Thermal FSI CFD Simulation Tutorial: Water-Lubricated Bearing

Marine ships and submarines rely on heavy propeller shafts spinning inside large stern bearings. Modern designs use clean water instead of industrial oil for lubrication. But water creates a very thin hydrodynamic film. The heavy rotating metal shaft easily pushes through this thin fluid layer and grinds against the soft rubber walls. This friction generates dangerous heat, forcing the materials to expand and warp under pressure.

The primary objective of this project is to use ANSYS Fluent to predict the exact heat generation and calculate the resulting rubber deformation using a fully coupled 2-way Thermal FSI solver. By understanding how fluid pressure and thermal expansion alter the physical clearance gap, we can prevent catastrophic overheating and avoid structural tearing.

• Reference [1]: Han, Yanfeng, et al. “A fluid–solid–heat coupling analysis for water-lubricated rubber stern bearing considering the deflection of propeller shaft.” Applied Sciences 11.3 (2021): 1170.

Figure 1: The physical geometry schematic defining the heavy propeller shaft, the thin water film, and the grooved rubber bushing. [1]

Simulation process: 2-Way Thermal FSI Setup for Marine Stern Tube

We model a steel shaft with a 90 mm diameter resting inside a solid rubber liner. The rubber geometry features 8 axial cooling grooves to allow water passage. We generate a highly dense boundary layer mesh inside the fluid domain to capture the ultra-thin water film. The solid mesh focuses heavily on the inner rubber surface where the mechanical contact occurs.

We utilize ANSYS System Coupling to connect the solvers. The shaft carries a massive 10 kN external load representing the weight of the propeller. As the shaft spins, the friction creates intense viscous heating. We apply a convection boundary condition to the outer wall to mimic cold seawater pulling heat away from the system. To capture the physics, we configure a 2-way communication loop. Fluent calculates the water pressure and temperature. It sends this fluid data to the Mechanical Coupled Field Static solver. Mechanical calculates the physical rubber bending and thermal expansion. It then feeds the newly warped shape back to the fluid solver. This constant data exchange ensures highly accurate fluid-structure interaction results that a rigid static model completely misses.

Figure 2: The computational mesh highlighting the solid rubber liner geometry and the internal cooling channels.

Post-processing: Thermo-Hydrodynamic Pressure, Deformation, and Heat Distribution

We evaluate the engineering safety of the bearing by deeply analyzing the generated pressure contours, structural deformation maps, and temperature flows. we examine the static pressure contours. The downward 10 kN mechanical load forces the metal shaft toward the bottom of the rubber liner. This squashes the water into a tight, converging wedge shape. The trapped fluid reacts by building a massive lifting force. The pressure hits a maximum peak of 1004.95 Pa in the red zone at the bottom. Conversely, the upper region of the bearing pulls apart. The stretched water causes the pressure to drop to a negative -126.05 Pa. This dark blue zone acts as a critical warning. Negative pressure creates a high risk for cavitation, where the liquid water violently boils into destructive vapor bubbles.

Figure 3: The static pressure map highlighting the massive load-carrying wedge at the bottom and the cavitation risk zone at the top.

Then, we analyze the structural integrity using the total deformation contour and the data table. The combined hydrodynamic pressure and thermal expansion force the soft rubber to bulge outward. The structure hits a maximum deformation of 5.45e-05 m (roughly 0.055 mm). This is an incredibly dangerous finding. The original design clearance between the shaft and the bearing is only 0.15 mm. The swelling rubber consumes over a third of this vital safety gap.

If we relied on a simple 1-way calculation, we would completely miss this severe shape change. The 2-way FSI proves the manufacturer must either widen the initial gap or select a stiffer material. The mechanical solver also verifies a minimum stress of 15.458 Pa and a maximum stress of 10435 Pa. While the deformation is high, this peak stress stays within the safe mechanical tearing limits of industrial rubber.

Finally, we track the temperature distribution. The extreme viscous friction generates harsh heat directly at the inner shaft interface. The temperature contour hits a maximum peak of 32.86 °C in this red zone. The heat travels outward through the solid rubber toward the cold boundary. The outer wall remains highly stable at a minimum of 20 °C, bringing the overall system average down to 21.39 °C. Standard industrial rubber begins to degrade and fail near 60 °C. These thermal contours prove that the 8 axial water grooves successfully channel enough cold fluid to keep the entire bearing well below the failure limit.

Figure 4: The temperature map visualizing heat generation at the friction interface and the cooling effect near the outer wall.

Finally, we must examine the most dangerous physical effect: the shape change. The total deformation contour reveals a massive physical shift in the solid rubber. The bright red zones located at the bottom show the highest structural displacement. The water pressure forces the soft rubber to push outward by exactly 5.45×10⁻⁵ m. This number seems small, but the original empty gap between the metal shaft and the rubber wall is only exactly 0.15 mm. This means the heavy pressure forces the rubber to stretch and consume more than exactly 30 % of the entire clearance space. If an engineer only ran a basic fluid simulation, they would completely miss this dangerous physical closure. Because the gap shrinks so severely, the water cannot flow properly. This exact visual proof guarantees that engineers must use the 2-way FSI method to accurately design safe marine bearings.

Figure 5: The total deformation map proving the soft rubber bulges outward under hydrodynamic pressure and heat.

FAQs About Thermal FSI Water-Lubricated Bearing Training

• Why is rubber deformation dangerous in this bearing?
• The bearing relies on a tiny gap to let the cooling water pass through. Because rubber is very soft, the high water pressure pushes it outward and makes it swell. This swelling consumes the tiny empty gap, choking the water flow and causing the bearing to overheat and break.
• Why must we use a 2-way FSI solver instead of a rigid fluid model?
• The pressurized water physically pushes the rubber, making it bulge. This bulging changes the shape of the tiny water gap. If we do not feed this new physical shape back into the fluid solver, our pressure calculations will fail completely.
• What causes the negative pressure zones inside the fluid domain?
• The heavy propeller shaft pushes down and creates a tight squeeze at the bottom. This leaves too much empty space at the top. The water stretches apart, causing the local pressure to drop below zero to -126.05 Pa.
• How does the heat safely escape the system in this CFD setup?
• Viscous friction generates heat at the inner rubbing wall. The solid rubber conducts this thermal energy outward. The cold seawater boundary at the outer wall pulls the heat away, safely dropping the outer temperature to 20 °C.