Hydrodynamic performance of a newly designed biplane-type hyper-lift trawl door for otter trawling

Abstract

A biplane-type hyper-lift trawl door is newly designed to achieve stable handling while maneuvering, deploying, and stowing with excellent hydrodynamic performances for both bottom and midwater trawling. Flume-tank experiments were carried out to investigate the effect of the aspect ratio (λ, defined as the span b divided by chord c) on the hydrodynamic characteristics of the biplane-type hyper-lift trawl door with a camber ratio of 20% within a wide range of angles of attack (α). The maximum lift coefficients (CLmax) were 2.10 (λ = 1.0, α = 42°), 2.11 (λ = 1.6, α = 37°), and 2.06 (λ = 2.0, α = 28°). With a decreasing stall angle, the maximum lift coefficient remained constant (greater than 2). Considering the compactness and maneuverability of the biplane-type hyper-lift trawl door, an aspect ratio of 2 was adopted for the following experiments. The gap-chord ratio (G/c, G the gap between the fore and rear wings perpendicular to the chord direction) and stagger angle (θ, the included angle between the gap and the line connecting the leading edges of the fore and rear wings) expressing the positional relationship between the fore and rear wings of the biplane-type hyper-lift trawl door were set as 0.75, 0.9, and 1.0 and 20°, 30°, and 40°, respectively, to investigate the hydrodynamic characteristics further. The maximum lift coefficients and stall angles for the five biplane-type models were approximately 2.05 and 30°, respectively. Interestingly, these values were larger than the maximum lift coefficient and stall angle for a monoplane hyper-lift trawl door with the same aspect ratio (= 2) (CLmax = 1.78, α = 22°). This implies that the biplane-type structure contributes to enhancing the lift force and increasing the stall angle. The hydrodynamic forces of the fore and rear wings of the biplane-type hyper-lift trawl door were then calculated using a computational fluid dynamics approach. The lift coefficient for the rear wing was significantly higher than that for the fore wing before an angle of attack of 30° was reached. Nevertheless, the lift force distribution ratio for both the fore wing and the rear wing relative to the entire otter board approached 50%. On the other hand, the stall angle for the rear wing was approximately 30°, and that of the fore wing was around 38°. The fluid between the fore and rear wings tended to flow downstream and to successfully inhibit flow separation on the suction side of the fore wing.