Home / Examples / Fluid Analysis [Bernoulli] / Example 15: Droplet Formation Analysis

Example 15: Droplet Formation Analysis

General

The change of droplet from a rectangle to a circle by surface tension is solved with VOF method.
Volume fractions of water and air each are solved.
Unless specified in the list below, the default conditions are applied.
The 3D model is also available. The change from cube to sphere is solved.

Results will vary depending on Femtet version and the PC environment.

Analysis Space

Item	Settings
Analysis Space	2D
Model Unit	mm

Analysis Conditions

Item

Tab

Settings

Solver

Fluid Analysis [Bernoulli]

Analysis Type

Fluid Analysis

Transient Analysis

Multiphase Flow Setting

Fluid Analysis
Multiphase Flow Setting

Execute free surface analysis (VOF method): Select

Phase Setting: Register [ 000_Air] and [100_Water].

Take into Account Surface Tension: Select

Phase Pair Setting:

Phase 1	Phase 2	Surface Tension	Contact Angle
000_Air	100_Water	0.07	90

Setup Details

Fluid Analysis

Setup Details

Control Volume Type: Cell-centered Base

Timestep

Transient Analysis

Setting Item	Settings
Timestep	Specified by 1 [ms]
Calculation Steps	100
Output Interval	10

Meshing Setup

Mesh

Setting Item	Settings
General Mesh Size	Specified by 1 [mm]
Element Type	Rectangle

Model

Body Attributes and Materials Setting

Body Number/Type	Body Attribute Name	Material Name
0/Face	Water	100_Water *
1/Face	Air	000_Air *

* Available from the material DB

Results

The volume fraction contours of phase 2 at 0 [s], 0.05 [s], and 0.1 [s] are shown below.

The part of phase 2 (water) is illustrated in red.

It is observed that the rectangle will change to a circle over time.

Time: 0[s]
Time: 0.05[s]
Time: 0.1[s]

The contour of the static pressure at 0.1 [s] and the graph of the static pressure distribution that has the x range of -20 to 20 and the z fixed at 0 are shown below.

The pressure at the center of the droplet is 13.9 [Pa].

The difference in pressure across the boundary between gas and liquid is theoretically calculated with the Young-Laplace equation (Difference in Pressure = Surface Tension Coefficient / Curvature Radius).

Volume of Initial Rectangle: S = 10 x 10 = 100 [mm2]

Radius of Circle: r = ( S / π )^2 = 5.64 [mm]

ΔP = 0.07 / 5.64 x 10^-3 = 12.4 [Pa]

This is close to the theoretical value and confirms that the analysis is correctly performed.