Drag on Hydroid-Fouled Nets – An Experimental Approach

• Published in: China ocean engineering 2015-06, Vol.29 (3), p.369-389
• Main Authors: Lader, Pål, Fredriksson, David W., Guenther, Jana, Volent, Zsolt, Blocher, Nina, Kristiansen, David, Gansel, Lars, Decew, Jud
• Format: Article
• Language: English
• Subjects: Coastal Sciences; Engineering; Fluid- and Aerodynamics; Marine & Freshwater Sciences; Numerical and Computational Physics; Oceanography; Offshore Engineering; Simulation; 养殖场地; 平均长度; 水螅; 生长特性; 田间试验; 缠绕结构; 速度测量; 验方
• ISSN: 0890-5487; 2191-8945
• DOI: 10.1007/s13344-015-0026-y

The present study investigated the drag increase on aquaculture nets due to biofouling of the colonial hydroid Ectopleura larynx. It had two main parts： firstly the growth characteristics of E. larynx were investigated by use of field tests at a Norwegian aquaculture site; secondly the hydrodynamic drag on the fouled twines was studied in a towing tank by using fabricated models of net twines with artificial hydroid fouling. In the field tests, the growth of the hydroids was first measured after three weeks of immersion and then again after six weeks. During this interval, the density of hydroids and the thickness of the hydroid stem were almost constant（1.4 hydroids/mm and 0.29 mm, respectively）, while the average length of the hydroids increased from 6.4 to 11.2 mm. The hydroid length followed a Rayleigh distribution, while the thickness was normal（Gaussian） distributed. Replicas of twines with three different levels of hydroid growth were made（1.5 hydroids/mm twine, hydroid length 9 mm, 16 mm and 20 mm）, and the drag on these twines was measured at different towing velocities（0.1 to 1.4 m/s） and with different twine configurations. For the twine with the shortest hydroids（9 mm）, the drag was from 1.5 times（Re=4000） to 2.2 times（Re=1000） the drag on a clean twine. For the longest hydroids（21 mm）, the drag was 2 times and 3.8 times, respectively.