## 1 Introduction
This can alter the distribution pattern of vortex strips in the draft tube, narrow the special pressure oscillation zone, and significantly reduce the dominant frequency of pressure oscillation; however, it increases the amplitude. It can also improve the internal flow field from the runner to the draft tube, prevent water flows from colliding with each other, and reduce hydraulic losses and pressure pulsation [16; 17]. Properly increasing the length of the runner cone in the axial direction can, to some extent, reduce the amplitude of pressure pulsation vibration in the draft tube and reduce the volume and eccentricity of the vortex strip in the draft tube. A too-short runner cone will cause serious mutual interference in water flow, while a too-long runner cone will generate significant lateral interference force. Extending the runner cone will reduce the pressure fluctuation amplitude in the draft tube, but the length and thickness of the vortex strip will increase [18]. Drilling on the runner cone can reduce the volume of the vortex strip, decrease the rotation intensity and the attachment of the vortex strip, and weaken or eliminate the pressure fluctuations caused by the vortex strip. However, drilling performed on the runner cone enhances the pressure fluctuations' peak value in the draft tube, while slotting on the runner cone decreases the amplitude of pressure fluctuation in the draft tube [19; 20]. Changing the draft tube structure is mainly achieved by changing the profile and adding grooves [21; 22; 23]. Changing the diffusion section of the draft tube to an inclined cone can eliminate vortex strips, reduce flow instability, disrupt the development of strong swirls, and reduce related pressure fluctuations [24; 25]. Modifying the profile of the elbow section of the draft tube can mitigate the equivalent amplitude associated with the root mean square of the plunging power spectrum that propagates along the hydraulic passage up to 25%, diminishing the equivalent amplitude on the narrow band around the fundamental frequency of the plunging component by 40-50% [26]. Efficiency is highest when the elbow profile is a hyperbolic logarithmic spiral, and the loss coefficient is lowest when it is a logarithmic spiral [27]. Adding J-grooves can reduce the turbulent dynamics level of the draft tube core, decrease vortices and turbulence, and reduce the pressure pulsation and vibration level caused by vortex shedding [28; 29]. In summary, although many researchers have conducted sufficient research on draft tube flow after changing the downstream flow structure of the runner, most of them have mainly focused on the simplified inherent hydraulic components of the draft tube and their structural changes in the draft tube on the flow state, pressure fluctuation law, and generation mechanism in the draft tube. After simplifying the draft tube, bolts, nuts, and other components were ignored [30; 31; 32]. However, the water turbine in this study belongs to small-size structures, and the influence of the nut downstream of the runner on the flow pattern of the draft tube cannot be ignored. Therefore, this study considered the influence of ignoring nuts and different nut structures on the flow pattern of the draft tube in order to obtain the influence of the locking nut on the flow pattern of the draft tube. In the future design of the turbine, the recommended parameters can be given according to different performance requirements. This paper has important theoretical significance and engineering application value. The research process of this paper is organized as follows. In Section 2, the numerical simulation scheme is selected. In Section 3, the hydraulic performance experiment is carried out, and the unsteady flow characteristics of the draft tube of the three runner downstream structures are discussed. In Section 4, the conclusions are given.