Lifting Surfaces

Acoustic Radiation from Cavitation

The noise resulting from cavitating flow past a 2D NACA 0015 was measured in a special low-noise water tunnel. Various cavitation regimes were studied. It was found that the strongest amplitude is present when sheet/cloud cavitation occurs at σ/2α = 3.5, where σ is the cavitation number and α is the angle of attack. This regime corresponds to an instability in cavity length that produces strong oscillations in both cavity length and lift. Although the strongest amplitudes are found to be on the order of the shedding frequency, the spectral content of the noise persists to relatively high frequencies. In the most unstable regime, cavitation noise also displays a strong correlation with the lift oscillations, indicating that vortices shed into the wake modulate the shedding of vapor bubbles and hence their collapse. Several types of cavitation phenomena have been studied, each radiating noise with unique spectral characteristics. It is shown that cavitation noise spectra can be mapped as a function of the parameter σ/2α. In addition, the noise appears to scale well with the dynamic pressure of the flow.

Figure 1. Spectral Characteristics of cavitation noise for different values of σ/2α. ENLARGE

With lift oscillation data as a guide, the spectral characteristics of the noise can be analyzed Figure 1 is a plot of noise spectra obtained at values σ/2α. In this figure and subsequent plots the spectra are normalized in the following manner:


where is the mean square acoustic pressure and Δf is the frequency bandwidth. As shown in Figure 2, this normalization process works reasonably well above U = 8 m/s.

Figure 2. Comparison of noise spectra measured at different values of velocity. ENLARGE

Noting that the spectral characteristics of the lift oscillations vary considerably with σ/2α, it is interesting to compare how the spectral characteristics of the noise vary over the same range. This is shown in Figure 3 where spectra at different values of σ/2α are compared. It is very clear that the noise maximizes in the same region as the lift oscillations are a maximum. The noise also persists to much higher frequencies.

Figure 3. Noise spectra plotted as a funtion of σ/2α. Color intensity denotes amplitude. ENLARGE