Dynamic Mesh Motion in Fluent - Piezoelectric Actuators

Hello everyone, this is Mohsen Seraj from the Ozen Engineering team. I wanted today to talk about dynamic mesh motion, the use of dynamic mesh motion in Fluent, and specifically about piezoelectric actuators. So, what is the piezoelectric effect?

Basically, when we have a piezoelectric material, if we apply an electrical field, we see deformation in the material or the other way around. If we apply displacement to the material, due to the deformation of the material, it could produce voltage.

We can use these features of these materials in piezoelectric actuators, and in this way, we can have controlled mechanical displacement on force. It could be repeatable in a precise manner and controlled manner.

Also, we have multiple layers of the materials, mostly these are crystals from ceramic materials that we can put together and build the actuators or motors. Some other applications for the use of piezoelectric materials, for example, we can have a piezoelectric pump. This is a schematic shown here.

Consider that when we have the motion, like a cyclic motion of the piezoelectric material, it could act like a vibrator, that we can use it to suck in the fluid from the inlet and then send out the flow from the outlet.

So, in this way, we can use it as a pump to send out the fluid from the inlet to the outlet. Another application of piezoelectric actuators could be, for example, an atomizer, like an ultrasonic atomizer that is used when we put a mesh that is comprised of many holes.

You can use this method in very high frequencies like ultrasonic frequencies in megahertz or in hundreds of kilohertz. We can use it for producing mist above here or use it for exact dosage for some pharmaceutical.

For example, with the fluid domain, then we have this moving wall between these nozzles, that we can apply the prescribed motion that we obtained from the piezoelectric actuator that we have. And in this way, we can use it either for the pump or actual atomizer or whatever application that we have.

So, as you can see, the boundary of the mesh is moving, so that sometimes we call it deforming mesh or moving wall for the mesh.

We in Fluent have features that we call dynamic mesh, that when activated, we can define motion for the boundaries of the mesh or for the specific zone inside the domain, and then if besides the dynamic mesh features, if we use it along with the multi-phase model, then we can go for the simulation of some applications of the piezoelectric actuators like for pump, atomizer, or other applications that we have.

A quick review of the main steps in Fluent when we want to use dynamic mesh motion. We need an initial mesh. Be sure that here that we are working with the transient solver. Activate the dynamic mesh.

We need some setup for the dynamic mesh motion like a method that we want to use, some options that we have that we can smooth out the mesh motion, and keep the quality of the mesh cells during the motion, because we don't want excessive distortion of the mesh shape or mesh size after the motion that we have.

And for sure, we need a way to define this prescribed motion that it could be either from a profile that one of the variables for sure is time and another one is x, if it is horizontal motion, y if it is vertical motion, or together if it is planar motion, and if it is three-dimensional, then for sure we can have XYZ and T components into the profile.

We can use UDF also if we have the data points and we can use it. And for sure, we need to preview the mesh motion. This is an example of the up and down motion of the moving wall of the mesh that we expected to see.

And as you can see, it is like a cyclic motion, it is like a sinusoidal wave that we can produce due to the displacement or deformation of the piezoelectric material applied to these walls that we can see.

So, at the beginning, it goes up and then goes down and then again goes up, going down, and after, and then back to the original position, so it is like a two-cycle.

The good thing about piezoelectric actuators is that we can produce really controlled motion with specified amplitude and specified frequency. So, a quick review of what steps you need to do in Fluent.

We need transient simulation, we need to activate the dynamic mesh feature in Fluent, it is not activated by default. These are the details that we need to work on that to set up the dynamic mesh motion.

We need to know what method we want to use, what options we want to use, if we need more setups for each of the methods, smoothing, layering, remeshing here. We need to define the moving zones where in the mesh, in what zone, in what region we want to apply the dynamic mesh motion.

And then we can do display and preview to mesh before starting the simulation to be sure that the motion is in a way that we like it and we don't see lots of distortion or unwanted distortion for the mesh element. We have some videos about that, about setting up the dynamic mesh motion in Fluent.

Please contact us at https://ozeninc.com/contact for more information. So now I will quickly show you the geometry and where we apply the wall motion for the mesh, and then I have to set up a model for the piezoelectric actuator. This is the geometry. These are the ten holes.

At the bottom, we have water, above is air. These are the holes, and I just partitioned each side for the water and air for per machine. So, we use named selection, outlets on top, and we have to choose where we have moving walls.

These are moving walls, and the walls that we have moving or sliding mesh, so this side, open, lower, and in the center. For meshing, Workbench Meshing is used. I use MultiZone, and the mesh is for CFD Fluent Meshing. You can see the mesh, which is refined above the holes that we have.

I use hexagonal mesh, and the matrix that it is based on orthogonality. You can see here the minimum is 0.82, which is very good. Mesh statistics, it is about half a million nodes and about 250,000 elements or cells. Okay, Fluent model, we have transient simulation, and this is the mesh.

