School of Mechanical Engineering Seminar
Wednesday, April 3, 2019 at 14:00
Wolfson Building of Mechanical Engineering, Room 206
Fishlike underwater imaging
Michael Nareznoy
MSc Student of Gregory Zilman
To perform their missions Autonomous Underwater Vehicles have to detect, to avoid and to identify obstacles. Mature technologies such as underwater video cameras and sonars are used currently for these purposes. However, in low light and turbid water the recognizing a plane wall, a corner, or a finite size body including planes may be difficult.
Blind Mexican Tetra fish, are capable to detect, to avoid and even to identify obstacles without vision or sonar. This phenomenon is known in the scientific literature by its figurative name “distant touch” or as hydrodynamic imaging. Distant touch is based on the alternations of the hydrodynamic field induced by a fish moving in the presence of other bodies and on sensing these alternations by the velocity and pressure sensors of the fish’s lateral line.
Mathematical modelling of the distant touch and understanding how distant and robust it can be is the aim of our work. Because of a huge diversity of obstacles shapes, only the very three representative examples are considered: an oblique approach a body-detector to a plane wall, motion of a detector in a corner, and passing by a detector of another body. A remarkable feature of the lateral line is that the density of the velocity and pressure sensors of a Mexican Tetra reaches the maximum on its forward part (head). For most of smooth bodies, the pressure on the forward (bow) part of them can be modeled as for an incompressible, inviscid and irrotational flow.
Within the framework of potential flow, a multibody problem is studied, where a simple layer of sources with unknown density replaces the surface of each body. Using the boundary condition of impermeability the Fredholm second type integral equation is derived. This integral equation is solved numerically using collocations combined with the panel method. Once the simple layer density is known, it becomes possible to calculate the pressure and the tangential velocity on the body-detector using the Bernoulli integral.
Systematic numerical calculations manifest two main features of the pressure distribution on the surface of the body-detector when it approaches to an obstacle: i) a sharp rise of pressure on its bow; ii) an essential shift of the stagnation point. While the pressure is proportional to the square of the detector velocity, the shift of the stagnation point is a purely kinematic characteristic. Notable, Mexican Tetra initiate a turning maneuver from an obstacle also regardless on its speed.
Finally, possible geometrical configurations of a body-detector and the associated detection distance for three considered examples is discussed.
