Ultrasonic properties of microbbulble-based contrast agents: Backscatter, attenuation and velocity

Abstract

In this paper, the tip leakage vortex was studied in a subsonic linear turbine cascade that was modeled from a typical high-pressure turbine stage. Due to a large streamwise adverse pressure gradient induced by the strong tip leakage vortex, vortex breakdown was observed and further examined. Vortex breakdown was accompanied by a sharp increase in swirl number and high mixing loss. In the vortex-vortex interaction simulations, a Rankine vortex flow was simulated numerically according to the complex turbine rotor inlet conditions. Ten quasi-steady, equally spaced phases between the incoming vortex and the blade in one pitch period were investigated for flow patterns and loss details. At a certain phase, the tip leakage vortex breakdown was significantly suppressed due to the vortex interactions. The upstream positive vortex was gradually entrained into the leakage flow. By improving the turbulent mixing between the leakage vortex and the incoming vortex, the streamwise momentum within the leakage core was enhanced. The vortex interactions weakened the adverse pressure gradient by eliminating the streamwise vorticity of the leakage vortex. By examining the vorticity transport equation, it's found that the viscous diffusion of vorticity dominated the leakage vorticity damping. The dilation effect also contributed to vorticity transportation as it operated in the subsonic, high-temperature, high-pressure turbine environment. It was surprising that the baroclinicity effect caused by the transportation of low temperature fluid was found to take part in the vorticity evolution in the current subsonic cascade. The associated loss was significantly reduced when vortex breakdown was suppressed.Different incoming vortex strengths were also tested. As the positive vorticity magnitude increased, the incoming flow tended to amplify the vortex interaction effect. When the vorticity magnitude was beyond a certain threshold, the vortex breakdown was no more observed. However, when the negative vortex was imposed upstream of the blade, the incoming vortex flow was transported radially downward and hardly interacted with the tip leakage flow. The leakage vortex breakdown was almost the same as that in the uniform inlet case. The vortex transportation was clearly captured with the help of passive scalar released from the incoming vortex core. By simulating a triple-vortex interaction, we developed a better understanding of vortex kinematic evolution.