PINNIPHYSICS
- May 13
- 5 min read
Tavishi
I miss taking math and physics classes. Luckily for me, we had a biomechanics class, and we did calculus during our kidney lectures.
However, and I might propose:
For starters, pinnipeds are basically just birds flying when they're swimming; they follow the same basic principles of aerodynamics, but with a side of being in Water.
Anyways, but pinnipeds follow the same basic principles of drag, lift, and thrust. According to the Coanda effect, fluids follow a curved path around seal rather than just straight into pinniped. This streamlined path creates a region of low pressure close to the surface of the seal called the boundary layer. This layer starts off laminar, or with non-turbulent flow, and turns turbulent about halfway through pinniped. The boundary layer also increases in thickness as it moves posterior- so the tail end of a seal will have a much larger, turbulent boundary layer in comparison to the nose.
The boundary layer is produced by viscosity and the no-slip condition. Viscosity tries to make water slower (slower seal) and then there's like a whole thing with shear forces. Tldr; how make boundary layer and speedy water layers (otherwise known as.... nonboundary layers!) go fast each other fast so seal can go fast.
If a seal was suspended in honey, it would have a lot of drag, because the boundary and nonboundary layers would have a harder time separating. The forces that cause those layers to separate from one another are shear forces, and are enacted on the boundary and nonboundary layers by each other. It is thus, a very good thing, that the ocean is not made of honey, because the seals would not be very good at swimming away fast.
Essentially, shear forces are like water friction.
There is also the no-slip condition which does not apply to seal because seal swim through water, and water does not swim in seal sink (?). (If drop seal in honey then seal sink to bottom with honey. if drop seal in water seal sink to bottom through water!) (We want seal to slip).
Anyways, but assuming the no-slip condition is not happening because the seal is in water, and that shear stress is minimized, seal swim.
sidebar 1: all my honey crystallized over Easter. this is. this is not meant to be occurring. bring me back my goop. it is not v iscous it is ijust solid. no solid. want liquid honey. want to make granola please and Thank you!!
Fluid flow is fastest near the fattest parts of the seal, and decreases in speed as the seal gets skinnier.
Near the tail end of the seal, flow slows down so much that it stops clinging to the seal, even reversing directions. This produces a lot of turbulent flow, which in turn, produces a lot of drag. This decrease in speed is caused by an increase in pressure (see: conservation of energy... ) Flow separation can be prolonged to decrease drag by switching boundary layers from laminar to turbulent flow at different points in the Creature.
Knowing all this, it follows logically to wonder how the structure of pinnipeds maximizes speed.
First, pinnipeds limit drag via a few different methods. There are two types of drag that must be mitigated: friction drag, which is produced by shear forces, and pressure drag, which is produced by flow separation and adverse pressure gradients.
The ratio of pinniped length to pinniped fatness (officially called the fineness ratio) is an important determining factor in this. By making a pinniped super long and skinny, pressure drag reduces, but friction drag increases. By making a pinniped super short and fat, friction drag reduces, but pressure drag increases.
Whereas phocids (earless seals) and walruses have a fineness ratio of around 3-4, otariids (sea lions and fur seals) tend to have a fineness ratio from 5-7.
Seals and walruses tend to have much more friction drag, whereas fur seals and sea lions tend to have much more pressure drag.
Pinnipeds propel themselves through the water by generating lift, by creating an area of low pressure above their flippers, and area of high pressure below their flippers. The morphology of the flippers itself controls how the pinniped moves. Pinniped flippers tend to have equal curve on both the top and bottom side, because they need to propel themselves forward, which requires both lift up and down.
The angle of attack is the angle between the direction of flow and the line cutting across the flipper:
As angle of attack increases, lift and drag both increase. However, if angle of attack goes too high (above 18 degrees), this results in stall and flow separation as mentioned above. (See: bugs). The ideal angle of attack is near 15 degrees, and reduces cost of transport such that hopefully, the piniped should barely have to move their flippers to propel themselves.
Phocids and odobenids, or seals and walruses, generate lift primarily through their pelvic flippers, whereas otariids, or fur seals and sea lions, use their pectoral flippers to generate lift. The shape of the flipper determines how efficiently it pinnipeds. A flipper that is longer and skinnier has a higher aspect ratio, in comparison to a short and stout flipper:
The way the edge of the flipper, or the leading edge, bends back (known as sweep) also affects drag. A lower sweep decreases drag and increases lift.
Phocids and odobenids swim by basically just moving their hind flippers from side to side (it's giving Shakira?), whereas sea lions properly flap their foreflippers to propel themselves, generating a lot of Thrust.
Also, otariids have much less maneuverability than phocids because of how disproportionately long and near the center of mass they are. Because of this, they're not great at turning, and rather, their motion depends on their flexible spine.
Phocids, on the other hand, have a lot of cool flexibility in their cervical vertebrae, meaning they can shoot their head out to grab fish. Body wise, because they're so freaking fat, they have less actual ability to twist- rather, their turning is dependent on their foreflippers.
Ok, this was definitely a weird but fun post to write! Highly recommend Marine Mammal Physiology: Requisites for Ocean Living- gotta be one of my favorite reads, and the book that really strengthened my knowledge of pinniphysics.
sidebar 2: i miss physics and math so much. pulling a sophia the way i've been doing diff eq for fun. do we need a diff eq post? or a more pure math/phys post....
i really want to write an entire post in latin but that would ragebait the hell out of certain classics enjoying white boys
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