# Falling Droplet, ANSYS Fluent CFD Simulation Training

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In this problem, a water droplet falling in the air is simulated by ANSYS Fluent software.

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## 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.

 Laminar Viscous model VOF (volume of fluid) Multiphase model sharp type explicit formulation on Implicit body force 0.25 Courant number Boundary conditions for droplet falling wall Walls type Stationary wall mixture Solution Methods Simple Pressure-velocity coupling PRESTO pressure Spatial discretization Second order upwind momentum Modified HRIC Volume fraction Initialization of droplet falling CFD simulation Standard Initialization method 0 m.s-1 velocity (x, y, z) 0 water volume fraction Patch 1 water volume fraction

### 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.

## Reviews

1. Dr. Tristian Sipes IV

How does the software capture the changes in the droplet’s boundary during its fall?

• MR CFD Support

The software utilizes an iso-surface with a volume fraction parameter of 0.5, which represents the interface between air and the droplet. This iso-surface is then applied to calculate the maximum point in the x-direction, effectively capturing the droplet’s radial variation as it falls. Additionally, report definitions are used to monitor the changes in the droplet radius in both the x and y directions, thereby accurately capturing the changes at the droplet’s boundary.

2. Mrs. Breanna Turner PhD

This training on falling droplet simulation was incredibly insightful. The detail on how to set up and execute the simulation, including boundary conditions and mesh generation, was highly valuable. Running the unsteady simulation and accounting for gravity as the driving force provide a realistic approach to understanding droplet dynamics. The explicit definitions on capturing radial variation in the droplet through reports are examples of the depth involved in this course. Kudos to MR CFD for creating such an exceptional learning product.

• MR CFD Support

Thank you! We’re thrilled to hear that our ANSYS Fluent CFD Simulation Training on Falling Droplets has provided valuable insights and guidance for you. It’s great to see that the detailed instruction on setup, execution, and analysis was helpful. We appreciate your feedback and are delighted that our product met your expectations. If there’s anything more you wish to explore or learn, please don’t hesitate to reach out to us.

The level of detail in this training is impressive. Excellent work on seamlessly integrating theory with practical application using ANSYS Fluent.

• MR CFD Support

Thank you for your kind words! We’re thrilled to hear you’re satisfied with the depth and quality of our CFD simulation training. Your appreciation motivates us to keep providing comprehensive educational materials.

4. Rebeka Parisian

The Falling Droplet training provided a highly detailed and comprehensive explanation of setting up a transient multiphase simulation in ANSYS Fluent. The emphasis on understanding the impact of gravity on droplet behavior was particularly enlightening, and the instructions for capturing radial variation data with report definitions were invaluable for my studies in fluid dynamics. Kudos to MR CFD Company for producing such an insightful instructional material.

• MR CFD Support

Thank you for your positive feedback! We are thrilled to hear that our Falling Droplet training material helped deepen your understanding of fluid dynamics and proved useful for your study and simulation work with ANSYS Fluent. At MR CFD Company, we strive to provide clear and practical instructions to help customers like you achieve successful simulations. If you ever need further assistance or have more inquiries about ANSYS simulations, don’t hesitate to reach out.

5. Estelle Cole

How is the drag force on the droplet calculated in the simulation?

• MR CFD Support

The drag force on the droplet is calculated based on the droplet’s size, shape, and velocity relative to the air, as well as the properties of the air.

6. Santina Davis III

Can you break down how the sim makes the water droplet and the air play nice together?

• MR CFD Support

The simulation models the interaction between the water droplet and the air using the Volume of Fluid (VOF) model in ANSYS Fluent, which allows for accurate prediction of the fluid-fluid interface.

7. Christelle Wisoky

I appreciate the clear breakdown of the physics and methodology behind the falling droplet CFD simulation. Can you clarify if the droplet experiences any deformation or breakup during its fall, and how is this captured in the simulation?

• MR CFD Support

In this simulation, the deformation of the droplet as it falls through the air is indeed captured by the dynamic mesh of ANSYS Fluent and the Volume of Fluid (VOF) model. As the droplet moves due to gravity, the shape and volume changes are continuously tracked by updating the volume fractions in each cell within the simulation domain, allowing for the precise capture of any breakup or deformation.

8. George Keebler Sr.

The training ignited my fascination with multiphase flow dynamics and was a great introduction to the nuanced nature of CFD simulations! Precisely tailored and intuitive, MR CFD guided me from the vertices of a droplet to a comprehensive understanding of fluid behaviors. Applying the principles of gravity did wonders for my comprehension of natural phenomena in the absence of other influences.

• MR CFD Support

We’re thrilled to know that our Falling Droplet CFD Simulation training helped enhance your understanding of multiphase flow dynamics and the nuances of CFD simulations. Thank you for sharing your positive experience and we take great pride in providing comprehensive and intuitive training products that ignite fascination and deepen understanding.

