Experimental investigation of forces and geometry of a net cage in uniform flow

A scale model of a flexible circular net with different weights attached to the bottom was tested in a flume tank. Global forces and net deformation were measured for different steady current velocities. Three different sizes of bottom weights were used in the tests. The results from these tests are presented and discussed with the emphasis on the dependency between the forces and the geometry. Comparison is also made to empirical based formulas for calculation of drag and lift forces on net structures. Findings show that i) the forces on, and deformation of a flexible net structure are mutually highly dependent on each other; ii) estimates of global forces on a flexible net structure calculated using simple drag formulas derived from stiff net panel experiments give large errors when compared to experimental measurements; iii) numerical models taking into account the dependency between force and deformation should be used to obtain accurate estimates of forces on flexible net structures; and iv) the forces on a flexible net structure are dependent on Reynolds number, and their dependency are similar to that of a regular cylinder.     

QUANTIFYING SIMILARITY FOR HANDLING INFORMATION IN KNOWLEDGE BASES

When constructing diagnostic systems or using knowledge-based systems,e.g.in analytical chemistry,features of different type and character,represented by numbers,trajectories or linguistic variables suchas intensities or colours,must be considered.To find neighbourhoods or to fill in missing values,thenotion of similarity is of essential importance.The paper presents a new fuzzy-set-theory-based approachto quantifying similarity and provides a system of rules to be implemented into the diagnostic part of theknowledge base to be used.     

Flow through nets and trawls of low porosity

In trawls intended for harvesting marine zooplankton the mesh size and twine thickness may be as small as O(10⁻⁴ m), the porosity less than 0.5 and the appropriate Reynolds number O(10⁰−10²). The flow locally through the meshes varies strongly with the Reynolds number in this range, and the entire flow field, filtered volume and drag of such nets therefore depend strongly on the net parameters and towing velocity.This paper presents a simplified model for the flow through and forces on inclined permeable screens based on pressure drop considerations. For conical nets the model provides simple expressions for the filtration efficiency and drag as functions of twine diameter, mesh opening, porosity, taper angle and flow (towing) velocity. Comparisons with test tank measurements of typical plankton nets show good agreement.