Mechanism Of Turbulent Drag Reduction And Spatiotemporal Identification Of Coherent Structures On Superhydrophobic Surface

The drag reduction control of turbulent flow is one of the important research topics in the field of engineering.The self-sustaining and self-developing processes between different types of coherent structures are the key to the generation and maintenance of turbulent flow,and are also the main source of high frictional drag in the turbulent boundary layer.Controlling the coherent structure can effectively change the basic properties of turbulent to achieve drag reduction control.Controlling the coherent structure can effectively influence the fundamental properties of turbulence to achieve drag reduction control.The research on the drag reduction mechanism of the structure and the basic properties of the flow field during this process can provide important theoretical support for the drag reduction control of the turbulent boundary layer.In this paper,the drag reduction mechanism of passive control of turbulent flow on superhydrophobic(SH)surface is mainly studied,and the flow field on hydrophilic(PH)and SH surface is compared by using Time-Resolved Particle Image Velocimetry(TRPIV)technique with high time resolution,then use Matlab to conduct in-depth analysis of the drag reduction mechanism in flow field.The influence of the SH surface on the migration of the vortex structure is analyzed.Since the coherent structure contain most of the energy of the flow field,the change of the migration path of the structure will also affect the energy stratification of the flow field,so the change of the flow field energy is carried out to analysis.The changes in the uniform momentum zones(UMZs)of the flow field and the TNTI are respectively studied,and the relationship between the two statistical characteristics and the resistance of the flow field is revealed.The modal decomposition method is used to study the energy curves and topographic changes of the modal characteristics of the flow field.According to the results of the energy analysis,it is proved that the SH surface has the inhibition of the large-scale coherent structure.The finite time Lyapunov exponents(FTLE)method is used to display the flow field under the lagrangian system.The development and evolution of coherent structures are tracked,and a coherent structure identification method based on FTLE is designed to count the number and distribution of structures,the influence of SH surface on the development of coherent structures in the flow field is studied,and the drag reduction mechanism of SH surface is determined.(1)The basic characteristics of the turbulent boundary layer on both surface are statistically based on experimental data.The average velocity profile and turbulence profile of the turbulent boundary layer are fitted.Compared with the PH surface,the logarithmic region of the average velocity profile on the SH surface has a significant upward shift,and the pulsation in flow direction has a certain weakening.The rate of drag reduction is calculated using the friction velocity,and the SH surface exhibited a drag reduction rate of about 11%when the friction Reynolds number is Re_x(28)630.(2)The migration process of the structure is traced.Firstly,the coherent structure is extracted and located by using the vortex identification method.In order to obtain the migration process of the structure on PH surface and SH surface,the structures are tracked by the correlation coefficient.The result of structure identification shows that combining the local velocity function with the traditional vortex identification method can effectively locate the structure.The calculation result of the correlation coefficient shows that the correlation coefficient can track the evolution of the flow field structure,and can display the structure which is similar to the target structure to a certain extent.The structure migration process on PH and SH surface shows that the structure on the SH surface has a faster migration speed and migrates farther in the same time.(3)The energy stratification of the turbulent boundary layer on two surfaces is investigated from the energy point using the local turbulent kinetic energy criterion and the modal decomposition method.The TNTI on both surfaces and the UMZs below the TNTI are calculated using the local turbulent kinetic energy criterion.The results show that the TNTI is located lower on the SH surface,but SH surface has a larger number of UMZs,and its energy distribution is more average.On both surfaces,the increase in the number of UMZs is synchronized with the decrease in the position of the TNTI,indicating that both statistical changes are related to the frictional drag of the flow field.The influence of SH surface on the turbulent boundary layer is mainly generated below 0.31δ,which means the frictional resistance in the near-wall region will be significantly affected.The POD calculation is performed on the positions below the TNTI of the 1800 sample flow fields,which shows that the SH surface will inhibit the large-scale structure and energy concentration,and at the same time inhibit the transfer of energy in the normal direction,and the inhibition of energy will affect the self-development process of the coherent structure.(4)The flow field is visualized using the FTLE method,taking into account the influence of the structure development.The original flow field is reconstructed using the modal decomposition method using 90%,95%and 98%energy to calculate FTLE,and the SH surface retained more details at the same energy ratio,small-scale structures are more abundant than that on PH surface.A coherent structure extraction algorithm based on the FTLE field is designed to obtain the probability distribution and the quantity distribution of FTLE structures at normal positions on both PH surface and SH surface.The results show that the number of small-scale structures identified on the SH surface increases,while the number of large-scale structures decreases,and their distribution changes significantly below the TNTI interface.The SH surface reduces the gradient of the average flow velocity of the viscous bottom layer and inhibits the transfer of energy in the normal direction,thus affecting the generation and development of large-scale structures,making the energy shift from large-scale structures to small-scale structures,and then reducing frictional resistance of the wall.