Flow simulation along a seal: the impact of an external device

Abstract

An increasing number of marine mammal studies on physiology, behaviour and ecology rely on data, which have been collected from back-mounted devices, such as bio-logging tags and satellite transmitters. However, external devices may influence an animal's hydrodynamics, behaviour and energy expenditure and, therefore, can impede the individual animal. To investigate the influence of external devices on seals, the water flow along a grey seal was simulated using computational fluid dynamics calculations. The simulations revealed several changes in forces and moments and thus balance, due to this device. The investigated satellite transmitter creates an average 12% increase of the drag coefficient. Additionally, there are significant relative transmitter-induced increases in pitching moment (32%) and lift (240%). The simulations also showed that the transmitter generates areas of decreased wall shear stress on the seal's back. The results of this study demonstrate that external devices can change the hydrodynamics of the seal, which is expected to alter the seal's physiology and behaviour and its use of the ecosystem. Long-term attachment may have adverse effects on the animal's welfare. It is important to take these effects into consideration when studying tagged seals; otherwise, the value of the data obtained will be poor. Therefore, interpretations and extrapolations regarding 'natural behaviour' of animals in their 'natural environment' should only be made with great caution.

Bottlenose dolphins modify behavior to reduce metabolic effect of tag attachment

“…A more streamlined urethane housing containing all of the tag elements (electronics, VHF and flotation) minimizes geometric disruptions in the flow around the housing, reducing drag forces. Similar to previous papers on tag design Culik et al, 1994;Hazekamp et al, 2010;McMahon et al, 2011;Jones et al, 2013), the study by Shorter et al (Shorter et al, 2014) suggests that tag designs should: (1) minimize frontal cross-sectional areas and maintain a smooth exterior to reduce drag; (2) cover suction cups or other exposed features to reduce flow stagnation and wake generation; and (3) reduce lift by minimizing the attachment area and by adding flow channels or spoilers to reduce differences in flow speed above and below the housing, or redirect flow to counter lift.…”

Effects of added drag on cetaceans : fishing gear entanglement and external tag attachment

“…A more streamlined urethane housing containing all of the tag elements (electronics, VHF and flotation) minimizes geometric disruptions in the flow around the housing, reducing drag forces. Similar to previous papers on tag design Culik et al, 1994;Hazekamp et al, 2009;McMahon et al, 2011;Pavlov and Rashad, 2012), the study by Shorter et al (2013) suggests that tag designs should: (1) minimize frontal cross-sectional areas and maintain a smooth exterior to reduce drag; (2) cover suction cups or other exposed features to reduce flow stagnation and wake generation; and (3) reduce lift by minimizing the attachment area and by adding flow channels or spoilers to reduce differences in flow speed above and below the housing, or redirect flow to counter lift.…”

“…CFD simulations assume static and uniform flow conditions across a rigid-body dolphin model (Chapter 8; Hazekamp et al, 2009;Pavlov and Rashad, 2012;Shorter et al, 2013) and therefore model passive drag forces only. Swimming dolphins interact with non-uniform and variable flow, interact with the water's surface, and generate additional (i.e., active) drag forces as they propel their bodies through the water (Lighthill, 1969;Webb, 1975b).…”

“…Both researchers and tag companies have taken active roles to work together in reducing device size Hazekamp et al, 2009;Pavlov and Rashad, 2012;Balmer et al, 2013), refining the tools on the market. For example, the transition from the DTAG2 to current DTAG3 model reduced the frontal area by~1/3, making it more suitable for deployments on smaller species ( Figure 8B; Shorter et al, 2013).…”

“…To improve environmental context, data from the tags themselves Aoki et al, 2011;Ware et al, 2016), along with estimated body parameters, have been used to estimate drag forces on free-swimming animals. In contrast with the various experimental methods described above, computational fluid dynamics (CFD) models are becoming an increasingly popular method to simulate the expected effects of instrumentation and to refine tag design (Hazekamp et al, 2009;Pavlov and Rashad, 2012;Balmer et al, 2013;Shorter et al, 2013;van der Hoop et al, 2014a).…”