于后者,表明并不是空间分辨率越高预报效果越好。
文摘提出了一种基于到达时间(time of arrival,TOA)和到达时间差(time difference of arrival,TDOA)的空中运动平台对目标高精度三维定位的无源定位方法。该方法使用3个辅站信号到空中运动平台的TOA以及辅站位置确定空中运动平台自身的位置,然后依据目标散射回波到达各个辅站与空中运动平台的TDOA确定目标的位置。分析了三维TDOA目标定位模糊产生的原因,提出了一种无模糊的高精度TDOA目标位置求解算法。仿真结果表明,该算法比经典的TDOA定位算法精度高,而且不存在定位模糊,从而验证了该空中运动平台对目标进行无源定位方法的有效性以及正确性。
文摘"雪龙"号极地科考船是推动我国极地科学考察事业发展的重要工具,"雪龙"号在数十次的极地科考过程中累积了大量的航迹数据,其中蕴含的巨大价值亟须挖掘。针对科考船的航迹分段是将科考船移动轨迹分为停留与行驶两部分,合理的分段方法可以分离出信息更丰富的航迹段,有利于航迹知识提取。然而,由于原始航迹信息密度分布不均等原因,现有的航迹分段方法往往会造成分段过多等问题,结果并不理想。本文针对该问题,提出了一种针对科考航迹整体的时域差分(Time Domain Difference,TDD)分段方法。本方法基于时间域对航速进行差分处理,有效降低了因为航速波动频繁对分段结果的影响。同时,考虑到该方法的差分步长在航迹处理过程中的不明确性,本文将差分后航迹的路程损失和航速波动幅值进行归一化处理,提出了航迹差分时间步长的动态确定方法,并以速率阈值对航迹进行分段。最后本文以第29次南极科考航迹数据为例,将本方法与经典的具有噪声的基于密度的聚类方法DBSCAN(Density-Based Spatial Clustering of Applications with Noise)进行了比较,实验结果表明本文提出的方法可有效降低航迹分段时分段过多的问题,在分段准确性和时间效率等方面结果更优。
