Modeling Water Flow in Underground Tunnels Using Open Channel Flow Model in Ansys Fluent
Hello everyone, this is Mohsen Seraj from Ozen Engineering, Inc. I'm a senior application engineer for ANSYS CFD. In this video, I want to talk about how to model open channel flow in ANSYS Fluent using the volume of fluid method. This is part 3 of the series of blogs for open channel flow modeling in ANSYS Fluent using the volume of fluid method.
Introduction
In this part, I want to show you how to model open channel flow in circular channels. This is a common application for the open channel flow model. Let's start with the open channel flow.
Open Channel Flow vs. Pipe Flow
- In pipe flow, there is no free surface, whereas in open channel flow, there is a free surface.
- The flow in pipes is typically driven by pressure, while in open channel circular channels, it is usually driven by gravity.
- The velocity profile in pipe flow is maximum at the center, but in open channel flow, it is usually just below the free surface.
Model Description
This model consists of two circular channels joining a main tunnel. The radius of each branch is 200 millimeters, and they join a main tube with a radius of 400 millimeters. The main tube is about six meters long.
Boundary Conditions
- Inlets are defined at the end of the larger tube.
- Walls are defined as boundary conditions in ANSYS Fluent.
Meshing and Simulation Setup
The model is meshed in ANSYS Fluent. The gravity is activated in the negative y-direction, and a transient simulation is run using the volume of fluid method.
Phase Setup
- Primary Phase: Air (due to its lower density).
- Secondary Phase: Water.
Material Properties
Water properties are defined using the Fluent database, with specific attention to density. A constant surface tension of 0.072 Newton per meter is considered.
Turbulence Modeling
- K-Omega SST model is used for turbulence modeling.
Boundary Conditions for Inlets and Outlet
- Pressure inlet with open channel option activated.
- Pressure outlet with the free surface level defined.
Simulation Execution
The simulation is initialized with water inside the domain using patching. Time steps are set to 5 milliseconds, and contours are defined for animations.
Visualization and Results
- Velocity and pressure distributions are visualized using contours and vectors.
- Pathlines are used to visualize flow development and interactions.
Conclusion
This video demonstrates how to set up a volume of fluid method to simulate open channel flow in circular tunnels, which can be used for modeling underground tunnels. The behavior of open channel flow is different from pipe flow, which is an internal flow.
Thank you for watching!
Modeling water flow in underground tunnels using open channel flow model in ANSYS Fluent Hello everyone, this is Mohsen Seraj from Ozen Engineering. I'm a senior application engineer for ANSYS CFD.
In this video, I want to talk about how to model open channel flow in ANSYS Fluent using the volume of fluid method. This is part 3 of the series of blogs for open channel flow modeling in ANSYS Fluent using the volume of fluid method.
In this part, I want to show you how to model the open channel flow that when we have flow in circular channels. This is a kind of application for the open channel flow model. So, let's start with the open channel flow. The tunnels in these underground tunnels can be used for water systems.
The difference between pipe flow and open channel circular flow is that in pipe flow, we don't have a free surface. However, in open channel flow, we have a free surface.
Another difference is that the flow speed in pipe flow increases by pressure, but in tunnels and open channel circular flow, the flow usually speeds up by gravity.
Additionally, the velocity profile is typically maximum at the center of the pipe flow, but in open channel flow in circular channels, it is usually a little below the free surface of the water. Here is the model I created. I have two circular channels joining together here to the main tunnel.
The radius for each of these branches is 200 millimeters, and they are joining to the main tube, which has a radius of 400 millimeters. This tube is about six meters long. I defined the inlets and walls as boundary conditions in ANSYS Fluent.
I meshed the model, and here are the circular tubes and the larger tube. I activated gravity in the negative y direction for the transient simulation. I chose the volume of fluid method and activated the open channel flow option. I used air for the primary phase and water for the secondary phase.
I defined the materials for the fluid water and specified the density. I considered constant surface tension and set the model for surface tension. For the inlet, I used a pressure inlet and activated the open channel option.
I defined the water level at the inlet and the velocity at the bottom level of the channel. For the outlet, I used a pressure outlet and defined the bottom of the larger tube.
The main difference between this case and previous cases in part two and part one is that we have a solid wall on the open channel, and we are using the volume of fluid method.
The top faces on those cases were open to the air, but here we have a solid wall around the water and air for the smaller branches and the main tube. I defined the animations based on the contours. After finishing the solution, I can check the animations.
Here, you can see the free surface over the water, the velocity increasing as the flow merges into the main tube, and the velocity decreasing in the almost dead zone. I can also define vectors for the magnitudes of the vector from inlet to outlet.
You can see that the velocity is higher when the flow from the branches gets in touch together. I can define new contours for pressure, turbulence, or other parameters. I can see the distribution of the pressure and the turbulence energy for intensity.
I can define vectors for the z velocity, which is the velocity in line with the main tunnel. You can see that the flow is coming back in the positive direction of z.
I can also use path line, which takes a little bit of time to draw for us the path line from the inlet to the outlet and over the free surface. Thank you for watching. I hope you can see the different behavior of the flow compared to pipe flow.
The open channel flow inside these tubes or tunnels is quite different compared to pipe flow.
