Testing otter board hydrodynamic performances in wind tunnel facilities

Abstract

The feasibility and potential advantages of wind tunnel testing of otter board designs is assessed. Traditional flume tank tests incur high operational costs and present some limitations in terms of flexibility and accuracy. Modern flume tanks, despite more flexible and accurate, are still expensive to operate or hire. Wind tunnel facilities are widespread, with a potential for low budget tests, and allow for an accurate control of velocity, angle of attack and sideslip as well as precise measurement of forces and moments in all three axes. A complete description of otter board hydrodynamics is paramount to optimising design and rigging and for the design of active control strategies that allow for stable trawling at a target speed and depth. We describe in detail the methodology of wind tunnel tests applied to general otter board designs, exemplify it with a commercial pelagic otter board and provide a comparison with existing flume tank results for the same design.

Otterboard hydrodynamic performance testing in flume tank and wind tunnel facilities

“…The effects of the upper and lower panel dimensions and respective dihedral angles on the definition of the roll reference are all encompassed by this single parameter. For details on how to relate the geometric parameters of the trawl door to φ e , see figure 1 in [25].…”

“…The wind tunnel is of the Göttingen construction type (also known as Prandtl type or closed return wind tunnel) with an open test chamber of square cross-section of 1.4 m side and provides an accurate three-axes positioning system and a six-component balance for forces and moments measurement. Tests were conducted on 20 October 2013, and a detailed account of the data analysis procedure and results for the Thyborøn type 15 vf is available at [25].…”

“…Note that the attitude angles are inferred from geometrical measurements in a slightly more subtle way than usual practice of straightforward parallel projection for better accuracy, although discrepancies are negligible. Warp and bridle forces are then expressed in flume tank reference frame (Earth frame of [25]) by projecting measured tensions according to attitude angles:…”

“…The hydrodynamic angles of the trawl door are the angle of attack (α) and sideslip angle (β), which measure the orientation of the door with respect to its velocity. These are obtained from measured attitude angles by first expressing the velocity axis (x-axis in the flume tank frame) in body coordinates [25] x ≡x…”

“…Modern flume tank facilities (SINTEF Fisheries, Hirtshals) have improved on traditional techniques by suspending the otter board from a 6-component balance (three-axes force and moment measurement) that can accurate orient the model in any desired attitude. The downside is their operation cost, as compared with general-purpose wind tunnel testing, which has been seldom used in analysing trawl door hydrodynamics [24,25], despite a number of evident advantages.…”