A vortex generator device (70) for mounting on a wind turbine blade (10) is disclosed. The device comprises: a base (71) having, when mounted on an exterior of the wind turbine blade (10), an inner side (72) for attaching on a surface, such as the exterior of the wind turbine blade (10), and an outer side (73) facing away from the exterior of the wind turbine blade (10). The device is provided with a vane vortex generator pair comprising a first vane (79) and a second vane (80) protruding from the outer side (73) of the base (71). The inner side (72) of the base (71) is provided with a recess (74) or undercut for obtaining an adhesive (81).