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Examples of calculation of the capacity-efficiency factor are presented in Figure 4 and in Tables 1 and 2. In Table 1 the data from Aleyev [ 6 ] crosses in Figure 4 have been used. Different colours of points correspond to the different values of the thickness ratio , as mentioned in the previous section.
In Aleyev [ 6 ] only data about animal length, the ratio , and are available. To calculate the velocities the average value was used. The volume was estimated with the use of Inaccuracy in the maximum velocity, the mass, and viscosity data is a reason for discrepancies in values obtained for the same animals in Tables 1 and 2.
Since the experimental drag data for live animals are very limited, expression 5 is here related only to rigid bodies of revolution. The body Dolphin was manufactured and tested by North American Aviation in see [ 16 ]. Tests revealed the minimal value of at ; 5 yields. The optimal shape X for an unclosed tail-boom body of revolution calculated in Parsons et al. For the same Reynolds number range the theoretical values of the minimum drag coefficient are see 5.
The higher vales of the drag can be explained by the presence of separation and turbulence on all the above-mentioned bodies of revolution. The theoretical drag values of different bodies of revolution calculated in Parsons et al.
This can probably be explained as due to different semiempirical criteria for the laminar-turbulent transition in the boundary layer. The values of calculated in Parsons et al. For example, in Zedan et al. A similar comparison can be performed for the critical values of the Reynolds number calculations 12 ; for the bodies Dolphin , Hansen and Hoyt, and X the critical Reynolds numbers are , , and , respectively. At the predictions of the laminar-to-turbulent transition coordinate presented by Parsons et al.
This very slight dependence on the Reynolds number can be explained by the use of the cross-section of the laminar separation as the transition point in Parsons et al.
Expressions 12 yield or for the bottlenose dolphin with body ratio. These estimations resolve the well-known Gray paradox, since the Reynolds number taken for estimations in Gray [ 22 ] corresponds to laminar flow on the dolphin see also [ 9 ].
Thus slender bodies of revolution can delay laminar-turbulent transitions on their surfaces and reduced skin-friction drag. It must be stressed that relations 12 are valid only for a flow pattern without separation. That is why the effect of the turbulization delay has not been achieved on standard separated slender bodies of revolution.
The difference in shape can be hardly perceptible see, e. It can be seen from Figure 4 and Tables 1 and 2 that the best swimmers are fish whose shape corresponds to the minimal possible values of. These are the Indo-Pacific sailfish, Mediterranean spearfish, narrow-barred Spanish mackerel, and wahoo. Some flying fish and some molluscs e. These animals are both perfect swimmers and rather good fliers.
In particular, the squid change their shape during flight to create lift forces see, e. The capacity-efficiency factor of the best swimmers is approximately times greater than that of common good swimmers. Applying expressions 11 and 15 and the average value yields where must be taken in metres.
The smallest values of are associated with nonstreamlined animals e. The shapes of these animals and vehicles obviously cannot ensure any attached flow pattern.
The low values of for submarines are both the result of the large supercritical Reynolds numbers at which they move there are huge differences in the theoretical values of both the laminar and the turbulent friction, shown in Figure 3 and of the separation that increases the drag 3- to 5-fold in comparison to the value possible for an attached flow pattern.
The main predator of whales—the killer whale Orcinus orca —has approximately twice as large a value of as the sei whale and comes close to the characteristics of its relative, the bottlenose dolphin Tursiops truncatus. Humans are not the best swimmers. For example, the world records men have a value similar to some turtles, sturgeons, and the blue whale Balaenoptera musculus , times smaller than the capacity-efficiency of the best swimmers.
The sharks have a very broad range of capacity-efficiency factor that decreases with increasing Reynolds number. For example, the juvenile blue shark belongs to the best swimmers, while barracuda and adult blue shark have 7—9 times smaller values of. The largest great white sharks that swim at supercritical Reynolds numbers have the smallest value of.
The same large difference can be seen in the case of birds. For example, the small and fast chinstrap penguin Pygoscelis antarcticus has approximately times greater value of than the large and slow emperor penguin Aptenodytes forsteri.
The capacity-efficiency factor can be sometimes very close for juvenile and adult animals e. In the case of the blue shark the large difference can be explained by the fact that juvenile animals swim at subcritical Reynolds number, whereas the adults swim at transitional and supercritical values of.
It can be concluded that the best swimmers have a streamlined shape that ensures an attached flow pattern and a laminar boundary layer at rather large values of the Reynolds number. The large difference in corresponding values see Figure 5 and Tables 1 and 2 shows that information about animal shapes and locomotion is not only of biological interest but very useful to improve the capabilities of robot fish and underwater vehicles as well. Better measurements of the maximum velocity, mass, and water temperature are necessary to determine the top swimmers among the animals.
Dolphins are mammals, but they look very different to mammals that live on land, as they are adapted to living in water. They have a streamlined shape and fins instead of legs. They also have blowholes on the tops of their heads. They use these to breathe, rather than through their mouths and noses. These adaptations enable the organism to regulate their bodily functions, such as breathing and temperature, and perform special functions like excreting chemicals as a defence mechanism.
Some marine mammals, such as whales , migrate over large distances and may spend time in a combination of arctic, tropical and temperate waters.
This means that they are able to maintain a constant body temperature that is not dependent on the surrounding water.
Slow-moving species have adaptations that help protect them from predators. For example, many marine organisms can only move slowly or not all. This means they cannot easily get away from mobile predators, and they have other adaptations to protect them from being eaten. These can include chemical defences in their skin, for example, sea stars. Behavioural adaptations are learned or inherited behaviours that help organisms to survive, for example, the sounds made by whales allow them to communicate, navigate and hunt prey.
Urban areas sometimes flush untreated sewage into streams, causing algal blooms that suffocate wildlife. Pollutants can wash into streams from farms and factories, and harm wildlife and humans who depend on the water. In response to these threats, scientists and conservationists are taking action. Hydrologists study streams and how water quality and flow change over time. Conservationists can work with communities to revive stream basins. Sandra Postel, a past National Geographic Freshwater Fellow, created a program that restored billions of gallons of water to river systems in North America.
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