Hydraulic Fluid Exchange CFD Simulation: Fluent Tutorial

Hydraulic machines do very heavy work in industrial factories. This hard work creates a lot of heat. Hot fluid inside a cylinder can easily destroy rubber seals and break the system. To stop this damage, machines must push the old hot fluid out and pull new cold fluid in. This physical process is called hydraulic fluid exchange. It keeps the cylinder at the correct temperature and pressure to last longer.

Our main goal in this CFD project is to analyze how the cold and hot fluids mix. We base our computational model on the academic study by Siwulski and Warzyńska. By recreating their moving cylinder experiment, this ANSYS tutorial teaches users how to manage fluid flow and protect mechanical parts from heat stress.

• Reference Paper [1]: Siwulski, Tomasz, and Urszula Warzyńska. “Numerical investigation of the influence of the inlet nozzle diameter on the degree of fluid exchange process in a hydraulic cylinder.” Engineering Applications of Computational Fluid Mechanics1 (2021): 1243-1258.
• Reference paper [2]: Siwulski, Tomasz, and Urszula Warzyńska. “Numerical simulations of the liquid exchange process in a chamber of a new hydraulic cylinder solution in the aspect of the angular nozzle arrangement.” Cuprum: czasopismo naukowo-techniczne górnictwa rud (2019).

Figure 1: The physical design and moving strokes of the hydraulic fluid exchange system.

Figure 2: Example of thermal degradation of the static seal of the hydraulic cylinder packing, visible splits of the sealing material [2]

Simulation Process: Dynamic Mesh Setup in Hydraulic Fluid Exchange

The physical model looks like a standard piston and cylinder system. When the machine operates, the solid piston moves back and forth. This means the fluid space inside the cylinder changes size. To calculate this changing space, we use a dynamic mesh. As the piston pushes, the grid must change its shape to follow the moving wall.

We apply specific smoothing and remeshing methods. These methods stretch and rebuild the grid so it stays clean during the movement. A User-Defined Function (UDF) mathematically controls the physical motion curve of the piston. You can get to know how to set up moving boundaries in our dynamic mesh simulation training section.

Figure 2: The dynamic mesh adapting to the fluid space inside the moving cylinder chamber.

Post-processing: Flow and Thermal Physics in the Cylinder

From our prespective, when we analyze the contours to understand how the new and old fluids physically mix, the new fluid enters through the inlet nozzle at a high speed. This creates a strong fluid jet inside the main chamber. The turbulence kinetic energy contour shows intense mixing happening directly in the center of the cylinder. This high turbulence acts like a blender. It is necessary to force the fluids to mix rapidly.

The temperature contour proves why this fast mixing is so important. The old fluid near the cylinder walls is very hot, reaching 353.16 K. The new fluid entering the nozzle is much colder, starting at 327.83 K. The fast, turbulent jet spreads the cold fluid evenly across the chamber. This rapid cooling stops dangerous hot spots from forming and protects the sensitive rubber seals from melting.

Figure 3: The turbulence kinetic energy and temperature distribution during the fluid exchange.

The pressure contour shows the mechanical stress on the cylinder body. The main chamber holds a very high fluid pressure of 1465.8 Pa. The inlet tube has a much lower pressure, dropping to -2.1 Pa. When the fluid exchange flows smoothly, the pressure balances evenly across the piston head. This reduces physical friction, lowers mechanical stress, and helps the hydraulic system last much longer.

Figure 4: The physical pressure pushing against the cylinder walls and moving piston.

FAQ About Hydraulic Fluid Exchange Modeling

• Why do we use a dynamic mesh for this CFD project? The piston inside the cylinder physically moves during operation. Because the solid piston moves, the fluid space grows and shrinks. The dynamic mesh automatically stretches and rebuilds the computational grid to match this changing physical space.
• Why is turbulence important in a hydraulic cylinder? Turbulence forces the fluid to mix rapidly. When the new cold fluid enters the cylinder, strong turbulence blends it with the old hot fluid. This stops heat from staying in one place and prevents thermal damage.
• How does high temperature break the hydraulic system? High heat directly damages the soft rubber seals inside the cylinder. If the hot fluid melts or cracks these seals, the high-pressure fluid leaks out, and the machine loses all its pushing power.