We revisit the classic DBSCAN algorithm by proposing a series of strategies to improve its robustness to various densities and its efficiency. Unlike the original DBSCAN, we first use the binary local sensitive hashing (LSH) which enables faster region query for the k neighbors of a data point. The binary data representation method based on k neighborhood is then proposed to map the dataset into the Hamming space for faster cluster expansion. We define a core point based on binary influence space to enhance the robustness to various densities. Also, we propose a seed point selection method, which is based on influence space and k neighborhood similarity, to select some seed points instead of all the neighborhood during cluster expansion. Consequently, the number of region queries can be decreased. The experimental results show that the improved algorithm can greatly improve the clustering speed under the premise of ensuring better algorithm clustering accuracy, especially for large-scale datasets.
During transit, a drillship moonpool creates added resistance that can be more than 50% of the total resistance and hence significantly increase the fuel consumption. This paper reports a real case study for reducing moonpool added resistance by model tests and Computational Fluid Dynamics (CFD) simulations. Model test data show that, unlike conventional ships that experience nearly constant resistance, a drillship resistance presents largely fluctuating behaviour. In CFD validation, the physics based CFD best practices are applied for resistance predictions. The largely fluctuating resistance is well captured in simulations. It is found from flow visualizations of the large amount of CFD simulation results that the high moonpool induced added resistance is mainly attributed to the vortices shed from the moonpool front wall, which enter into the moonpool and impinge on the rear wall. The CFD predicted mean resistance are in good agreement with the model test data, within 3% difference for a wide range of speeds. The well validated CFD tool is applied to study the effects of moonpool dimensions on the added resistance. The results of parametric study reveal a design principle that smaller moonpool dimension results in smaller added resistance. Through comprehensive CFD parametric study, a universal design principle and CFD best practice are established for industry applications.
Aiming at the disadvantage of slicing STL model in rapid prototyping, a method of slicing CAD model directly is proposed to advoid the precision damage during converting CAD model to STL file. The effect of the staircase error to the part surface is analyzed, and an algorithm of compensating the section contour is proposed to eliminate the layers’ volumetric difference and improve the part surface precision. The test shows that the methods can obtain the accurate sections of CAD model and improve the part’s shape and size precision.
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