Understanding of remora's "hitchhiking" behaviour from a hydrodynamic point of view.

Symbiotic relationships have developed through natural evolution. For example, that of the remora fish attached to the body of a shark. From the remora's perspective, this could be associated to an increased hydrodynamic efficiency in swimming and this needs to be investigated. To understand the remora's swimming strategy in the attachment state, a systematic study has been conducted using the commercial Computational Fluid Dynamics (CFD) software, STAR-CCM + to analyse and compare the resistance characteristics of the remora in attached swimming conditions. Two fundamental questions are addressed: what is the effect of the developed boundary layer flow and the effect of the adverse pressure gradient on the remora's hydrodynamic characteristics? According to the results, the resistance of the remora can generally be halved when attached. Besides, the results have also demonstrated that the drag reduction rate increases with the developed boundary layer thickness and can be estimated using the boundary layer thickness ratio and velocity deficit. The paper demonstrates that the most frequent attachment locations are also the areas that provide the maximum drag reduction rate.