# Reduction of the Cavitation Noise in an Automotive Heater Core
## 3 Identification of the Cavitation Noise Generation Mechanism
Additionally, vibration magnitudes in the tanks are greater than those in the pipes. The difference in vibration magnitudes between cavitation and non-cavitation conditions is also more pronounced in the tanks than in the pipes. A particularly interesting observation is that, in the absence of cavitation, vibration magnitudes remain relatively uniform across different locations in both the pipes and tanks. However, under cavitation conditions, significant location-dependent variations in vibration magnitude are observed. Specifically, Figure 15a indicates that among the points on the pipes, point 3 exhibits the highest vibration magnitude, while Figure 15b shows that among the points on the tanks, point F has the highest vibration magnitude. Considering that point 3 on the outlet pipe and point F on the upper tank are adjacent, it can be inferred that cavitation bubbles collapse near the junction of the outlet pipe and the upper tank. To investigate why cavitation bubble collapse occurs near this junction, computational fluid dynamics (CFD) simulations were conducted. The simulations were performed using the commercial software Ansys Fluent 2021 R1 [29] to analyze the velocity and pressure distributions of the coolant within the heater core. Figure 16 shows the CFD mesh model representing the coolant flow domain within the heater core. The mesh comprises structured and unstructured elements designed to capture the complex heater core geometry  
Figure 15: Vibration levels measured by the accelerometer mounted at point F on the upper tank: (**a**) engine speed 4000 RPM—cavitation not observed, and (**b**) engine speed 5000 RPM—cavitation observed. and resolve coolant flow characteristics accurately. This model was employed to analyze the fluid velocity and pressure distributions within the heater core. A coolant temperature of 1 \({}^{\circ}\)C was set, and pressures of 101 kPa and 58 kPa were applied at points I and O of Figure 16, respectively, corresponding to the inlet and outlet pipe ends. These pressure values were obtained from actual vehicle measurements using pressure gauges. A 'no-slip wall' boundary condition was applied at the interface between the fluid and the structure. The Shear Stress Transport (SST) turbulence model was used to capture turbulence characteristics near the walls while maintaining robustness in separated flow regions. A steady-state analysis was performed to obtain a converged solution and assess flow behavior under the specified conditions. The computational domain consisted of approximately 2 million cells. To resolve near-wall flow structures accurately, especially around the junctions and the outlet pipe where complex flow behavior is expected, five prism layers (inflation layers) were applied. A mesh sensitivity analysis was conducted using meshes with 1.5 million, 2.0 million, and 2.5 million cells. The results showed that the local velocity and pressure distributions became stable with increasing cell count, confirming mesh independence. Accordingly, the mesh with 2 million cells was selected for the final simulations. The y+ values near the walls were maintained below 1, satisfying the resolution requirements of the SST turbulence model used in the analysis. The CFD simulations provided detailed velocity and pressure distributions of the coolant within the heater core. To verify the accuracy of these results, the simulated flow rate of 56.1 L/min was compared with the experimentally measured value of 57.0 L/min. The resulting discrepancy was only 1.6%, demonstrating the reliability of the CFD simulation model. Figure 17a,b present the velocity and pressure distributions, respectively. Note that in Figure 17a, although the velocity appears to be non-zero at the wall surfaces, this is due to the visualization method used in the CFD software (Ansys Fluent 2021 R1). The displayed values are interpolated from the first layer of cells adjacent to the wall, not from the wall itself, where the velocity is zero due to the no-slip boundary condition.