Effect of dimensionless roughness on cascade flow separation

Abstract

This study examines the impact of dimensionless roughness on flow separation structures and flow characteristics through a combination of experimental and numerical simulation methods. The findings indicate that a specific dimensionless roughness k+ exists for the fixed airfoil that minimizes cascade loss. Furthermore, increasing surface dimensionless roughness can alter the flow separation structure and effectively prevent flow separation. The numerical simulation results indicate that as dimensionless roughness increases, the impact of open separation and separation bubbles on the flow state over the blade surface is reduced or eliminated. In cases where separation occurs in the cascade, increasing dimensionless roughness results in a reduction in the boundary layer thickness and the width of the wake jet, thereby leading to a smoother outlet angle and improved efficiency. Conversely, when there is no separation, increasing dimensionless roughness leads to the accumulation of low-energy fluid at the trailing edge of the cascade. This results in an increase in the boundary layer thickness and the width of the wake loss, a larger outlet angle of the blade, and a decrease in the efficiency of the cascade. By analyzing the influence of dimensionless roughness on flow separation in cascades, engineers can quickly identify the geometric roughness height to suppress flow separation.