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When the momentum of the acoustic source changes, the oscillation around the average vertical position can have both frequencies mentioned above. For both cavities, the particle oscillates with the frequency of the sound source and its harmonics, and in some cases there is a much smaller second dominant frequency. This distance increases linearly as the density ratio between the solid particle and fluid grows. With no gravitational force, a particle oscillates around a pressure node in the presence of gravity the oscillation is shifted a small vertical distance below the pressure node. Using the lattice Boltzmann equation method, we find the acoustic force acting on a rounded particle for two different single-axis acoustic levitators in two dimensions, the first with plane waves, the second with a rounded reflector that enhances the acoustic force. When the acoustic force inside a cavity balances the gravitational force on a particle, the result is known as acoustic levitation. The acoustic spectra of various natural rainfall events on wet surfaces are comparable to those of discrete drops recorded in the laboratory, but upshifted in frequency. A Helmholtz resonator model for the collapse of an air cavity describes the observed acoustic signature well. For example, in the case of water drops impacting a deep pool of water, we observe two distinct sound events: one as the drop breaks through the free surface of the pool with frequency content centered around 10 kHz and then approximately 50 ms later at the time of air cavity collapse and Worthington jet formation with frequency content around 4 kHz. Each impact type results in unique features in the spectra from 1-20 kHz that can be related to the impact and subsequent fluid phenomena. ![]() These airborne sound pressure versus time measurements are converted to power spectra using the fast Fourier transform. Longer audio recordings are also collected for natural rainfall on wet concrete surfaces. Audio recordings of the airborne sound emitted by single drop impacts on deep pools, thin liquid films, and dry aluminum and masonry surfaces are captured in the laboratory. In this work, we experimentally study the acoustic response of water drop impacts onto a variety of surfaces using both audio recording and high-speed imaging. However, an oscillation of the cavity bottom can not be observed in the multiphase neither in the acoustic outputs of the airborne signal. The acoustic pressure shows a significant rise in the vicinity of the bubble detachment within both phases. A coupled drop impact test case corresponds with equivalent experiments until the drop detachment. It is shown, that the methods are suitable for simple test cases. The results are compared with numerical and experimental data. The acoustic part is simulated with the linearized Euler equations which are valid in each phase but need to be adapted in the interface region. A high resolution in space and time is essential and therefore the method is parallelized by domain decomposition. For the curvature computation a standard finite difference method within the continuum surface force model is employed, including some necessary improvements. For this an in-house block-structured finite-volume solver with the volume-of-fluid method is used. First the multiphase flow needs to produce the correct physical mechanisms, e.g. ![]() In this work the feasibility of numerical methods for simulating this challenging test case is evaluated. The sound which is produced when a water drop impacts into a water pool is a prominent example for acoustics produced by multiphase flow.
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