Novel herringbone riblets for mitigating flow separation in a linear diffuser cascade

Herringbone riblets (or convergent-divergent riblets) can generate strong secondary flows with small geometric dimensions, while their effects on the boundary layer vary along the spanwise direction, making it possible for flow control. In the work presented of this paper, novel herringbone riblets are applied to the suction blade surfaces of a linear diffuser cascade and their effectiveness in mitigating flow separation losses is evaluated numerically at a Reynolds number of 150,000 for a blade incidence angle of α=0°, 2° and 4°, respectively. The uRANS results shown that this type of herringbone riblets can decrease losses near the suction surface by up to 14.5%, at α=0° with a riblets height of 0.1 blade boundary layer thickness, while the conventional flow control methods of the same scale typically achieve an efficiency improvement of less than 10% and are only effective within a limited angle-of-attack range. These herringbone riblets induce a spanwise motion within the boundary layer from the diverging line (DL) to the converging line (CL) of the riblets, altering both the velocity magnitude and spanwise vorticity distribution. Additionally, the riblets can advance the laminar to turbulent transition near the CL and slightly delay it near the DL thereby modifying the size and location of the laminar separation bubble and the trailing edge separation vortex. Although riblets can significantly suppress flow separation, since the secondary flow caused by taller riblets results in higher parasitic drag a suitable riblet height should be chosen carefully to achieve a net reduction in pressure losses.