Effect of spanwise slit on hydrodynamic characteristics of biplane hyper-lift trawl door

To improve the lift performance of the biplane-type otter board with a high aspect ratio, a monoplane-type hyper-lift trawl door (HLTD) with a spanwise slit h was designed by vertically assembling a pair of square HLTD models. Flume experiments were conducted using four models with slit ratio s (= h/c, c: chord) changed to 0, 0.1, 0.2, 0.3. Experiment results coupled with a CFD analysis revealed that the wingtip flow generated from the 0.2c slit is beneficial for suppressing the flow separation at moderate to high angles of attack, associated with increased lift and drag coefficients. Based on these results, a monoplane HLTD with a slit ratio of 0.2 was adopted for the fore wing, rear wing, and both wings of the biplane HLTD (composed of two monoplane HLTDs), denoted as model A, model B, model C. Flume experiments showed that model B was superior with a maximum lift coefficient of 2.18 (stall angle: 32°) in the free stream and 2.22 (28°) near the bottom. A bottom-trawling sea trial for the new biplane HLTD design (model B) was conducted. The new biplane HLTD with 80% of the wing area spread about 1.3 times more than the comparative biplane-type otter board.