# Reduction of the Cavitation Noise in an Automotive Heater Core
## 3 Identification of the Cavitation Noise Generation Mechanism
As shown in Figure 17a, the highest coolant velocity occurs at the junction between the upper tank and the outlet pipe, highlighted by a red circle. Correspondingly, Figure 17b reveals that the lowest pressure is also observed at this same location, marked with a blue circle. The specific point, exhibiting both the highest velocity and the lowest pressure, corresponds to the location with the lowest cavitation index [30]. The cavitation index is a dimensionless parameter that indicates the potential for cavitation in a fluid system; lower values signify  
Figure 16: CFD mesh model illustrating the coolant flow domain within the heater core. “I” and “O” indicate the inlet and outlet pipe ends, respectively. a greater likelihood of occurrence. Cavitation is expected when the cavitation index drops below 1. Figure 18 illustrates the cavitation index distribution in the midplane of the heater core. At the junction between the upper tank and the outlet pipe--marked by a circle--the cavitation index is 0.876, which is below the threshold of 1. This confirms a high probability of cavitation forming in this region. The cavitation index is defined as  
\[\sigma=\frac{p-p_{v}}{\rho u^{2}/2} \tag{1}\]  
where \(p\) is the fluid pressure, \(p_{v}\) is the saturated vapor pressure, \(\rho\) is the fluid density, and \(u\) is the velocity. According to this equation, the cavitation index decreases as the flow velocity increases or as the pressure difference between the pipe and the vapor pressure decreases. Both of these conditions increase the likelihood of cavitation. Additionally, a higher flow rate or lower coolant temperature results in a lower cavitation index, further increasing the probability of cavitation. To further validate this relationship, experimental data measured in the actual vehicle were analyzed. At an engine speed of 5000 RPM, the measured flow rate was 57.0 L/min, while at 4000 RPM, the flow rate was 45.1 L/min. Cavitation noise was observed at the  
Figure 17: CFD simulation results in the heat core: (**a**) velocity distribution and (**b**) pressure distribution of the coolant. Figure 12:0 Cavitation index distribution on the cross-sectional plane of the heater core. The red circle marks the junction between the upper tank and the outlet pipe where cavitation is likely to occur. higher flow rate of 57.0 L/min but not at 45.1 L/min. This supports the conclusion that as the flow rate (and consequently the velocity) increases, the cavitation index decreases, thereby increasing the likelihood of cavitation. Moreover, the relationship between the coolant pressure and cavitation index was analyzed. At 5000 RPM, when cavitation noise occurred, the saturated vapor pressure was 1 kPa, and the pressure at the outlet pipe was 50 kPa. When no noise was observed, the saturated vapor pressure was 19 kPa, and the outlet pipe pressure was 100 kPa. Therefore, when cavitation noise occurred, the pressure difference between the pipe and the saturated vapor pressure was 49 kPa, whereas when no noise occurred, the difference was 81 kPa. This further supports the conclusion that the cavitation index is lower when noise is present, which aligns with the observed presence or absence of cavitation. To understand why the coolant velocity reaches its maximum around the junction between the upper tank and the outlet pipe, the velocity distribution in this region is magnified and presented in Figure 12:1 As shown in Figure 19a, the area marked with a circle indicates the location of the highest velocity. To examine the velocity distribution more clearly, a magnified view of the velocity profile in the cross-section of the outlet pipe at the junction is provided in Figure 19b. The red regions in Figure 19b represent a high-velocity area, revealing that the velocity increases significantly only in a specific portion of the cross-section of the outlet pipe. This localized high velocity results from the abrupt reduction in cross-sectional area at the junction between the upper tank and the outlet pipe.