The dynamic mechanism of the vortex generation and evolution process in a fully developed turbulent boundary layer with Reθ=97-194 is experimentally investigated.In this study,a moving single-frame and long-exposure(...The dynamic mechanism of the vortex generation and evolution process in a fully developed turbulent boundary layer with Reθ=97-194 is experimentally investigated.In this study,a moving single-frame and long-exposure(MSFLE)imaging method and a moving particle image velocimetry/particle tracing velocimetry(M-PIV/PTV)are designed and implemented for measuring the temporal and spatial evolution of vortex cores in both qualitative and quantitative ways,respectively.On the other hand,the Liutex vector,which is a new mathematical definition and identification of the vortex core proposed by Liu’s group,is first applied in the experiment for the structural visualization and quantitative analysis of the local fluid rotation.The results show that an intuitional process of vortex evolution can be clearly observed by tracking the vortex using MSFLE and verify that the roll-up of the shear layer induced by shear instability is the origin of vortex formation in turbulence.Furthermore,a quantitative investigation in terms of the critical vortex core boundary(size)and its accurate rotation strength is carried out based on the Liutex vector field analysis by M-PIV/PTV.According to statistics of the relation between vortex core size and the rotation strength during the whole process,the physical mechanism of vortex generation and evolution in a turbulent boundary layer of low Reynolds number can be summarized as a four-dominant-state course consisting of the“synchronous linear segment(SL)-absolute enhancement segment(AE)-absolute diffusion segment(AD)-skewing dissipation segment(SD)”.展开更多
This work is motivated by previous experimental and numerical studies which reveal that the hairpin vortex could be formed by the interaction between spanwise adjacent low-speed streaks. To prove that such an interact...This work is motivated by previous experimental and numerical studies which reveal that the hairpin vortex could be formed by the interaction between spanwise adjacent low-speed streaks. To prove that such an interaction mechanism is still applicable in the normal direction, two sinuous low-speed streaks with the same streamwise phase are set to be in the upper half and bottom half of a small size channel, respectively, and their evolution and interaction are investigated by direct numerical simulation. A new kind of hairpin-like vortical structure, distributed in the normal direction and straddled across both halves of the channel, is found during the cross-interaction process of the low-speed streaks. The influence of such a normal-distributed hairpin-like vortex （NHV） on the turbulent statistical regularity is also revealed. It is observed that the NHV can lead to a sudden surge of wall skin friction, but the value of the normal velocity as well as the streamwise and spanwise vorticity sharply decrease to zero in the center of the channel.展开更多
The receptivity of plane Poiseuille flow to local single-period micro-vibration disturbances with different phases at the top and bottom walls was investigated through direct numerical simulation of three-dimensional ...The receptivity of plane Poiseuille flow to local single-period micro-vibration disturbances with different phases at the top and bottom walls was investigated through direct numerical simulation of three-dimensional incompressible Navier-Stokes equations. Results show that the disturbance presents a symmetrical distribution in the spanwise direction when the micro-vibration on the wall ends, and the initial disturbance velocities and spatial distribution of the disturbance structure are different at the top and bottom walls. The disturbance＇s velocity, amplitude, and high- and low-speed streaks increase with time, and the amplitude of streamwise disturbance velocity is larger than those of spanwise and vertical disturbance velocities. However, no significant Tollmien-Schlichting wave was found in the flow field. The number of disturbance vortex cores gradually increases with the disturbance area. High-speed disturbance fluid concentrates near the wall and its normal velocity largely points to the wall, while low-speed disturbance fluid largely deviates from the wall. Furthermore, the streamwise velocity profiles near the top and bottom walls both become plump because of the existence of the disturbances, and the streamwise velocity profiles show a trend of evolving into turbulent velocity profiles. The shear stress near the wall increases significantly. The local micro-vibration disturbance on the wall in plane Poiseuille flow can induce the development of a structure similar to turbulent spots.展开更多
基金supported by the National Natural Science Foundation of China(Grants Nos.51906154,51576130)the National Science and Technology Major Project(Grant No.2017-V-0016-0069).
