Cylindrical wave cloaking to reduce wave velocity

June 5, 2018

Cloaking has been a theoretical concept by many researchers in many field. However, most of the studies and experiments conducted have been focused for linear and controlled waves such as those in electromagnetic wave, light spectrum as well as temperature spectrum.

 

In the case of cloaking in light spectrum, a device called the Rochester Cloak allows the light to bend around and away from the main object rendering it “invisible” to the eye incident to the direction of the cloak. This concept could be used and adopted in the military and defense sector . However, the disadvantage to the Rochester Cloak device is that after a certain angle, the device is no longer able to bend the light waves and the object is no longer cloaked. Hence, limits the light spectrum cloaking to a very small angle.

 

An arrangement of a constant temperature copper heat reservoir to be used for temperature cloaking is developed by Zhang et al. (2014). The operating principle for this cloaking device is that of redirecting the heat away from the centroid of the symmetrical arrangement. This allows for heat transfer to occur around the cloaked object and thus leaving the main center to be free of the heat flux. The purpose of this is for the protection of heat sensitive electric components from high temperature fluxes. This paves way for cloaking to be used in different mediums aside from the light spectrum.


Cloaking in water waves on the other hand, have multiple setbacks as well as complications and hence the limited study and lack of experimental analysis in this area. For oceanic waves, only theoretical studies and research has been done mainly because oceanic waves are a 3-dimensional problem. Oceanic waves are made up of irregular and regular waves. On top of that, there are internal and external waves where the prior only exists if there is a stratification layer due to the change in density that is caused by the difference in salinity and temperature.

 

However, in order to reduce impact from ocean waves, it is required to reduce the energy of the encountering wave. This can be achieved by creating a cloaking mechanism to reduce the encountering wave energy to as low as possible. This is particularly important for the protection of offshore structures, trawlers at seas as well as beaches from being hit with massive high energy waves like those of tsunamis.

In this study, cloaking is done by cylinders of different geometries, arranged in a torus, to disrupt the flow of fluid along a fluid domain by adopting several concepts such as breakwaters as well as Bragg resonance. The velocity of the flow is captured after it encounters the cylinders and the behavior of the flow is then studied. The results of the simulation prove the effectiveness of this method of cloaking accurate to the theories listed within. Among the results, it is found that the hexagonal cylinder has the best cloaking property however due to the spacing in between each cylinder, the fluid flow is unstable despite a drop in fluid velocity (picture on the left below). As such, a further improvement is done on the initial hexagonal cylinder design..

As shown in the figure on the right hand side, a secondary torus of cylinder has been created in the inner part of the primary torus. The flow entering the improved hexagonal cylinder is relatively slower than the initial arrangement. The flow in the internal diameter in improved design is recorded to be half the speed of that in the initial arrangement at about 20 m/s.

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© 2018 by M A Hanann: All rights reserved.