Study On Drag Reduction Mechanism Of Grooved Surface Based On Entropy Analysis

The influence of the grooved surface on the drag reduction is of important research value,and it received continuous attention due to its mere change of the turbulent flow characteristics,the low energy consumption and high efficiency.In present paper,the numerical simulation software is used to analyze the mechanism drag reductionThe energy dissipation in the wall turbulence is mainly composed of the deformation motion and rotational motion in the fluid micro cluster.The present paper starts with the influence of the grooved surface on face deformation,and the distribution of pressure drag and shear stress on the grooved surface is analyzed.Then,in order to further explore the mechanism of drag reduction,the influence of groove structure on the rotational motion of fluid in the wall is investigated taking the vortex structure in turbulent flow as a starting point.The results show that the groove structure leads to the generation of two vortex,which is beneficial to decrease the viscous drag,while it would result in the increase of the differential pressure resistance.Secondly,due to the vortex structure,the velocity gradient in the near wall region is significantly reduced,and the shear stress between the velocity layer decreased.The size of vortex is directly related to the velocity gradient.Compared with the flat surface,the velocity gradient in the near wall region of the grooved surface is smaller,which leads to a significant reduction of the vorticity.Due to the weakening effect of the tensile shear layer,the degree of vortex tube was lower in the groove surface,and the band structure in the viscous sublayer is more stable and not easily broken,which reduced the turbulent burst frequency.At the same time,the direction of the vortex structure in the normal direction is reduced by the change of the groove surface method,which leads to the decrease of the intensity of the vortex rotation.With the method of the entropy analysis the mechanism of drag reduction on the groove surface is analyzed from another angle.Taking the main source of entropy generation into account,instantaneous velocity distribution of turbulent kinetic energy and boundary layer in the flow of the first groove structure is investigated.Then,by analyzing the entropy produced by viscous dissipation and turbulent dissipation,the influence law of the groove structure on the size and distribution of entropy production is studied.It is found that the turbulent kinetic energy on the surface of the groove is decreased,the chaos is suppressed,and the instantaneous velocity distribution is more close to the average velocity distribution than the flat surface.In a certain range,the value of entropy production caused by viscous dissipation near the surface of the groove is significantly reduced and the distribution of the distribution is not uniform.At the same time,due to the existence of the groove structure,the entropy produced by the turbulence dissipation is improved in this region,but its value is not as the same as the entropy produced by the viscous dissipation.The entropy production at the tip of the trench structure is large,and its numerical value is more than one order of magnitude,and the entropy produced by the turbulence dissipation is the main flow entropy.