SMALL-SCALE MODELING RULES OF TRAWL NETS

Small-scale modelling rules of trawl nets are derived from first principles taking into account the porous two-dimensional nature of a net surface. It is shown that under certain circumstances the dynamic and body force balance ratios are characterised respectively by the Reynolds and Froude numbers based on the twine diameter and that both dynamic similarity and geometric similarity are achieved when these quantities are held constant. Owing to practical considerations it is not usually possible to enforce this optimal scaling regime and so a hierarchy of regimes is proposed. In particular, the vertical Reynolds number is introduced, although it is made clear that it remains uncertain whether this is an accurate representation of the dynamic force balance in the proximity of the net. The importance of understanding the effect of change of the Reynolds number based on the twine diameter is stressed and the need for exact and controlled experimentation stated.