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Falling Objects into Water CFD Simulation by Dynamic Mesh

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In this tutorial, the fluid flow around two cubes falling into the water was simulated by ANSYS Fluent software.

This product includes a Mesh file and a comprehensive Training Movie.

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Description

Project description

The fluid flow around two cubes falling into the fluid was simulated by ANSYS Fluent software, in this tutorial. In general, it is essential to study the motion of objects in liquids. The fall of the cubes due to the acceleration of gravity helps a lot to understand the phenomenon of sloshing. Sloshing occurs when a partially filled reservoir with fluid is subjected to permanent or transient external forces. The liquid’s free surface moves and hits the tank walls and exchanges forces with its wall. These forces may cause problems such as malfunctions of spacecraft. Therefore, it is essential to study the effect of the sloshing phenomenon on different objects in different fluids.

Falling Objects Geometry and Mesh

The two-dimensional geometry created by ANSYS Design Modeler software can be seen in following Figure. Two cubes are placed in a square space and are supposed to fall into the fluid during gravity acceleration. A completely unstructured computational grid is produced by ANSYS Meshing software, and the total of its elements is 8727.

falling objectsfalling objects

Numerical Setup for Falling Objects CFD Simulation:

In this simulation, the following assumptions are considered:

  1. The solver type is assumed Pressure Based.
  2. Time formulation is assumed unsteady.
  3. Gravity effects are considered in the Y direction equal to –9.81 m/s2.

Complete information on boundary conditions is presented in Table 1 and the following Figure.

Table 1. Table of Boundary conditions

Inlet 29 m/s * Normal to Boundary

Turbulence intensity: 1%

Hydraulic diameter(m): 1

Outlet Pressure outlet
Tank Stationary wall
Box 1 & Box 2 Stationary wall
Tank outlet Symmetry

falling objects

We use the Dynamic Mesh with the smoothing and remeshing method to simulate the cubes’ motion.  Also, we apply a UDF in the dynamic mesh options. We apply the Coupled algorithm in the pressure-velocity coupling, and also we choose k-epsilon standard model with standard near-wall treatment as a turbulence model. In this analysis, the two-phase VoF model investigates the two-phase flow of air and water when the cubes fall. Other settings such as the discretization method and initial conditions are listed in the table below:

Multiphase( Volume of fluid)
No. of Eulerian phases 2
Formulation Implicit
Body force formulation Implicit body force
discretization
Gradient Least square cell-based
Pressure Body force weighted
Density First-order upwind
Momentum First-order upwind
Volume fraction Compressive
Modified turbulent visc. First-order upwind
Initialization method (Falling Objects)
Method Standard
Gauge pressure 0
X-Velocity 0
Y-Velocity 0
Z-Velocity 0
k 1
Epsilon 1
Air volume fraction 1

Results & Discussions

Although the tutorial video thoroughly explains how to extract the results, In this section, as in the last section, we present the contours of Pressure, kinetic energy, velocity, vectors, and streamlines.

As can be seen from the contours, due to the cubes’ movement and the sloshing phenomenon, the Pressure along the cubes’ movement length has increased. The Pressure in the sharp corner of the tank and the cubes has its maximum value and has a minimum value in the cubes’ vertical lengths (due to the separation of the flow due to the cubes’ movement in the fluid). The energy kinetics contour also shows that the average kinetic energy due to vortices (vortices formed due to cubes’ movement in the fluid) around the cubes is higher than in other areas. The cubes’ movement has led to more turbulence in the flow and the formation of vortices and separation in areas of the flow.  We can easily see the separation areas in streamlines and vectors.

There are a Mesh file and a comprehensive Training Movie that presents how to solve the problem and extract all desired results.

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