## 3 CFD Model of the Large Diameter Modular Drill Bit
### Fluid Domain and Meshing  
Due to unpredictable formation conditions, time-varying stress state of drilling tools, irregular borehole bottom, and wear of drill bit, simplification was introduced when the drill bit simulation model was built. Firstly, since the linear velocity of a drill bit is much lower than the flow velocities of the drilling fluid, drill bit rotation was not considered when constructing the model to study flow field characteristics [21; 22]. Secondly, in order to decrease the calculation time and enhance the convergence rate, the fillets, chamfers, and other structures that did not affect the calculation results, were ignored. Meanwhile, for the convenience of establishing the fluid domain, the isolation casing, torque ring, the rock at the drill bit bottom, and the external bit, were combined into one part.  
A physical model of a modular drill bit was built using CAD software (Inventor education version) and imported into COMSOL software, and the fluid domain of the modular bit was created using the geometric component construction module by Boolean operation, as shown in Figure 2. The fluid domain of the coring bit includes the clearance between external bit and internal bit, the radial holes on the isolating casing, the chamber between the external bit and the torque ring, the nozzles of the external bit, the water gaps on the external bit, the chamber between the isolating casing and internal bit, the passageways between the torque ring and the passageways between the torque ring and the internal bit, the water gap on the internal bit, and the annulus clearance between the external bit and borehole wall. The fluid domain of the non-coring bit includes the clearance between the external bit and the internal bit, the radial holes of the isolating casing, the chamber between the external bit and the torque ring, the nozzles on the external bit, the water gaps on the external bit, the chamber between the isolating casing and the internal bit, the passageways between the torque ring and the internal bit, the water gaps on the internal bit, the central channel of the internal non-coring bit, and the annulus between the external bit and borehole wall.  
Because the waterways on this modular drill bit are very complex and narrow, dividing high quality meshes is quite difficult and demanding, and reasonable mesh density is an important prerequisite for improving the accuracy of flow field calculation [23; 24; 25]. For the drill bit simulation model, unstructured tetrahedral meshes were introduced to speed up calculation and increase precision of the results. To ensure the accuracy of the simulation, this study includes an analysis of mesh independence. For example, for the coring drill bit, when the inlet flow rate was set to 300 L/min and the clearance between the external bit and the internal bit was 9 mm, the flow field model was simulated using three different mesh divisions, as shown in Table 1. Finer meshes resulted in longer computation times but provided more accurate results. Therefore, within the limits of hardware capabilities, a refined mesh was used for the calculations to ensure the accuracy of the model.