## 5 Conclusions  
Drilling is one of the most commonly used techniques in hydrogeological exploration and is employed to obtain rock samples and build boreholes. During the drilling process, depending on the specific drilling purpose, it is sometimes necessary to switch between coring drilling and non-coring drilling. To switch between these two drilling methods without lifting the whole set of drilling tools, a modular drill bit is proposed, designed according to the structure of wireline coring tools. The drill bit includes an external bit, an internal coring bit, and an internal non-coring bit. When switching from coring drilling to non-coring drilling, it is necessary just to pull out the inner tube assembly and change the internal coring bit to the internal non-coring bit, without lifting all the drilling tools from the borehole. Flow field characteristics of the modular bit were studied, with factors of inlet mass flow rate, clearance between external bit and internal bit for coring drilling, and layout of the bottom nozzles of the non-coring bit considered.  
Figure 10: Velocity and pressure with inlet mass flow rate increasing for the large diameter modular drill bit: (**a**) velocity and pressure of the coring bit; (**b**) velocity and pressure of the non-coring bit.  
According to the results, the clearance between the external bit and the internal coring bit has little effect on drilling fluid flow velocity when it flows upward and out of the modular drill bit, while pressure at the bit inlet decreases as the clearance increases. When the outside diameter of the modular bit is 216 mm, the recommended clearance value is 9 mm or 10 mm to obtain lower pressure loss and larger diameter cores. From the analysis of nozzle layout on the non-coring bit, it can be found that velocity of drilling fluid at the outlet of the simulation model remains largely the same, while pressure at the inlet of the simulation model decreases when the number of the nozzles increases. To generate low pressure loss and ensure bit strength, the layout of four nozzles on the internal non-coring bit is recommended for the drill bit with an outside diameter of 216 mm. Based on the result of the inlet flow rate analysis, velocity at the bit outlet increases linearly and the pressure at the bit inlet increases parabolically, as the flow rate increases. With the same input flow rate, similar velocities and lower pressure loss can be obtained in non-coring drilling compared with the using the coring bit, which is suitable for drilling conditions when more cuttings are produced in non-coring drilling than in coring drilling for a borehole drilled with the same diameter.  
Conceptualization, Y.S. and C.L.; methodology, Y.S.; software, C.L.; validation, Y.S.; formal analysis, C.L.; investigation, C.L.; resources, Y.S.; data curation, C.L.; writing--original draft preparation, C.L. and Y.S.; writing--review and editing, Y.S. and C.L.; supervision, Y.S.; project administration, Y.S. All authors have read and agreed to the published version of the manuscript.  
The research was funded by Anhui Provincial Natural Science Foundation (grant number: 2108085 QE210); Natural Science Research Project of Anhui Educational Committee (grant number: KJ2019A0102); Academician Workstation in Anhui Province, Anhui University of Science and Technology (grant number: 2022-AWAP-05); Anhui Provincial Major Science and Technology Project (grant number: 202203a07020009); and the Scientific Research Foundation for High-Level Talents of Anhui University of Science and Technology.  
Not applicable.  
The raw data supporting the conclusions of this article will be made available by the authors on request.  
The authors declare no conflicts of interest.