An empirical velocity scale relation for modelling a design of large mesh pelagic trawl

Physical models of fishing nets are used in fishing technology research at scales of 1:40 or smaller. As with all modelling involving fluid flow, a set of rules is required to determine the geometry of the model and its velocity relative to the water. Appropriate rules ensure that the model is subject to similar forces and behaves in a similar way to the full-scale net. It is not possible however, to choose a completely compatible set of modelling rules and a compromise is necessary. The common practice is to assume that similarity is achieved when a constant Froude Number is maintained. This is often found to be inadequate in that drag is increasingly overestimated as the scale is reduced. A new empirical relation between net drag coefficient and Reynolds Number at constant net geometry for one design of large mesh pelagic trawl is derived which applies over a wide range of Reynolds Numbers (based on mean twine thickness) from 63 to 1.67 x 104. Six sizes of the same net design from full-scale to 1:40 were investigated. From this relation a new velocity scale relation has been proposed which relates velocity scale to linear scale for this particular trawl design. It is argued that not only the net mouth but also all the individual netting panels will maintain the correct geometry over this range of model sizes. The traditional Froude similarity law states that the velocity scale is equal to the linear scale to the power of 0.5. The new relation suggests a power of approximately 0.6. Lower velocities may be necessary to compensate for the increase in drag coefficient as Reynolds Number decreases at constant Froude Number. Until more experiments are done on radically different net designs it will not be possible to assess how widely this new relation may apply