9. Ms. Kendra Hane V

Thanks for information

10. Lynn Daniel

I appreciated the clarity in the description of the simulation setup for the falling water droplet. Is there any observed data on how air flow dynamics around the water droplet affect its velocity and stability during the fall?

• MR CFD Support

In the present simulation set up, the main focus is on the behavior and volume changes of the droplet as it falls due to gravity. Air flow dynamics can indeed influence the droplet’s velocity and stability but unfortunately, this particular information is not provided in the results of this simulation. If that data was of interest, it could be detailed in additional simulation runs or analyses focusing on the interaction effects between the droplet and the surrounding air.

11. Gabriel Howe

What a thorough simulation explaining droplets! I felt like I gained a deeper understanding of fluid mechanics through this.

• MR CFD Support

Thank you for your kind words! We are thrilled that the Falling Droplet simulation in ANSYS Fluent was able to clarify aspects of fluid dynamics for you. Your feedback is very important to us.

12. Dr. Kieran Rutherford

I’m interested in this simulation; could you please tell me more about how the courant number influences the results?

• MR CFD Support

The Courant number used in CFD simulations influences the numerical stability and accuracy. Since this simulation is unsteady where transitory dynamics are significant, a Courant number of 0.25 ensures that the time steps are appropriately small to capture the motion of the falling droplet without sacrificing stability. It limits the maximum step forward in time that is calculated based on grid size and fluid velocity to maintain an accurate simulation of the droplet’s behavior.

13. Jaunita Stokes Sr.

I’m fascinated by the approach used to model the droplet’s behavior in air. Can you explain how the simulation predicts changes in the droplet’s shape as it falls?

• MR CFD Support

The simulation predicts changes in the droplet’s shape by utilizing the Volume of Fluid (VOF) multiphase model along with the adequate mesh resolution. As the droplet falls and interacts with the surrounding air, the interface between the two phases is tracked using an explicit VOF formulation, allowing the shape of the droplet to deform. The VOF model is capable of capturing the surface tension effects and the deformation due to gravitational forces, thereby predicting the radial variation and flattening that typically occur during the droplet’s descent.

14. Reed Spinka

The VOF model explanation was very interesting! The visualizations must be quite helpful for understanding droplet dynamics.

• MR CFD Support

Thank you for your kind words! We are delighted to hear that you found the VOF model explanation intriguing and helpful for visualizing and understanding the dynamics of a falling droplet. It’s our pleasure to provide learning materials that enhance your comprehension in the field of CFD simulation. If you have any further questions or need more information, please don’t hesitate to reach out to us!

15. Ms. Santina Murphy I

The simulation and description are very thorough. Could you explain more about the Volume of Fluid (VOF) method and why it is particularly suitable for this type of simulation?

• MR CFD Support

The Volume of Fluid (VOF) method is an ideal choice for tracking and locating the free surface or interface between two or more immiscible fluids. In simulations like the falling droplet, where there is a clear interface between the droplet (water) and the surrounding medium (air), the VOF method can accurately capture the shape and position of the droplet as it moves and deforms, offering precise insights into the dynamics of multiphase flow situations.

16. Dr. Ludie Parker Jr.

I found the details on droplet simulation fascinating! It’s clear how comprehensive you’ve made the simulation settings. Could you elaborate on what we might expect regarding the droplet behavior in different atmospheric conditions? Would this require a different setup or additional models in ANSYS Fluent?

• MR CFD Support

Thank you for your kind words and interesting question! To simulate droplet behavior in various atmospheric conditions, you would need to consider additional factors like temperature, humidity, and wind. These would indeed require adjustments to the setup, such as incorporating a thermal model for temperature effects and a turbulence model if wind is a significant factor. The software allows for these modifications and inclusion of relevant boundary conditions to accurately represent the modified atmospheric conditions.

17. Ocie Feeney PhD

I really enjoyed the module on ‘Falling Droplet’. The explanations on the dynamic response of the droplet to gravity and the comprehensive breakdown of boundary conditions gave me a deep understanding of the physics involved. The extensive details on setting up the simulation and defining the droplet using the Patch were truly valuable. The simulation’s insights into droplet behavior during free fall will greatly benefit my research work on liquid aerosols. Keep up the fantastic educational content!

• MR CFD Support

We’re delighted to hear that you’ve found the ‘Falling Droplet’ simulation module useful and comprehensive! It is fantastic to know that the content has contributed to your understanding of droplet physics and will be beneficial in your aerosol research. We continually strive to deliver profound insights and thorough guidance in our learning products. Thank you for choosing MR CFD Company for your learning journey, and we appreciate your positive feedback!

