Borehole Flow, ANSYS Fluent CFD Simulation Training

Description

Project Description

The present problem simulates borehole water flow using ANSYS Fluent software. Borehole or drilling is making a hole or drilling a well on the ground, which is used to identify the type of soil in the area. Pre-construction drilling or borehole is used to determine the soil type and groundwater status and agriculture and oil and gas drilling operations. A borehole is a hole of 5cm to 25cm in diameter that is drilled into the ground, and the exact size of this diameter depends on the type of soil under study. A borehole is a small-diameter well that is important in soil tests.

These boreholes can be drilled vertically, horizontally, or sloping and have an essential role in the industry to sampling soil materials, checking surface layers, sending equipment and tools, studying the soil condition of the area, etc. Now the water flow inside the borehole or well causes the soil grains to separate from the well wall and enter the water flow. Of course, the degree of separation of soil grains from the well wall depends on the grain’s diameter and the intensity of soil grain adhesion. Since this project simulates a mixture of water flow and soil grains, a multiphase flow must be defined. Therefore, the Eulerian multiphase model is used to determine the liquid-solid two-phase flow.

Project Description

So that the primary phase is water flow, and secondary phase is soil grain. In general, from the eulerian multiphase model in applications such as particle-laden flows with a volume fraction of more than 10% for the dispersed phase, or fluidized bed, or slurry flows including liquid and solid, is used. The calculation area is designed as a section of a well in the form of a vertical cylinder. The inner part of the domain is related to the area of ​​water flow, and the outer part of the domain is related to the area of ​​soil. Water flow with a speed of 1.6 m.s-1 along with soil particles with a velocity of 1 m.s-1 enter the central part of the well and affect the surrounding soil particles.

Borehole Geometry & Mesh

The present model is designed in three dimensions using Design Modeler software. It is assumed that the well is a perfectly symmetrical cylinder. Therefore, to reduce the computational cost, only one segment of this well has been modeled. This sector is composed of two separate sections in the radial direction. The inner or central part of the sector is related to the fluid flow area, and the outer part of the sector is related to the location of soil grains. Inlet and outlet boundaries are defined only for the water flow area in the central part of the model.

We carry out the model’s meshing using ANSYS Meshing software, and the mesh type is structured. The element number is 413658. The following figure shows the mesh.

Borehole Flow CFD Simulation

We consider several assumptions to simulate the present model:

• We perform a pressure-based solver.
• The simulation is unsteady.
• The gravity effect on the fluid is equal to -9.81 m.s-2 along the Z-axis.

The following table represents a summary of the defining steps of the problem and its solution:

Models
Viscous		k-epsilon
	k-epsilon model	standard
	near wall treatment	standard wall functions
	turbulence multiphase model	dispersed
Multiphase Model		Eulerian
	formulation	implicit
	number of eulerian phases	2 (water & soil)
Boundary conditions
Inlet		Velocity Inlet
	velocity magnitude – water	1.6 m.s-1
	velocity magnitude – soil	1 m.s-1
Outlet		Outflow
	flow rate weighting	1
Wall of Soil Zone & Fluid Zone		Wall
	wall motion	stationary wall
Symmetry of Soil Zone & Fluid Zone		Symmetry
Methods
Pressure-Velocity Coupling		Phase Coupled SIMPLE
	Pressure	PRESTO
	momentum	first order upwind
	turbulent kinetic energy	first order upwind
	turbulent dissipation rate	first order upwind
	volume fraction	first order upwind
Initialization
Initialization methods		Standard
	gauge pressure	0 Pascal
	velocity (x,y,z) for water	0 m.s-1
	soil volume fraction	0
	velocity (x,y,z) for soil	0 m.s-1

Results & Discussions

At the end of the solution process, we obtain two-dimensional contours related to the volume fraction and the velocity of water flow and soil particles. The results show that some soil particles are separated from the wall of the drilled well and join the water flow, and also some water flow penetrates the soil of the drilled well. This indicates that the amount of shear stress created between the water flow and soil particles prevails over adhesion between the soil grains.