Fabrication of flexible blade models from a silicone-based polymer to test the effect of surface corrugations on drag and blade motion

Macrocystis blades develop longitudinal corrugations in regions with strong current and wave action. This study examined the effect of corrugations on blade motion and blade drag by constructing flexible blades with different corrugation amplitude and a control blade with no corrugation. The models were designed to be dynamically and geometrically similar to natural blades. Acrylic molds were etched using a laser cutter and filled with a silicone-based polymer to create flexible model blades with sinusoidal corrugations. The corrugated and flat model blades were tested in a water channel using drag force measurements and video analysis. The corrugated blades experienced a drag per surface area reduction of up to 60% compared to the flat blade. Additionally, the corrugated models exhibited smaller motion, as quantified by the maximum vertical displacement. The reduction in drag may explain why corrugations are observed in exposed regions of high current and wave action, where a reduction in drag provides important protection against breakage. Through phenotype plasticity, the morphology of kelp blades changes in response to environmental stimuli, such as variations in nutrient availability and hydrodynamic conditions (Hurd 2000). By transplanting live plants to sites with different environmental conditions, studies have shown that the blade geometric parameters such as length, width, thickness, and overall shape can change within the timescale of the kelp life cycle (Norton 1969; Druehl and Kemp 1982). Since kelp and other macroalgae are frequently the dominant producers in coastal regions, providing shelter and nutrients to other organisms in their ecosystem (Hurd 2000), there is incentive to understand how specific morphologies enhance plant survival in various environmental conditions. The environmental settings associated with morphological change are commonly divided into “sheltered” and “exposed” conditions, referring to relatively lower and higher current and wave exposure, respectively (e.g., Hurd et al. 1996). The morphological changes are hypothesized to either enhance nutrient flux to the kelp surface or reduce hydrodynamic drag (Koehl et al. 2008). For example, Koehl et al. (2008) observed that Nereocystis blades exhibit ruffles or undulations in sheltered regions, compared to exposed regions where blades are flat. The ruffles increase blade movement, which can renew the water near the blade surface, which in turn can enhance nutrient flux (Koehl and Alberte 1988; Huang et al. 2011). However, the ruffles also increase drag force, which can put the blade at risk of breaking in exposed regions (Koehl and Alberte 1988). Although differences between sheltered and exposed blade morphology are not always observed in Macrocystis (Hepburn et al. 2007), generally the blades are thicker and have longitudinal corrugations in exposed regions (Fig. 1), and are thinner and exhibit little or no corrugations in sheltered regions (Hurd and Pilditch 2011, summarized here in Table 1). Hurd et al. (1997) suggested that the corrugations reduce skin friction, because similar longitudinal grooves (called riblets) have been observed to reduce skin friction by 7–8% on rigid surfaces by reducing the impact of span-wise boundary-layer vortices for a particular range of riblet spacing (Djenidi et al. 1994). Previous studies of corrugation in metal sheets have also shown that corrugation can enhance rigidity (Briassoulis 1986). Based on this, Rominger and Nepf (2014) hypothesized that the kelp corrugations enhance blade rigidity, which in turn reduces peak forces. Specifically, Rominger and Nepf (2014) showed that in some flow conditions, increasing blade rigidity significantly reduced drag by limiting the blade motion that occurs in response to flow oscillation (e.g., turbulence or waves). The degree of blade motion in response to flow can be characterized by the following non-dimensional force ratio of blade rigidity (resistance to bending) to fluid forcing (Michelin et al. 2008): *Correspondence: [email protected] 1 LIMNOLOGY and OCEANOGRAPHY: METHODS Limnol. Oceanogr.: Methods 00, 2015, 00–00 VC 2015 Association for the Sciences of Limnology and Oceanography doi: 10.1002/lom3.10053