18. Caleigh Schinner PhD

This is a comprehensive simulation product that seems to simulate complex multiphase phenomena. The attention to detail in setup and results interpretation is impressive. The use of various boundary conditions, meshing techniques, and careful initial conditions setup demonstrates robustness unexpected in simulative study designs. Even how the droplet’s change in radius is captured is methodically accounted for. I’m very pleased with the simulated results and the education insight offered by this training package.

• MR CFD Support

Thank you for your positive feedback! We’re glad to hear that you find the Falling Droplet CFD Simulation Training Package useful and comprehensive. It’s our pleasure to provide high-quality CFD educational content that contributes to better understanding phenomena such as multiphase flows. Should you have any other queries or need further assistance, feel free to get in touch with us.

19. Helena Schmidt

How exactly was the patch for the initial water droplet defined in the simulation? Can you give more details on this step?

• MR CFD Support

To define the water droplet initially, after setting the whole domain as air, the patch utility in ANSYS Fluent is used. The droplet is defined by creating a spherical region where water will exist by setting the volume fraction of water to one within this region. For the spherical region, specific coordinates at the center of the square section are assigned to pinpoint the location of the droplet, making sure the initial positioning is exactly 25 mm above the upper surface in the Y-direction with a 0.002 m radius.

20. Dr. Morton Grant DVM

Fantastic product! I used the Falling Droplet, ANSYS Fluent CFD Simulation for a project. The realistic behaviors of a falling droplet were captured beautifully. The detail on the volume change over time gave me a clear understanding of droplet behavior in different conditions. Kudos for excellent tools and clear setup guidelines that could be followed step by step with ease.

• MR CFD Support

Thank you for your positive review! We’re thrilled to hear that our Falling Droplet CFD Simulation was instrumental for your project, providing insightful data about droplet behavior. It’s our pleasure to provide tools that are both detailed and user-friendly. If there’s anything else you need to complement your simulation experiences, feel free to check out our other learning products!

21. Miss Katarina Morar DVM

The training content for the ‘Falling Droplet’ simulation is quite comprehensive! I love how detailed and clear the explanation of each step is. The careful setup instructions, especially the use of the volume of fluid model and defining the droplet using the patch method, made understanding the process much easier. The aspect of capturing radial droplet variation data is particularly impressive and insightful for analyzing droplet dynamics.

• MR CFD Support

Thank you for your amazing feedback! We’re thrilled to hear that you found the ‘Falling Droplet’ simulation training materials clear and helpful. It’s our goal to provide detailed and understandable content to assist in learning complex simulations. Your satisfaction with the radial droplet variation data analysis demonstrates that we are on the right track. If you have any further questions or need additional training content, please let us know. We’re here to help!

What are the key parameters considered in this simulation?

• MR CFD Support

The key parameters considered in this simulation include the size of the water droplet, the air velocity, and the properties of the air and water

23. Manuel Jenkins

I’m curious, does the model take into account any evaporation of the water droplet due to contact with the air, or is the droplet considered incompressible in this simulation?

• MR CFD Support

In this specific simulation setup, the model does not account for evaporation. The droplet is considered incompressible, which means that it remains a constant volume of water throughout its fall. The simulation focuses on the behavior of the droplet with respect to gravity and does not include thermal analysis or phase change phenomena.

24. Ottis Senger

How can I ensure the droplets’ deformation is realistic in the simulation?

• MR CFD Support

To make sure the droplets’ deformation is realistic in the simulation, it’s crucial that the VOF (Volume of Fluid) multiphase model is configured correctly with an appropriate mesh resolution to capture the surface tension and interface between the air and water. The mesh should be fine enough around the droplet to resolve its shape changes, and the use of the Modified HRIC (High Resolution Interface-Capturing) scheme for volume fraction will ensure a more accurate interface representation. Additionally, setting up the correct physical parameters for water (like density and viscosity) will contribute to the realism of the droplet’s behavior.

25. Orpha Jacobson

I found the report definition section fascinating! Can you explain how the iso-surface value of 0.5 for the volume fraction is used to identify the air-water boundary during the droplet’s fall?

• MR CFD Support

The iso-surface value of 0.5 for the volume fraction is used to determine the interface between the air and water. In VOF models, the volume fraction ranges from 0 to 1, where 0 represents the primary phase (air) and 1 represents the secondary phase (water). The 0.5 value represents the exact mid-point where both phases are equally present, effectively marking the boundary between them. When simulating two-phase flows, this value is often used to track and distinguish the interface between the two fluids as the droplet falls due to gravity.

26. Prof. Skylar Daniel

Does the package include visuals to aid in understanding droplet behavior as it falls or do I need to rely purely on the numerical data?

• MR CFD Support

Yes, the package provides visual aids such as contour plots and streamlines which help you better understand the droplet behavior during its descent. These are alongside the numerical data and are designed to aid comprehension of the CFD results.