Cylinder Piston Motion CFD Simulation, Dynamic Mesh

Description

This project is related to the numerical simulation of the Cylinder Piston Motion using ANSYS Fluent software. Suppose we have a four-stroke engine. So we will move with four steps. The rotational movement of the crankshaft is transmitted to the piston through the connecting rod, creating a vertical reciprocating motion of the piston in the cylinder. Now, this reciprocating movement includes four stages.

This product is the second chapter of the Dynamic Mesh Training Course.

First, the piston inside the cylinder moves down. Simultaneously, the intake valve opens, and flow enters the cylinder.

Second, the piston rises to compress the flow inside the cylinder.

Third, the piston reaches the highest possible point (top dead center) for the explosion to occur.

Finally, the piston inside the cylinder moves down again. At this step, it is time to open the exhaust valve to direct the resulting flow out.

We do not intend to investigate chemical reactions, combustion, or explosion in this simulation. We want to model the reciprocating motion process of the piston in the cylinder.

We modeled the geometry of the project using Design Modeler software. The geometry is related to the cylinder and piston. The computational domain of the model includes the interior of a cylinder, where a piston is placed at the bottom, and two intake and exhaust paths with valves are placed on top of it. Then we meshed the model with Ansys Meshing software. The model mesh is unstructured, and the number of cells equals 16,623.

Cylinder Piston Motion Methodology

In this project, we used the Dynamic Mesh Model. We generally use a dynamic mesh whenever we have a moving boundary or a deforming zone. Here, a piston surface has a transitional motion inside a cylinder. So this causes the mesh to deform over time. Therefore, we define the In-Cylinder option to define the reciprocating motion of the piston inside a cylinder.

In the in-cylinder model, we determine parameters such as crank radius, connecting rod length, piston stroke cutoff, etc. Using the parameters we defined, the Fluent software defines an in-build function called full-piston. We use this full-piston function to define the movement of the piston (that is, the boundary of the piston).

Now, we must define the reciprocating motion of the piston surface and intake and exhaust valves. We have to define the boundary related to the piston surface and valves as a rigid body; so that we define motion for these rigid bodies.

To define the movement of the piston surface, we use the function of the full piston. But to define the movement of intake and exhaust valves, we use profiles that describe the changes in valve lift in terms of crank angle.

According to the reciprocating motion of the piston and valves as rigid bodies, the mesh zone inside the cylinder is deformed. So, for this zone, we use the Deforming option. Input and output paths are not affected by mesh changes. So for these areas, we use the stationary option.

Due to the nature of this modeling, flow behavior is time-dependent. Hence, we use the unsteady (Transient) solver.

Conclusion

After the solution, we obtained pressure and velocity contours. In addition, we obtained an animation of velocity and pressure contours.

The results show that the cylinder-piston system is working correctly. The piston surface constantly moves up and down over time. Also, the intake and exhaust valves are opened and closed in turn and sync with the piston motion.

Finally, we got an animation of momentary mesh changes.