To investigate the complex flow and vortical structure of a centrifugal pump with low specific speed, the numerical simulation approach DDES is used to capture the unsteady flow fields when the blade periodically sweeps the volute tongue. Attentions are attracted on unsteady vortical structures on the midspan of the model pump, and quantitative data of z-vorticity(axial component of vorticity) at the impeller exit are extracted to discuss the corresponding evolution process. Besides, three dimensional vortices within the impeller are also identified at different working conditions. Results show that at the nominal and high working conditions, evident vorticity sheets are generated within the pump. High vorticity sheets are developed on the blade suction side and the trailing edge, which are also significant in the volute area caused by the vortex shedding from the impeller. The interactions between the shed wake flow and the tongue are not identical at various working conditions, and it is more evident at high working condition. With the blade moving away from the tongue, high vorticity is generated around the blade suction side. Especially at high working condition, several vorticity sheets with alternative negative and positive values are formed near the blade suction side. The vorticity sheet tends to stretch towards the blade pressure side with the impeller passing the tongue. Based on the 3D vortical structures, it is found that the hairpin vortices are generated within the blade channels, which could not be captured at low working condition. Finally, the obtained vorticity sheet and 3D vortex results will contribute to the understanding of the complex flow structures in pumps. (C) 2020 Elsevier Ltd. All rights reserved.