A numerical simulation is performed to find out a key vortical structure in the laminar-turbulent transition. A low-speed streak is generated inside a laminar boundary layer using an isolated cuboid roughness, aimed a...A numerical simulation is performed to find out a key vortical structure in the laminar-turbulent transition. A low-speed streak is generated inside a laminar boundary layer using an isolated cuboid roughness, aimed at providing an environment unstable to outer disturbances. Then, a short duration jet is issued into the boundary layer. When the jet velocity is low, some vortices appear in the boundary layer, but the transition of the boundary layer does not take place.However, when the jet velocity exceeds a certain threshold, two vortices newly appear above the elongated legs of a V-shaped vortex and only one of them is stretched and survives. After that,vortices are generated one after another around the survived one. By comparing the decayed and the survived vortices, it is found that the difference in their heights is the key characteristic which leads to the transition.展开更多
The micro-characteristics of the streaky marks and the corresponding substrate surface of galvannealed （GA） steel sheets have been investigated using SEM and FIB.The streaky mark area seems to have many cavities,sim...The micro-characteristics of the streaky marks and the corresponding substrate surface of galvannealed （GA） steel sheets have been investigated using SEM and FIB.The streaky mark area seems to have many cavities,similar to craters,which scatter light and make the surface appear darker when compared with the surrounding normal area.Moreover,it is found that the irregularity or difference in steel grain size tends to affect the surface quality of the GA steel sheets.During alloying,the diffusion speed of Zn-Fe on the grain boundary is faster than that at the center of the grain,which results in the selective formation of crater-like morphology in the coating area with larger steel grains.Due to the clear grain boundary of IF steel,apparent streaky marks will be formed on the coating,if the IF steel surface consists of grains of irregular size.展开更多
文摘A numerical simulation is performed to find out a key vortical structure in the laminar-turbulent transition. A low-speed streak is generated inside a laminar boundary layer using an isolated cuboid roughness, aimed at providing an environment unstable to outer disturbances. Then, a short duration jet is issued into the boundary layer. When the jet velocity is low, some vortices appear in the boundary layer, but the transition of the boundary layer does not take place.However, when the jet velocity exceeds a certain threshold, two vortices newly appear above the elongated legs of a V-shaped vortex and only one of them is stretched and survives. After that,vortices are generated one after another around the survived one. By comparing the decayed and the survived vortices, it is found that the difference in their heights is the key characteristic which leads to the transition.
文摘The micro-characteristics of the streaky marks and the corresponding substrate surface of galvannealed （GA） steel sheets have been investigated using SEM and FIB.The streaky mark area seems to have many cavities,similar to craters,which scatter light and make the surface appear darker when compared with the surrounding normal area.Moreover,it is found that the irregularity or difference in steel grain size tends to affect the surface quality of the GA steel sheets.During alloying,the diffusion speed of Zn-Fe on the grain boundary is faster than that at the center of the grain,which results in the selective formation of crater-like morphology in the coating area with larger steel grains.Due to the clear grain boundary of IF steel,apparent streaky marks will be formed on the coating,if the IF steel surface consists of grains of irregular size.
