Falling Droplet Description

In this problem, a water droplet falling in the air is simulated by ANSYS Fluent software. Therefore, the two-phase flow model is used to simulate the initial air phase and the secondary water phase. This simulation only includes fluid analysis and does not discuss thermal analysis. The purpose of the simulation is to investigate the drop behavior during a downward slope and the extent of its volume changes. No external factor as a boundary condition affects the droplet, and the downward movement is based solely on the force of gravity. The time taken to process the downward movement of water droplet within the air space is assumed to be 0.26 seconds.

CFD Simulation Assumptions

Problem-solving is based on a pressure-based perspective.

The simulation is unsteady (transient) because the problem deals with the downward droplet over time.

The effect of the Earth's gravity on the model is considered because gravity is the sole cause of the droplet falling.

Geometry and Mesh

The 3-D geometry of the present model is designed by Design Modeler software. Specific air zone is defined as a cube with a square cross-section of 1 cm and a height of 30 cm.

The meshing of the present model is performed by ANSYS Meshing software. The mesh is structure using face meshing and the element number is 1086822.

Water Droplet Falling CFD Simulation Set-Up

A summary of the problem definition and problem solving steps are presented in the table.

How to define a droplet using Patch

To define the existence of a drop and move it along the Y-direction in the air, we must define the shape of the drop in the appropriate coordinates in the air using the adapt region and select the sphere, and then mark it so that the software Identify the sphere that contains our definition sphere within the main geometry of the model. After initializing the model with a zero volume fraction for water (ie, there is only air in the model), using the patch option, the volume fraction for the water phase in the defined spherical region is equivalent to one. This means that there is air within the whole model space and there is only a defined spherical area of the water droplet. The initial droplet formation is in the middle of a square section at a distance of 5 mm in the x and z directions and 25 mm above the upper surface in the Y-direction and the defined radius for the drop is 0.002 m.

Capture radial droplet variation data using report definition

To investigate the magnitude of the drop radius changes (in the x and y directions) during the downward drop in the air we proceed as follows: In the X-direction, the central point of the drop is always fixed. The center of drop is (at x = 0.005 mm). We now need to obtain the change in the air and water droplet boundary, which is the maximum point in the x-direction. To do this, we first create an iso-surface for the drop surface, so that we use the volume fraction parameter of 0.5, which is the air and droplet interface. Then, using the Definition option and the Surface report mode, we select the Facet maximum command for the mesh and x-coordinate at the Iso-surface defined lateral surface, meaning that at the lateral Drop surface to obtain points that are in the maximum X-direction. Thus, by finding the value of the drop radius and the center point of the drop, we achieve the length of the drop radius changes in X-direction. To check the radius change in the direction of Y we also do the following: To study the radius change at the boundaries we do the same as before with the facet maximum command, while the center point coordinate in the direction of Y is opposite to X-direction. The direction of x increase as the drop in the direction of y decreases. We have to select the Area-weighted average in the mesh and Y-coordinate at the previously defined lateral level using the Define option and the Surface report mode, since moving down, the weighting effect is effective in averaging.

Explicit Formulation

If we don't use this method, it may result in a not reasonable drag in drop when moving downward in the air.

Implicit Body Force

Since in the present model, the effect of gravity is defined as the main cause of the downward motion of the drop, the effect of the volume forces in the model must be applied because in this case, the effect of the pressure gradient terms and the volume forces on the momentum equation is significant compared to the viscosity and transport terms.