Stability Analysis Of Wake Generation Mechanisms And Its Applications In Turbine Flow

Turbines play an important role in modern aero engines,its efficiency has important influence on the efficiency of the whole engine.High pressure turbines are located behind the burning room and work at a very severe environment.Turbines are always designed with high inlet gas temperature for power output and efficiency consideration,thus it is necessary to adopt a thick trailing edge for heat transfer and lifetime reasons.Vortex shedding from a blunt trailing edge is known as Von Kármán vortexs treet and often increase blade profile loss and lead to unsteady forces on turbine blade.To have a better understanding of turbine performance,it is important to understand trailing vortex generation mechanics.The onset of vortex shedding could be explained from flow stability theory.In this article,we would try to study key factors that have major impact on the vortex shedding behind blunt body by using stability analysis theory.First of all,we introduced a family of simplified velocity profiles to represent real turbine wake to study the influences of asymmetry effects on the stability of wakes.The temporal stability analysis shows that the sinuous mode is the most unstable mode,and the maximum growth rate of the sinuous mode is dominated by the shear layer thickness of the thinner side and the maximum growth rate of the varicose mode is dominated by the thicker side.The spatio-temporal analysis shows that absolute frequency is mainly determined by the total shear thickness,and increasing the degree of asymmetry generally has a stabilizing effect.Then we combined local stability analysis and global stability analysis to assess the effect of Reynolds number,shear layer thickness and trailing edge shapes on the free/-confined wakes.The results show that vortex shedding frequency has slight dependence on the Reynolds number and trailing edge shapes.The effect of increasing shear layer thickness on wake stability is not always stabilizing.Increasing boundary layer thickness acts in two ways to modify the global stability characteristics: it increasesthe length of the absolute unstable region and it makes the flow less locally absolutely unstable in the near wake region,these two effects work against each other to destabilize or stabilize the ow.Similarly,increasing the degree of confinement also acts in multiple ways to affect global stability.When the wake is not strongly confined,increasing confinement would increase the length of absolutely unstable region and make the wake more locally absolutely unstable,thus the flow would become more unstable.However,if the degree of confinement is increased beyond a critical point,the flow would be greatly stabilized due due to the decrease of absolute growth rate in the near wake region.We found that the confinement that maximize the flow instability is independent of Reynolds number of shear layer thickness.We also studied the effect of compressibility on the wake instability.The local stability analyses of compressible wakes behind flat plates at both low and high Reynolds number are performed.It is found that compressibility act in a complex way to modify the instability characteristics.In the subsonic region,increasing Mach number would lengthen the recirculation zone and reduce the absolute growth rate.These two mechanisms work against each other,which explains why the linear global growth rate varies slightly as Mach number changes in the subsonic region.Further increasing Mach number to 1would greatly reduce the length of the absolute unstable region due to the occurrence of ex-pansion waves around the trailing edge corner.As a result,the wake is strongly stabilized and the shedding frequency is greatly modified.In the last chapter of this article,both stability analysis method and delayed detached eddy simulations are performed to study wake flows of a turbine blade at a high subsonic exit Mach number.It is found that a slight change in the trailing suction profile would have a big influence on the formation of wake vortex street and thus would greatly influence the wake mixing losses.local spatial-temporal stability analyses of the wake velocity profiles suggest that the flow is stabilized due to shorter absolutely unstable region and smaller absolute growth rate.It proves that flow stability analysis could be used as flow diagnose tools and could provide us intuition on how to control vortex shedding strength.