Radial fluid machines with conventional impeller often lead to flow separation in the blade passage in case of high flow deflection. Tandem blades can be used to counteract this problem. Due to their properties, a new thin boundary layer forms at the rear blade profile, which benefits the deflection of the flow. The present investigation focuses on a comparison between numerical and experimental streamline patterns on the impellers blades suction side. For this purpose, a conventional impeller is compared with a tandem impeller. Furthermore, a method for creating oil painting patterns in a rotating system is presented. The numerical investigations were performed using a full model (including inlet section, impeller, volute, outlet section and impeller side gaps). A total of four operating points were examined in the range from high partial load to overload. The same operating points were run in a pump loop during the experiment. The results of the present work show that the numerically and experimentally generated streamline patterns agree well with exceptions. It could be shown that a flow separation of the blades suction side can be prevented using tandem blades.