Catalina Stern

Hydrodynamics of an oscillating water column seawater pump. Part II: tuning to monochromatic waves

Flume experiments with a scale-model of a wave driven seawater pump in monochromatic waves are described. A tuning mechanism optimises the pump performance by keeping it at resonance with the waves. The pumping process itself was found to de-tune the system because of the reduced gravity restoring force due to spilling in the compression chamber. A perturbation analysis of the pump equations shows that performance of the system can be increased by optimising the shape of the pump intake to minimise losses due to vortex formation. An algorithm is derived, using a numerical model of the pump, which accurately determines the required volume of air in the compression chamber to induce resonance given variations in the wave frequency, the wave height and the tides. A sustainable development project to use a seawater pump to manage fisheries at a coastal lagoon in Mexico is described.

New Experiments on the Kaye Effect

When a jet of a thinning fluid is poured over a perpendicular surface covered with the same fluid, under certain circumstances it is observed that a small jet seems to bounce out of the surface very close to the incident jet. This phenomenon was first described by Kaye in 1963. In this work, the same type of fluid was used but the surface of incidence was inclined between 10 and 45 degrees. To better visualize the flow, the fluid was seeded with metallic flakes and illuminated with a sheet of light. The speed of the incident jet was kept constant. For each angle of incidence several videos at 240 frames per second were captured. It was observed that initially, a column of the falling liquid is formed. When the column breaks down, a jet comes out of it. The length of contact between the surface and the jet gets longer as the angle of inclination is increased. A critical angle \( \theta c \) was found for which the terminal speed is larger than the falling speed of the incident jet. This cannot be explained by any of the mathematical models that have been proposed. The thinning characteristic of the fluid is not taken into account in any of the models.

The Saline Oscillator: Visualization Using Shadowgraphs and Particle Image Velocimetry

A saline oscillator is a device that consists of a small container with salt water and a tiny hole in the bottom. The small container is partially submerged into a larger container with distilled water. First, as expected, a jet of salt water falls into the larger container, but after a certain time the jet becomes unstable, a jet of distilled water starts going up, and an apparently periodic motion is installed. The global structure of the flow has been visualized using a technique sensitive to the density gradients, called shadowgraph. The velocity fields have been measured using particle image velocimetry (PIV). The flow rate has been measured as a function of time. Not all instabilities lead to a change of direction of the flow, so different flow patterns can be observed. With a pair of electrodes inserted in each container, a voltage signal with the same period of oscillation as the flow has been obtained. The interval of time when the flow goes down is always larger than the interval of time going up. Some experimental results that do not agree with other authors are presented.

Using Schlieren imaging to estimate the geometry of a shock wave radiated by a trumpet bell

The Schlieren method has been used before to visualize weak shock waves radiated from the open ends of brass instruments, but no attempt has previously been undertaken, however, to measure the geometry of the radiated wavefronts using the Schlieren images. In this paper Schlieren visualization is used to estimate the geometry of the two-dimensional shock wavefronts radiated from the bell of a trumpet at different frequencies. It is observed that the geometry of the shocks does change with frequency, in the expected manner. The propagation speeds of these shocks are also calculated, and they too exhibit the anticipated behavior.

Evaluation of periodgrams as a tool to study spectral densities of heterodyne signals

The use of periodgrams as a tool to study heterodyne signals is evaluated. The heterodyne signal carries information about density fluctuations from light scattered by a turbulent jet. The goal of the analysis is to be able to separate the information about density fluctuations of acoustics origin (peaked at about 2 MHz) from fluctuations in the density due to entropy irregularities (wide peak centered at the origin). There are frequency regions where both overlap. Various methods are compared: Bartlett, Welch, Yule–Walker, Burg and others. (To be presented in Spanish.)

Statistical analysis and measurement of thermodynamical properties of a supersonic air jet

In this work, we present a new method for measuring the pressure and the temperature of a supersonic airflow. These quantities are necessary to evaluate mathematical models applied to the study of shock waves. By means of a microphone and a thermocouple we measure these quantities through perturbations produced inside the flow by shock waves. We applied Fourier transform and several statistic methods in order to analyze and evaluate the signals obtained.

Rayleigh scattering and heterodyne detection as a nonintrusive microphone

Heterodyne detection of a monochromatic laser beam scattered by molecules in a transparent gas gives a signal that is proportional to the spatial Fourier transform of density fluctuations for a wave vector determined by the optics. In a turbulent air jet, density fluctuations are due to either acoustic waves or to entropy fluctuations. This technique captures both. The method can then be used as a nonintrusive microphone to study the propagation of acoustic waves inside the jet. At each point in the flow the direction of propagation of the acoustic wave is determined, and the acoustic field can be described. This will eventually help to localize the sources of the waves in the flow and hopefully determine the hydrodynamic event that generates these waves.