WWW.WAS.ORG • WORLD AQUACULTURE • DECEMBER 2014 41 small-scale models mimicking a simplified aquaculture net pen. The models were manufactured using the low-cost, rapid prototyping capabilities offered by modern 3D-printing technology, capable of a spatial resolution of 20 µm. This method guarantees precision and regularity of twines and mesh, allowing us to modify the geometry by microns. Thirty circular pen models, with a surface porosity varying from 0.56 to 0.90, were manufactured (Fig. 1). As a point of reference, typical aquaculture nets have a porosity of about 0.80. Data were acquired using dye visualization, Particle Image Velocimetry and unidirectional load cell measurements. Observations Despite using a small-scale model at low inflow velocity, the observed flow behavior was in good agreement with larger-scale experiments and numerical simulations (Klebert 2013). Varying the net-pen porosity serves to vary the bleed flow velocity (Wood 1967), designated as U1, which is the reduced velocity flow in the wake of the model as a result of the flow through the porous surface at the back side of the obstruction. Our models are able to recreate the types of flow expected in the wake of this kind of geometry: • Regime 1 flows - A laminar steady wake with a bleed flow velocity (U1) that is close to the free flow velocity (U∞). • Regime 2 flows - A steady wake of finite length that evolves into an oscillating shear layer and subsequent vortex shedding. • Regime 3 flows - The bleed flow velocity is a small fraction of the free flow velocity, resulting in high velocity gradients and a shear layer developing into the classic von Kármán vortex shedding pattern (Williamson 1996). The mechanism for generation of a particular flow regime is related to the porosity of the net pen. A very porous pen will have regime 1 flow (Fig. 2 top) and a nearly solid pen will generate a regime 3 flow (Fig. 2 bottom). However, porosity is not the only parameter determining the wake, as two net-pen models of identical porosity but different twine/mesh ratio can produce different flow dynamics and significantly different drag. Environmental Aspects The netting density of a pen, usually imposed by the need to retain fish of a specified size, plays an important role in the operation of a net pen. It dictates the oxygen supply rate to the pen from inflowing water and the rate that wastes are flushed through the pen. The dispersion of biosolids is directly related to the flow regime and the magnitude of the bleeding flow. Netpen models with the first flow regime have the fastest diffusion time, whereas models with the third flow regime tend to trap particles in their wake for an extended period. To consider the issue of waste, this may have an impact on fish health, as in the case of waste returning to a net pen with a turning tide, which is not preferable. Impact on Drag We conducted multiple experiments, each varying only one parameter: twine thickness, the mesh void or porosity. The results indicate the following trends: • An increase in the mesh void tends to result in a reduction of drag. However, a void that is too small induces more drag than a completely opaque net (Fig. 3). • A reduction of twine thickness does not necessarily lead to a reduction of drag. Regardless of twine thickness, it is always possible to identify a net-pen model porosity with a greater drag than an opaque net (Fig. 4). • An increase in net porosity does not necessarily induce less drag. The same drag was obtained with models of surface porosity varying by over 40 percent (Fig. 5). Based on these results, we infer that the induced drag on a net pen produced by the net cannot be determined by the surface porosity parameter alone. Although mesh void is the dominant parameter driving the mechanism, generating drag, the effect of twine thickness cannot be overlooked. We are currently revising the mathematical model which, if verified on a larger-scale aquaculture net-pen model, could (CONTINUED ON PAGE 42) FIGURE 5. Drag of various net models as a function of the surface porosity E. FIGURE 4. Drag of various net models as a function of the twine thickness. FIGURE 3. Drag of various net models as a function of the mesh void.
RkJQdWJsaXNoZXIy MjExNDY=