文摘The dynamic mechanism of the vortex generation and evolution process in a fully developed turbulent boundary layer with Reθ=97-194 is experimentally investigated.In this study,a moving single-frame and long-exposure(MSFLE)imaging method and a moving particle image velocimetry/particle tracing velocimetry(M-PIV/PTV)are designed and implemented for measuring the temporal and spatial evolution of vortex cores in both qualitative and quantitative ways,respectively.On the other hand,the Liutex vector,which is a new mathematical definition and identification of the vortex core proposed by Liu’s group,is first applied in the experiment for the structural visualization and quantitative analysis of the local fluid rotation.The results show that an intuitional process of vortex evolution can be clearly observed by tracking the vortex using MSFLE and verify that the roll-up of the shear layer induced by shear instability is the origin of vortex formation in turbulence.Furthermore,a quantitative investigation in terms of the critical vortex core boundary(size)and its accurate rotation strength is carried out based on the Liutex vector field analysis by M-PIV/PTV.According to statistics of the relation between vortex core size and the rotation strength during the whole process,the physical mechanism of vortex generation and evolution in a turbulent boundary layer of low Reynolds number can be summarized as a four-dominant-state course consisting of the“synchronous linear segment(SL)-absolute enhancement segment(AE)-absolute diffusion segment(AD)-skewing dissipation segment(SD)”.
基金Project supported by the Natural Science Foundation of Zhejiang Province,China(Grant No.LQ18A020002)the National Natural Science Foundation of China(Grant No.41776105)the Foundation of the Educational Committee of Zhejiang Province,China(Grant No.Y201737053)
文摘This work is motivated by previous experimental and numerical studies which reveal that the hairpin vortex could be formed by the interaction between spanwise adjacent low-speed streaks. To prove that such an interaction mechanism is still applicable in the normal direction, two sinuous low-speed streaks with the same streamwise phase are set to be in the upper half and bottom half of a small size channel, respectively, and their evolution and interaction are investigated by direct numerical simulation. A new kind of hairpin-like vortical structure, distributed in the normal direction and straddled across both halves of the channel, is found during the cross-interaction process of the low-speed streaks. The influence of such a normal-distributed hairpin-like vortex （NHV） on the turbulent statistical regularity is also revealed. It is observed that the NHV can lead to a sudden surge of wall skin friction, but the value of the normal velocity as well as the streamwise and spanwise vorticity sharply decrease to zero in the center of the channel.
基金supported by the National Natural Science Foundation of China(Grant No.51179075)the Natural Science Foundation of Jiangsu Province(Grant No.BK20131256)
文摘The receptivity of plane Poiseuille flow to local single-period micro-vibration disturbances with different phases at the top and bottom walls was investigated through direct numerical simulation of three-dimensional incompressible Navier-Stokes equations. Results show that the disturbance presents a symmetrical distribution in the spanwise direction when the micro-vibration on the wall ends, and the initial disturbance velocities and spatial distribution of the disturbance structure are different at the top and bottom walls. The disturbance＇s velocity, amplitude, and high- and low-speed streaks increase with time, and the amplitude of streamwise disturbance velocity is larger than those of spanwise and vertical disturbance velocities. However, no significant Tollmien-Schlichting wave was found in the flow field. The number of disturbance vortex cores gradually increases with the disturbance area. High-speed disturbance fluid concentrates near the wall and its normal velocity largely points to the wall, while low-speed disturbance fluid largely deviates from the wall. Furthermore, the streamwise velocity profiles near the top and bottom walls both become plump because of the existence of the disturbances, and the streamwise velocity profiles show a trend of evolving into turbulent velocity profiles. The shear stress near the wall increases significantly. The local micro-vibration disturbance on the wall in plane Poiseuille flow can induce the development of a structure similar to turbulent spots.
