Enhancing diffuser performance using transverse grooves to delay flow separation

A flow-control method is applied to enhance the efficiency and flow homogeneity of three-dimensional diffusers used in open-jet wind tunnels. Suitably shaped grooves are introduced in the diffuser diverging walls. The grooves promote the relaxation of the non-slip condition along the streamlines bounding the small recirculation regions forming passively inside the grooves. That reduces momentum losses and results in a downstream boundary layer with higher momentum, which is more separation-resistant. The proposed flow-control device has been successfully validated for plane diffusers [Mariotti et al., "Separation control and efficiency improvement in a 2D diffuser by means of contoured cavities," Eur. J. Mech.-B 41, 138-149 (2013); and Mariotti et al., "Control of the turbulent flow in a plane diffuser through optimized contoured cavities," Eur. J. Mech.-B 48, 254-265 (2014)]. In this study, we examined circular and square-section diffusers with different degrees of flow separation. Given that the investigated diffusers are part of open-jet wind tunnels, the entire wind tunnel geometry was included in the numerical simulation. The grooves significantly enhanced performance in circular diffusers by reducing the extent of separation and promoting an axisymmetric and spatially uniform flow. However, negligible benefits were observed for square-section diffusers. In these cases, since flow separation originates from one of the four inclined edges of the diffuser, placing grooves along the diverging walls does not effectively reduce the separation extent. Nonetheless, the grooves become effective again in diffusers with rectangular cross sections of high aspect ratio.