American biomechanists have modeled the flow of fluid around a swimming blue whale and identified the places on its body that are most favorable for attaching clingfish. It turned out that in the area of the blowhole, as well as the dorsal and pectoral fins, the hydrodynamic resistance is significantly reduced — it can be 84 percent less than in an open stream. This allows fish not only to firmly attach themselves to the body of the whale, but even for a short time to detach themselves and swim near it, scientists write in the Journal of Experimental Biology.
Fish from the clingfish family (Echeneidae) use large marine animals — sharks, turtles, or whales — as transport for long-distance migrations. With the help of a sucker, which in the course of evolution has become a dorsal fin, they attach themselves to the body of a large animal and can move with it in search of food or a partner. Clingfish are not so small: their body length is several tens of centimeters, while the host animal usually swims at a fairly high speed (for example, the maximum speed of a blue whale reaches five meters per second), so the liquid creates quite a lot of drag. This resistance of the liquid must be overcome, including the adhesive, and to make it easier to hold on, you need to carefully choose the place of attachment. However, detailed studies of why some points on the body of the host animal stick to others, have not been conducted.
Biomechanists from the United States and Spain, led by Brooke E. Flammang of the new Jersey Institute of Technology, decided to determine which places on the body of the blue whale (Balaenoptera musculus) are most often chosen by clingfish for attachment and how this choice can be explained in terms of hydrodynamics. To do this, the researchers first analyzed 3.5 hours of video of blue whales swimming, and then modeled the swimming of whales using the computational fluid dynamics method (CFD, computational fluid dynamics) and found areas with the lowest hydrodynamic resistance.
On video recordings, scientists recorded 61 points on the bodies of three whales, to which were attached sticky species of Remora australis with a length of 20 to 40 centimeters. It turned out that almost all the attachment points are located in three areas on the whale's body: directly behind the blowhole, in the dorsal fin area, and to the side of the pectoral fins. At the same time, in the area behind the blowhole and around the dorsal fin, the sticks were fixed in groups.
Interestingly, the fish were not always attached to the host's body: for food, they can temporarily detach, and then attach themselves back — to the same place or to a new one. While the fish is untethered from the whale, it is located at a very short distance from the surface of the whale's body, in the area of the trace, where the whale's body drags the liquid behind it. In this area, it is possible to significantly reduce the energy consumption for swimming by sliding in the stream and it is quite easy to swim directly near the whale.
To explain the choice of these particular areas for attaching adhesives and sliding along the surface of the whale's body, scientists built a computer model of the floating animal based on the finite element method. The geometry of the whale's body for the calculation model was recreated from a three-dimensional replica, the average speed of the whale was 1.5 meters per second, and the body length was 18 meters. Taking into account the viscosity Reynolds number it amounted to about 23 million. The entire calculation cell included almost 42 million elements, and the calculation was carried out on 720 cores of the Barcelona supercomputer for two days. After the calculation of scientists have built a velocity field of fluid flow around swimming whales, vorticity of flow and pressure distribution and coefficients of resistance and friction in the flow.
The researchers modeled flows around sticks of four sizes: height from 1 to 5.3 centimeters. It turned out that in those areas where most adhesives are attached, the coefficient of drag can be reduced by 71-84 percent compared to the open flow. It is in these areas that the flow is divided into several parts, which leads to a decrease in resistance. The areas of the hydrodynamic track where fish prefer to glide in the stream, having broken away from the body of the host animal, are located to the side of the attachment areas or behind them. Such "stagnant" areas in the flow are created precisely thanks to the protrusions on the whale's body due to Venturi effect-pressure drop in a narrowed flow-where the resistance of the liquid is reduced by 50-72 percent. In the same areas on the body of the whale where the fish did not attach at all, and according to modeling data, the gain in hydrodynamic resistance was negligible or zero.
According to scientists, the data they have obtained not only explains the behavior of clingfish, but is also useful for improving the effectiveness of environmental monitoring. Understanding the pattern of fluid flows around the whale's body will allow you to attach sensors or tags to it, which will last longer on the animal.
The study of hydrodynamic features of swimming of large animals helps to find optimal ways to move in a viscous environment. For example, American scientists have foundthat the maximum speed of movement of large fish and marine mammals is optimal if the ratio of the amplitude of vibrations of the animal's tail to the length of its body is from 10 to 30 percent. And before that, biologists have shownthat fish can also save energy by swimming on their side.