Study On Drag Reduction Characteristics And Flow Field Structure Of Bionic Non-smooth Surface Mimicked Puffer Spines

The excellent drag reduction surfaces and structures of natural creatures play an increasingly important role in energy conservation,ship speed increase and so on.The study of resistance characteristics and flow field structures of the bionic non-smooth surface,inspired by the spines of puffer skin,at medium and low velocities has been carried out.At the first place,habitus and features of the puffer have been studied and that of related parameters were collected.Then,based on the analysis of the migratory flow field of the puffer,the relationship between the distribution of the spines and the flow field at the maximum profile of the fish back was resolved into two factors: the arrangement position and the inflow velocity.In addition,according to the boundary layer thickness,the size parameters value of drag reduction elements were set.Following,the non-smooth surface area was defined,and the distribution of drag reduction elements was designed.Orthogonal experimental scheme of drag reduction influencing factors and the overall scheme of single-factor investigation were designed.Validation of the simulation scheme was done followed by solution method and simulation scheme of the flow field have been drafted.The relative error between the simulation drag coefficient and the theoretical estimation drag coefficient is less than 5%.It indicated that the simulation results error can be guaranteed in a certain range.By analyzing the orthogonal experimental results,the main influencing factors of drag reduction were obtained.The characteristics of drag reduction of the main factors were gained by computer simulation.It can be concluded that the main factors includes the distribution and arrangement position of drag reduction elements,the length of the element axis and the radius of the element bottom surface.The total drag reduction effect of the non-smooth surface can reach more than 30% and the viscous drag reduction rate is more than 40%.The results show that the viscous drag reduction rate and total drag reduction rate of drag reduction element change in stages,with the increment of the axis length and bottom radius.The drag reduction rate vary linearly as the column numbers of elements raising and the position of the non-smooth area moving backward.The viscous drag reduction rate and the total drag reduction rate increase like a parabolic curve as the row number of drag reduction elements growing.The velocity,pressure and vorticity of the flow field on the non-smooth surface were analyzed.It is found that the boundary layer on the non-smooth surface is thicker and there is a low-speed reflux fluid,which aroused by vortex in the spacing of the drag reduction elements.The pressure oscillation occurs in the non-smooth region and attenuates along the flow direction.The pressure difference of the first three rows of the selected surface accounts for more than 88%.There is a strong exchange of fluid with large vorticity above the nonsmooth region.The vorticity near the wall and in the following region of the non-smooth region is smaller than that of the smooth surface.Above all,the drag reduction mechanism of bionic non-smooth surface can be expressed as follows: the non-smooth surface can reduce the velocity gradient of near-wall fluid and produce reflux fluid,so as to reduce or offset the surface viscous drag.Besides,drag reduction elements can reduce the energy dissipation by reducing the rotational motion of fluid particle in the near-wall region.Finally,the non-smooth surface components printed with resin materials were tested,and the flow field information was obtained,with the use of the self-manufactured water tunnel and PIV(Particle Image Velocimetry)system.The comparison of the flow fields shows that the non-smooth surface can make the boundary layer of the flow field thicker,produce vortex around the element and low-speed backflow in the spacing.However,the eddy structure of the simulative flow field is different from that of the experimental flow field,which indicates that the current simulation cannot be used to predict the actual flow field.