As you see the nozzles, up, we will assign air material, at the bottom water. About the model, I use VOF or Volume of Fluid method here.

Two phases that we have, one phase that is primary is air, another phase is water-liquid, and the phase interaction I just consider constant surface tension between air and water. For materials, I have air and water liquid. We start with air, constant density for the air.

We can work on other models for the available material modeling for available for air like ideal gas. But we start with just constant density, and same for water liquid constant properties. So, for here, as you can see, I already activated dynamic mesh and using smoothing and remeshing.

Here, I have many zones that I already created in Ansys.

Then I use it here, as you can see, in general, I have two types of moving zones, one is rigid body, that when we have the moving walls, and the forming zone, it means that they are adjacent to the moving walls, and it is in this way we can allow the mesh to slide along these walls.

The details about this setup are fully explained in another video that I already created. This is the Ozen Engineering channel partner, channel here that we have on YouTube.

If you go to the videos, okay, these are the videos, part 3, part 2, and part 1. These are the videos that I created, and you can see the details above about the setting of the dynamic mesh motion for this model.

Back to Fluent, be sure that here, for the method of solutions, we use PISO for coupling of velocity and pressure, and also PRESTO for pressure.

We initialize the model, initialize that, one more to be sure that it is initialized, and then we patch water because it is already initialized with air when the volume of fluid is zero. So, I need to patch the water into the domain at the bottom below these nozzles.

So, instead of mixture, I go to the phase water, volume fraction, I define this region by cell register that includes this part of the domain, and the below the nozzles, below the holes, between air and top parts and bottom parts. Patch it.

Before starting, we can check the volume of the fluid to be sure this is for water. And you can see, in this way, I just consider half of each hole or nozzle filled with water, and half is empty or it is air. That here we have it. We can check the volume of fluid method.

No warning, the setup is correct. Before running the calculation, we can check the case.

Just about the general recommendation for improving mesh quality, we already know that the mesh quality is good because the orthogonality is well above 0. 15. And this time step depends on the frequency of the prescribed motion that we have.

I already, in physics, you can see that here, you can define the profile, I already defined a profile that is being assigned already to moving walls, and we have two parameters, time and y components, which belong to this cyclic motion, this sinusoidal wave.

So, I hit the calculation button, and the solution is starting. I use the K-Omega method, just the default value, this is the first time step, you see the iteration within the first time step. Now, the second time step.

I defined already some monitors for solution reports, for example, this is for the velocity at the hole, the nozzles, max velocity, and also max.

And visit the material modeling that we are using for water and for air, and also be sure about the mesh, maybe we need a better mesh resolution compared to the prescribed motion that we have for moving walls, in terms of the amplitude of the prescribed motion and frequency.

And this is for, you can see that the air is sucking into the domain, it means that these walls here, okay, between the walls, between the holes, is going up, and as the walls are going up, it means that the air is sucking into the bottom parts, and in the next time steps, when these moving walls are going up, okay, sorry, going down, then you can see that we can have like a water jet that exists.

The velocity we see the sinusoidal variation, it is in line with the cyclic motion that we have for the walls, and this is the average value, and then this is the max value of the velocity in the domain, and this is the max value for the holes or the nozzles.

You can see that the velocity, max velocity, and so we have to think about some improvement that we need to do here. Let's look at the contours that based on that I created some animations. And you see that it is just going to get into the almost very low velocity.

Let's look at the volume of fluid contours that I created an animation on that.

That, okay, if we decrease the speed for the play, you can see that the wall is going up, and so sucking in the air, and now in going down the wall, and we see the ejection of the or like water jets from the from this nozzle or from these holes, and you can see that the air entering into the lower part of the domain is not the same for these holes, it depends on the water conditions that we have, and here we can see how far the droplets are going.

So, we are in the second cycle, that still the wall is going up or down, and we see that again another ejection of the water like a water jet is going up, and these are like droplets of the air packets that are spreading into the air.

And this is all the use, so we can model this from the dynamic mesh motion.

Now, we pass the time for the dynamic mesh motion, so as you can see, there is no change here in the entering of the air into the water or ejection of or sending out the waters from the nozzles, and we just see the just the traveling of the droplets.

If you then have to set up the model with dynamic mesh motion for this, due to the prescribed motion that it can be from a piezoelectric actuator, and for this type of application that it is for forming the droplets and forming the mist above the, here above the nozzles, and you can see the number of the holes that we are having here, that can be used in a variety of applications like a pharmaceutical application for specified dosage of the drug by inhaling and prescription for inhaling for the patient.

So, this is the first part of the video, in the next part, I'm going to show you some possible improvement to the model, that how we can decrease this hike in the max velocity and improving the model. Please contact us at https://ozeninc.com/contact for more information.