Immersed Body Valve Simulation Using Ansys CFD: Part 3
Introduction
The next analysis we can perform involves introducing a rigid body motion or a six-degree freedom calculation. To do this, I'm going to duplicate our current setup and call it the 6 Degrees of Freedom Solver. This analysis will involve the same type of geometry connection and solution, but with adjusted immersed body settings.
Immersed Body Settings
Currently, we have settings for speed and direction, but I will change this to a general rigid body motion. We need to specify the following parameters:
- Mass: 100 grams
- Moment of Inertia: Set to 1 for all axes
We will only free up the translational degree of freedom along the y-axis and prevent rotation. Additionally, gravity will be added, making the mass significant.
Defining Forces and Torques
We can define multiple forces and torques, such as:
- Spring Force: Define a spring position and a linear spring constant, or assign a constant value or equation.
- Stopping Force: Create an expression to increase force significantly when a stopper or valve reaches a certain position.
- Magnetic Force: Specify a stroke value if a solid is driving the motion.
In this setup, there are no forces in the x and z directions. We will apply a constant force of 14 newtons downward, balancing the spring force with the fluid hydrodynamic pressure to find an equilibrium position. Smaller time steps may be necessary depending on the force magnitude.
Running the Simulation
Before running the simulation, I will save the setup in my immersed boundary examples. Let's proceed with the simulation and examine the results.
Results
The simulation has completed. Here's a summary of the results:
- The initial time step shows the system moving downward.
- After a tenth of a second, the system stabilizes with minor vibrations.
- After approximately 76 steps, the system reaches a stable position due to the balance of fluid dynamic and spring forces.
Conclusion
This example demonstrates the capability of using immersed body modeling to simulate moving valves. We can adjust the valve's motion through remeshing and mesh stretching. Ansys CFX supports remeshing, and Fluent offers additional mesh deformation and remeshing capabilities. While the immersed boundary method is the easiest approach, it is the least accurate.
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The next analysis we can do is we can put in a rigid body motion or a six degree freedom calculation. I'm going to duplicate this again. We'll call this "6 degrees of freedom solver." Same type of analysis, we have geometry connected as well as a solution.
But I'm going to adjust the immersed body settings a little bit. Here are the immersed body settings we have right now, speed and direction. I can change this to a general rigid body motion. We have to specify mass. Let's say this is 100 grams and moment of inertia.
I will just set one for all of these. And then we can do the other two exercises later. What's interesting here is I can specify degrees, translational degrees of freedom. So I'm going to free up only the y axis and not allow this to rotate.
And add gravity as well, which means the mass will become important. We can define multiple forces and torques. So for example, we can define a spring force. And the spring force could be a spring. So I can define a spring position and a linear spring constant. Or we can assign a value.
So I can say value of this. The value of the force can be a constant or an equation. And I can define multiple forces. So I can, for example, define a stopping force.
And I can create an expression such that when the stopper, when the spring, when the valve comes down far enough, it stops it, I have a large increase in force to stop it from going further. I can create, for example, a magnetic force.
So if I have a solid one driving this, I can specify a stroke value. So all those things are optional. Here we can say we have no forces in the x and z direction. And here we have, let's say, a minus 10-unit force. So I can say, I have a constant force pushing down at 14 newtons.
Or actually, I think when we fully seal it, it's about 14 newtons. So why don't we put in a 14 newtons of force here? So it's like we have a constant force pushing down at 14 newtons. And in this case, the force of the spring will balance out the fluid hydrodynamic pressure.
So we can use this pressure on the valve to find an equilibrium position. We may need to have smaller time steps here, depending on how big the force is. But let's go ahead and run the simulation and see how it works. Before I do that, I'm going to save it.
So I will save it in my immersed boundary examples. This is the simulation. And then I can go ahead and set it up. And this is the result. Take a look at the results. Okay, this simulation has completed. Let's take a look at the results. Let's see what it looks like. Let's see what it looks like.
It looks pretty good. It's the final result. Let's animate this and see. Oh no, I think I ran this as a... Let me double check here. Is this the right one? This is. So, output control... Oh, I am saving transient results. So, it means we can't edit this. Huh. Let's see what happened here.
So, the very first time step looks like this. Okay, so we are going down. After a tenth of a second, it's pretty much down here. Let's see if it bounces up. And then the rest is just kind of maintaining. The small gap flow. Maybe a little bit of vibration up and down.
So, this shows you the ability to do this type of analysis. There we go. Alright, we won't see anything different. It's because after about... Okay. About 76 steps here, it just stays the same. It reaches a position and it stays because of the fluid dynamic forces and the spring force balance out.
Alright. So, that's a few examples of how we can use immersed body modeling to model moving valves. We can certainly move the valve as it goes up and down through remeshing, through mesh stretching. In fact, CFX has the ability to do remeshing. If we open up this. Let's see.
We have the option to specify mesh deformations. Fluent has additional mesh deformation and remeshing capabilities. So, we can support a wide range of motion analysis. But, by far the easiest, although it's the least accurate way to do this is using this immersed boundary method.
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